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Ministry of Energy Mines and Responsible for Core Review

Q - Gems and Semi-Precious Stones

(Example Deposits)

BC Profile # Deposit Type Approximate Synonyms USGS Model #
Q01 Jade - - - -
Q02 Rhodonite - - - -
 Q03* Agate - - - -
 Q04* Amethyst - - - -
 Q05* Jasper - - - -
Q06 Columbia-type emerald - - 31c
Q07 Schist-hosted emerald Exometamorphic emerald deposit - -
Q08 Sedimentary rock-hosted opal Australian-type opal - -
Q09 Gem corundum in contact zones - - - -
Q10 Gem corundum hosted by alkalic rocks - - - -
Q11 Precious opal in volcanic sequences

Volcanic opal

- -

 

JADE (Nephrite)

Q01
by Z.D. Hora
Retired, British Columbia Geological Survey, Victoria, B.C., Canada

IDENTIFICATION

 

SYNONYMS: Nephrite, nephrite jade, jadestone, greenstone (in New Zealand).

 

COMMODITY: Jade

 

EXAMPLES (British Columbia - Canadian/International):  Jade King (092HSW097), Birkenhead (092JNE063), Marshall Creek (092JNE064), Noel Creek (092JNE118), O`Ne-ell Creek (093K 005),  Mt. Sidney Willians (093K 043), Mt.Ogden (093N157,165), Wheaton Creek (104I082,085,104), Provencher Lake (104I064,065,066,078,111), Cassiar (104P005) - Yukon, Alaska, Wyoming, California  China, Taiwan, New Zealand, Australia, Siberia, Transvaal.

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Nephrite is an exceptionally tough, bright green to nearly black massive aggregate of fine grained fibrous amphibole – tremolite or actinolite. Nephrite jade occurs as lenticular bodies. They are associated with serpentinites that are intrusive into or in fault contact with suites of greenstones, chert, pelite and limestone.

 

TECTONIC SETTING: Alpine type serpentinites, ophiolite complex preceding a formation of island arc. In British Columbia the significant occurrences are found in Bridge River, Southern Cache Creek, Slide Mountain, and Northern Cache Creek terranes.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: The initial geological setting requires the formation of ultramafic rocks that form in proximity of ribbon chert and argillites or are structurally emplaced next to these types of sediments. The ultramafic rocks are frequently associated with regional fault systems where serpentinites are in contact with cherts and siliceous sediment or siliceous intrusive rocks. Subsequent geological processes, in particular serpentinization, is the main factor in forming nephrite jade.

 

AGE OF MINERALIZATION:  Globally, nephrite occurrences are hosted by Precambrian serpentinites in Zimbabwe to Tertiary ultramafic rocks in New Zealand. In B.C. jade is hosted by mid-Pennsylvanian oceanic crustal rocks and Permo-Triassic sediments. Deformation with resulting greenschist metamorphism took place during Middle to Late Jurassic time and was followed by further deformation and dextral strike slip in Late Cretaceous to Early Tertiary. It is thought that the jade formation occurred during the later stages of deformation. At least on some sites, nephrite is contemporary with origin of rodingite.

 

HOST/ASSOCIATED ROCKS: Serpentinized ultramafic rocks (harzburgite, dunite, pyroxenite) in contact with argillite and cherty sediments. Associated rocks are rodingite and similar heterogeneous calc-silicate rocks (in BC frequently called “whiterock”).

 

DEPOSIT FORM: Lenticular nephrite bodies typically less than 10 metres wide, 100 metres long at surface occur near contact of major serpentinite mass or in smaller satellite serpentinite slices.  In general parallel to the contact with sediments or granitic intrusives and along shear zones.

 

TEXTURE/STRUCTURE: Massive, very fine grained rock that is light to dark green or almost black in colour. Under the microscope nephrite is characterized by microfibrous tremolite that occurs as interlocking, twisted and felted bundles, tufts and sheaf-like aggregates. Accessory minerals, like magnetite, picotite or garnet, are frequently present in the otherwise fine grained amphibole matrix.

 

ORE MINERALOGY: Actinolite – tremolite; chromite, magnetite, picotite,uvarovite (chromium garnet).

 

GANGUE MINERALOGY [Principal and subordinate]: Rodingite – a heterogeneous mixture of zoisite/clinozoisite, serpentine, actinolite/tremolite, wollastonite, prehnite and garnet)  / talc, chlorite, sphene, magnetite.

 

ALTERATION MINERALOGY: Nephrite jade is an alteration product resulting from a metasomatic reaction between serpentinite and a source of silica, usually cherty metasediments.

 

WEATHERING: Jade is resistant to weathering in contrast to the common host rocks, serpentine and metasediments, and frequently forms boulder trains along its outcrops. Jade may be a common component in alluvial and colluvial deposits.

 

ORE CONTROLS: Serpentinization of ultramafic rocks in contact with high silica and calcium rocks to produce the formation of tremolite. Key element is large volumes of fluids moving through the serpentinized ultramafics capable to produce the metasomatic reaction with chemically right environment.

 

GENETIC MODELS: The studies on serpentinites indicate, that nephrite is a result of the desilication and calcium metasomatism produced in contact of serpentinites with sedimentary rocks under close to the blueschist metamorphic conditions.The reaction zone develop at a time when larger masses of ultramafic mantle are tectonically emplaced into the base of the crust or moved tectonically higher into it. This may produce a metasomatic reaction resulting in nephrite lenses surrounded by tremolite/chlorite alteration zone (usually called "whiterock"). Ultramafic rock must be below olivine stability field for the reaction to occur. Monomineralic and fine grained jade is stable in a variety of thermodynamic conditions. Evidence is pointing to conclusion, that nephrite jade forms during highly dynamic tectonic activity with fast changing local conditions in temperature, pressure and circulating fluids. Formation of nephrite is considered to occur at temperatures between 500ºC and 290ºC with pressure between 4 and 8 Kb where the water pressure was nearly equal to the total pressure.

 

ASSOCIATED DEPOSIT TYPES: Ultramafic hosted talk-magnesite (M07), ultramafic hosted asbestos (M06).  The Cassiar asbestos deposit produced jade periodically as a by-product.

 

COMMENTS:  There are two types of jade - nephrite and jadeite. While nephrite jade is mineralogically a variety of amphibole, jadeite is a variety of pyroxene. All known Canadian jade occurrences are the nephrite variety.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Not normally used.
 

GEOPHYSICAL SIGNATURE:  Magnetic highs can be used to identify ultramafic and serpentinite bodies.

 

OTHER EXPLORATION GUIDES: Prospecting for boulder trains and boulders in creeks. The boulders typically have a smooth surface and need to be broken or drilled to see the jade quality. Presence of “whiterock” or rodingite in serpentine outcrop.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Grades depend on colour, impurities, and presence of fractures. The highest quality jade has a uniform bright green translucent colour (emerald) with little or no impurities and limited fractures.  Also the stone`s fabric, i.e. orientation of individual grains is important. In situ deposits range in size from a few tonnes to more than 4000 tonnes, although not all of the deposit will be commercial jade. Jade cobbles and boulders are also exploited and can be up to tens of tones in weight.

 

ECONOMIC LIMITATIONS: Top grade jade (without flaws and of good colour) is routinely flown out to the nearest good road, otherwise road access is required. Non-standard colours can come into vogue at times.

 

END USES: Semiprecious stone used for carving, tiles and ornamental applications. In primitive cultures, jade has been used for weapons, tools and gemstones. Some cultures particularly value jade as a precious stone like Chinese, Maori in New Zealand or Maya Indians in Mexico and Guatemala.

 

IMPORTANCE:  Annual exports of B.C. jade are approximately 100 to 200 tonnes. In 1991, a 32 tonne jade boulder was sold for $CDN 350,000 to be carved into a Buddha statue in Thailand. In 1996 the best quality raw jade sold for $100 per kilogram. Most of the jade produced in British Columbia is regularly exported to Far East countries, some to New Zealand. BC is leading world exporter of nephrite jade.

 

REFERENCES

 

 

Coleman, R. (1967): Low-Temperature Reaction Zones and Alpine Ultramafic Rocks of California, Oregon and Washington. U.S.Geological Survey, Bulletin 1247, 49 pages.

Fraser, M. (2000): Nephrite Jade; Western Canadian Gemstone Newsletter, Winter 2000 Edition, Volume 2, Number 1, 5 pages.

Gunning, D.F. (1995): Exploring British Columbia`s Stone Industry; Stone World, Volume 12, Number  10, pages 40-50.

Holland, S.S. (1961): Jade in British Columbia; British Columbia Minister of Energy, Mines and Petroleum Resources, Annual Report for the Year 1961, pages 119-126.

Keverne, R., Editor (1995): Jade; Lorenz Books, London, UK, 376 pages.

Leaming, S.F. (1984): Jade in British Columbia and Yukon Territory, in Guillet, G.R. and Martin, W., The Geology of Industrial Minerals in Canada, The Canadian Institute of Mining and Metallurgy, Special Volume 29, pages 270-273.

Leaming, S.F. (1978): Jade in Canada, Geological Survey of Canada, Paper 78-19, 59 pages.

Makepeace, K. and Simandl, G.J. (2004): Jade (Nephrite) in British Columbia, Canada;in G.J.Simandl, W.J.McMillan and N.D.Robinson, Editors, 37th Annual Forum on Industrial Minerals Proceedings, Industrial Minerals

with emphasis on Western North America,British Columbia Ministry of Energy and Mines, Geological Survey Branch, Paper 2004-2, pages 287-288.

Scott, A. (1996): Jade. The Mystical Mineral; Equinox, Number 89, September/October 1986, pages 63-69.

Simandl, G.J., Riveros, C.P. and Schiarizza, P. (2000): Nephrite (Jade) Deposits, Mount Ogden Area, British Columbia (NTS 093N 13W); British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 2000-1, pages 339-347.

Simandl, G.J., Paradis, S. and Nelson, J. (2001): Jade and Rhodonite Deposits, British Columbia, Canada; in Proceedings of the 34th Forum on Industrial Minerals, Salt lake City (1998); Utah Geological

 

Survey Miscellaneous Publication 01-2, pages 163 – 171.

Thompson, B., Brathwaite, B. and Christie, T. (1995): Mineral Wealth of New Zealand; Institute of Geological and Nuclear Scienes Limited, Information Series 33, 170 pages.

Ward, F. (1987): Jade. Stone of Heaven; National Geographic, Volume 172, Number 3, pages 282-315.

 

 

RHODONITE

Q02
by Z.D. Hora
Retired, British Columbia Geological Survey, Victoria, B.C., Canada

IDENTIFICATION

 

SYNONYMS:  Manganese spar, manganolite (pyroxmangite)

 

COMMODITIES/BY-PRODUCTS:  Rhodonite / jasper

 

EXAMPLES (British Columbia - Canadian/International):  Hill 60 (092B 027), Hollings (092B 074), Rocky (092C 113), Arthur Point (092M015), Clearcut (082ESE241), OrofinoMtn (082ESW009), Olalla (082ESW017), Ashnola Pinky (082ESW208), Joseph Creek (092P148), Snowy Creek (104P067), / Evelyn Creek (Yukon).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION:  Lenses of bedded or massive rhodonite hosted by laminated cherts and cherty tuffs at the base of turbiditic sandstone, siltstone and argillite sequences, usually in proximity to mafic volcanics or greywackes. Red jasper beds with laminated hematite, pyrite and magnetite are sometimes associated with deposits of rhodonite.

 

TECTONIC SETTING:  Island arcs, back arc basins, sea floor spreading areas. Epicratonic or continental margin marine basins associated with oceanic faults or rifts.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:  Deep to shallow marine basins. The widespread soft-sediment structures present in the host rock and within the rhodonite bodies suggest deposition of an unstable, gel-like sediment. Submarine hydrothermal activity in mafic environment, abundant silica to feed radiolarian population in the area. Similar modern environments have been described from the Galapagos, southwest Pacific island arc, Gulf of Aden and Mid-Atlantic Ridge.

 

AGE OF MINERALIZATION:  In British Columbia described as mostly Mississippian, Carboniferous or Permian in age, also Upper Triassic.  Recently found in a Lower Cretaceous unit.  Globally, this type probably may occur in oceanic units of any age.

 

HOST/ASSOCIATED ROCKS:  Ribbon cherts, laminated cherts and cherty tuffs with argillite interbeds; pillow basalts, mafic tuffaceous sediments and greywacke turbidites in the lower part of sequence; sandstone, argillite, calcarenite and limestone in the upper part. Rhodonite is also found in the metamorphosed equivalents of these rocks; typically prehnite-pumpellyite to greenschist grade.

