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Characterization of Eocene Paleogeography, Magmatism, and Ore Deposition of the Piñon Range, Nevada: New Insights into an Integrated Carlin Gold Deposit Model
AdvisorGordon, Stacia M.
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Carlin-type deposits of Nevada constitute the second largest concentration of gold on earth. However, discovery and study of these systems is difficult due to the fine-grained nature of the alteration and ore mineralogy. Compounding this difficulty is the fact that for exploration in general, most easily recognizable deposits in the near surface have already been discovered. As the depth to concealed targets increase over time, so does the obscurity of their distal surficial expressions. To help overcome the challenges of deeper exploration, it is necessary to build a more integrated conceptual models of deposit formation that includes their tectono-magmatic settings and their relationship to crustal architectural controls at the global, province, district, and deposit scales. The Piñon Range, or Rain-railroad mining district, of the southern Carlin trend is an ideal location for studying the various geological aspects required to generate such an integrated model for Carlin-type deposits. Geochronology and geochemistry results show that all high- and low-temperature deposits throughout the Piñon Range were produced during the Middle Eocene. They are associated with the most intense pulse of magmatism from 38.5-37.5 Ma within the Robinson Mountain volcanic field (RMVF). Emplacement of the 38.2 Ma Bullion granite stock exposed at the core of the Piñon Range is directly related to adjacent high-temperature polymetallic skarn and porphyry-style mineralization. Slightly more distal, low-temperature, and precious metal-rich, Carlin-style mineralization also overlaps with the most intense pulse of RMVF magmatism, and is likely related to one or more coeval intrusive phases. Depth estimates from district-wide paleosurfaces and minor thermochronology modeling reveal that all high- and low-temperature deposits were emplaced in the near surface environment of the ‘Nevadaplano’ plateau. The porphyry-skarn system formed at depths of <3 km (likely ~1.5 km), and the classic Au-only and atypical Ag-enriched Carlin systems formed mostly at depths <500 meters. A district-wide geochemical zonation was identified from multielement and precious metals geochemical data. Combined, the above spatial, temporal, and geochemical overlap confirm a genetic link between the various deposit types. These results reveal a larger, interconnected magmatic-hydrothermal system composed of a central high-temperature porphyry-skarn hydrothermal source, an adjacent atypical Ag-enriched Carlin-type intermediate stage, and a final low-temperature classic Au-only Carlin-type distal expression. The magmatism present within the Piñon Range consists of hydrous, high-K calc alkaline, metaluminous to peraluminous intrusive and extrusive rock derived from a metasomatized sublithospheric mantle. The one exposed cupola (i.e., Bullion stock) directly responsible for both porphyry-skarn and Carlin mineralization in the central RMVF is a highly-differentiated, K-rich granite with elevated fluorine values. The fluorine content of this evolved melt is interpreted to be an important component of the underlying magmatic system responsible for the metals-rich mineralization throughout the study area. Recognition of the above genetic and magmatic links was facilitated by the shallow emplacement depth of the high- and low-temperature deposits and the source igneous center within the Piñon Range study area. The sharp geochemical and physiochemical gradients inherent in the near surface environment greatly condensed the spatial distribution of alteration and metals zonation into an observable district-scale footprint. The presence of an overlying coeval lake associated with the Eocene Elko Basin likely enhanced these gradients. The lake additionally provided a means to suppress boiling that facilitated the transformation of the initial high-temperature, chloride-rich magmatic brine into a distal, low-temperature, bisulfide-rich hydrothermal fluid; a process necessary for producing both porphyry-skarn and Carlin-type mineralization from a common hydrothermal source within the larger magmatic-hydrothermal system. Conceptual strategies for finding future Carlin systems moving forward would be to identify regions that can produce similar tectono-magmatic conditions, locate highly-differentiated, Fluorine-rich igneous centers, determine the emplacement depth of the potential magmatic-hydrothermal system of interest, and scale subsequent porphyry-skarn through Carlin-style exploration targets accordingly.