TESTING SILVER MOBILITY: AN INVESTIGATION INTO SUPERGENE SILVER ENRICHMENT AT THE ROCHESTER MINE IN PERSHING COUNTY, NEVADA

Supergene silver enrichment has been considered to be similar to processes that form chalcocite enrichment blankets in porphyry copper deposits. However, the immobility of silver halides raises questions whether this comparison is valid, and, if present, may simply be due to residual enrichment. Supergene silver enrichment at Rochester has been investigated by observing patterns in supergene oxidation, structures, and ore mineralogy and paragenesis relative to silver grade. A supergene silver enrichment blanket, similar to a porphyry copper deposit, does not exist at Rochester; however, evidence suggests high grade zones are enriched, not by residual processes but by transport of silver. High silver grades (>5 opt Ag) are largely controlled by faults, as well as the lithologic contact between rhyolites of the Weaver and Rochester Formations. Macroscopic characteristics most closely associated with high silver grades include structurally produced gouge zones, dark sulfides, and goethite-rich rocks. Acanthite is the dominant supergene Ag-bearing mineral recognized to date. Though silver does occur in immobile silver-halides (e.g., iodargyrite, chlorargyrite), as well as native silver and argentojarosite, abundant acanthite rims relict hypogene sulfides, including sphalerite, pyrite, and tetrahedrite strongly suggesting silver migrated downward along faults below the water table at which point fluids became reduced and precipitated acanthite. How much silver becomes fixed in situ as silver-halides, native silver, and argentojarosite likely depends on environmental conditions during weathering, which likely changed over time. Environmental conditions determining the mobility of silver include the availability of halides due to the salinity of groundwater, length and periodicity of weathering, and tectonic activity causing uplift thus down dropping the water table. While significant silver may have been fixed in the completely oxidized upper portion of the supergene profile, high grade silver mineralization at Rochester occurs in the mixed oxide-sulfide zones within faults, likely from downward moving fluids. Recognition of this fault-controlled enrichment is very important in mining and expanding low-grade disseminated silver deposits like Rochester.