 

DEPOSIT FORM: Stratiform lenses of rhodonite that are metres thick and intermittently extend laterally some hundreds of metres. Frequently discontinuous and pinching out over tens of metres or less.  Thin rhodonite bands less than 10 centimetres across often pinch out over several metres. Thicker rhodonite layers can be followed for distances of more than 50 metres.

 

TEXTURE/STRUCTURE: The texture of rhodonite varies from laminated to patchy.  Massive rose-pink layers alternate with darker bands and minor yellow and grey chert.  Soft sediment deformation features are frequent, particularly along the hanging wall of major rhodonite lenses.  Embedded fragments of roof sediment in chert rhodonite and small "ghost" radiolarians infilled by microcrystalline quartz are common.  Crosscutting and bedding parallel rhodonite veinlets up to a few millimetres thick are typical.  Some rhodonite lenses adjacent to footwall contain layers and round shaped metallic, black nodules of braunite (Mn,Si)2O3 from 5 to 10 millimetres in diameter that are overgrown with

 

concentric pink rhodonite rims. The predominant minerals in massive rhodonite-rich zones are usually microcrystalline quartz interbedded with disseminated, mottled and banded rhodochrosite and rhodonite, interlayered with hematite and garnet-rich bands. Manganese oxide is invariably present lining hairline fractures and as surface coatings. Rhodonite occurs as intergrowths with massive or crystalline rhodochrosite as euhedral elongate tabulate crystals, stellated crystal masses or sheath-like bundles enclosed within and encroaching upon a microcrystalline quartz matrix.  Rhodonite also forms spongy porphyroblasts up to 500 microns in length.  Some rhodonite exhibits mammillary growth textures.  Rhodochrosite ranges from a massive carbonate to a mixture of with microcrystalline quartz, rhodonite or disseminated hematite.  At higher metamorphic grades the rhodonite exhibits more granoblastic to porphyroblastic texture.

 

ORE MINERALOGY [Principal and subordinate]:  Rhodonite, rhodochrosite, pyroxmangite / microcrystalline silica, barite, spessartine garnet, bustamite, tephroite, palenzonaite, penninite, clinochlore, stilpnomelane, adularia.

 

GANGUE MINERALOGY [Principal and subordinate]: Jasper, chert and manganese oxide / argillite interbeds.

 

ALTERATION MINERALOGY: Effects of metamorphism:  amphibole, graphite, phlogopite, garnet, plagioclase in higher metamorphic grades; garnet - tremolite – adularia – prehnite - epidote quartz association in lower metamorphic grades.

 

 

WEATHERING: Widespread development of black oxidized manganese secondary products as veinlets, on fractures and coating surfaces.

 

ORE CONTROLS:  The primary control on rhodonite deposits are a favourable stratigraphic package of deep water marine sedimentary and volcanic rocks with a significant sequence of cherts containing manganese-rich layers.  Secondary controls are synsedimentary and later deformation that can create thicker or thinner lenses and later processes, such as cross-cutting veinlets, that reduce the value of the rhodonite or rhodochrosite.

 

GENETIC MODELS:   Manganese is deposited as a distal member of volcanogenic products from hydrothermal solutions generated in rifting environment where the thermal fluids percolated through sea-floor basalts and associated rocks of similar chemical composition. These solutions would be hot, slightly acid, strongly reducing, and enriched in Mn, Fe, Si, Ba, Ca, K, Li, Rb and trace metals. Separation of Fe from Mn may occur within the seafloor at depth with the formation of sulphides like pyrite and on the seafloor when the hydrothermal solution mixed with cold, alkaline and oxygenated seawater and precipitated chemical sediments. Nucleation and precipitation kinetics of Mn and Si oxides are sluggish; therefore, they are often distributed distally from the hydrothermal vent. At lower temperatures the manganese precipitates as amorphous oxyhydroxides and Mn4+ oxides, moderately

 

higher temperatures could promote syndepositional Mn silicate and carbonate. The hydrothermal sediments can form mounds and retain level of semi-plasticity for extended time which can lead to synsedimenatary deformation and slumping. Alternatively, Mn silicate and carbonate may form during subsequent diagenetic or metamorphic episodes.

 

ASSOCIATED DEPOSIT TYPES:  Red jasper with taconite iron mineralization (G01), all manganese oxide deposits associated with volcanogenic origin (G02) and volcanogenic massive sulphides (G04, G05, G06).

 

COMMENTS:  The name "Rhodonite" is rather a misnomer.  The rock is a mixture of pink manganese silicates and a carbonate with fine grained silica.  There are many manganese deposits described from all parts of world that are considered of volcanogenic origin. Deposits of rhodonite outside of the Canadian Cordillera are rather uncommon. One possible explanation is that glaciation may have exposed primary deposits which could be deeply weathered with a thick zone of manganese oxides in non-glaciated areas.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Anomalous in manganese, enriched in base metals, barium and strontium.

 

GEOPHYSICAL SIGNATURE:  Not used, probably reflecting the small size of the orebodies and similarities to the associated chert.

 

OTHER EXPLORATION GUIDES:  Oceanic terrains, chert beds, particularly with red jasper, and black staining.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Lenses from up to 3 metres thick and 15 metres long on surface.  In most economic deposits, rhodonite and rhodochrosite account for at least 45% of the lens.   


However, rock hounds often contribute some supply from lenses with lower contents of the semi-precious stone. Two properties in Western Canada were active producers for 20 years – one in Yukon and the other on the British Columbia coast. After the initial major production period, both have been largely inactive for more than 10 years.

 

ECONOMIC LIMITATIONS:  Physical/chemical properties affect the end use. Brighter colours due to rhodochrosite and attractive dendrite black veining increase aesthetic value. Dense fracturing decreases the value and uncemented microfractures restrict carving of larger statues. The relatively high unit value for rough rhodonite allows for processing far away from the original source, frequently overseas.

 

 

END USES:  Semiprecious stone used in lapidary, jewellery and carvings.

 

IMPORTANCE:  The North American market is very small - only a few tonnes annually. Semi-precious stone used to produce jewellery and carvings. The industry considers rhodonite as a potential substitute for pink coral when environmental considerations prevent its production.

 

 

REFERENCES

 

Cowley, P. (1979): Correlation of Rhodonite Deposits on Vancouver Island and Saltspring Island, British Columbia, Bachelor thesis, Department of Geological Sciences, The University of British Columbia, 54 pages.

Crerar, D.A., Namson,J., Chyi, M.S., Williams, L. and Feigenson, M.D. (1982): Manganiferous Cherts of the Franciscan Assemblage: I. General Geology, Ancient and Modern Analogues, and Implications for Hydrothermal Convection at Oceanic Spreading Centers, Economic Geology, volume 77, Number 3, pages 519-540.

Danner, W.R. (1976): Gem Materials of British Columbia, in Montana Bureau of Mines and Geology, Proceedings Volume, Eleventh Forum on the Geology of Industrial Minerals, Special Publication 74, pages 156-169.

Flohr, M.J.K. (1992): Geochemistry and Origin of the Bald Knob Manganese Deposit, North Carolina, Economic Geology, volume 87, pages 2023-2040.

Flohr, M.J. and Huebner, J.S., (1990): Microbanded Manganese Formations: Protoliths in the Franciscan Complex, California, U.S. Geological Survey, Professional Paper 1502, 72 pages.

Glasby, G.P. (1977): Marine Manganese Deposits, Elsevier Scientific Publishing Company, Amsterdam, 465 pages.

Hancock, K.H. (1992): Arthur Point (Sea Rose) Rhodonite, Ministry of Energy, Mines and Petroleum Resources, Exploration in British Columbia 1991, pages 89-98.

Hora, Z.D. (1988): Industrial Minerals in Island Arcs, in Metallogeny of Volcanic Arcs, BC Ministry of Energy, Mines and Petroleum Resources, Short Course Notes, Open File 1998-5, section B, pages L1-L41

Hora, Z.D., Langrova, A. and Pivec, E.(2005): Contribution to the Mineralogy of the Arthur Point Rhodonite Deposit, Southwestern British Columbia; in Geological Fieldwork 2004, BC Ministry of Energy, Mines and Petroleum Resources, Paper 2005 – 1, pages 177-185.

Hora, Z.D., Langrova, A. and Pivec, E. (2007): Rhodonite from the Bridge River Assemblage, Downton Creek (NTS 092J/09), Southwestern British Columbia; in Geological Fieldwork 2006, BC Ministry of Energy, Mines and Petroleum Resources, Paper 2007-1, pages 39 – 43.

Leaming, S.F. (1966): Rhodonite in British Columbia, The Canadian Rockhound, pages 5-11.

Massey, N.W.D. (1992): Geology and Mineral Resources of the Duncan Sheet, Vancouver Island 92B/13, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1992-4, 112 pages.

Nelson, J.A., Hora, Z.D., and Harvey-Kelly, F. ( 1990): A New Rhodonite Occurrence in the Cassiar Area, Northern British Columbia (104P/5), B.C. Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork 1989, Paper 1990-1, pages 347-350.

Sargent, H. (1956): Manganese Occurrences in British Columbia, in J.G.Reyna, Editor, Symposium Sobre Yacimientos de Manganeso, XX Congreso Geologico Internacional, Mexico, Tomo III., pages 15-34.

Simandl, G.J. and Church, B.N. (1996): Clearcut Pyroxmangite/Rhodonite Occurrence, Greenwood Area, Southern British Columbia (82/E2), in Geological Fieldwork 1995, BC Ministry of Energy, Mines and Petroleum Resources, Paper 1996-1, pages 219-222.

Simandl, G.J., Hancock, K.D., Nelson, J.A., and Paradise, S. (1997): Rhodonite Deposits in British Columbia, Canada, Canadian Institute of Mining and Metallurgy, Annual Meeting, Paper CIM MPM2-F3, abstract.

Simandl, G.J., Paradis, S. and Nelson, J. (2001): Jade and Rhodonite Deposits, British Columbia, Canada; in Proceedings of the 34th Forum on Industrial Minerals, Salt Lake City (1998), Utah Geological

 

Survey, Miscellaneous Publication 01-2, pages 163 – 171.

Snyder, W.S. (1978): Manganese Deposited by Submarine Hot Springs in Chert-Greenstone Complex, Western United States, Geology, volume 6, pages 741-744.

Sorem, R.K. and Gunn, D.W. (1967): Mineralogy of Manganese Deposits, Olympic Peninsula, Washington, Economic Geology, volume 62, pages 22-56.

 

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COLOMBIA-TYPE EMERALDS


Q06
by G.J. Simandl1, S. Paradis2 and T. Birkett3

1 British Columbia. Geological Survey, Victoria, B.C., Canada
2 Geological Survey of Canada, Pacific Geoscience Centre, Sidney, B.C., Canada
3 SOQUEM, Quebec City, Quebec, Canada

 

Simandl, G.J., Paradis, S. and Birkett, T. (1999): Colombia-type Emeralds; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines, Open File 1999-10.

 

IDENTIFICATION

 

SYNONYMS: Emerald veins, Muzo and Chivor-type emerald deposits.

 

COMMODITY: Emeralds (pale-green and colorless beryl gemstones).

 

EXAMPLES (British Columbia - Canadian/International): No Colombia-type emerald deposits are known in British Columbia. Chivor, La Mina Glorieta, Las Cruces, El Diamante, El Toro, La Vega de San Juan, Coscuez and Muzo (Colombia).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION:  Colombia-type emerald deposits consist mainly of carbonate-pyrite-albite quartz veins forming "en échellon" or conjugate arrays and cementing breccias. So called "stratiform tectonic breccias" may also contain emeralds. Emeralds are disseminated in the veins as clusters, single crystals or crystal fragments; however, the best gemstones are found in cavities. Country rocks are black carbonaceous and calcareous shales.

 

TECTONIC SETTING: Probably back arc basins (shales deposited in epicontinental marine anoxic environments spatially related to evaporites) subjected to a compressional tectonic environment.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: The deposits are controlled by deep, regional decollements, reverse or thrust faults; hydraulic fracture zones, intersections of faults and by permeable arenite beds interbedded with impermeable black shales.

 

AGE OF MINERALIZATION:  Colombian deposits are hosted by Cretaceous shales. Ar/Ar laser microprobe studies of Cr-V-K-rich mica, believed to be penecontemporaneous with the emerald mineralization, indicate 32 to 38 Ma for Muzo area and 65 Ma for Chivor district. It is not recommended to use these age criteria to constrain the exploration programs outside of Columbia.

