Carbonate Hosted Disseminated Gold Silver - Mineral Deposit Profiles, B.C. Geological Survey

Schroeter, Tom and Poulsen, Howard (1996): Carbonate-hosted Disseminated Au-Ag, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T, Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 9-12.

IDENTIFICATION

SYNONYMS: Carlin-type gold, sediment-hosted micron gold, siliceous limestone replacement gold, invisible ("no-seeum") gold.

COMMODITIES (BYPRODUCTS): Au (Ag). In rare cases Ag dominates over Au.

EXAMPLES (British Columbia (MINFILE #) - Canada/International): Golden Bear (104K 079); parts of Brewery Creek (Yukon, Canada), Carlin, Getchell, Cortez, Gold Acres, Jerrett Canyon, Post and Gold Quarry (Nevada, USA), Mercur (Utah, USA), Mesel? (Indonesia),Guizhou (China).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: Very fine grained, micron-sized gold and sulphides disseminated in zones of decarbonated calcareous rocks and associated jasperoids. Gold occurs evenly distributed throughout hostrocks in stratabound concordant zones and in discordant breccias.

TECTONIC SETTINGS: Passive continental margins with subsequent deformation and intrusive activity, and possibly island arc terranes.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Host rocks to the Nevadan deposits were deposited in shelf-basin transitional (somewhat anoxic) environments, formed mainly as carbonate turbidites (up to 150 m thick), characterized by slow sedimentation. These rocks are presently allochthonous in thrust fault slices and have been overprinted by Miocene basin and range extension. There are Mesozic to Tertiary felsic plutons near many deposits.

AGE OF MINERALIZATION: Mainly Tertiary, but can be any age.

HOST/ASSOCIATED ROCK TYPES: Hostrocks are most commonly thin-bedded silty or argillaceous carbonaceous limestone or dolomite, commonly with carbonaceous shale. Although less productive, non-carbonate siliciclastic and rare metavolcanic rocks are local hosts. Felsic plutons and dikes are also mineralized at some deposits.

DEPOSIT FORM: Generally tabular, stratabound bodies localized at contacts between contrasting lithologies. Bodies are irregular in shape, but commonly straddle lithological contacts which, in some cases, are thrust faults. Some ore zones (often higher grade) are discordant and consist of breccias developed in steep fault zones. Sulphides (mainly pyrite) and gold are disseminated in both cases.

TEXTURE/STRUCTURE: Silica replacement of carbonate is accompanied by volume loss so that brecciation of hostrocks is common. Tectonic brecciation adjacent to steep normal faults is also common. Generally less than 1% fine-grained sulphides are disseminated throughout the hostrock.

ORE MINERALOGY (Principal and subordinate): Native gold (micron-sized), pyrite with arsenian rims, arsenopyrite, stibnite, realgar, orpiment, cinnabar, fluorite, barite, rare thallium minerals.

GANGUE MINERALOGY (Principal and subordinate): Fine-grained quartz, barite, clay minerals, carbonaceous matter (late-stage calcite veins).

ALTERATION MINERALOGY: Strongly controlled by local stratigraphic and structural features. Central core of strong silicification close to mineralization with silica veins and jasperoid; peripheral argillic alteration and decarbonation (“sanding”) of carbonate rocks common in ore. Carbonaceous material is present in some deposits.

WEATHERING: Nevada deposits have undergone deep supergene alteration due to Miocene weathering. Supergene alunite and kaolinite are widely developed and sulphides converted to hematite. Such weathering has made many deposits amenable to heap- leach processing.

GENETIC MODELS:

a) Epithermal model: Once widely accepted but now discounted for most deposits. Mineralization was thought to result from shallow Miocene magmatism related to basin and range extension. New discoveries of deep orebodies, overprinting basin and range deformation, and recognition of a supergene origin of alunite have cast doubt on this model.
b) Distal skarn model: Currently very popular because many deposits occur near intrusions, skarns and calcsilicate rocks. Carbonate-hosted disseminated gold is thought to be related to collapse of intrusion-centred porphyry-type hydrothermal systems. Although compelling for many deposits, this model fails to explain several districts (e.g., Jerritt Canyon; Guizhou, China) where no related magmatism has been observed.
c) Deep crustal fluid model: Recently proposed to account for inferred deep mixing of different fluids from different reservoirs as demanded by light stable isotopic and fluid inclusion data. Variants of this model imply only indirect links to magmatism, suggest a single Paleogene age for the Nevadan deposits and relate them to a unique period of pre-basin and range crustal extension and associated faults that are controlled by pre-existing Paleozoic and Mesozoic structures.

ORE CONTROLS:

1. Selective replacement of carbonaceous carbonate rocks adjacent to and along high-angle faults, regional thrust faults or bedding.
2. Presence of small felsic plutons (dikes) that may have caused geothermal activity and intruded a shallow hydrocarbon reservoir or area of hydrocarbon-enriched rocks, imposing a convecting geothermal system on the local groundwater.
3. Deep structural controls are believed responsible for regional trends and may be related to Precambrian crystalline basement structures and/or accreted terrane boundaries.

