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Upper Devonian stratiform barite-lead-zinc-silver mineralization at Tom claims, Macmillan Pass, Yukon Territory Carne, Robert Clifton

Abstract

The Macmillan Pass area is underlain by Hadrynian to Middle Devonian fine grained sedimentary strata and volcanic rocks of Selwyn Basin. Widespread occurrences of Upper Devonian debris flows and turbidites record uplift and erosion of older rocks to the west or northwest. Deposition of overlying, locally derived coarse clastic assemblages are related to subsequent formation of a graben-like, fault-bounded trough in the study area. Continued slow subsidence of the down dropped basin is reflected by anomalously large thicknesses of an overlying siliceous black shale unit. Upper Devonian strata are unconformably overlain by Mississippian(?) peri-tidal or shallow water clastic sedimentary rocks which record a gradual, northward sea level transgression. Paleozoic and older rocks are intruded by Cretaceous granitic bodies. Stratiform barite-lead-zinc mineralization on the Tom claims is contained in two tabular zones separated by a fault. Both zones occur at the transition between Upper Devonian locally derived coarse clastic rocks and overlying basinal shales. The two mineralized bodies together contain nine million tons of ore grade material averaging 8.6% Pb, 8.4% Zn and 2.8 oz/ton Ag, based on initial development work. The Tom West Zone, studied in detail, consists of seven stratiform mineral horizons, each with distinctly different characteristics. Ore textures vary from massive quantities of poorly bedded galena, sphalerite and pyrite to finely laminated barite and cherty argil lite with disseminated sulphide minerals. A mineralized and altered breccia body underlies the stratiform massive sulphide mineralization. Time-stratigraphic reconstructions of a cross-section through the Tom West Zone, with accompanying mineralogical and assay data, predicate a multi-stage genetic model. Each mineralizing event is time related to localized tectonic activity which resulted in the formation of depressions on the seafloor. Ore forming constituents were carried by geothermal fluids, ascending along cross-stratal permeability provided by deep-seated faults and venting to the seafloor through the breccia body. Exhalative fluids were initially relatively high temperature, cooling gradually through the life of the geothermal system. Observed metal and mineralogical zonation within the stratiform mineralization reflects these processes.

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