 

HOST/ASSOCIATED ROCKS:  Emerald-bearing veins and breccias are hosted mainly by black pyritiferous shale, black carbonaceous shale and slate. Claystone, siltstone, sandstone, limestone, dolomite, conglomerate and evaporites are also associated. Two special lithologies described in close association with the deposits are albitite (metasomatized black shale horizons) and tectonic breccias ("cenicero"). The latter consist of black shale and albitite fragments in a matrix of albite, pyrite and crushed black shale.

 

DEPOSIT FORM: The metasomatically altered tectonic blocks may be up to 300 metres in width and 50 km in length (Beus, 1979), while individual productive zones are from 1 to 30 metres in thickness. Emeralds are found in en échelon and conjugate veins that are commonly less than 10 centimetres thick, in hydraulic breccia zones and in some cases in cenicero.

 

TEXTURE/STRUCTURE:  Emeralds are found disseminated in veins as clusters, single crystals or crystal fragments, however, the best gemstones are found in cavities. Quartz is cryptocrystalline or forms well developed hexagonal prisms, while calcite is fibrous or rhombohedral. In some cases, emerald may be found in black shale adjacent to the veinlets or cenicero.

 

ORE MINERALOGY: Emerald; beryl specimens and common beryl.

 

GANGUE MINERALOGY [Principal and subordinate]: Two vein stages are present and may be superimposed, forming composite veins. A barren stage 1 consisting mainly of fibrous calcite and pyrite and a productive second stage with associated rhombohedral calcite and dolomite, albite or oligoclase, pyrite, ± quartz and minor ± muscovite, ± parisite, ± fluorite, ± barite, ± apatite, ± aragonite, ± limonite and anthracite/graphite-like material. Some pyrite veins also contain emeralds. Cavities within calcite-rich veins contain best emerald mineralization.  Solid inclusions within emerald crystals are reported to be black shale, anthracite/graphite-like material, calcite, dolomite or magnesite (?), barite, pyrite, quartz, albite, goethite and parisite.

 

ALTERATION MINERALOGY: Albitization, carbonatization, development of allophane by alteration of albite, pervasive pyritization and development of pyrophyllite at contacts between veins and host rocks has also been reported.

 

WEATHERING:  In Columbia the intense weathering and related alteration by meteoric water of stratiform breccias and albitites are believed to be responsible for the formation of native sulfur, kaolinite and gypsum. Albite in places altered to allophane.

 

ORE CONTROLS: Deep, regional fault systems (reverse or thrust); intersections of faults; breccia zones; permeable arenites interbedded with impermeable shales.

 

GENETIC MODELS:  The hypotheses explaining the origin of these deposits are fast evolving. The most recent studies favor a moderate temperature, hydrothermal-sedimentary model. Compressional tectonics result in formation of decollements that are infiltrated by alkaline fluids, resulting in albitization and carbonatization of shale and mobilization of Be, Al, Si, Cr, V and REE. The alkaline fluids are believed to be derived from the evaporitic layers or salt diapirs. As the regional compression continues, disharmonic folding results in the formation of fluid traps and hydrofracturing. A subsequent decrease in fluid alkalinity or pressure could be the main factor responsible for emerald precipitation. Organic matter is believed to have played the key role in emerald precipitation (Cheilletz and Giuliani, 1996, Ottaway et al., 1994).

 

ASSOCIATED DEPOSIT TYPES: Spatially associated with disseminated or fracture-related Cu, Pb, Zn, Fe deposits of unknown origin and barite and gypsum (F02) deposits.

 

COMMENTS: Colombia-type emerald deposits differ from the classical schist-hosted emerald deposits (Q07) in many ways. They are not spatially related to known granite intrusions or pegmatites, they are not hosted by mafic/ultramafic rocks, and are emplaced in non-metamorphosed rocks. Green beryls, where vanadium is the source of colour, are described at Eidsvoll deposit (Norway) where pegmatite cuts bituminous schists. Such deposits may be better classified as pegmatite-hosted.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Black shales within the tectonic blocks are depleted in REE, Li, Mo, Ba, Zn, V and Cr. The albitized zones contain total REE<40 ppm while unaltered shales have total REE values of 190 ppm. Stream sediments associated with altered shales have low K/Na ratio. Soils overlying the deposits may have also low K/Na ratio.

 

GEOPHYSICAL SIGNATURE:  Geophysics may be successfully used to localize major faults where outcrops are lacking. The berylometer, has applications in ground exploration.

 

OTHER EXPLORATION GUIDES: Regional indicators are presence of beryl showings, available sources of Cr and Be and structural controls (decollement, reverse faults, fault intersections). In favourable areas, exploration guides are bleached zones, albitization and pyritization. White metasomatic layers within black shale described as albitites, and stratiform polygenetic breccias consisting of black shale fragments cemented by pyrite, albite and shale flour are closely associated with the mineralization.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Distribution of emeralds within the mineralized zones is erratic; therefore, pre-production tonnage estimates are difficult to make. The official grade reported for Colombian deposits is approximately 1 carat/m3. All stones are valued according to size, intensity of the green colouration and flaws, if

 

present. Tonnages for individual deposits are unknown; however, Chivor reportedly produced over 500,000 carats between 1921 and 1957.

 

ECONOMIC LIMITATIONS: The earliest developments were by tunneling. To reduce mining costs benching, bulldozing and stripping of mountainsides were introduced. Recently, apparently to reduce environmental pressures, underground developments have been reintroduced at Muzo. Physical and chemical properties of high-quality synthetic emeralds match closely the properties of natural stones. There is currently uncertainty if synthetic emeralds can be distinguished from the high-quality, nearly inclusion-free natural specimens. Recent attempts to form an association of emerald producers may have a similar effect on emerald pricing as the Central Selling Organization has on diamond pricing.

 

END USES: Highly-valued gemstones.

 

IMPORTANCE: Currently, world production of natural emeralds is estimated at about $US 1 billion. In 1987 ECONOMINAS reported emerald production of 88,655,110 carats worth US$ 62,910,493. Colombia is the largest producer of natural emeralds by value; most of the gemstones come from the Muzo and Chivor districts. The other major producing countries are Brazil, Zambia, Zimbabwe, Pakistan, Afghanistan, Russia and Madagascar which have schist-hosted emerald deposits (Q07). Brazil is the world’s largest producer of emeralds by weight.

 

REFERENCES

 

Beus, A.A. (1979): Sodium: A Geochemical Indicator of Emerald Mineralization in the Cordillera Oriental, Colombia; Journal of Geochemical Exploration, Volume II, pages 195-208.

Cheilletz, A. and Giuliani, G. (1996): The Genesis of Colombian Emeralds: a Restatement; Mineralium Deposita, Volume 31, pages 359-364.

Cox, D.P. (1986): Descriptive Model of Emerald Veins; in Mineral Deposit Models, D.P. Cox and D. Singer, Editors, United States Geological Survey, Bulletin 1693, page 219.

Escovar, R. (1975): Geologia y Geoquimica de las Minas de Esmeraldas de Gachalà, Cundinamaraca; Boletin Geologico, Volume 22(3), pages 116-153.

Giuliani, G., Rodriguez, C.T. and Rueda, F. (1990): Les Gisements d’émeraude de la Cordillère Orientale de la Colombie: Nouvelles Données Métallogéniques; Mineralium Deposita, Volume 25, pages 105-111.

Giuliani, G., Cheilletz, A., Arboleda, C., Carrillo, V., Rueda, F. and Baker, J. (1995a): An Evaporitic Origin or the Parent Brines of Colombian Emeralds: Fluid Inclusion and Sulfur Isotope Evidence; European Journal of Mineralogy, Volume 7, pages 151-165.

Kazmi, A.H., and Snee., L.W. (1989): Emeralds of Pakistan. Geology, Gemology and Genesis, in: Kazmi, A.H., and Snee, L.W., Editors; Geological Survey of Pakistan. Van Nostrand Company, New York, 269 pages.

Koslowski, A., Metz, P. and Jaramillo, H.A.E. (1988): Emeralds from Somondoco, Colombia: Chemical Composition, Fluid Inclusion and Origin; Neues Jarhrbuch für Mineralogie Abhandlungen, Volume 159, pages 23-49.

Oppenheim, V. (1948): The Muzo Emerald Zone, Colombia, S. A; Economic Geology, Volume 43, pages 31-38.

Ottaway, T.L., Wicks, F.J., Bryndzia, L.T., Kyser, T.K. and Spooner, E.T.C. (1994): Formation of the Muzo Hydrothermal Emerald Deposit in Colombia; Nature, Volume 369, pages 552-554.

Sinkankas, J. and Read, P. (1986): Beryl; Butterworths Gem Books, USA, 225 pages.

Van Landgham, S.L. (1984): Geology of World Gem Deposits; Van Nostrand Reinhold Co., Publishers, USA. 406 pages.

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SCHIST-HOSTED EMERALDS


Q07
by G.J. Simandl1, S. Paradis2 and T. Birkett3

1 British Columbia. Geological Survey, Victoria, B.C., Canada
2 Geological Survey of Canada, Pacific Geoscience Centre, Sidney, B.C., Canada
3 SOQUEM, Quebec City, Quebec, Canada

 

Simandl, G.J., Paradis, S. and Birkett, T. (1999): Schist-hosted Emeralds; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines, Open File 1999-10.

 

IDENTIFICATION

 

SYNONYMS: Emerald deposits commonly described as "suture zone-related", "pegmatite-related schist-hosted" or "exometamorphic", "exometasomatic", "biotite schist-type", "desilicated pegmatite related" and "glimerite-hosted" are covered by this model.

 

COMMODITIES (BYPRODUCTS): Emerald (industrial grade beryl, other gemstones, such as aquamarine, chrysoberyl, phenakite, tourmaline).

 

EXAMPLES (British Columbia - Canadian/International): Socoto and Carnaiba deposits (Brazil), Habachtal (Austria), Perwomaisky, Mariinsky, Aulsky, Krupsky, Chitny and Tsheremshansky deposits (Russia), Franqueira (Spain), Gravelotte mine (South Africa), Mingora Mines (Pakistan).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Emerald deposits principally related to mafic and ultramafic schists or unmetamorphosed ultramafic rocks in contact with felsic rocks, either pegmatoid dykes, granitic rocks, paragneisses or orthogneisses. Such contacts may be either intrusive or tectonic.

 

TECTONIC SETTING: Found in cratonic areas as well as in mobile belts. In many cases related to major Phanerozoic or Proterozoic suture zones that may involve island arc-continent or continent-continent collision zones. The lithological assemblages related to suture zones commonly form a "tectonic mélange" and in some areas are described as "ophiolitic melange".

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Mainly in greenstone belts, but also in other areas where Cr-bearing rocks may be adjacent to pegmatites, aplites, granites and other felsic rocks rich in beryllium.

Metamorphic grade is variable; however, it typically reaches green schist to amphibolite facies.

 

AGE OF MINERALIZATION: The deposits are hosted by Archean age rocks or younger. The age of mineralization is typically linked to either a period of tectonic activity or a time of pegmatoid emplacement.

 

HOST/ASSOCIATED ROCKS: Biotite schists ("biotites", "phlogopitites" and "glimerites") are a particularly favourable host. Other favourable hosts are metamorphosed mafic volcanic rocks, such as epidote-chlorite-actinolite-bearing rock, chlorite and chlorite-talc schists, talc and talc-carbonate schists, white mica schists, mafic schists and gneisses and amphibolites. Less commonly emeralds occur in unmetamorphosed mafic or ultramafic rocks and possibly listwaenites. Pegmatites or quartz veins in the contact zone between granitic rocks and mafic rocks may in some cases host emeralds. A wide variety of rocks can be associated with schist-hosted emerald deposits, including granite, syenite, tonalite, granodiorite, a variety of orthogneisses, marbles, black phyllites, white mica schists, mylonites, cataclasites and other metasedimentary rocks.

 

DEPOSIT FORM: Most of the mineralization is hosted by tabular or lenticular mafic schists or "blackwall zones". Favourable zones are a few metres to tens of metres wide and follow the contacts between felsic and mafic/ultramafic lithologies for distances of tens to hundreds of metres, but economically minable portions are typically much smaller. For example, minable bodies in the Urals average 1 metre in thickness and 25 to 50 metres in length. Pegmatoids, where present, may form horizontal to steeply dipping pods, lens-shaped or tabular bodies or anastomosing dykes which may be zoned.

