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Gataga District - Cirque, Akie, Elf, Driftpile, Bear |
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British Columbia, Canada |
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Zn Pb Ag Ba
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Cirque deposit is the most significant of several stratabound barite-zinc-lead occurrences that have been found in Devonian carbonaceous shales of the Aikie district in north-eastern British Columbia. It was discovered within a group of claims pegged in 1977, during a regional joint venture exploration program undertaken by Cyprus Anvil mining Corporation and the Hudson Bay Oil and Gas Co (Jefferson, etal., 1983).
Published resource figures for the main Cirque deposits include:
40 Mt @ 7.8% Zn, 2.2% Pb, 47 g/t Ag (Reserves, 1981, Jefferson, et al., 1983).
33 Mt @ 8% Zn, 2% Pb, 57 g/t Ag (Reserves, 1983, Goodfellow, et al., 1993).
The adjacent South Cirque deposit contains:
15.5 Mt @ 6.9% Zn, 1.4% Pb, 32 g/t Ag
Both contain 45 to 50% BaSO4. In addition to Cirque, a number of other less significant Zn-Pb, pyrite and/or barite occurrences have been found, making up the larger Gataga District of north-eastern British Columbia. These include Driftpile and Bear approximately 80 and 65 km respectively to the north of Cirque, and Fluke, Akie and Elf within 40 km to the south.
Click here for a regional setting image.
These deposits are hosted by middle Ordovician, early Silurian and late Devonian rocks in a 180 km long belt within the north-west trending Kechika Trough, the south-eastern extension of the Selwyn Basin (McIntyre, 1983). These deposits, prospects and occurrences are hosted by sequences range in age from the middle Ordovician rocks of the Kechika Group, to the early Silurian and upper Devonian of the Road River Group1.
Cirque is a lensoid, stratabound barite-sulphide body, some 1000 m long, by 300 m wide and 2 to 70 m thick. Its north-eastern margin is exposed at the surface. The South Cirque deposit is a similar body approximately 1 km to the south-east of the main Cirque body which had been drilled over an area of 700 x 250 m with thicknesses of 2 to 30 m, although it had not been closed to the north, south and east in 1983 (Jefferson, et al., 1983).
Geology - Sediments in the Akie district range in age from Cambrian to Triassic. Within the Ordovician to Silurian Road River sequence, there is a lateral transition from shale basin sedimentation to the south-west, to carbonate platform facies by around 25 km to the north-east of Cirque. The Road River Formation is represented by a uniform basinal succession in the Cirque area. In contrast, sediments of the Devono-Carboniferous (Mississippian) "Black Clastic" or Earn Group sequence indicates more localised north-west trending depositional troughs. The Cirque deposit is hosted within the Earn Group (Jefferson, et al., 1983).
Structurally, Cirque falls within the Rocky Mountain Fold and Thrust (Foreland) Belt, the southern continuation of the Mackenzie Thrust Belt. The dominant structural style consists of tight, asymmetric, north-east verging folds, bounded by north-east directed reverse faults. The Earn Group is preserved in north-west trending synformal keels and thrust panels. The main deformation is considered to have taken place in the late Cretaceous to Tertiary, ie., Laramide (Jefferson, et al., 1983).
Both the Cirque and South Cirque deposits are within an upright, south-west dipping limb of Earn Group strata which has been overthrust by a structurally complex panel of highly imbricated Road River Group rocks. Primary bedding within the slightly imbricated Earn Group dips moderately to the south-west. In contrast the overlying, older, Road River Formation is more strongly deformed (Jefferson, et al., 1983).
Mineralisation at Cirque - Cirque is a banded deposit which comprises a continuum from nearly 100% barite to near 100% sulphides. It also contains interbedded siliceous, carbonaceous, shale and siliceous siltstone which may locally be correlated between drill holes. Overall these beds account for about 10% of the deposit. Most of the siltstones are actually intra-formational breccias with a shale-siltstone matrix (Jefferson, et al., 1983).
Contacts between sulphide bearing rocks and clastics are sharp, except for those between intraformational breccias which include clastic clasts in a sulphide rich matrix. These breccias generally grade into banded sulphides. Locally the contact between sulphidic and enclosing siliceous shales is a gouge or shear zone. Stockworks, disturbed bedding intervals or alteration halos are not visible within the surrounding Gunsteel Formation, although minor barite and sulphide veins have been intersected in the footwall at two localities. The major minerals in decreasing order of abundance are barite, pyrite, sphalerite and galena. No silver or copper minerals have been identified, while little carbonate is evident within the deposit. It is assumed that the silver is contained in solid solution within the pyrite, sphalerite and/or galena (Jefferson, et al., 1983).
Three mineral facies are recognised within the deposit (Jefferson, et al., 1983), namely:
(1) Baritic facies - which comprise pale grey to white, diffusely laminated, fine to medium grained barite with less than 40% sulphides. The sulphides occur as discontinuous, 1 to 5 cm thick, wavy, lenticular laminations of pyrite, sphalerite and minor galena. Sulphide laminae are composed of framboidal pyrite in a matrix of interlocking barite and sphalerite grains. Framboids range from single grains to 25 µm clusters. Galena is remobilised and commonly occurs at grain boundaries and as fracture fillings in barite. A small proportion is present as a breccia with barite clasts in a shale and sulphide matrix. Baritic facies rocks are most common on the southern end of the deposit.
