Geology and Fluid Inclusion Properties of the Buffalo Canyon Intrusion-Related and Grantsville Epithermal/Mesothermal Systems, Nye County, Nevada
AdvisorMuntean, John L
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The Union district, located in northern Nye County, Nevada, hosts a variety of ore deposit types in strongly deformed and locally altered Triassic and Tertiary metasedimentary and volcanic rocks. Cretaceous mesothermal Ag-(As-Hg) mineralization at the Grantsville Fury Pit occurs as fine-grained pyrite, illite, and mm- to cm-scale straight-walled quartz veins and massive silicification of carbonate breccias with proximal Cu-Pb-Zn sulfides. The mineralization exploits permeability in folded and brecciated carbonates of the Triassic Luning and Grantsville Formations. These units are part of the northern, upright limb of a northwest-trending overturned anticline. The open pit is capped by northwest-trending jasperoid bodies and parallel bull quartz veins. Epithermal alteration in the Fury pit is centered around the core of the mesothermal deposit, zoning out into illite-illite/smectites-smectite over ~100 m laterally and vertically, strongly affecting Oligocene volcanics overlying the deposit; swarms of thin, sulfide-rich quartz veins accompany the clay alteration. Fluid inclusions in epithermal quartz are 2-phase, liquid-rich with vapor bubbles, and display rounded/equant to negative crystal forms, indicating formation at or above 230 ºC and 2-3 km paleodepth; those in mesothermal quartz were trapped at greater pressures, likely at >4 km paleodepth. A Ag-Cu-Pb-Zn-F-W skarn body occurs roughly 50 m beneath the epithermal system, overlapping the base of the mesothermal silicification. Jasperoid-hosted F-Ag mineralization with massive botryoidal/crystalline fluorite is found on the south side of the anticline. Four miles to the east, the Grantsville Summit epithermal Au-Ag-Hg-Sb deposit is hosted as coarser quartz veins in variably dolomitized limestones and limestone breccias with bladed calcite, extending into overlying bleached Oligocene tuffs. The highest grades occur in hematite-bearing dolomites. 2-phase liquid-rich fluid inclusions are highly abundant in the epithermal quartz at Grantsville Summit, commonly following primary growth zones in quartz grains with feathery rims. Comparison of fluid inclusions from these deposits with those in the Oligocene Buffalo Canyon reduced intrusion-related gold (RIRG) deposit indicates that the Grantsville-area mesothermal deposits formed much earlier and at greater depths, but farther from the parent intrusion than Buffalo Canyon. Epithermal alteration and mineralization at Grantsville and Grantsville Summit formed at similar paleodepths and in the same 21-25 Ma age range as Buffalo Canyon, but at lower temperatures, with distinct geochemical signatures, and from different parent intrusions than the intrusions associated with the Buffalo Canyon deposit to the north. Although the parent intrusions for the epithermal deposits were not identified, the exploration concept of identifying satellite intrusions in proximity to known intrusion-related mineralization, ideally with strong structural controls, is still valid in other mineral districts and in the western Nevada volcanic field. Fluid inclusions may be used to assess deposit relations and refine genetic models, particularly in well-preserved epithermal and mesothermal quartz-rich systems, but are not a universally applicable tool.