 

TEXTURE/STRUCTURE: In blackwall or schists lepidoblastic texture predominates. The individual, discrete emerald-bearing mafic layers within the favourable zones may be complexly folded, especially where the mineralization is not spatially associated with pegmatites. Emeralds are commonly zoned. They may form porphyroblasts, with sigmoidal orientation of the inclusion trails; beryl may form the rims separating phenakite form the surrounding biotite schist; or emerald crystals may be embedded in quartz lenses within the biotite schist. Chrysoberyl may appear as subhedral porphyroblasts or skeletal intergrowths with emerald, phenakite or apatite.  Where disseminated beryl crystals also occur within pegmatites, they are short, commonly fractured, prismatic to tabular with poor terminations; but may be up to 2 metres in length and 1 metre in cross section. Long, prismatic, unfractured crystals occur mainly in miarolitic cavities.

 

ORE MINERALOGY: Emerald and other beryls (in some cases aquamarine or morganite), ± chrysoberyl and industrial grade beryl. Spodumene gems (in some cases kunzite) may be found in related pegmatites.

 

GANGUE MINERALOGY [Principal and subordinate]: In the schist: biotite and/or phlogopite, talc, actinolite, plagioclase, serpentine, ± fuchsite, ± quartz, ± carbonates, ± chlorite, ± muscovite, ± pyrite, epidote, ± phenakite, ± milarite and other beryllium species, ± molybdenite, ± apatite, ± garnet, ± magnetite, ± ilmenite, ± chromite, ± tourmaline, ± cassiterite.  In the pegmatoids: feldspars (commonly albite), quartz, micas; ± topaz, ± phenakite , ± molybdenite, ± Sn and W-bearing minerals, ± bazzite, ± xenotime, ± allanite, ± monazite, ± phosphates, ± pollucite, ± columbite-tantalite, ± kyanite, zircon, ± beryllonite, ± milarite and other beryllium species. Emerald crystals may contain actinolite-tremolite, apatite, biotite, bityite, chlorite, chromite, columbite-tantalite, feldspar, epidote, fuchsite, garnet, hematite, phlogopite, pyrrhotite, rutile, talc, titanite and tourmaline inclusions.

 

ALTERATION MINERALOGY: Limonitization and pyritization are reported in the host rocks. Kaolinite, muscovite, chlorite, margarite, bavenite, phenakite, epidimyte, milarite, bityite, bertrandite, euclase are reported as alteration products of beryl.

 

WEATHERING: Weathering contributes to the economic viability of the deposits by softening the matrix, and concentrating the beryl crystals in the overlaying soil or regolith.

 

ORE CONTROLS:
1) The principal control is the juxtaposition of beryllium and chromium-bearing lithologies along deep suture zones. Emerald crystals are present mainly within the mafic schists and in some cases so called "blackwall zones" as described ultramafic-hosted talc deposits (M07). In this settings it may be associated with limonite zones.
2) This often occurs near the contacts of pegmatoids with mafic schists. Emerald crystals are present mainly within the mafic schists, although in some cases some of the mineralization may be hosted by pegmatoids.
3) Another prospective setting is along fracture-controlled glimmerite zones.
4) Mineralization may be concentrated along the planes of regional metamorphic foliation, especially in cores of the folds where the relatively high permeability favors chemical exchange and the development of synmetamorphic reaction zones between chromium and beryllium-bearing lithologies.
5) Serpentinite roof pendants in granites are prospective.

 

GENETIC MODELS: The origin of schist-hosted emerald deposits is controversial as is the case with many deposits hosted by metamorphic rocks. All emerald deposits require special geological conditions where chromium (± vanadium) and beryllium coexist. Where pegmatoids or plagioclase-rich lenses occur within ultramafic rocks, the crystalization of emeralds is commonly explained by interaction of pegmatites or pneumatolytic-hydrothermal, Be-bearing fluids with Cr-bearing mafic/ultramafic rocks. In other cases, emeralds in schists form by syn- or post-tectonic regional metamorphic chemical exchange (metasomatism) between felsic rocks, such as felsic gneisses, garnet mica schists or pre-metamorphic pegmatoids, with the adjacent Cr-bearing rocks such as schists, gneisses or serpentinites. Contacts between Cr- and Be-bearing source rocks may be tectonic, as is the case for "suture zone-related" deposits.

 

ASSOCIATED DEPOSIT TYPES: Feldspar-quartz and muscovite pegmatites (O03, O04). Mo and W mineralization may be associated with emeralds. Some porphyry W deposits (L07) have associated beryl. Tin-bearing granites are in some cases associated with emeralds. Gold was mined at Gravelotte Emerald Mines (no information about the gold mineralization is available).

 

COMMENTS: Recently, microprobe studies have shown that the green color of some beryls is due to vanadium rather than chrome. In most cases both Cr and V were detected in the beryl crystal structure. There are two schools of gemmologists, the first believes that strictly-speaking the vanadium-rich beryls are not emeralds. The second school believes that gem quality beryls should be named based on their physical, and more particularly, color properties. It is possible that pegmatoid-related or suture zone-related emerald deposits hosted by black shales or other chromium and/or vanadium-bearing rocks will be discovered. In those cases it will be difficult to decide if these deposits are schist-hosted or Columbia-type (Q06) emeralds.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: The presence of beryl in eluvial and alluvial deposits is good pathfinder. The distribution of beryllium in stream sediments proved to be useful in Norway when coupled with identification of the individual drainage basins and knowledge of the geological environment.

 

GEOPHYSICAL SIGNATURE:  A portable field detector that uses 124Sb as a gamma radiation source, the

 

berylometer, is used to detect Be in outcrop. The instrument should be held less than 4 cm from the sample. Radiometric surveys may be useful in detecting associated radioactive minerals where pegmatites are involved. Magnetic and electromagnetic surveys may be useful in tracing suture zones where ultramafic rocks and felsic rocks are faulted against each other.

 

OTHER EXPLORATION GUIDES: Any Be occurrences in a favorable geological setting should be considered as positive indicators. If green, chromium and/or vanadium-bearing beryls are the main subject of the search then ultramafic rocks, black shales or their metamorphic equivalents represent the most favorable host rocks. If exploration is focused on a variety of gem-quality beryls (not restricted to emerald), or if the targeted area is not mapped in detail, then Be occurrences without known spatial association with Cr- or V-bearing lithologies should be carefully considered. Minerals associated with emeralds in the ores may be considered as indirect indicators. A wide variety of field-tests based on fluorescence, alkalinity, staining, density and refractive index have been used in the past to distinguish beryl.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: The grade and tonnage of these deposits is difficult to estimate due to erratic emerald contents (gram/tonne), episodic nature of the mining activity which often results in high grading, and variability in the quality of gemstones (value/carat). For example, at the Mingora mines in Islamia Trench two, 15 to 30 centimetres thick layers of talc-rich rock surrounding quartz lenses contained 1000 to 5000 carats of good stones up to 30 carats in size. Some of the individual pits in the area produced less than 1000 carats. The cumulative production of the Mingora emerald mines was reported between 20 000 to over 50 000 carats/year between 1979 and 1988. At Gravelotte Emerald Mine, at least 23 000 kg of emeralds of varying grades have been produced since 1929 from several zones. For the same mine promotional literature states that " conservative estimates" of ore within the Cobra pit are 1.69 million tonnes that could result in production of 17 000 kg of emeralds ( approximately 1gram /tonne). It is estimated that about 30% of the emeralds could be sold, but only 2-3% of these are believed to be gem quality. In the Urals the Mariinsky deposit was explored to a average depth of 500 metres by boreholes and underground workings. To determine emerald content, bulk samples as large as 200 tonnes are taken systematically at 100 metres interval along the favourable zone. No grade and tonnage are available.

 

ECONOMIC LIMITATIONS: Mining of precious stones in underdeveloped countries and smaller deposits is done using pick and shovel with limited use of jackhammers and bulldozers. Larger schist-hosted emerald deposits, may be successfully exploited by a combination of surface and underground mining. The Mariinsky deposit was mined by open pit to the depth of 100 metres and is exploited to the depth of 250 metres by underground methods. "Low impact" explosives, expanding plastics or hydraulic wedging are used to break the ore. The ore is milled, screened and manually sorted.

 

END USES: Transparent and colored beryl varieties, such as emerald, morganite and aquamarine, are highly valued gemstones. Industrial grade beryls commonly recovered as by-products are a source of Be oxide, Be metal alloys used in aerospatial and defence applications, Be oxide ceramics, large diameter berylium-copper drill rods for oil and gas, fusion reactors, electrical and electronic components. Berylium metal and oxides are strategic substances, and may be substituted for by steel, titanium and graphite composites in certain applications. Phosphor bronze may replace beryllium-copper alloys. However, all known substitutes offer lower performance than Be-based materials.

 

IMPORTANCE: Schist-hosted deposits are the most common source of emeralds, although the largest and most valuable gemstones are most frequently derived from the Colombia-type deposits. Besides schist-hosted deposits and pegmatites, beryl for industrial applications may be also be present in fertile granite and syenite complexes that may be parent to pegmatites. A major portion of the beryl ore used in the U.S.A. as raw material for beryllium metal is recovered as a byproduct of feldspar and quartz mining from pegmatites.

 

REFERENCES

 

Beus, A.A. (1966): Geochemistry of Beryllium and Genetic Types of Beryllium Deposits; W.H. Freeman, San Francisco, 401 pages.

Brinck, J.W. and Hofmann, A. (1964): The Distribution of Beryllium in the Oslo Region, Norway - a Geochemical, Stream Sediment Study; Economic Geology, Volume 59, pages 79-96.

Frantz, G., Gilg, H.A., Grundmann, G. and Morteani, G. (1996): Metasomatism at a Granitic Pegmatite-Dunite Contact in Galicia: The Franqueira Occurrence of Chrysoberyl (alexandrite), Emerald, and Phenakite: Discussion; Canadian Mineralogist, Volume 34, pages 1329-1331.

Giuliani, G., Silva, L.J.H.D. and Couto, P. (1990): Origin of Emerald Deposits of Brazil; Mineralium Deposita, Volume 25, pages 57-64.

Grundmann, G. and Morteani, G. (1989): Emerald Mineralization during Regional Metamorphism: The Habachtal (Austria) and Leydsdorp (Transvaal, South Africa) Deposits; Economic Geology, Volume 84, pages 1835-1849.

Kazmi, A.H., Anwar, J. and Hussain, S. (1989): Emerald Deposits of Pakistan; in Emeralds of Pakistan, Geology, Gemology and Genesis, A.H. Kazmi and L.W. Snee, Editors, Van Nostrand Reinhold Co., New York, USA. Pages 39-74.

Kazmi, A.H., Lawrence, R.D., Anwar, J., Snee, L.W. and Hussain, A.S. (1986): Mingora Emerald Deposits (Pakistan): Suture-associated Gem Mineralization; Economic Geology, Volume 81, pages 2022-2028.

Kramer, D.A., Cunningham, L.D. and Osborne, S. (1997): Beryllium Annual Review-1996; Mineral Industry Surveys; United States Geological Survey, 7 pages.

Laskovenkov, A.F. and Zhernakov, V.I. (1995): An Update on the Ural Emerald Mines; Gems and Gemology, Summer issue, pages 106-113.

Martin-Izard, A., Paniagua, A., Moreiras, D., Aceveddo, R.D. and Marcos-Pasqual, C. (1995): Metasomatism at a Granitic Pegmatite-Dunite Contact in Galicia: The Franqueira Occurrence of Chrysoberyl (alexandrite), Emerald and Phenakite; Canadian Mineralogist, Volume 33, pages 775-792.

Martin-Izard, A., Paniagua, A., Moreiras, D., Aceveddo, R.D. and Marcos-Pasqual, C. (1996): Metasomatism at a Granitic Pegmatite-Dunite Contact in Galicia: The Franqueira Occurrence of Chrysoberyl (alexandrite), emerald, and phenakite: Reply; Canadian Mineralogist, Volume 34, pages 1332-1336.

Muligan, R. (1960): Geology of Canadian Beryllium Deposits, Geological Survey of Canada; Economic Geology Report, Number 23, 109 pages.

Robb, L.J. and Robb, V.M. (1986): Archean Pegmatite Deposits in the North-eastern Transvaal; in Mineral deposits of South Africa, C.R. Anhaeusser, and S. Maske, Editors, Geological Society of South Africa, Johannesburg, Volumes 1 and 2, pages 437-449.

Sinkankas, J. (1959): Gemstones of North America; D. Van Nostrand Company, Inc., Princeton, 75 pages.

Sinkankas, J. (1981): Emerald and other Beryls; Chilton Book Company, Radnor, Pennsylvania, pages 1-665.

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SEDIMENTARY ROCK-HOSTED OPAL


Q08
by S. Paradis1, J. Townsend2 and G J. Simandl3

1 Geological Survey of Canada, Pacific Geoscience Centre, Sidney, British Columbia, Canada
2 South Australia Department of Mines and Energy
3 B.C. Geological Survey, Victoria, British Columbia, Canada

 

Paradis, S., Townsend, J. and Simandl, G.J. (1999): Sedimentary Rock-hosted Opal; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines.

 

IDENTIFICATION

 

SYNONYMS: Australian opal deposits.

 

COMMODITY: Gem quality opal (precious and common).

 

EXAMPLES (British Columbia - Canadian/International): Lightning Ridge and White Cliffs (New South Wales, Australia) , Mintabie, Coober Pedy, Lambina and Andamooka (South Australia) Yowah, New Angledool (Queensland, Australia).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION:  Most of the Australian opal occurs in cracks, partings, along bedding planes, pore spaces and other cavities in strongly weathered sandstones generally underlain by a subhorizontal barrier of reduced permeability. The barriers consist mainly of claystones, siltstones and ironstone strata.

 

TECTONIC SETTINGS:  The tectonic setting at the time of deposition and lithification of the opal-bearing lithologies is not indicative of favourable environment for opal. However, the presence of a terrestrial (non-marine) environment at the time of intense weathering is essential.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:  Clastic sediments were deposited in the shallow inland basins. Subsequently, these areas were affected by climatic/paleo-climatic changes (transformation into desert environment) that have resulted in rapid fluctuation in water table levels and entrapment of silica-rich waters.

 

AGE OF MINERALIZATION:  In Queensland, Australia the host rocks are Cretaceous or Paleozoic and have been affected by deep weathering during the Early Eocene and Late Oligocene. The latter period is believed by some to be related to opal precipitation. Similar conditions favourable for opal deposition could have prevailed in different time periods in other parts of the world.

 

HOST/ASSOCIATED ROCKS:  Sandstones, conglomerate, claystone and silty claystone. Associated lithologies are feldspathic rocks weathered to kaolinite, silcrete and siliceous duricrust, shales and shaley mudstones, limestones, dolostones and ironstones. Exceptionally, precious opal may be found in weathered crystalline basements stratigraphically underlying the lithologies described above.

 

DEPOSIT FORM:  Opal occurrences are stratabound. Favorable subhorizontal, precious opal-bearing intervals can exceed 10 m in thickness, and are known to persist for distances of one to over 100 km. The distribution of individual precious opal occurrences within favorable areas is erratic. Veins are subhorizontal to subvertical and locally up to 10 cm thick. They pinch and swell, branch or terminate abruptly. A single vein can contain chalky to bony to blue, gray or milky common opal and precious opal.

 

TEXTURE/STRUCTURE:  Opal occurs as veinlets, thin seams in vertical and horizontal joints, desiccation cracks in ironstone layers, lenses and concretions, and replacing fossils (shell and skeletal) and wood fragments. Opal also forms pseudomorphs after glauberite4. In places opal seems to follow cross bedding. In unusual cases opal pieces eroded from the original host are incorporated into younger sediments. In silicified sandstones precious opal may form the cement around detrital quartz grains, in other areas, the opal may be cut by gypsum or alunite-filled fractures. The lithologies above the opal may contain characteristic red-brown, gypsiferous silt-filled tubules.

4 Glauberite: 4[Na2 Ca(SO4 )2 ], widespread as a saline deposit formed as a precipitate in salt lake environments,

 

also occurs under arid conditions as isolated crystals embedded in clastic sediments.

 

ORE MINERALOGY: Precious opal.

 

GANGUE MINERALOGY [Principal and subordinate]: Host rock, common opal, gypsum and gypsum-shot opal, alunite, hematite, limonite/goethite.

 

ALTERATION MINERALOGY:  N/A.

 

WEATHERING:  Feldspathic rocks strongly altered to kaolinite typically overly the Australian precious opal-bearing deposits. Opal exposed to arid weathering environments may desiccate, crack and lose its value; however, gem quality opal may be found at depth.

 

ORE CONTROLS: 1) Regional configuration of impermeable layers permitting groundwater pooling. 2) Local traps within regional sedimentary structure, such as bedding irregularities, floored by impermeable layers, porous material (e.g. fossils) or voids where opal can precipitate.

 

GENETIC MODELS:  Australian opal is hosted mainly by strongly weathered sandstones which are underlain by claystone, siltstone and ironstone that form relatively impermeable barriers. Periods of intense weathering are evidenced by indurated crust horizons. Silica-transporting solutions derived from intense weathering of feldspar within sandstones percolated downward to the contact between the porous sandstone and the underlying impermeable layers. During a subsequent dehydration (dry) period silica was progressively concentrated by evaporation. The last, most concentrated solutions or colloidal suspensions were retained within bedding irregularities at the permeable/impermeable rock interface, in joints and in other traps. Gem-quality opal was formed by ordered settling and hardening of silica microspheres of uniform dimensions. Disordered arrangement of silica microspheres or variability in microsphere size results in formation of common opal.

 

ASSOCIATED DEPOSIT TYPES:  Possibly clay deposits (B05).

 

COMMENTS: There is good reason to believe that a similar mode of opal formation could also take place in porous terrestrial and waterlain pyroclastic rocks, assuming favorable geological and paleo-climatic setting.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: N/A

 

GEOPHYSICAL SIGNATURE:  Most opal fluoresces brightly if exposed to ultraviolet light. Limited success was achieved using magnetic field and resistivity to find ironstone and ironstone concretions that commonly contain precious opal in Queensland.

 

OTHER EXPLORATION GUIDES:  Unmetamorphosed or weakly metamorphosed areas known for:

1) prolonged periods of deep chemical paleoweathering characterized by rock saturation and dehydration cycles;
2) broad sedimentary structures permitting shallow underground solution pooling;
3) local traps where opal could precipitate from nearly static, silica-bearing ground waters; and
4) presence of common opal.

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: No reliable estimates of grade or tonnage are available for individual deposits. Until 1970 the only records of production were annual returns submitted by opal buyers. Miners fear that reporting the true production would be used for taxation purposes. As with other gemstones, reporting the grades in terms of grams or carats per tonne may be strongly misleading. Large and exceptional quality stones command very high prices. Precious opal may be transparent, white, milky-blue, yellow or black. It is characterized by the internal play of colors, typically red, orange, green or blue. The best opal from Lightning Ridge was worth as much as $Aus. 10 000.00 per carat in cut form and Mintabie opal varied from $Aus. 50.00 to 10 000.00 per ounce of rough. Most of the white to milky colored opal from Coober Pedy was worth $Aus. 10.00 to 100.00 per ounce of rough, but the prices of top quality precious black and crystal opals exceeded $Aus. 5 000.00 per ounce. The value-added aspect of the gem industry is fundamental. An opal miner receives 1 to 50% of the value of cut and polished stone.

 

ECONOMIC LIMITATIONS: In Australia mining is largely mechanized, either underground or on surface. Opal-bearing seams are generally found at shallow depths (< 30 metres). Opal is still recovered from old tailings by hand sorting over conveyer belts using ultraviolet light. Large and exceptional quality stones command very high prices and the unexpected recovery of such stones may change an operation from losing money to highly profitable. Stones from these deposits are believed to have better stability under atmospheric conditions than opal from most volcanic-hosted deposits.

 

END USES: A highly priced gemstone that is commonly cut into solid hemispherical or en cabochon shapes. Doublets are produced where the precious opal is too thin, needs reinforcement or enhancement; plastic cement, a slice of common opal or other support is added to the back of the opal.

 

IMPORTANCE:  Australian sedimentary-hosted opal deposits account for most of the opal produced today. The situation is likely to continue since these deposits recently attracted important Japanese investment. In 1990, the Coober Pedy, Andamooka and Mintabie produced opal worth over $Aus. 47 million. Total production estimates for Australia are in the order of $Aus. 100 million annually.

 

REFERENCES

 

Barnes, L.C., Towsend, I.J., Robertson, R.S. and Scott, D.C. (1992):  Opal, South Australia’s Gemstone; Handbook No.5 (revised edition), Department of Mines and Energy, Geological Survey of South Australia, 176 pages.

Cipriani, C. and Borelli, A. (1986): Simon & Schuster’s Guide to Gems and Precious Stones; K. Lyman, Editor, Simon & Schuster Inc., New York, 384 pages.

Daragh, P.I., Gaskin, A.J. and Sanders, J.V. (1976): Opals; Scientific American, Volume 234, pages 84-95.

Downing, P.B. (1992): Opal Identification and Value; Majestic Press, 210 pages.

Hiern, M.N. (1976):  Precious Opal-South Australia; in Economic Geology of Australia and Papua New Guinea, Volume 4, Industrial Minerals and Rocks, C.L. Knight, Editor, Australian Institute of Mining and Metallurgy, Monograph Series, Volume II, pages 322-323.

Jones, J.B. and Segnit, E.R. (1971): The Nature of Opal. Nomenclature and Constituent Phases; Geological Society of Australia Journal, Volume 18, pages 57- 68.

Keeling, J.L. and Farrand, M.G. (1984): Origin and Formation of Matrix Opal from Andamooka; South Australia Geological Survey, Quarterly Geological Notes, Volume 90, pages 3-10.

Nichol, D. (1975): Opal Occurrences near Granite Downs Homestead; Mineral Resources Review, South Australia, Volume 135, pages 164-168.

Towsend, I.J., Wildy, R.L., Barnes, L.C. and Crettenden, P.P. (1988): The Opal Industry in South Australia 1984-1986; Mineral Resources Review, South Australia, Volume 156, pages 106-107.

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ULTRAMAFIC-RELATED CORUNDUM


Q09
by G.J. Simandl1 and S. Paradis2

1 British Columbia Geological Survey, Victoria, B.C., Canada
2 Geological Survey of Canada, Mineral Resources Division, Sidney, B.C., Canada

 

Simandl, G.J. and Paradis, S. (1999): Ultramafic-related Corundum (Contact Metamorphic/Metasomatic); in Selected British Columbia Mineral Deposit Profiles, Volume 3, G.J. Simandl, Z.D. Hora and D.V. Lefebure and T. Höy, Editors, British Columbia Ministry of Energy and Mines, Open File 1999-10.

 

IDENTIFICATION

 

SYNONYMS: Plumasite and marundite deposits, contact-metamorphic corundum and emery, "desilication" or metasomatic sapphire.

 

COMMODITIES (BYPRODUCTS): Rubies, sapphires, industrial grade corundum and emery.

 

EXAMPLES (British Columbia - Canadian/International): Corundum Hill (North Carolina, USA), Emery Hill (New York, USA), Natal and Birdcage camp (South Africa), Umba (Tanzania), Kinyiki Hill and Penny Lane ruby mine (Kenya).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Sapphire, ruby and industrial grade corundum occur within, or adjacent to, aplite, pegmatite, albitite, plumasite or marundite dykes, sills and rarely plugs cutting mafic and ultramafic rocks and their metamorphosed equivalents. Industrial grade corundum is also found commonly along contacts of mafic/ultramafic intrusions with metapelites or other felsic country rocks. It may occur both within country rock and the intrusion.

 

TECTONIC SETTINGS: These deposits occur in orogenic belts where felsic rocks are thrust against silica-undersaturated rocks and within the stable cratons.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Corundum is commonly found in quartz-free reaction zones located along contacts of silica-deficient rocks, such as ultramafic and mafic rocks, with pegmatite, paragneiss, syenite gneiss or other felsic rocks. Country rocks are typically affected by medium to high grade regional metamorphism.

 

AGE OF MINERALIZATION: Archean or younger. Abrasive-grade corundum deposits are commonly contemporaneous with contact metamorphism, while gem-quality corundum may post-date metamorphism and the peak of the tectonic activity.

 

HOST/ASSOCIATED ROCKS: Common host rocks are vermiculite ± chlorite ± asbestos-bearing rocks, plumasite (coarse grained rock consisting of anhedral corundum crystals in an oligoclase matrix), marundite (corundum in margarite matrix), syenite, pegmatite, aplite or hornfels. Associated rocks are ultramafics, a variety of mafic lithologies including gabbro, amphibolite, anorthosite, serpentinite, anthophyllite-chlorite-talc schist, peridotite and dunite and peraluminous orthogneisses or paragneisses.

 

DEPOSIT FORM: Most of the dyke-associated or fracture-controlled deposits that crosscut ultramafic and mafic rocks are planar or lens-shaped; rarely forming vertical plugs. They are less than a metre to 10 metres in thickness and may extend from few metres to several tens of metres along strike. These deposits exhibit several types of mineralogical zoning from the center of the deposit outwards:

  1. Corundum-chlorite > spinel - chlorite > enstatite > talcose rock > friable dunite > dunite;
  2. plumasite > biotitite > pegmatite > serpentinite;
  3. aplite> plumasite>spinel-magnetite rock > vermiculite and/or chlorite > actinolite >
    talc>serpentinite;
  4. barren pegmatite> marundite > talc-chlorite zone>amphibolite (pegmatite may not be present).


