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Geology, geochemistry, and copper occurrences of the Mid Proterozoic Gateway Formation (member A of Kintla formation), Purcell Supergroup, SW. Alberta and SE. British Columbia, Canada
Abstract
The lower 650 to 2000 feet of the Gateway Formation of the Mid Proterozoic Purcell Supergroup in SW Alberta and SE British Columbia consists of oxidized, alluvial, red, fine to medium-grained sandstones. A 4 to 35 foot thick gray-green bed located in the upper middle part of the redbed sequence is the focus of this study. The gray-green unit may be subdivided into three subunits based on mineralogy, geochemistry and, therefore, rock type. Subunit A, the lower part of the unit, is 0 to 4 feet thick and marks the beginning of a reducing environment. This subunit consists of slightly metamorphosed, calcarous siltstone and argillites. Subunit B consists of 2 to 8 feet of argillaceous dolomite, siltstone and micrite. Subunit C represents the upper part of the reduced unit and is composed mostly of argillaceous, micritic dolomite and some siltstone. Sedimentary structures such as ripple marks, intraclasts, peloids, desiccation cracks, and possibly stromatolites indicate a sedimentary regime that comprised a tidal flat environment fringing an arid shoreline. Other common sedimentary features such as water escape structures, load casts, and heavy minerals sagging are the result of late stage sediment deformation. Cu mineralization ranges from background ($<$100 ppm) to about 0.5% and is hosted by the gray-green unit, particularly subunit B, where Ba and Mo concentrations are high. Within the unit P$\sb2$O$\sb5$ increases upward, but Zr, Fe, Hg, Zn, and V decrease. Se show no significant variation. Geochemical study shows that TiO$\sb2$, Hg, Zn and B are concentrated in the mica fraction of the rock. Ore petrography reveals veinlets and disseminated chalcocite (Cu$\sb2$S) and/or digenite (Cu$\sb{1.8}$S); bornite (Cu$\sb5$FeS$\sb4$), covellite (CuS) and blue-remaining covellite (Cu$\sb{1.28}$S). Electron microprobe analysis shows that chalcocite is intergrown with djurleite (Cu$\sb{1.96}$S) and anilite (Cu$\sb{1.75}$S). Commonly, chalcocite replaces bornite and covellite and sulfides replace primary quartz and carbonate minerals. Sulfide distribution, replacement textures, and relationship of sulfides to the host rocks support a diagenetic model for the origin of copper sulfides in the Gateway Formation.
Subject Area
Geology
Recommended Citation
Al-Khirbash, Salah A, "Geology, geochemistry, and copper occurrences of the Mid Proterozoic Gateway Formation (member A of Kintla formation), Purcell Supergroup, SW. Alberta and SE. British Columbia, Canada" (1987). ETD collection for University of Nebraska-Lincoln. AAI8803738.
https://digitalcommons.unl.edu/dissertations/AAI8803738