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Origin and Evolution of the Green Prospect Antimony Deposit, Churchill County, Nevada
AuthorSchumer, Benjamin Nathan
AdvisorMcCormack, John K
Geological Sciences and Engineering
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The Green Prospect is located in the Mopung Hills, northwestern Churchill County, Nevada, in an area previously covered by Pleistocene Lake Lahontan. It was worked for antimony during World War II and has lain idle since. Field mapping and petrographic analyses show that the Green Prospect mineral deposit occurs at a structural boundary between recent Miocene lithic rhyolite ash-flow tuff and basalt, and Triassic-Jurassic carbonate rocks tentatively correlated to the Auld Lang Syne group of Burke and Silberling (1973). Mineralization is controlled by and hosted in a high angle reverse fault structure within carbonate that likely formed as the result of Basin and Range extension. Petrographic and geochemical analyses demonstrate that the Green Prospect ore deposit is a low sulfidation epithermal antimony deposit characterized by quartz-sericite-pyrite (QSP) alteration of volcanics and pervasive silicification and patchy fluoritization of carbonate. Wet chemical assays show selenium concentrations of over 4% in the antimony ore, incorporated into stibnite by replacing sulfur in the structure. Preliminary sulfur isotope analyses show depletion of δ34S, interpreted as mineralization being deposited from magmatic fluids with contribution from crustal rocks. The deposit is no older than surrounding, hydrothermally altered, presumably Miocene basalt, and may be genetically tied to the nearby highly felsic Mopung Porphyry intrusive, described herein. Supergene oxidation of the deposit took place in two separate events, producing an earlier suite of secondary antimony oxides, antimonates, and native elements, and a second later suite of sodium-rich oxides and sulfates. The lack of any features indicating that the deposit formed while open to the surface, and the magmatic origin of the hydrothermal fluids as shown by sulfur isotope data indicate that no interaction between hydrothermal fluids and lacustrine waters of Lake Lahontan took place during the formation of the deposit. The deposit has geochemical and mineralogical characteristics that are similar to the Khaidarkan Sb-Hg deposit of Kyrgyzstan, and the Le Cetine di Cotorniano and Pereta deposits of Tuscany, Italy. Samples from the Green Prospect contain trace gold, silver, and arsenic concentrations that may indicate the possibility of associated Carlin-type precious metal mineralization.