Lenticular or irregularly shaped, corundum-bearing pockets may be also present along the tectonic contacts between gneiss and serpentinite. Some of the gem-quality and most of the industrial grade corundum and emery deposits occur near the contacts of mafic and ultramafic intrusions with country rocks. Emery may form veins, layers and irregular or lens-shaped masses within both endo- and exometamorphic reaction zones. Most of the corundum is typically found in metapelites adjacent to such intrusions.

 

TEXTURE/STRUCTURE:  Sapphire and ruby may form rhombohedral or hexagonal prisms or they may occur as clear portions of large, poikilitic corundum crystals that may exceptionally reach over a metre in length. In South African plumasites the corundum crystals commonly vary from 3 millimetres to 10 centimetres. In marundite, corundum occurs as coarse hexagonal crystals embedded in scaly or rosette-shaped aggregates of margarite. Emery rock is typically equigranular, fine-grained (<1mm). It may form layers, veinlets or lenses and irregular zones of massive ore in intrusive and country rock.

 

ORE MINERALOGY:


In plumasite and marundite: sapphires, rubies, specimen- quality or industrial grade corundum.


Within contact metamorphic zones of mafic and ultramafic intrusions: mostly emery or silimanite-corundum rock or coarse industrial-grade corundum.


Along tectonic contacts: rubies, sapphires, specimen and industrial grade corundum.

 

GANGUE MINERALOGY [Principal and subordinate]:


In plumasites: mainly plagioclase, ± biotite, ± amphibole, ± fuchsite, ± tourmaline. Some of the solid inclusions identified within sapphires and rubies are zircon, rutile, apatite, boehemite, monazite, hematite, mica, calcite, pyrrhotite and graphite.


In marundites: margarite, ± feldspar, ± biotite, ± apatite, ± garnet, ± tourmaline, ± fuchsite, ± kyenite (?), ± talcose material and possibly anthophyllite.


In metasomatic zones cross-cutting ultramafic rocks without plumasite core: vermiculite, ± chlorite. The main solid inclusion in gem corundum is vermiculite.


In contact metamorphic deposits: a) In emery ores: Hercinite, pleonaste, magnetite, hematite/ ilmenite, ilmenohematite. hypersthene, sapphirine, sillimanite, cordierite, garnet, biotite, feldspar, staurolite, gahnite. Some of the minor constituents in emery ore may be due to hostrock inclusions. b) In sillimanite-corundum rock: rutile and ilmenite are trace constituents.

 

ALTERATION MINERALOGY: Corundum may retrograde into diaspore or mica. In marundites it is commonly partially replaced by gibbsite and margarite.

 

WEATHERING: Some uneconomical primary gemstone and industrial grade deposits may form viable residual or placer deposits.

 

ORE CONTROLS: There are three major spatial controls: 1) fracture zones control metasomatic and plumasite mineralization within the mafic/ultramafic rocks; 2) tectonic contacts control mineralization pockets located between gneisses and serpentinites; and 3) contact metamorphic zones around mafic intrusions are also favourable.

 

GENETIC MODELS: A number of theories explaining the origin of these deposits have been proposed over the years. The three models that appear the most likely are:
a) Desilication of granitic pegmatites or pegmatitic fluids by interaction with silica-undersaturated country rocks. This is particularly popular theory to explain the origin of fracture-controlled mineralization associated with marundite, plumasite, vermiculite rock, pegmatite or aplite crosscutting ultramafic country rocks.
b) In the contact metamorphic/metasomatic settings, the high ratio of Al2O3/(CaO+Na2O+K2O) and low silica content that favor corundum formation may be achieved by magmatic interaction of mafic or ultramafic rocks with metapelites or by partial melting of the pelitic country rocks.
c) Where the felsic rocks were thrust against ultramafic rocks, reaction zones may have formed under open system conditions during regional metamorphism.

 

ASSOCIATED DEPOSIT TYPES: Placer-type corundum deposits (C01 and C02) and corundum-bearing residual soils. Vermiculite (M08), nepheline syenite (R13) and pegmatites (O01, O02, O03 and O04) may be genetically related to some of the corundum deposits covered by this profile. Corundum-bearing metapelites (P06) may also be present in the same geological setting.

 

COMMENTS: Emery is a black granular rock formed by intergrowths of corundum with magnetite, hercinite or hematite. Emery deposits may also form during regional metamorphism of aluminous sediments, such occurrences are described in the profile P06 (this volume). It is used mainly as an abrasive or for anti-skid surfaces. "Anolite", a highly-priced ornamental stone formed from a famous ruby-bearing zoisite amphibolite from Longido (Tanzania), is closely associated with serpentinites (Keller, 1992). Due to the lack of outcrops, it is not clear if this deposit belongs to the metasomatic type of mineralization described in this profile. Marble and skarn-hosted ruby/sapphire deposits, such as those described by Okrush et al. (1976), also may be similar in origin. Some of these deposits may have formed by essentially isochemical regional metamorphism, while others may be pegmatite and aplite-related metasomatic zones. Marble hosted deposits should be considered as a distinct deposit type.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Corundum-bearing lithologies are silica-undersaturated and characterized by their high Al2O3/(CaO+Na2O+K2O) ratio. Saphire, ruby, corundum or emery may be found in heavy mineral concentrates from stream sediments or tills. As well, the solid inclusions within corundum crystals, corundum texture, and associated minerals in the concentrates may be indicative of the type of primary source, such as gem corundum hosted by alkalic rocks (Q10), corundum in aluminous metasediments (P06) and gem corundum in marbles.

 

GEOPHYSICAL SIGNATURE: Ultramafic rocks associated with this deposit type may be detected and possibly delimited by magnetic or electromagnetic surveys. Magnetite-bearing emery deposits may be detected using a magnetometer.

 

OTHER EXPLORATION GUIDES: Some vermiculite occurrences may be worth examining for gem corundum.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: Grades are rarely reported for hard rock-hosted sapphire and ruby deposits. They are difficult to determine as these deposits are often high-graded and mined sporadically. A substantial proportion of the production is sold on the black market. Grades of up to 2000 carats of rough gems per ton are reported from the weathered extension of sapphire and ruby rock occurrences at Umba (Tanzania). In another portion of the same property 100 000 carats were recovered from soil above apparently barren veins, but the grade is not reported. In South Africa, plumasites contain 5 to 80% corundum with typical grades around 30 to 40%. Larger deposits may contain 5 to 10 thousand tonnes, but average tonnage is more likely less than 2 thousand tonnes. These deposits were mined in the first half of the 19th century to about 40 metres. Typical content of eluvial deposits associated with plumasites varies from 10 to 20% by volume. The emery deposits of Emery Hill (Peekskill area) consisted of veins (some less than 2cm thick), pods and thin layers parallel to the schistosity. The emery consisted of varying proportions of spinel (0 to 65%), magnetite (20-30%) and corundum (15 to 65%).

 

ECONOMIC LIMITATIONS: Together with emerald, red beryl and diamond, ruby and sapphire are the most valuable gemstones. The most valuable rubies are dark purplish red ("pigeon's blood red"). The most desirable color for sapphire is "Kashmir blue". Star rubies and sapphires exhibit asterism better than any other gems. The color of many natural corundum gems is artificially enhanced by heat treatment. . Due to the highly variable grades and relatively small deposit size, these hard rock deposits are commonly mined by open-cast methods and in some cases by primitive underground methods.

 

END USES: Depending on quality, corundum may be used as a gemstone, abrasive or friction material on non-slip surfaces. Sillimanite-corundum rock is a relatively highly priced material for refractory applications. Some corundum-bearing rocks are used as ornamental stones.

 

IMPORTANCE: Most corundum gems are recovered from regoliths, residual soils or gravels, and as byproducts of placer mining (C01, C02). They may be also found in alkali basalts, lamprophyres (Q10) and rarely in aluminous metamorphic rocks (P06) and marbles. However, deposits of this type, remain worthwhile targets for prospectors and small exploration companies. Clear, nearly inclusion-free corundum crystals are produced synthetically, and compete with natural gems. Silicon carbide and artificial corundum manufactured from bauxite has largely replaced corundum and emery in most industrial abrasive applications. Today, the combined consumption of industrial grade corundum and emery in the USA is estimated to be less than 10,000 tonnes/year.

 

REFERENCES

 

Andrews, P.R.A. (1991): Summary Report No. 15: Minor Abrasives-Corundum, Emery, Diatomite, Pumice, Volcanic Ash and Staurolite; Canada Centre for Mineral and Energy Technology, Division Report MSL 91-110 ®, 91 pages.

Barker, F. (1964): Reaction Between Mafic Magmas and Pelitic Schist, Cortland, New York; American Journal of Science, Volume 262, pages 614-634.

De Villiers, S.B. (1976): Corundum; in Mineral Resources of the Republic of South Africa, Coetze, C.B., Editor, Geological Survey of South Africa, Volume 7, pages 341-345.

DuToit, A.L. (1918): Plumasite (Corundum-aplite) and Titaniferous Magnetite Rocks from Natal; Transactions of the Geological Society of South Africa, Volume 21, pages 53-73.

Grant, J.A. and Frost, B.R. (1990): Contact Metamorphism and Partial Melting of Pelitic Rocks in the Aureole of Laramie Anothosite Complex, Morton Pass, Wyoming; American Journal of Science, Volume 290, pages 425-427.

Hall, A.L. (1920): Corundum in Northern and Eastern Transwal; Union of South Africa Geological Survey, Memoir 15.

Keller, P.C. (1992): Gemstones of East Africa; Geosciece Press Inc., 144 pages.

French, A.E. (1968): Abrasives; in Mineral Resources of the Appalachian Region; United States Geological Survey, Professional Paper 580, pages 261-268.

Game, P.M. (1954): Zoisite-amphibolite with Corundum from Tanganyika; Mineralogical Magazine, London, Volume 30, pages 458-466.

Gillson, J.L. and Kania, J.E.A. (1930): Genesis of the Emery Deposits near Peekskill, New York; Economic Geology, Vol. 25, pages 506-527.

Hughes, R.W. (1990): Corundum. Butterworth-Heinmann, London, 314 pages.

Keller, P.C. (1990): Gemstones and Their Origin; Van Nostrand Reinhold, New York, 144 pages.

Larsen, E.S. (1928): A Hydrothermal Origin of Corundum and Albitite Bodies; Economic Geology, Volume 23, pages 398-443.

Okrusch, M., Bunch, T.E. and Bank, N. (1976): Paragenesis and Petrogenesis of a Corundum-bearing Marble at Hunza (Kashmir); Mineralium Deposita, Volume 11, pages 278-297.

Pattison D.R.M. and Harte, B. (1985): A Petrogenic Grid for Pelites in the Ballachulish Aureole and other Scottish Thermal Aureoles; Journal of Geological Society of London, Volume 142, pages 7-28.

Pattison D.R.M. and Tracy, J. (1991): Phase Equilibria and Thermobarometry of Metapelites; in Contact Metamorphism, D.M. Kerrick, Editor, Reviews in Mineralogy and Petrology, Volume 26, Mineralogical Society of America, pages 105-204.

Robb, L.J. and Robb, V.M. (1986): Archean Pegmatite Deposits in the North-Eastern Transwaal; in Anhausser, C.R. and Maske, S. Editors, Mineral Deposits of Southern Africa Volume I, Geological Society of South Africa, Johannesburg, pages 437-449.

Rossovskiy, L.N. and Konovalenko, S.I. (1977): Corundum Plagioclasite of the Southwestern Pamirs; Doklady Academie Science. U.S.S.R., Earth Science Section, Volume 235, pages 145-147

Sinkankas, J. (1959): Gemstones of North America; D. Van Nostrand Company Inc, New York, 675 pages.

Solesbury, F.W. (1967): Gem Corundum Pegmatites in NE Tanganyika; Economic Geology, Volume 62, pages 983-991.


ALKALI BASALT AND LAMPROPHYRE- HOSTED SAPPHIRE and RUBY


Q10
by G.J. Simandl1 and S. Paradis2

1 British Columbia Geological Survey, Victoria, B.C., Canada
2 Geological Survey of Canada, Mineral Resources Division, Sidney, B.C., Canada

 

Simandl, G.J. and Paradis, S. (1999): Alkali Basalt and Lamprophyre-hosted Sapphire and Ruby; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines, Open File 1999-10.

