Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 36-2
Presentation Time: 9:00 AM-3:30 PM


BLAKEMORE, Daniel, Department of Geology & Environmental Earth Sciences, Miami University, 250 S. Patterson Avenue, 118 Shideler Hall, Oxford, OH 45056, MCLEOD, Claire, Department of Geology & Environmental Earth Sciences, Miami University, Oxford, OH 45056 and KREKELER, Mark P.S., Geology & Environmental Earth Sciences, Miami University Hamilton, Hamilton, OH 45011

Round Mountain is a large open pit, low sulfidation epithermal deposit type gold mine, located in northern Nye County Nevada. Gold occurs as macrocrystalline deposits in the Paleozoic metasedimentary bedrock at the base of the mine, and as disseminated deposits in overlying volcaniclastic strata. This strata is specifically comprised of several heterogeneous pumice-bearing, variably welded, lithic-rich rhyolitic tuffs. This study focuses on the mineral associations within the altered non-welded, pumice rich rhyolitic tuff, designated as Type 2 ore. Thin sections of the Type 2 gold ore from Round Mountain were studied using polarized light microscopy, scanning electron microscopy, and backscattered electron microscopy. In addition, elemental analysis via X-ray energy dispersive spectroscopy was completed in house at Miami University. Preliminary observations of the Type 2 ore reveal a complicated geologic history and preserve clear evidence of late-stage hydrothermal alteration. Thin sections of the Type 2 ore have a pyroclastic texture with heterogeneous distribution of lithic clasts interpreted to be derived from the underlying Paleozoic metasedimentary strata. To date, identified lithologies include fine grained subarkosic sandstones, meta-sandstones, altered phyllites, and variably altered dolostones. Collectively, these lithics comprise <30% of all studied sections thus far. In addition, quartz grains of highly angular to subrounded shape constitute ~15% of the tuff with less abundant, significantly altered plagioclase feldspar grains (<5%). Euhedral pyrite is the most abundant heavy mineral in the Type 2 sections studied so far and small galena specks are consistently present within the pyrite crystals. Other minor phases include arsenopyrite, sphalerite, monazite, zircon, and apatite. The encompassing fine grained, ash-dominated, matrix constitutes ~50%. Future work will entail in situ analysis of chronologically significant minerals. Acquired ages could enhance the understanding of the timing of gold mineralization at Round Mountain.