Au-Cu-Ag skarn and replacement mineralization in the McLaren Deposit, New World District, Park County, Montana
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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.Andradite
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Abstract Cu and Fe skarns are the world's most abundant and largest skarn type deposits, especially in China, and Au‐rich skarn deposits have received much attention in the past two decades and yet there are few papers focused on schematic mineral deposit models of Cu–Fe–Au skarn systems. Three types of Au‐rich deposits are recognized in the Edongnan region, Middle–Lower Yangtze River metallogenic belt: ∼140 Ma Cu–Au and Au–Cu skarn deposits and distal Au–Tl deposits. 137–148 Ma Cu–Fe and 130–133 Ma Fe skarn deposits are recognized in the Edongnan region. The Cu–Fe skarn deposits have a greater contribution of mantle components than the Fe skarn deposits, and the hydrothermal fluids responsible for formation of the Fe skarn deposits involved a greater contribution from evaporitic sedimentary rocks compared to Cu–Fe skarn deposits. The carbonate‐hosted Au–Tl deposits in the Edongnan region are interpreted as distal products of Cu–Au skarn mineralization. A new schematic mineral deposit model of the Cu–Fe–Au skarn system is proposed to illustrate the relationship between the Cu–Fe–Au skarn mineralization, the evaporitic sedimentary rocks, and distal Au–Tl deposits. This model has important implications for the exploration for carbonate–hosted Au–Tl deposits in the more distal parts of Cu–Au skarn systems, and Fe skarn deposits with the occurrence of gypsum‐bearing host sedimentary rocks in the MLYRB, and possibly elsewhere.
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Skarn deposits occur throughout the world and have been mined for a variety of elements. This paper describes the basic stages of skarn formation and the main causes of variation from the general evolutionary model. Seven major classes of skarn deposits (Fe, W, Au, Cu, Zn, Mo and Sn) are briefly described, and relevant geological and geochemical features of important examples are summarized in a comprehensive table. The important geochemical and geophysical parameters of skarn deposits are discussed, followed by a summary of important petrologic and tectonic constraints on skarn formation. Finally, exploration models are presented for several major skarn types, with a plea for field mapping as a fundamental basis for future studies.
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