 

IDENTIFICATION

 

SYNONYMS: Alkali basalt-hosted, lamprophyre-hosted or volcaniclastic-hosted gem corundum deposits.

 

COMMODITIES (BYPRODUCT): Sapphire and ruby (zircon).

 

EXAMPLES (British Columbia - Canadian/International): Mark diatreme (082N 089); Yogo Gulch (Montana, USA) Braemar, Stratmore and Kings Plains Creek (New South Wales, Australia), Changle (China).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Sapphires and rubies are found as xenocrysts in some hypabyssal or eruptive alkalic rocks. The residual soil or regolith overlying these rocks can be enriched in sapphires and rubies due to intense weathering which liberates the megacrysts from the matrix.

 

TECTONIC SETTINGS: Host rocks occur in continental and pericontinental settings related to rifts, deep faults and/or hot-spots. In some cases they are interpreted to be subduction zone-related.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Corundum gems are brought to the surface by alkali basalt eruptions. The highest grades are associated with diatreme and base surge lithologies that erode quickly unless capped by weathering-resistant rocks, such as lava flows. Significant corundum can also be present in lava flows and hypobysal equivalents of these corundum-rich volcanic pulses.

 

AGE OF MINERALIZATION: Post-dates tectonic and metamorphic events. Typically hosted by Cenozoic or younger rocks. Oligocene and Miocene in New South Wales, Australia.

 

HOST/ASSOCIATED ROCKS: Lava flows, hypabyssal intrusions and volcaniclastic rocks of alkali basalt, lamprophyre, nephelinite, basanite or phonolite composition. Highly altered and/or weathered volcaniclastic rocks, including reworked lahar flows and base surge and ash-fall deposits commonly have the highest gem corundum content. Mantle and crustal rock xenoliths, including lherzolites, peridotites and in some cases corundum-bearing gneiss, occur in the above lithologies. There are little or no restrictions as to the lithology of the wallrock.

 

DEPOSIT FORM: With the exception of diatremes and volcanic necks, host igneous rocks are generally tabular bodies (dykes, lava flows, pyroclastic flows). The flows and their erosional remnants vary from less than a metre to several metres in thickness and extend from hundreds of metres to more than several kilometres. Extensive, thin, heavy minerals-enriched layers can carry higher grades. They form volcaniclastic aprons around diatremes and are possibly produced by base surges. High grade zones may also form thin blankets associated with unconformities or recent erosional surfaces. The lamprophyre dykes, such as Yogo, may consist of several en echelon segments from less than a metre to several metres thick and hundreds of metres in length.

 

TEXTURE/STRUCTURE: In extrusive rocks, sapphire and ruby occur as megacrysts that are typically bi-pyramidal, stepped and tapering or barrel shaped. The corundum crystals can be corroded and etched. Some crystals are zoned, contain a variety of solid inclusions and can be intergrown with other minerals. They may have spinel reaction rims. In New South Wales they are typically less than 1 carat in weight (about 5 mm or less). In Thailand the typical size of sapphires from alluvial sediments is 3-6 mm, but crystals up to 9.5 x 6 x 5.5 cm are also reported. Rubies of about 1-1.5 cm in diametre were found in some localities. In Yogo lamprophyre dykes, most of the sapphire occurs as subhedral to anhedral grains. The most common shape is a wafer with etched surfaces and a thin spinel crust. The host lithologies may contain numerous mantle or crust xenoliths, some of them corundum-bearing gneisses.

 

ORE MINERALOGY [Principal and subordinate]: Sapphire, ruby; ± zircon.

 

GANGUE MINERALOGY [Principal and subordinate]: In alkali volcanic rocks the gangue minerals are feldspar (mainly anorthoclase), pyroxene, ± analcime, ± olivine, amphiboles, such as kaersutite, ilmenite, ± magnetite, ± spinel, ± garnet, with minor biotite/phlogopite, spinel and chrome diopside and zircon ± rutile. Vesicles may contain amorphous silica, andesine and zeolites.

In lamprophyre hypabyssal rocks, pyroxene, phlogopite, ± calcite (mainly in veins), ± olivine, ± analcime are major constituents. Minor constituents are magnetite, apatite, chlorite, serpentine, amphibole, brucite and feldspar.

The main solid inclusions reported within the corundum in volcanic rocks are: spinels (hercynite, gahnite), ilmenite, rutile, ilmeno-rutiles, columbite, uranopyrochlore-betafite, zircon, akali feldspar, plagioclase, mica, thorite, sulphides and glass.

 

ALTERATION MINERALOGY: Volcaniclastic rocks that host gem corundum are commonly clay-altered and ferruginized due to combination of alteration and weathering.

 

WEATHERING: Palagonitic clasts and "clast in clast" structures are visible in weathered volcaniclastic rocks that host gem corundum. Weathering can greatly enhance the gem corundum grade and transform a low grade occurrence into a deposit of economic interest. The near surface portion of Yogo dike was weathered to a yellowish clay. Ore from the Yogo Gulch deposit was left on surface to weather for few months to reduce the need for crushing.

 

ORE CONTROLS: Primary controls are sapphire and ruby-bearing alkali basalt, lamprophyre, nephelinite, basanite or phonolite dikes, flows, pyroclastics or possibly diatremes. Unconformities, paleoregoliths or current errosional surfaces intersecting sapphire/ruby-bearing lithologies provide a vector for identifying secondary deposits.

 

GENETIC MODELS: Several hypotheses have been proposed to explain the origin of the sapphire-bearing lithologies. Most of proposed models involve alkali volcanic or hypabyssal rocks incorporating previously formed sapphires and/or rubies as xenocrysts and transporting them to the surface in a similar way to diamonds in kimberlites (N02). Any volcanic rock type with the potential to host sapphires (alkali basalts, kimberlites, lamproids, lamprophyres) must originate at greater depth than that required for the formation of sapphire. There is no consensus about the source lithology or magma for gem corundum. Corundum gems may have formed by metamorphism of aluminous sediments; crystallization in deep-seated syenitic melts or from undersaturated fractionated felsic melts; contact reactions between ultramafic/mafic intrusions and alumina-rich metasediments in deep continental crust; metamorphism of aluminous sediments contained in subducting oceanic crust, etc.

 

ASSOCIATED DEPOSIT TYPES: Can be the source for placer corundum ± zircon ± diamond deposits (C01, C02, C03, C04) and corundum-bearing regolith.

 

COMMENTS: Syenite-hosted corundum deposits, such as the Blue Mountain deposit (Ontario, Canada), may also be a source of corundum. These occurrences are described as nepheline syenite deposits (R13). Corundum is also known to occur as discrete crystals in diatremes of carbonatitic and kimberlitic affinity.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: "Zircospilian" association (zircon-corundum-spinel-ilmenite-anorthite) can be considered characteristic of these deposits. Corundum gemstones and indirect indicator minerals, such as kaersutite and chrome diopside (derived from lherzolite xenoliths), in heavy mineral concentrates from stream and lake sediments or from tills. Blue-green-yellow zoned corundum is particularly characteristic of Australian and Asian deposits. These corundums contain up to 0.04 wt% Ga2O3 and have low Cr/Ga and Ti/Ga ratios.

 

GEOPHYSICAL SIGNATURE: Electromagnetic and magnetic surveys may be effectively used in delimiting sapphire/ruby-bearing host rocks, assuming good contrast with surrounding lithologies.

 

OTHER EXPLORATION GUIDES: Pipes, dikes, plugs and diatremes of alkali lithologies are positive indicators. In some localities there appears to be a positive correlation between the abundance of mantle-derived xenoliths and corundum (Guo and O’Reilly,1996). Unexplained sapphire, ruby or corundum occurrences in favourable tectonic settings warrant follow-up.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: No reliable grades in terms of carats recovered or dollars per metric tonne are available for most of the hard rock-hosted gem deposits. In New South Whales, corundum is typically present as trace constituent in basalts, but volcaniclastic sediments may contain as much as 12 kg of corundum per cubic metre of material (Peacover, 1994). The Yogo dike yielded about 10 carats/ton between 1897 and 1929. The grade varied probably from 0 to 70 carats/tonne. It supplied about 16 million carats valued in the rough at about $2.5 US million. About 2.25 million carats were gem quality. Approximately 675 000 carats of cut sapphires, worth $ US 20-30 million were obtained (Claubaugh, 1952). More recently, the production for 1984 was 4 000 carats with $US 3 million in sales for finished jewelry (Voynock, 1985). According to Brownlow and Komorowski (1988), the weight of average stone is less than 1 carat.

 

ECONOMIC LIMITATIONS: Red beryl, emerald, diamond, ruby and sapphire are the most valuable gemstones. The colors of corundum reflect variations in trace element contents. The most valuable rubies are dark purplish red ("pigeon's blood red"). The most desirable color for sapphire is Kashmir blue. The color and clarity of many natural corundum crystals is commonly artificially enhanced by heat treatment to increase the proportion of stones suitable for faceting (Turnovec, 1987). For example, the treatment of material from Laos can increase the proportion of stones suitable for faceting by 20% by weight. Star rubies and sapphires exhibit asterism better than any other gems. Synthetic corundum competes with natural crystals in gem applications and has replaced natural corundum crystals in most high technology applications. Nevertheless, the "magic" of the natural stones persist in the gem industry.

 

END USES: Gemstones, specimen samples, industrial grade abrasives and friction surfaces.

 

IMPORTANCE: Primary (hard rock) sapphire-bearing deposits of this type are relatively rare. Most of the corundum gems are recovered from associated residual soils or placer deposits. Sapphire-bearing, alkali volcanic rocks are source rocks for some of the large alluvial sapphire deposits, such as the Kings Plain deposits in the Inverell-Glen Innes and Anakie districts of Eastern Australia, Pailin gem fields in Cambodia and Bo Rai deposits of Thailand. In 1993-94, the sapphire production in eastern Australia was estimated at A$ 20 to 25 million (65-75 million carats).

 

REFERENCES

 

Aspen, P., Upton B.G.J. and Dickin, A.P. (1990): Anorthoclase, Sanadine and Associated Megacrysts in Scottish Alkali Basalts: High -pressure Syenitic Debris from Upper Mantle Sources?; European Journal of Mineralogy, Volume 2, pages 279-294.

Brownlow, A.H. and Komorowski, J-C. (1988): Geology and Origin of Yogo Sapphire Deposit, Montana, Economic Geology, Volume 83, pages 875-880.

Claubaugh, S.E. (1952): Corundum Deposits of Montana; U.S. Geological Survey, Bulletin 983, 100 pages.

Coenraads, R.R. (1992): Surface Features of Natural Rubies and Sapphires Associated with Volcanic Provinces. Journal of Gemmology, Volume 23, pages 151-160.

Guo, J., Wang, F. and Yakoumelas, G. (1992): Sapphires from Changle in Shadong Province, China; Gems and Gemmology, Volume 28, pages 255-260.

Guo, J. and O’Reilly, S.Y. (1996): Corundum from Basaltic Terrains: a Mineral Inclusion Approach to the Enigma; Contributions to Mineralogy and Petrology, Volume 122, pages 368-386.

Harlan, S.S. (1996): Timing of Emplacement of the Sapphire-bearing Yogo Dike, Little Belt Mountains, Montana; Economic Geology, Volume 91, pages 1159-1162.

Hughes, R.W. (1990): Corundum. Butterworth-Heinmann, London, 314 pages.

Jobbins, E.A. and Berrangé, J.P. (1981): The Pailin Ruby and Sapphire Gemfield, Cambodia; Journal of Gemmology, Volume XVII, Number 8, pages 555-567.

Levinson, A.A. and Cook , F.A. (1994): Gem Corundum in Alkali Basalt: Origin and Occurrence, Gems & Gemology, Winter 1994, pages 253-262.

Meyer, H.O.A. and Mitchell, R.H. (1988): Sapphire-bearing ultramafic Lamprophyre from Yogo, Montana: A Ouachitite; Canadian Mineralogist, Volume 26, pages 81-88.

Oakes, G.M., Barron, L.M. and Lismund, S.R. (1996): Alkali Basalts and Associated Volcaniclastic Rocks as a Source of Sapphire in Eastern Australia; Australian Journal of Earth Sciences, Volume 43, pages 289-298.

Peacover, S.R. (1994): Exploration Licence 38867. Final report. Great Northern Mining Corporation N.L., New South Wales Geological Survey, Files 1994/114 (unpublished).

Sutherland, F.L. (1996): Alkaline rocks and Gemstones, Australia: A Review and Synthesis. Australian Journal of Earth Sciences, Volume 43, pages 323-343.

Turnovec, I. (1987): Termické Zušlechtovaní Laoských Safiru; Geologický Pruzkum, no.1, pages 27-28.

