A PETROGRAPHIC STUDY OF THE SHOSHONE GRANITE, CENTRAL NEVADA: NEW INSIGHTS IN MESOZOIC ARC MAGMATISM AND OLIGOCENE MINERALIZATION AT THE ROUND MOUNTAIN GOLD MINE

The Cretaceous-aged Shoshone Granite of Central Nevada provides important constraints on the nature of Mesozoic arc magmatism and the geological evolution of the North American continent. The Shoshone Granite is located within the Toquima Mountain Range and defines the Type 8 unit in the Round Mountain Gold Mine (RMGM). The RMGM an open pit gold mine that has an exploration and operational history of over 100 years. Here, gold is extracted from Oligocene-aged, mineralized, pyrite-bearing andesitic to rhyolitic tuffs (Types 2,3,4) which unconformably overlie the Shoshone Granite. In this context, the Shoshone Granite is considered waste rock. This study focuses on samples of the Shoshone Granite collected externally to the RMGM and aims to constrain the extent of Oligocene mineralization as well as the timing of Mesozoic-arc related magmatism.

Fieldwork was conducted in March 2022 and defined a sampling transect away from the RMGM to the east and southeast. Six hand samples and thin sections from from this sampling locations were investigated to assess potential mineralogical and compositional variations. The modal minerology of samples include: 1) 40% quartz, 40% microcline, 20% muscovite, <1% plagioclase feldspar, trace oxides (biotite is notably absent); 2) 30-40% quartz, 25-30% microcline, 10% plagioclase feldspar+biotite, 5% muscovite, sanidine, and minor oxides, <1% apatite+zircon; 3) 30-40% quartz+ microcline, 5-10% biotite, 5% plagioclase feldspar, muscovite, and oxides, and <1% zircon+apatite; 4) 50% quartz, 15% microcline, 10% biotite, <5% plagioclase+oxides, <1% chlorite, apatite, and zircon; 5) 30-35% quartz, 20-25% microcline and plagioclase feldspar, 10% biotite, 5-10% muscovite, <5% oxides, <1% apatite+zircon; 6) 30-40% quartz and microcline, 5-10% plagioclase feldspar and muscovite, <5% oxides, and <1% biotite+apatite.

Ongoing and future work involves a detailed SEM-EDS study to identify growth zones in apatite and zircon for potential in-situ U-Pb analyses, and to investigate the potential occurrence of pyrite which has been documented in samples of the Shoshone Granite from the RMGM pit. These datasets will help constrain the timing of Mesozoic arc-related magmatism and further characterize the nature of Oligocene-aged mineralization.