Date of Award
Master of Science
The Wind Mountain (WM) and Willard-Colado (WC) deposits, respectively of Washoe and Pershing Counties, Nevada, represent recent episodes of low-sulfidation, epithermal gold-silver mineralization. They have the principal features of amagmatic epithermal deposits including 1) young ages (<7 Ma), with Willard-Colado at 6.03 ± 0.04 Ma and Wind Mountain at a Pliocene or younger age, 2) absence of associated coeval igneous rocks, 3) development along range-front fault zones, and 4) proximity to modern geothermal systems. Such deposits in the Great Basin are generated by deeply convecting meteoric hydrothermal fluids that derive their thermal energy from the region's high heat flow and metals and volatiles from the traversed rock packages or shallow mantle. Hydrothermal alteration assemblages at both WM and WC are zoned outward from controlling structures along stratigraphic horizons and subsidiary structures, grading through silica, silica-kaolinite, kaolinite, and hematite-rich zones. Silicic and argillic alteration represents main stage hydrothermal activity; significant steam-heated alteration lithocaps are absent. Weathering-related, acid-sulfate fluids overprinted mineralized rocks at both deposits, oxidizing pyrite, remobilizing Au, and producing widespread hematite-goethite-kaolinite-alunite alteration. Gold mineralization is unique at each deposit but formed in several general paragenetic stages. Element maps generated by laser ablation-inductively coupled plasma-mass spectrometry demonstrate that primary Au is in solid solution with pyrite or arsenian pyrite forming patchy concentrations and enriched rims. Secondary and possibly remobilized Au occurs as disseminated electrum and native gold along fractures in silicified rocks. Elevated concentrations of Ag, As and Sb spatially correlate with primary and secondary Au. A final stage is represented by gold in Fe-oxide/hydroxide rims on pyrite crystals or aggregates. The hydrothermal fluids at WM and WC dominantly consisted of meteoric water, based on their low δ18OH2O and δDH2O values calculated from quartz δ18O, kaolinite δ18O and δD, and calcite δ18O data and assumed temperatures characteristic of the epithermal environment. Black calcite prominently fills the range-front faults where at WM it displays a banded texture and contains acicular Mn-oxide inclusions and at Colado is part of a vein sequence grading from centrally distributed quartz, through quartz-calcite and calcite to distal gypsum. Calcite δ13C values are similar to those of igneous and some sedimentary carbon but distinct from values for tufas along the WM range-front fault and at Pyramid Lake. Alunite and gypsum have negative δ34S values indistinguishable from those of pyrite, as well as from H2S in the San Emidio geothermal fluids near WM. This isotopic correspondence results from kinetically inhibited, nonequilibrium isotopic exchange during the low temperature oxidation of reduced sulfur species. It is consistent with the surficial and near surface formation of gypsum, particularly at WM where lenticular gypsum beds are in stratigraphic proximity to sinter containing reed casts. Alunite δ34S values, combined with a psuedocubic habit and association with quartz, support an origin by weathering rather than steam-heated processes. The absence of steam-heated alteration at both deposits is likely related to a high water table during hydrothermal activity, which at WM was expressed by springs depositing sinter or gypsum. Collectively, the data for Wind Mountain and Willard-Colado are consistent with the amagmatic genetic model. Meteoric hydrothermal fluids circulating along range front faults assimilated C, S and potentially Au and other metals from organic bearing sedimentary±volcaniclastic and metasedimentary host rock packages and (or) sediments in adjoining Cenozoic basins. Although they lack a steam-heated alteration lithocap, such as those prominently displayed at the Hycroft and Florida Canyon amagmatic deposits, weathering pervasively altered the tops of the deposits following a substantial drop in the water table due to a change to a drier climate or uplift of the ranges due to local tectonism.
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