Voyinik, S.M. (1985): The Great American Sapphire: Missoula, Montana; Mountain Press Publishing Co., 199 pages.

 

 

PRECIOUS OPAL IN VOLCANIC SEQUENCES


Q11
by S. Paradis1, G.J. Simandl2 and A. Sabina3

1 Geological Survey of Canada, Pacific Geoscience Centre, Sidney, B.C., Canada
2 British Columbia Geological Survey, Victoria, B.C., Canada
3 Geological Survey of Canada, Ottawa, Ontario, Canada

 

Paradis, S., Simandl, G.J. and Sabina, A. (1999): Opal Deposits in Volcanic Sequences; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines.

 

IDENTIFICATION

 

SYNONYMS: Hydrothermal or "volcanic opal".

 

COMMODITIES (BYPRODUCTS): Precious opal (common opal, chalcedony, jasper, agate).

 

EXAMPLES (British Columbia - Canada/International):   Klinker (082LSW125), Northern Lights (093E 120), Whitesail Range (maps 93E10W and 93E11E) and a precious opal occurrence near Falkland, Eagle Creek (093K 095); pale green and apple green common opal occurs at Savona Mountain (092INE158); Queretaro Mines (Mexico), Virgin Valley (Nevada, USA), Tepe Blue Fire Opal Mine (Idaho, USA).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Opal occurs commonly in seams of volcanic ash or lahars sandwiched between successive lava flows. It occurs mainly as open space fillings and impregnations. Common opal, opalized wood and to some extent "fire opal" are widespread within Triassic or younger volcanic sequences, but precious opal is rare. Where opal occurs in massive volcanic rocks, it occurs also as open space fillings, however the opal-bearing areas are much smaller. Regardless of volcanic hostrock, the precious opal occurrences are discrete, whereas common opal occurs over large areas.

 

TECTONIC SETTINGS:  Volcanic arcs, rifts, collapsed calderas, hot spot related volcanism and others.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:  Volcanic sequences formed in subaerial or shallow marine environments where porous, pyroclastic or lacustrine rocks are interbedded with lava flows.

 

AGE OF MINERALIZATION: Tertiary or younger, commonly Miocene.

 

HOST/ASSOCIATED ROCKS: Common host rocks are rhyolite, basalt, andesite and trachyte lavas, lahars and other volcaniclastic rocks. Associated rocks are perlite, bentonite, scoria, volcanic ash and diatomite; volcanic rocks may be intercalated with lacustrine sedimentary rocks.

 

DEPOSIT FORM: Favourable opal-bearing horizons are commonly stratabound. Occurrences of precious opal within these horizons are commonly considered as erratic, controlled by permeability at the time of opal deposition. Individual precious opal-bearing fractures or lenses may grade into common opal and agate over distances of centimetres.

 

TEXTURE/STRUCTURE:  Opal occurs as open space fillings in irregular cavities, narrow discontinuous seams, partially-filled pillow tubes, fractures, vesicles, matrix in volcaniclastic rocks and replacing wood fragments and logs. Common opal may form miniature stalagmites and stalactites within cavities, nodules in clay or diatomite beds and "thunder eggs".

 

ORE MINERALOGY [Principal and subordinate]: Precious opal; "fire opal", chalcedony, agate, common opal.

 

GANGUE MINERALOGY [Principal and subordinate]: Common opal, agate, fragments of host rock, clays, zeolites, quartz, jasper, celadonite, manganese and iron oxides.

 

ALTERATION MINERALOGY:  Opal-bearing cavities may have zeolite and celadonite coatings, but so do the barren cavities. There is no known alteration which is specific to precious opal.

 

WEATHERING: In arid environments, opal in surface outcrops may desiccate, become brittle and crack. Such material is not suitable as a gemstone. However, these opal bodies may be gem-quality at depth.

 

ORE CONTROLS: Open spaces and other permeable zones open to the silica-bearing solutions.

 

GENETIC MODELS: In many large opal districts, it is believed that during the longer periods of volcanic inactivity, shallow lakes developed. Forests grew along the lake-shores and driftwood accumulated in the lakes. Volcanic eruptions covered everything with pyroclastic materials capped by lava flows resulting in aquifers, perched water tables, and anomalies in the thermal gradient. This in conjunction with subsequent brittle tectonic deformation resulted in ideal conditions for the formation of hydrothermal systems. A variety of silica forms, including silica sinter, opaline silica, chalcedony and common opal are believed to have formed by deposition of silica-bearing fluids. The dissolved SiO2 content in water is well known to be temperature dependent with the maximum dissolution at around 325°C, however, the conditions needed for the precipitation of precious opal in volcanic environment are not well understood. At least a portion of the opal-CT in volcanic rocks is believed to precipitate directly from supersaturated solutions. The temperatures of formation for precious opal are expected to be relatively low by analogy to sedimentary-hosted precious opal deposits, but temperatures as high as 160°C are reported from fluid inclusion studies. No precious opal is reported from active hydrothermal fields, such as Geyser Valley, Yellowstone or Whakarewarewa (New Zealand). This suggests that the precious opal forms only under very specific physico-chemical conditions. Eh and definitely pH may be important. Chemical composition of hydrothermal fluids in terms of silica concentrations, as well as Na, K, Cl, Ca, SO4, HCO3, B, Li and other elements may be important. The composition of the silica-bearing fluid is probably modified during migration through the permeable host rock, specially if the latter contains zeolites and/or clays. Zeolites act as molecular sieves and are well known for their cation exchange properties.

 

ASSOCIATED DEPOSIT TYPES:  Associated deposits can be beds of diatomaceous earth (F06), volcanic ash (E06), zeolite deposits (D01, D02), perlite and a variety of semi-precious or ornamental silica gemstones, such as jasper (Q05), moss agate (Q03), and chalcedony. Other deposit types occurring in the same setting are hot-spring Au-Ag (H03), hot-spring Hg (H02), agate (Q03) and hydrothermal Au-Ag-Cu: high sulphidation (H04). It is possible that these deposit types are the source of primary amorphous silica.

 

COMMENTS: Precious opal is characterized by a play of color. The term common opal, as used here, covers any opal that does not show this play of colors. Some common opal specimens may be used as gemstones, but in general they have substantially lower value than precious opal. The term "Fire Opal" describes a common opal having a transparent orange to red-orange base color. Such opal is commonly faceted. Precious and common opal coexist within the same deposits.

 

Common opal and opaline silica are also commonly associated with the spectacular hydrothermal systems characterized by hot springs pools and geysers, mud pots, geyser terraces and fumaroles where it may be deposited as common opal, opaline silica or silica sinter. The well known examples of such systems are: Yellowstone hot springs; Geyser Valley in Kamchatka and now inactive Waimangu Geyser (Taupo volcanic zone, New Zealand). It is possible that some of the precious opal is formed by the dissolution of the previously formed common opal, silica sinter in the same conditions as sedimentary rock-hosted precious opal deposits.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Mn oxide fracture coating was observed in the proximity of the Klinker deposit. In some cases the indicator elements used in exploration for epithermal metalliferous deposits such as Hg, Sb and As may be indirectly applied to precious opal exploration.

 

GEOPHYSICAL SIGNATURE: N/A, except for detecting perched water tables and faults (mainly VLF and resistivity). Thermometry may have use where precious opal is associated with recent hydrothermal activity.

 

OTHER EXPLORATION GUIDES:  Boulder tracing is commonly used in opal exploration. Unmetamorphosed or weakly metamorphosed (zeolite facies) terrains (gem opal deteriorates and becomes brittle if subject to moderate temperatures); Tertiary or younger volcanic rocks. Areas containing known occurrences of precious or common opal, opalized wood and possibly chalcedony. Opal occurrences hosted by volcaniclastic rocks are commonly confined to the same lithologic unit over a large area. The presence of warm springs in an appropriate setting may also be considered as an indirect exploration indicator.

 

At the MINFILE (Klinker) 082LSW125 deposit, mineralogical zoning within vesicule fillings may be used to delimit the most favourable areas. For example the common opal occurs only within broad areas of agate mineralization and precious opal only in small areas within the common opal mineralization.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE:  Grade and tonnage for volcanic-hosted opal deposits are not well documented, largely because the opal extraction is done by individuals or family type businesses. The precious opal distribution within most deposits is erratic, "Bonanza-type". The deposits at Querétaro were discovered in 1835 and are still in production. Furthermore, the term "grade" as commonly used for metalliferous deposits is much harder to apply to gemstone deposits and especially to opal deposits. For example "fire opal" ranges in value from $CDN 5 to 300 per gram. Average commercial precious opal will sell probably around $CDN40 per gram, the top quality stones may sell for $CDN 1400.00 per gram.

 

ECONOMIC LIMITATIONS: Some of the common opal specimens may be used as semi-precious or ornamental stones, but in general they have substantially lower value than precious opal. Gem opal contains up to 10% water, which contributes to the translucency of the specimens. Precious opal from some localities, such as Virgin Valley in Nevada, are generally not suitable for gems because they crack too easily; however the opal from many other volcanic-hosted occurrences is as stable as that from the Australian sedimentary-hosted deposits. Deposits located in intensely weathered terrains are easier to mine than deposits in unaltered rocks. Prices of the best quality opal have risen steadily since 1991. There is a relatively good market for precious opal, nevertheless strong marketing and value-added processing are considered essential parts of successful opal mining operations.

 

END USES:  Precious opal is highly priced gemstone; "fire opal" may be faceted, opalized wood is a speciality ornamental stone commonly used for book ends.

 

IMPORTANCE: Volcanic rock-hosted opal deposits are numerous, but most of today's high quality opal production comes from Australian sedimentary-hosted deposits.

 

REFERENCES

 

Broughton, P.L. (1974): A Recent Discovery of Blue Opal in Western Idaho; Lapidary Journal, October, pages 1102-1106.

Cuthbert, D.L. (1969):  Precious Opal in Idaho; Lapidary Journal, October, pages 928-930.

Downing, P.B. (1992): Opal Identification and Value; Majestic Press, 210 pages.

Leechman, F. (1961): The Opal Book, Ure Smyth Pty. Ltd, 255 pages.

Panczner, B. (1994):  Mexican Opal, Rock & Gem, October, pages 58-62.

Paradis, S. and Simandl, G.J. (1998): Precious Opal in British Columbia, Wat on Earth, Volume 11, pages 8-10.

Rienhart, J.S. (1980): Geysers and Hydrothermal Energy, Springer-Verlag, New York, 223 pages.

Sanders, J.V. (1975):  Microstructure and Crystallinity of Gem Opals; American Mineralogist, Volume 60, pages 749-757.

Sinkankas, J. (1959):  Gemstones of North America, D.Van Nostrand Commpany, Inc., Princeton, New Jersey.

Simandl, G.J., Hancock, K.D., Callaghan, B., Paradis, S. and Yorke-Hardy, R. (1997): Klinker Precious Opal Deposit, South Central British Columbia, Canada - Field Observations and Potential Deposit-scale Controls; British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork 1996, pages 321-327.

Simandl, G.J., Paradis S., Diakow, L.J., Wojdak, P. and Hartley, A. (1999): Precious Opal in White Sail Range, West-Central British Columbia, Canada. British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork 1998, pages 285-293.

Spencer, R.J., Levinson, A.A. and Koivula, J.I. (1992):  Opal from Querétaro, Mexico: Fluid Inclusion Study; Gems & Gemology, Volume 28, pages 28-34.

Spendlove, E. (1991): Idaho Opal; Rock & Gem, October, pages 32-39.

Zeitner, J.C. (1979): The Opal of Querétaro; Lapidary Journal, July, pages 868-880.

Zeitner, J.C. (1987): Querétaro Opals; Lapidary Journal, June, pages 20-26.

 

*  Note:  All BC deposit profile #s with an asterisk have no completed deposit profile.  USGS deposit model #s with an asterisk had no published model in the late 1990s. 

 

Examples of Gem and Semi-Precious Stone Deposits

BC Profile # Global Examples B.C. Examples
Q01 - - Cry Lake, Ogden Mountain
Q02 - - Hill 60, Arthur Point, Cassiar
  Q03* - - - -
  Q04* Thunder Bay (Ontario), Artigas (Uruguay), Maraba (Brazil) - -
  Q05* - - - -
Q06 Chivor and Muzo districts (Columbia) - -
Q07 Habachtal (Austria), Leysdorp (South Africa), Socoto (Brazil) - -
Q08 Coober Pedy (Australia) - -
Q09 Umba (Tanzania), Kinyki Hill (Kenya) - -
Q10 Yogo Gulch (Montana) - -
Q11 - - - -