ASSOCIATED DEPOSIT TYPES: Porphyry (L04, L05), Au, W or Mo skarns (K04, K05, K07), polymetallic manto (J01).

COMMENTS: B.C.: 1. Limestone fault slices (part of accreted Stikine terrane) which have been intruded by felsic plutons, especially near high-angle fault zones, may host deposits (e.g., Golden Bear mine area). 2. Interior Plateau region - if carbonate units present - potential basin and range setting.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE: Two geochemical asemblages - Au+As+Hg+W or ? Mo and As+Hg+ Sb+Tl or Fe. NH3 important in some deposits. Au:Ag 10:1 or greater. Anomalous values in rock: As (100-1000 ppm); Sb (10-50 ppm); Hg (1-30 ppm).

GEOPHYSICAL SIGNATURE: Resistivity lows for some deposits. Aeromagnetic surveys may highlight spatially associated intrusions, skarns if present and possibly regional trends.

OTHER EXPLORATION GUIDES: In Nevada the deposits exhibit regional alignments or trends. Satellite imagery is useful to identify regional structures.

ECONOMIC FACTORS

TYPICAL GRADE AND TONNAGE: Grades range from 1 to 35 g/t Au and deposit sizes from 1 to 150 Mt of ore. For 43 significant deposits the median tonnages and grades for low-grade oxide and higher grade hypogene deposits are 20 Mt grading 1.2 g/t Au and 6 Mt containing 4.5 g/t Au, respectively. Supergene deposits amenable to heap leaching typically grade 1-2 g/t Au; whereas, production grades for deposits with hypogene ore typically grade 5 to 10 g/t or greater.

ECONOMIC LIMITATIONS: Parts of deposits are amenable to open-pit mining and heap leaching (especially oxidized zones), but roasting and autoclave extraction is required for more refractory ores. New discoveries of high-grade hypogene ore have resulted in increased underground mining.

IMPORTANCE: Between 1965 and 1995, deposits along the Carlin Trend (70 x 10 km), have yielded approximately 750 t of Au. Deposits that are unquestionably of this type are not presently known in Canada but may be present.

REFERENCES

Bagby, W.C. and Berger, B.R. (1985): Geologic Characteristics of Sediment-hosted, Disseminated Precious-metal Deposits in the Western United States; in Geology and Geochemistry of Epithermal Systems, Berger, B.R. and Bethke, P.M., Editors, Reviews in Economic Geology, Volume 2, Society of Economic Geologists, pages 169-202.

Bakken, B.M. and Einaudi, M.T. (1986): Spatial and Temporal Relations Between Wallrock Alteration and Gold Mineralization, Main Pit, Carlin Gold Mine, Nevada; in Proceedings of Gold '86 Symposium, Macdonald, A.J. , Editor, Geological Association of Canada, pages 388-403.

Berger, B.R. and Bagby, W.C. (1991): The Geology and Origin of Carlin-type Gold Deposits; in Gold Metallogeny and Exploration, Foster, R.P., Editor, Blackie, Glasgow and London, pages 210-248.

Christensen, O.D., Editor (1993): Gold Deposits of the Carlin Trend, Nevada; Society of Economic Geologists, Guidebook Series, Volume 18, 95 pages.

Cunningham, C.G., Ashley, R.P., I.-Ming, C., Huang, Z., Wan, C. and Li, W. (1988): Newly Discovered Sedimentary Rock-hosted Disseminated Gold Deposits in the People’s Republic of China; Economic Geology, Volume 83, pages 1462-1467.

Kuehn, C.A. and Rose, A.W. (1992): Geology and Geochemistry of Wall-rock Alteration at the Carlin Gold Deposit, Nevada; Economic Geology, Volume 87, pages 1697-1721.

Radtke, A.S., Rye, R.O. and Dickson, F.W. (1980): Geology and Stable Isotope Studies of the Carlin Gold Deposit, Nevada; Economic Geology, Volume 75, pages 641-672.

Romberger, S.B. (1988): Disseminated Gold Deposits, in Ore Deposit Models, Roberts, R.G. and Sheahan, P.A., Editors, Geoscience Canada, Ore Deposits #9, Volume 13, No. 1, pages 23-31.

Sawkins, F.J. (1990): Sediment-hosted Gold Deposits of the Great Basin; in Metal Deposits in Relation to Plate Tectonics, Springer-Verlag, pages 159-162.

Schroeter, T.G. (1986): Muddy Lake Project; in Geological Fieldwork, 1985, B. C. Ministry of Energy, Mines and Petroleum Resources, Paper 1986-1, pages 175-183.

Sillitoe, R.H. and Bonham, H.F. Jr. (1990): Sediment-hosted Gold Deposits: Distal Products of Magmatic-Hydrothermal Systems; Geology, Volume 18, pages 157-161.

March 28, 1996

[E02] [E03] [E04] [E05] [E06] [E07] [E08] [E09] [E13] [E14] [E15] [E16] [E17] [Published Profile Index]
[Deposit Profiles]

Last Updated September 13, 2007