Abstract

The McLaren deposit is one of five sediment-hosted Au-Cu-Ag skarn and replacement deposits that lie within the New World district near Cooke City, Park County, in south-central Montana. The deposit is hosted by gently dipping micrite and dolomitic and calcareous shale of the Cambrian Meagher Formation and occurs along the southwestern contact zone of the Tertiary, dacitic Fisher Mountain intrusive complex.Hydrothermal alteration in the McLaren deposit is concentrically zoned relative to the Fisher Mountain intrusive complex and was controlled by sills, faults, and lithologic contacts. The earliest alteration is biotite hornfels in shale and recrystallization of limestone. This was followed by proximal potassic alteration and distal propylitic alteration of intrusive rocks and early epidote alteration of sedimentary rocks. Early epidote alteration consists largely of epidote (Ps (sub 24-33) ) and K feldspar with lesser amounts of amphibole, andraditic garnet (Ad (sub 32-99) 99), and diopsidic pyroxene (Hd (sub 10-30) ). Early epidote alteration occurs predominantly in the Park Formation where it formed an impermeable barrier to later Au-Cu-Ag-bearing hydrothermal fluids that replaced the underlying Meagher Formation.Pervasive sericitic alteration of intrusive rocks, genetically related to late epidote alteration, quartz-pyrite alteration, and magnetite-rich replacement assemblages in adjacent sedimentary rocks, postdates the early alteration assemblages. Late epidote alteration consists predominantly of epidote, amphibole, pyrite, magnetite, carbonate, and chalcopyrite. Quartz-pyrite alteration postdates late epidote alteration and is associated with chalcopyrite and phyllosilicates. Quartz-pyrite rock occurs as extensive replacements in the Meagher dolomitic limestone (with biotite, chlorite, and talc) and veins and disseminations in the underlying Wolsey shale (with muscovite).Gold mineralization of >1.37 ppm occurs with quartz-pyrite alteration, with lesser amounts related to late epidote alteration and magnetite-rich replacements. Native Au and electrum correlate with abundant pyrite (>30%), abundant chalcopyrite (>0.10%), and to a lesser extent, acicular magnetite. Although Au mineralization may occur both proximal and distal to the contact of the Fisher Mountain intrusive complex, magnetite/pyrite ratios, metal contents, and magnetite textures are systematically zoned within the Meagher Formation with respect to the intrusive complex in the northern part of the McLaren deposit.Fluid inclusions in epidote, garnet, and quartz indicate that early epidote alteration formed at temperatures >600 degrees C and from fluids with high salinities (>26 wt % NaCl equiv), whereas late epidote, quartz-pyrite, and magnetite-rich replacements, and the bulk of sulfide and gold mineralization, formed at lower temperatures between 240 degrees and 415 degrees C (avg 328 degrees C) from fluids with lower salinities (generally between 3.6-11.8 wt % NaCl equiv). The preponderance of pyrite and magnetite in the mineralized rocks suggests that the main stage of Au deposition was characterized by high sulfur and oxygen fugacities.

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