Bulletin 105 is the first
comprehensive report published on the rich, metal endowed Tagish area in
northwestern British Columbia, and it is the first Bulletin to be produced
digitally for Web posting. The Bulletin is accompanied by Geoscience Map
1997-1 at 1:100 000 scale.
Bordered to the north by
the Yukon and straddling the Coast - Intermontane Belt contact, the Tagish
area overlaps a gold-antimony-arsenic geochemical province which is
anomalous on a provincial scale. Bulletin 105 illuminates the tapestry of
events that created mineralizing systems and reveals a billion years of
geological history within the broader context of Cordilleran evolution. In
addition to previously undocumented geological relationships, Bulletin 105
presents critical new data sets including more than 20 new isotopic age
dates, 60 new fossil ages, hundreds of analytical results, petrofacies
data, paleoflow measurements, age date compilations, and hypothetical
crustal cross sections.
Bulletin 105 covers the geology and mineral
deposits of the Tagish area, which is located in the northwest corner of
British Columbia. The area is bounded by the Yukon border to the north,
rugged Coast Mountains to the west, and Atlin Lake, British Columbia’s
largest natural water reservoir, to the east. It is an area with a colourful
mining history that blossomed during the Klondike gold rush and discovery of
the Atlin placers in 1898. It is richly endowed with mineral showings and
one mine, the Engineer, having produced over 560 000 grams of gold. A belt
of anomalously high regional gold-arsenic and antimony geochemistry extends
the length of the area, coextensive with the crustal-scale Llewellyn fault.
Three crustal fragments of strikingly different
character that converge in the Tagish area dominate the geology. In the east
are weakly metamorphosed, Carboniferous to Triassic oceanic plateau remnants
of the northern Cache Creek Terrane, here known as the Atlin complex. In the
west are two suites of metamorphic rocks that comprise a polydeformed belt
belonging to the Yukon-Tanana Terrane: a pre-Mississippian, quartz-rich
clastic succession of pericratonic origin; and a Devonian to Permian,
heterolithic suite interpreted to correlate with volcanic arc strata of the
Stikine Terrane. Sandwiched in between are Triassic arc, clastic arc apron,
and overlying Jurassic basinal strata of the Whitehorse Trough. They are
juxtaposed across two crustal-scale faults, the Nahlin to the east and
Llewellyn to the west, that brought the crustal fragments together, mainly
in Triassic to Middle Jurassic times. Geological interrelationships are
complicated by structural intermixing and by voluminous Late Cretaceous and
Eocene intrusion of the Coast Plutonic Complex. Pre-Jurassic deformational
histories of each crustal fragment are distinctive, but all are affected by
early Middle Jurassic, predominantly south and west-verging folds and
thrusts that shortened and stacked the fragments. Reactivation of major
faults and subsidiary splays is apparent from dextral offsets that affect
rocks as young as Eocene.
Quartz-rich clastic rocks of the Yukon-Tanana
Terrane are rifted relicts of ancestral North American margin across which
the Paleozoic and Mesozoic arc complexes of Stikinia and Quesnellia were
linked, similar to the manner in which the Aleutian and Japan arcs are
joined across ensialic crust of Kamchatka. Stikinia was rotated
counter-clockwise throughout the Early Mesozoic, entrapping relicts of
oceanic plateau of the Atlin complex prior to collision with Quesnellia in
early Middle Jurassic time. Near orogen-parallel fault displacements
modified the terrane distribution into the Early Tertiary.
Yukon-Tanana terrane, Whitehorse Trough and
Atlin complex display different styles of mineralization. In the Atlin
complex, gold-quartz veins are developed in mafic and ultramafic rocks.
Although important placer production has been has been attributed to this
source, no significant past gold production has come from lodes. Oceanic
mafic rocks are also prospective for Cyprus-type massive sulphide
copper-zinc mineralization, but none is known here.
Old Yukon-Tanana Terrane rocks were deposited in
part during rifting of the North American continental margin, a setting in
which sedimentary exhalative deposits accumulated elsewhere in the
Cordillera. Younger Yukon-Tanana terrane rocks were deposited in a volcanic
arc environment. The arc package includes felsic submarine volcanics, which
may have correlatives in the Tulsequah area where such rocks host
Kuroko-style volcanogenic massive sulphide accumulations like at the
Tulsequah Chief deposit. Upper Triassic arc rocks of the Whitehorse
Trough are lithologically and temporally equivalent to those hosting
important copper-molybdenum-gold porphyry deposits in southern British
Columbia. Although they are not voluminous, synsedimentary volcanic rocks in
the Early Jurassic trough strata may hold potential for shallow subaqueous
hot spring deposits rich in gold and silver like those at the Eskay Creek
mine.
Cretaceous and
younger plutons cross the crustal fragment boundaries. Cretaceous plutons
produce copper skarn mineralization where they cut Upper Triassic carbonates
in the Whitehorse Copper Belt, and the southern end of the belt may extend
into the Tagish area. Tertiary plutons and coeval volcanic rocks are
associated with gold skarn mineralization in the northern Tagish area, and
with epithermal gold mineralization in both the Tagish area and southern
Yukon.
Crustal-scale
faults, as well as related secondary faults, provide conduits for pluton
emplacement and subsequent mineralizing hydrothermal systems. Thus, they are
important environments for thermal aureole gold deposits.
Deep epithermal gold mineralization at the
Engineer Mine developed adjacent to splays of the Llewellyn fault, and is
probably coeval with a nearby Eocene volcanic centre. High mineral
potential exists in the Tagish area for a number of deposit types.
Juxtaposition of three disparate crustal fragments has created mineral
exploration opportunities as varied and challenging as the geology.
