Testing Silver Mobility: An investigation into supergene silver enrichment at the Rochester Mine in Pershing County, Nevada
AuthorAnderson, Tracy Loren
AdvisorMuntean, John L.
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Supergene silver enrichment, comparable to chalcocite supergene enrichment blankets in porphyry copper deposits, has long been called upon to explain high grades in silver deposits that diminish in grade with depth. Rochester is a large-tonnage, low-grade disseminated silver-(gold) deposit hosted in Early Triassic rhyolites located in northwestern Nevada, where the role of supergene processes in controlling the distribution of ore and silver grade remained unclear prior to this study. The effect of supergene processes at Rochester was investigated by observing patterns of silver occurrences in structures, weathering zones, and ore and gangue mineral paragenesis. The overall tabular geometry of the ore body straddling the Weaver-Rochester lithologic contact is defined by high grade zones in the lower Weaver oxide and upper Rochester oxide and mixed oxide-sulfide zones. Grades drop off significantly in the underlying protore sulfide zone. Protore hydrothermal alteration is dominated by quartz-sericite-pyrite with minor relict K-feldspar. Hypogene sulfides include pyrite, sphalerite, galena, tetrahedrite-tennantite, chalcopyrite, arsenopyrite, and rare molybdenite. Silver phases in the protore include silver-bearing tetrahedrite-tennantite; acanthite; and rare stromeyerite, pearceite, and polybasite. Geologic characteristics most closely associated with higher silver grades in oxide and mixed oxide-sulfide zones include faults, acanthite, and goethite-rich rocks. High grade zones (≥2 opt Ag) are controlled by high- and low-angle faults cutting the Weaver-Rochester contact. Acanthite is the dominant silver phase. Much of the acanthite formed by supergene enrichment based on its common occurrence with covellite as rims on hypogene sulfides in mixed zones. Chlorargyrite and native silver are commonly found in the oxide and mixed zone with acanthite. Other silver-halides, such as iodargyrite, are found in the oxide zone. Based on these observations, coupled with published phase equilibria, early oxidation of protore caused the formation of silver-halides resulting in a decrease in silver mobility, particularly when iodargyrite was stable. Upon oxidation, a small amount of silver is able to migrate to lower portions of the supergene profile forming acanthite. However, if iodide (I-) was not continually replenished the I/Cl ratio would have decreased and the chlorargyrite field would have expanded at the expense of iodargyrite, which would have allowed more downward transport of Ag+, as chlorargyrite is more soluble than iodargyrite. A dropping water table caused by uplift, erosion, and climatic changes, controlling silver deportment resulted in continued oxidation. As the water table dropped due to uplift related to Basin and Range extension, as well as decreasing precipitation from a cooling and drying climate, enriched zones would have oxidized. When groundwater was not saline chlorargyrite released Ag+ and upon oxidation acanthite released silver as AgHSo for downward transport. New acanthite could have then formed at the deeper water table.The amount of silver that became fixed as silver-halides and native silver in the oxide zone controlled the amount of aqueous silver in downward moving supergene fluids, which in turn controlled the extent and grade of supergene enrichment expressed as zones of abundant supergene acanthite. This was dependent on environmental conditions, including the salinity of groundwater, duration and periodicity of weathering, tectonic activity resulting in uplift and dropping of the water table, and composition of gangue minerals. The possibility that the overall tabular geometry of the Rochester ore body that sits just above the sulfide zone, resulted from supergene enrichment processes cannot be ruled out at this time without further work. Nevertheless, supergene silver enrichment certainly occurred at Rochester and resulted in high grade zones (≥2 opt Ag up to 70 opt) along high- and low-angle faults. Identifying the controls on such high grades is important for future exploration and mine planning.