A Genetic model for the Sustut copper deposit, North-Central British Columbia

Host of the Sustut Copper deposit, 230 miles (370 km) northwest of Prince George, is a 600 m thick sequence of volcaniclastic rocks of the upper member of the Upper Triassic Moosevale Formation. This unit strikes northwesterly and dips about 20 degrees to the south. Regional stratigraphic nomenclature has been rather confused but the latest subdivision by Monger and Church (1977) was used for this report. The Moosevale Formation is mostly conglomerate and volcanic breccia with local discontinuous and thin layers of tuffaceous rocks. Fragments in the unit are mainly basic to intermediate volcanic rock. Six relatively common types of fragments are recognizable as being derived from coeval volcanism. Logging of 10,000 ft of diamond drill core representing four cross-sections through the main ore zones was done in a rigorous manner on a coding form designed for input into a computer. Computer output led to a rabid visual and quantitative evaluation of the data, particularly as regards to physical characteristics and internal stratigraphy of the host unit. No stratigraphic subdivision within the upper member of the Moosevale Formation was accomplished. Thin section study of metamorphic mineral assemblages indicated the metamorphic grade of the host rocks is prehnite-pumpellyite facies. This grade is higher than that in other rocks of the same lithologic units at equivalent stratigraphic levels elsewhere. Copper-rich zones with economic potential appear to be restricted to the upper 60 m of the 600 m thick, coarse-grained volcaniclastic host, though there are minor occurrences throughout. 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. Veins are negligible in total volume of mineralized rock and formed mainly by open space filling; both open space filling and metasomatism were operative to variable degrees in tabular mineralized zones. A crude vertical zoning is apparent in some tabular zones, where the idealized sequence from margin to core is: pyrite-chalcopyrite-bornite-chalcocite-native copper. Most tabular zones show neither the complete sequence nor a perfectly symmetric distribution of zones about a native copper core, and in some cases there is no semblance of a zonal distribution of ore minerals. Specific ranges of textural parameters (such as sorting, grain size, etc.) of the host rock, have been found to have been particularly amenable to the occurrence of copper minerals. These quantified textural parameters have been.compared statistical1y with copper mineralization and illustrate the correlation of copper minerals with volcaniclastic rocks that have sedimentary features indicating a high porosity and probably a high permeability prior to mineralization. These, porous units within the vol caniclastic pile are represented by the tabular copper zones. Copper minerals and non-metallic metamorphic minerals were precipitated from the same solutions, both in veins and relatively porous tabular zones. A limited geothermometry, based on sulphide phase relations and an interpreted origin to intergrowth textures, is consistent with temperatures of deposition of 250 - 300*C indicated by gangue mineral assemblages. 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. Age of mineralization is uncertain but vertical mafic dykes that cut the Moosevale Formation stratigraphically above the main mineralized zones are cut by veins and thus are premineralization. The upward flow of the hot solutions within a structural zone defined now by swarms of Cu-bearing veinlets produced a confined geothermal high that resulted in a local node of prehnite-pumpel1yite facies. metamorphism and con-comittent development of copper minerals and pyrite. Tabular zones with interstitial ore fluid represent loci of f1uid-wal1 rock reaction and ore-mineral zoning may be attributed in-part to diffusion outward from these loci. Sequential development of ore minerals as indicated by paragenetic studies and the observed zonal distribution requires an ore fluid initially more oxidizing than the basaltic country rock. Continued reaction to produce first native copper followed" successively by chalcocite, bornite, chalcopyrite and finally pyrite, resulted in the ore solution nearing equilibrium with the host.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat