MINERALOGICAL AND GEOCHEMICAL STUDY OF A SULFIDE-RICH BRECCIA FROM THE HILLTOP GOLD DEPOSIT, LANDER COUNTY, NEVADA

The goals of this study were to mineralogically characterize a sulfide-rich breccia located on the eastern flank of the Hilltop Au deposit, Lander County, Nevada and to determine the conditions under which the breccia formed.

The matrix-supported breccia consists of variably-sized, angular, and altered clasts of Ordovician Valmy Formation quartzite, chert, siltstone, and argillite within a sulfide-rich matrix. The rock clasts were bleached and recrystallized prior to breccia formation and sulfide mineral deposition. The funnel-shaped breccia body trends N, dips vertically, and is offset normally by the Independence fault and floored by the stratigraphically-lower Hilltop mine fault; approx. breccia body dimensions: 300m x 50m x 300m.

Samples of breccia material were collected in-situ from the Hilltop deposit. Breccia matrix mineralogy was determined by transmitted and reflected light microscopy and electron microprobe analysis; sulfide minerals include, in order of abundance, chalcopyrite, pyrite, galena, fahlore ±Ag, sphalerite, and arsenopyrite; gangue minerals include quartz and barite. Based on textural evidence, mineral deposition within the breccia consists of at least eight distinct stages: 1. (oldest) fracturing of host rock, 2. pyrite, 3. fracturing, 4. pyrite ± galena, 5. fracturing, 6. chalcopyrite + pyrite + sphalerite + galena + fahlore + quartz + arsenopyrite + barite, 7. fracturing, 8. (youngest) Ag-fahlore + quartz.

Primary fluid inclusions (n=13) along stage 6 quartz growth zones exhibit no evidence of boiling and measured Th data are pressure corrected for 1km depth under lithostatic conditions (salinity ranges from 0.7-2.6 wt% NaCl; corrected Th values range from 217.0-268.9ºC, average 234.4ºC). Calculated δ¹⁸O signature of stage 6 quartz source fluid composition (based on fluid inclusion Th and corresponding δ¹⁸O_meas data from the stage 6 quartz) ranges from -5.1‰ to -2.7‰ and indicates that both meteoric and magmatic fluids were present during breccia formation; δ³⁴S_meas of sulfide S (chalcopyrite, pyrite, galena; n=4) range from +2.8‰ to +5.26‰ and probably also indicate the contribution of a magmatic component to breccia formation.

We believe that fluid inclusion data, textural evidence, and isotopic data support breccia formation within an epithermal-type environment.