Bulletin 103: Geology, Geochemistry and Mineral Deposits of the
Akie River Area, Northeast British Columbia
(NTS 094F01, 02, 07, 10, 11)
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The Akie River area of the northeast Rocky Mountains has the potential to be one of the most important future Zn-Pb-Ag producing mineral districts in British Columbia. Here, large tonnage sedimentary exhalative barite-sulphide mineral deposits such as the Cirque, Mt. Alcock, Elf and the recently discovered Akie occur within an 80 kilometre long belt of Late Devonian basinal facies sedimentary rocks of the Kechika Trough.
Bulletin 103 summarizes the results of 3 years of mapping in the districts and covers all or part of the 94F/1,2,7,10 and 11, 1:50,000 scale NTS map sheets. Also included are updates on recent exploration activity and research in the district. The report describes in detail the geology of the district with emphasis on regional stratigraphy and structure. Separate chapters discuss the results of lithogeochemistry and sulphur isotope analyses, important mineral deposits and the regional metallogeny. In addition to the 22 figures, 8 plates and 7 tables that accompany the text, the Bulletin also includes a coloured 1:100,000 scale geological map with interpretive structural cross sections and appendices of geochemical analyses, fossil identifications, and mineral occurrences located during the course of mapping.
Thick westward prograding clastic wedges that were deposited along the margin of ancestral North America from Mid Proterozoic to Mid Paleozoic time. Periodic rifting and tectonic subsidence resulted in the formation of starved sub-basins along this otherwise passive continental margin. These sub-basins host important sedimentary exhalative barite-zinc-lead-silver deposits.
In northeast British Columbia, a thick succession of Paleozoic basinal facies clastic rocks is preserved within the northwest trending, Kechika Trough. Within the trough, sedimentary exhalative barite and barite sulphide deposits occur in starved basin sediments of Middle Ordovician, Early Silurian and Late Devonian age. The latter are the most economically significant and are hosted by carbonaceous cherty argillites and siliceous shales of the Middle to Late Devonian Gunsteel formation of the Lower Earn Group. The largest deposit is the Cirque, with reserves in excess of 35 million tonnes averaging 10 percent combined lead-zinc and 47 grams per tonne silver. Other potentially significant deposits include Driftpile Creek, Bear, Mt. Alcock, Fluke, Pie and Elf. Stratiform barite deposits are also common, particularly near the basin to shelf transition zone.
Late Devonian barite-sulphide deposits are typically zoned with interlaminated barite-sphalerite-galena-pyrite occurring near suspected vent areas and grading outward into bedded barite. Beds of laminar-banded pyrite typically occur in hangingwall siliceous shales, particularly above the inferred vent zone. Epigenetic stringer sulphide zones are not recognized and footwall alteration is generally absent, implying that the deposits formed from ponded brines at relatively low temperature. Away from the main deposits the favourable stratigraphic interval is recognized by the presence of thin beds of nodular and laminated barite and thin laminae of pyrite in the shales. Thin tuff beds also occur locally in this stratigraphic interval and are evidence for a Late Devonian volcanic event.
A paleogeographic reconstruction of the Kechika Trough in Late Devonian to Mississippian time suggests barite-sulphide deposits formed in third order starved basins between uplifted, northwest-trending horst blocks that were capped by shallow water carbonate banks. The presence of intraformational breccias suggests fault movement accompanied barite-sulphide precipitation. This tectonic activity may have allowed the escape of heated, overpressured metalliferous brines from permeable reservoirs within the sedimentary pile. Sedimentary exhalative deposits formed where these brines were exhaled onto the seafloor and ponded in anoxic seafloor depressions.
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