The Volcanic Stratigraphy and Varietal Au-Ag-Cu-Hg-S° Epithermal Deposits in the Monitor-Mogul Mining District, Alpine County, California
Loading...
Authors
McKnight, Grant Cameron
Issue Date
2022
Type
Thesis
Language
Keywords
epithermal , Leviathan , Morning Star , volcanic stratigraphy , Walker Lane , Zaca
Alternative Title
Abstract
The Monitor-Mogul mining district hosts diverse Au, Ag, Cu, Hg, and S° epithermal deposits in a volcanic field dominated by Miocene to early Pliocene flows, intrusions, debris flows, and volcaniclastic rocks associated with both ancestral Cascades continental arc and slab window magmatism. Previous studies have focused on similar arc- and slab window-associated epithermal deposits in the Walker Lane, but few mining districts of this type contain the varietal Au-Ag-Cu-Hg-S° resources and the distinct alteration and mineralization styles present in the Monitor-Mogul district. As such, this district provides a unique opportunity to characterize the volcanic stratigraphy that hosts epithermal deposits not typically observed together in one mining district.At least six million years of Tertiary volcanic activity are recorded in the Monitor volcanic field. These mineralized Miocene to early Pliocene volcanic rocks are common features of ancestral Cascades arc and slab window magmatism in the Walker Lane, a northwest trending zone of transtensional tectonism. In the district, basaltic andesite flows and shallow intrusions (~11 Ma) and related volcaniclastic rocks correlated with the Relief Peak Formation are overlain by highly potassic trachyandesites (~10 Ma) correlated with the Stanislaus Group. Dacite domes (8.1 Ma) correlated with the Disaster Peak Formation intrude these older volcanic strata. Last, slab window-associated rhyolite plugs (~4.8 Ma) intrude the Relief Peak Formation basaltic andesites. Production in the Monitor-Mogul district totaled around 16,404 oz. Au and 728,275 oz. Ag with associated Cu, Zn, and Pb, tens of flasks of Hg, and 0.5 Mt S° from a diverse suite of deposits. These deposits include the Mogul Peak Hg mine, various Au-Ag-Cu deposits such as the Morning Star and Curtz mines, the Leviathan S° mine, and the Zaca Au-Ag mine and its associated Au-Ag deposits found in the mineralized rhyolite dome known as Colorado Hill. This study primarily focuses on the Mogul Peak Hg, Morning Star Au-Ag-Cu, Leviathan S°, and Zaca Au-Ag mines as they best characterize each of the different types of deposits in the district. Temporal, stratigraphic, and mineralogic relationships suggest at least two periods of alteration and mineralization at ~9.1 and ~4.8 Ma. The earlier event may include the Mogul Peak Hg mine, Morning Star and other Au-Ag-Cu mines, and the Leviathan S° mine, all of which are hosted in Relief Peak Formation and Stanislaus Group volcanic and volcaniclastic rocks. The Mogul Peak Hg mine is subjacent to silicified apical zones in trachyandesite pervasively altered to alunite, kaolinite, and dickite. Cinnabar occurs with kaolinite and dickite and in a steam-heated zone of residual quartz with coarse alunite filling voids. The Morning Star mine appears to be a small but dynamic mineralized breccia pipe subjacent to a silicified apical zone with enargite, famatinite, and pyrite occurring in small extensional veins, breccia voids, and hydrothermal sedimentary strata primarily in basaltic andesites and dacites altered to kaolinite and dickite. Stratiform S° and pyrite at the Leviathan mine occur in a thick sequence of basaltic andesite and porous volcanic tuff primarily altered to quartz, opal, kaolinite, and montmorillonite. A H2S- and Fe-rich hydrothermal vapor and permeable strata allowed for this voluminous S° deposit to form. These mines characterize high-sulfidation epithermal alteration, but some mineralized zones do not contain typical high-sulfidation sulfide assemblages. The later mineralization event includes the Zaca Au-Ag mine and other prospects on Colorado Hill, where illite ± quartz is the dominant alteration zone. Ore assemblages are represented primarily by pyrite, sphalerite, galena, chalcopyrite, Ag-tetrahedrite, pyrargyrite, and electrum that occupy fracture fillings and disseminated pockets in the flow-banded rhyolite. Deposits associated with this rhyolite intrusion, emplaced ~4.8 Ma and altered shortly thereafter, mostly align with intermediate-sulfidation epithermal systems. These varietal and asynchronous deposits may still be intimately related as one long-lived eruptive center with multiple pulses of mineralization, similar to the Bodie Hills volcanic field and other deposits across the Walker Lane.