Abstract

The Sustut copper deposit of north-central British Columbia occurs in the upper member of the Upper Triassic Moosevale Formation, Takla Group, a unit composed of volcanic conglomerate with local tuffaceous layers. The metamorphic grade of the host rocks is of the prehnite-pumpellyite facies. This grade is higher than that of the same unit at equivalent stratigraphic levels elsewhere in the area. The copper minerals, mainly chalcocite, bornite, chalcopyrite, and native copper, are epigenetic and occur with quartz, epidote, prehnite, and carbonate in veinlets and in tabular zones parallel to bedding. Pyritized rock surrounds some copper-bearing tabular zones. A crude vertical zonation is apparent in the tabular zones. It grades from margin to core as pyrite-chalcopyrite-bornite-chalcocite-native copper. The tabular copper zones and local sets of veinlets appear to have formed interconnected channelways permeable to ore fluids that presumably were derived from below. Sequential development of ore minerals, as indicated by paragenetic studies and distribution of the ore minerals, requires an ore fluid initially more oxidizing than the basaltic country rock. The upward flow of the hydrothermal solutions within a structural zone, now defined by swarms of Cu-bearing veinlets, produced a confined geothermal high that resulted in a local node of prehnite-pumpellyite facies metamorphism and a concomitant development of copper minerals and pyrite. The tabular zones represent loci of fluid-wall rock reaction, and the ore-mineral zoning may be attributed to diffusion outward from these loci.

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