Northeastern (46th Annual) and North-Central (45th Annual) Joint Meeting (20–22 March 2011)

Paper No. 2
Presentation Time: 8:00 AM-12:00 PM


MORRIS, Evan1, KELSON, Christopher R.1 and DARLING, Robert S.2, (1)Department of Geology, State University of New York, College at Potsdam, 44 Pierrepont Avenue, Potsdam, NY 13676, (2)Department of Geology, SUNY College at Cortland, Cortland, NY 13045,

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 δ18Omeas 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; δ34Smeas 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.