Abstract

In the Rocky Mountains of northeastern British Columbia, a Lower and Middle Devonian platform carbonate succession of six formations and graphic total thickness gives way laterally north and west to shales, siltstones, and carbonaceous limestones of the Besa River Formation, of deeper water origin. Promising mineralization, dominantly sphalerite with some galena, is located in carbonates near the platform margin, within the Muncho–McConnell–Stone, Pine Point – Sulphur Point, and Slave Point Formations. Mineralization is not obviously related to structure, unconformities, or evaporite solution. Sphalerite and galena are closely associated with white sparry dolomite and commonly accompanied by quartz and thermally altered reservoir bitumen. Sulphur isotope ratios of galena and sphalerite range from + 5.0 to + 17.5 δ34S‰, consistent with a reduced sulphate or petroleum-derived hydrogen sulphide source for sulphur.At Robb Lake, mineralization occurs within broadly conformable breccias in the Stone, Muncho–McConnell interval; sparry dolomite forms the breccia matrix; and bitumen is common although rare elsewhere in the interval. Maximum paleotemperatures of host carbonates at Robb Lake appear to have been about 200–230 °C, as determined by bitumen characteristics, fluid inclusion filling temperatures in quartz, and illite crystallinity from illite in Besa River Formation shales. The Paleozoic and Mesozoic sedimentary section in the region suggests that the Middle Devonian of the Robb Lake area reached a maximum burial depth of about 5 km during mid-Cretaceous time. Physical proximity of bitumen and sulphide mineral deposits suggests that similar processes were involved in petroleum maturation–migration, and metallic mineral precipitation. Besa River shales are seen as probable sources for both petroleum and metals; sulphide sources appear to have been hydrogen sulphide generated within carbonate host rocks. A genetic association of petroleum and metals implies that mineralization took place relatively late (?early Mesozoic), and under deep subsurface conditions.Ground preparation in the form of brecciation and fracturing is problematical; at Robb Lake it may be a consequence of hydraulic fracturing due to high pore pressures generated by dewatering of the fine clastic facies that envelope the host dolomites. A better knowledge of time of origin of these deposits will contribute greatly to an understanding of their genesis. Meanwhile, consideration of the burial and thermal history of the host rocks offers a useful framework within which the mineral deposits and their genesis may be assessed.

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