Abstract

Three separate stratiform Pb-Zn deposits occur on the Jason property in the Macmillan Pass area of the Yukon, Canada. The deposits are contained within chert conglomerates and finer grained clastic rocks of Middle to Late Devonian age. The deposits are on the eastern margin of the Cambro-Ordovician Selwyn basin and are associated with a fault-bounded, second or third-order subbasin, referred to as the MacPass graben.Three different facies of mineralization and associated sedimentation have been defined on the basis of field relationships, drill core descriptions and correlations, and detailed petrography, Facies A comprises massive to thick-bedded galena and sphalerite with abundant pyrrhotite and pyrite. This facies is extensively crosscut by galena-bearing quartz siderite and quartz ankerite veins and has the highest combined Pb-Zn-Ag concentrations. Facies A is proximal to the hydrothermal source represented by the veins. Facies B has a mineralogic assemblage and textures common to facies A but is considerably thinner. Abundant soft sediment deformation features indicate facies B formed from slumping of unstable accumulations of material from facies A. Facies C contains well-bedded sphalerite, chert, and barite laminae and is distal to the hydrothermal source. Secondary alteration and pore-filling minerals found in facies C include celsian, kaolinite, ankerite, and a complex suite of rare Ba carbonates.Fluid inclusions in quartz, siderite, and ankerite within veins crosscutting facies A indicate that the ore-forming fluids were NaCl brines (9 equiv. wt %) with temperatures of approximately 250 degrees C. Sulfur isotope data indicate that the sulfides within the three separate deposits had a common sulfur source, probably reduced seawater sulfate. Barite in the laminated facies C has delta 34 S values consistent with coeval Devonian seawater. The sulfide minerals are approximately 10 per mil lighter than coeval seawater.A maximum water depth of 420 m for venting of hydrothermal fluids was determined by combining fluid inclusion data with boiling curves for H 2 O-NaCl. Brine densities indicate the hydrothermal plume may have reached the seawater surface and dispersed laterally to form the barite in facies C. Accumulations, on unstable slopes, of thick deposits of sulfide slumped to cause thinning and soft sediment deformation of facies B. Since facies B is a sedimentary feature, not necessarily proximal or distal to the vent, it may or may not contain barite.

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