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Petrography and Field Mapping of Eocene Intrusions and Adjacent Breccia Zones at the Scraper Springs Prospect, Elko County, Nevada
AuthorCantor, Bradford M.
AdvisorThompson, Tommy B.
Geological Sciences and Engineering
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The Scraper Springs project, held by Cordilleran Exploration Inc. (Cordex), is a multi-target prospect located in northwestern Elko, County, Nevada, approximately 8 miles northeast of the Midas district. Regionally, the property can be considered to be along trend with the northern-most projection of the Carlin trend, approximately 25 miles north of the Dee/Rossi area. The eastern limit of Northern Nevada Rift (NNR) is also considered to border the western margin of the Scraper Springs property. The geology at Scraper is characterized by an exposure of Paleozoic rocks from the upper-plate Vinini Formation above the Roberts Mountains thrust through a window of Eocene and Miocene volcanic cover. Additionally, there is an exposure of intrusive Eocene diorite, dated at 38.9 ± 1.0 Ma by K/Ar techniques, adjacent to the Paleozoic window. Exploration efforts at Scraper Springs date back to 1983; various exploration efforts since 1983 reveal three types of mineralized targets at Scraper Springs: Carlin-type mineralization hosted at depth below the Roberts Mountains thrust, epithermal vein-style mineralization hosted in volcanics, and polymetallic Au-bearing skarn mineralization hosted in Paleozoic sedimentary rocks in contact with the diorite intrusion. The present study employs field mapping techniques to document the geology and alteration of the diorite intrusion and adjacent silicified/advanced argillic lithocap breccias. Petrography and sodium-cobaltinitrite staining supplement field observations to aid in understanding alteration events. Geochemical data from topaz-rich samples are considered to help classify the hydrothermal system present at Scraper in addition to soil and rock chip samples. Finally, drill-hole data from 9 holes were logged and drawn in cross-section to offer one three-dimensional possibility of the hydrothermal system at Scraper. Results suggest the Eocene diorite intrusion has undergone significant potassic alteration. A separately-mapped unit of quartz syenite is also documented, revealing potassic alteration haloes around its margins. Petrography indicates this type of potassic alteration is overprinted by propylitic and ultimately sericitic events. Advanced argillic alteration in the breccia zones adjacent to the diorite intrusion is also profound and exhibits several different events. Early vuggy quartz-alunite alteration is common throughout both breccia zones and is followed by an advanced argillic assemblage containing zones of pyrophyllite, topaz, kaolinite, and alunite. Late chalcedony overprints much of the earlier alteration. In general, each pulse of advanced argillic mineralization is separated by a breccia event. Overall, the system at Scraper Springs appears to reflect that of a porphyry-epithermal transition zone. This evidence, coupled with high fluorine and molybdenum geochemistry, and alteration zoning patterns documented in the present study suggest future exploration at Scraper could credibly target a Climax-type molybdenum porphyry similar to the Mount Hope deposit in Eureka County, NV.