(2) Pyritic facies - are distinguished from the baritic facies by their greater sulphide, >40%, content. They range from diffusely intercalated sulphides and barite to almost 100% sulphides. The constituent mineralogy is pyrite, barite, sphalerite and galena. Pyrite occurs as framboidal clusters with subhedral pyrite overgrowths. Fractures in the large pyrite grains are filled with galena and sphalerite. Irregular colloform aggregates of 20 to 50 µm pyrite with galena and sphalerite interlayers and sub-spherical atoll structures with pyrite rimmed cores of galena or sphalerite are also seen. Sphalerite occurs as interlocking grains with pyrite and as fine grained laminations within the pyrite beds. Crosscutting, sharp edged veins to diffuse pods of coarsely crystalline barite with patches of coarsely crystalline galena are restricted to the pyrite facies. The pyritic facies predominates on the northern end of the deposit, and transgresses from the bottom of the mineralised zone to the east, to the top in the west.
(3) Laminar banded pyrite facies - are made up of 1 to 15 mm inter-laminae of pyrite and black siliceous shale. Individual pyrite laminae are densely disseminated 20 µm, spheroidal, granular, framboids of pyrite in a matrix of siliceous shale, with sparse granular sphalerite and galena. This facies occurs as a fringe to the deposit and as a minor constituent. It is primarily located to the east of and above the main mineralisation.
The axis of the thickest barite and sulphide development is aligned nearly north-south, parallel to the elongation of the deposit. The axis of the best grade and greatest thickness coincide in the northern part of the deposit, although overall the thickest part of the deposit lies just to the west of the highest grade portion of the mineralisation. Zinc predominates on the western margin while the lead content increases to the east (Jefferson, et al., 1983).
OTHER DEPOSITS
Driftpile and Bear - are about 15 km apart and approximmately along strike in a recessive section of the Gusteel formation expressed at surface by a zone of ferricrete, barite beds and/or barite "kill zones". Driftpile comprises several mineralised units localised within at least five different thrust bound panels which appear to represent different stratigraphic levels. Rocks underlying the mineralised units include variably silicified and carbonaceous black shale, argillite and non-siliceous mudstone. The mineralised bands consist of pyrite ± sphalerite, galena facies and barite-pyrite ± sphalerite, galena facies which total up to 100 m in cumulative thickness. In the central part of the mineralised unit these facies are massive, but towards the top they contain upwardly increasing amounts of interbedded black and grey shale to a hangingwall of turbiditic sediments including these same litologies with lesser calcite, barite and pyrite nodules. The Bear deposit has a similar stratigraphy but has only one mineralised unit of finely laminated barite and pyrite hosted by black carbonaceous and siliceous mudstone-shale.
The Fluke, Akie andElf deposits are 18, 20 and 35 km south of Cirque respectively. Fluke consists of stratiform, laminar banded pyrite with minor galena and sphalerite in siliceous and carbonaceous black shale of the Gunsteel formation. At surface it is a 1 m thick west dipping barite-pyrite band, which 200 m down dip is laminated pyrite with minor laminae of sphalerite and galena in a 20 m section of siliceous and carbonaceous black shale. Elf comprises massive barite and galena with minor pyrite and sphalerite at surface, while drilling encountered mainly laminar banded pyrite interbedded within siliceous and carbonaceous shale starting 40 m above the base of the Gunsteel Formation in the sub-surface. Mineralisation extends over a strike length of 500 m, with the best intersection being 11 m @ 13.8% combined Pb+Zn, 27 g/t Ag. This mineralisation is in a thrust panel structurally overlain by the Road River Group. Akie is a sheet like body with dimensions of 1600 m in strike length, 800 m down dip and a true thickness of 30 m composed of laminated to massive pyrite and barite with minor sphalerite and galena in a siliceous and carbonaceous black shale of the Gunsteel Formation at a simialr stratgraphic position to Elf.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Carne R C, Cathro R J 1982 - Sedimentary exhalative (sedex) zinc-lead-silver deposits, northern Canadian Cordillera: in CIM Bull v75, no. 840 pp 99-113
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Insley M W, 1991 - Modification of sedimentary barite textures during deformation, Gataga District, NE British Columbia : in Ore Geology Reviews v6 pp 463-473
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MacIntyre D G 1982 - Geologic setting of recently discovered stratiform barite-sulphide deposits in north-east British Columbia: in CIM Bull v75 pp 99-113
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McClay K R, 1991 - Deformation of stratiform Zn-Pb(-barite) deposits in the northern Canadian Cordillera : in Ore Geology Reviews v6 pp 435-462
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Paradis S, Nelson J L, Irwin S B E 1998 - Age constraints on the Devonian shale-hosted Zn-Pb-Ba deposits, Gataga District, northeastern British Columbia, Canada: in Econ. Geol. v93 pp 184-200
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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