Michael Say—Friday, April 17 at 1:00 PM
Curtis Johnson—Monday, April 20 at 10:00 AM
James Ingraffia—Tuesday, April 21 at 3:30 PM
Gilberto Martinez—Monday, April 27 at 3:00 PM

MSc Thesis Defense of Michael Say: Late Miocene Transition between Basin and Range Extension and Walker Lane Tectonics, Northern Pine Nut Mountains, Nevada: New Insights from Geologic Mapping and 40Ar/39Ar Geochronology

Advisor: Andrew Zuza
Time: Friday, April 17th, 2020 1:00 PM, PST
Please email msay@nevada.unr.edu for Zoom Meeting ID and password.
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Abstract: The westward encroachment of Basin and Range extension on the relatively stable Sierra Nevada block occurred during the Miocene. To better bracket the timing, magnitude, and kinematics of this transition, new geologic mapping, 40Ar/39Ar geochronology, and geochemical analyses was conducted in the northern Pine Nut Mountains, NV, which are the westernmost structural and topographic expression of the Basin and Range extensional province. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation in the Carson Domain. Newly obtained 40Ar/39Ar radiometric dates collected from 30-36° NW-dipping intermediate to felsic (~55-65% SiO2) volcanic and sedimentary rocks in the northern Pine Nut Mountains show that deformation initiated after 7.15 ± 0.10 Ma. Tilting of the range was accommodated by a major east-dipping normal fault that defines the eastern flank of the range. Extension magnitude recorded in the northern Pine Nut Mountains (14% extension) and westward towards the rigid Sierra Nevada is significantly less than the highly extended Singatse and Wassuk Ranges (~150-180% extension) to the east. Subsequently, Walker Lane transtension initiated and dextral shear in the Carson Domain induced clockwise rotation of structural blocks bounded by northeast-striking left-slip faults orthogonal to the dextral shear zone.

This resulted in a northeast-striking oblique-slip and transtensional structure in the northern Pine Nut Mountains called the Bull Canyon fault. Although this oblique left-slip normal fault system is covered by young alluvium and inactive today, it is likely this structure served as a major kinematic component of left-lateral shear similar to other left-slip faults identified in the Carson Domain to the north and south. It may have become inactive because it became mechanically unfavorable as the Carson Domain rotated and slip was accommodated on the other parallel left-slip faults. Recently active, north-striking east-dipping curvilinear faults on the western flank of the range show dip-slip to oblique right-slip normal kinematics that may have accommodated some dextral shear as conjugate Riedel shears of the Carson Domain.

PhD Dissertation Defense of Curtis Johnson: The Relationship between Eocene Magmatism and Gold Mineralization in the Great Basin, USA

Advisors: Mike Ressel (NBMG/CREG) and Philipp Ruprecht (DGSE)
Time: Monday, April 20, 2020 at 10:00 AM, PST
Please email curtisj@nevada.unr.edu for Zoom Meeting link and password.
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Abstract: Late Eocene (~42-34 Ma) ore deposits of the Great Basin, U.S.A. have contributed the majority of the region’s precious- and base-metal production. Mineralization is strongly partitioned, with Au-rich systems in the west in Nevada, and Cu-rich systems to the east in Utah, however, both share a strong spatial and temporal link to the broad southwestward sweep of Eocene subduction-related, high-K magmatism. Coupled with the bimodal metal distribution is a distinct shift in the Paleozoic passive margin from relatively oxidized subaerial to platform carbonate units in the east, to up to 10 km thick reduced carbonaceous slope and basin units in the west. The character of mineralization associated with Eocene magmas mirrors this change in crustal redox, with dominantly oxidized porphyry Cu, Ag-Pb-Zn carbonate-replacement, and high-sulfidation Au-Cu epithermal systems in Utah, and reduced, intrusion related Au (Cu-Ag-Bi-Te), Au-Ag-Pb-Zn carbonate replacement, and various styles of Au-rich Carlin-type mineralization. Further, Eocene magmas have been shown isotopically to have interacted with significant volumes of reduced crust in Nevada, potentially impacting the magmatic redox state, which has profound control on Au/Cu in magmas and subsequent fluids. Here, this magmatic redox hypothesis is tested by detailed field and analytical studies of the Phoenix-Fortitude reduced, Au-rich porphyry-skarn and redox evolution of the associated intrusive complex in Nevada, and analytical studies of 21 regional intrusions associated with mineralization throughout the Great Basin to understand province-scale variability in redox and volatile contents.

Detailed study of the Phoenix-Fortitude system has revealed a time-space sequence of early, more oxidizing P1 porphyry intrusions (~fayalite-magnetite+quartz, FMQ -1 to +1; Ce4+/CeTotal dominantly >90%) associated with Au-Cu pyrrhotite-chalcopyrite mineralized garnet-pyroxene skarn in carbonate rocks and quartz-chalcopyrite-pyrrhotite veinlets and biotite-K-feldspar alteration in aluminosilicate units. A second, later pulse of more evolved and reduced P2 porphyry intrusions (dominantly <FMQ; Ce4+/CeTotal <90%) associated with widespread Au-As-Sb-Bi-Te -rich pyrite-arsenopyrite mineralized actinolite-chlorite skarn in carbonate rocks and quartz-sericite-pyrite-arsenopyrite veins in aluminosilicate rocks. This new understanding of alteration and mineralization was coupled with a novel application of hyperspectral imaging to production blast hole cuttings at the Phoenix Mine to derive geometallurgical models capable of optimizing mill recovery and throughput. Hyperspectral data were also useful for defining detailed bench-scale alteration zoning for improved exploration targeting.

To understand the regional variation of magmatic redox and volatiles, apatite and zircon from 21 regional Eocene, Eocene-early Oligocene, and one unexpected Miocene intrusion associated with mineralization were studied in detail. Analyses of Ce speciation (Ce3+/Ce4+) and determination of magmatic oxidation state from igneous zircon show initially oxidized, mantle-derived arc magmas (FMQ +1-2) were systematically reduced to ~FMQ -1 as they evolved in Nevada, whereas magmas in Utah retained, and in some cases increased their oxidation state (~FMQ +2-3). Nevada intrusions exhibit a wide range of 0-100% Ce4+/CeTotal, while Utah magmas are dominantly >70% with substantially more 100% Ce4+/CeTotal measured in zircon. In conjunction, igneous apatite in magmas from Utah is shown to be more enriched in S, dominantly 0.1-1 wt. % SO3, than in Nevada where apatite is dominantly <0.1 wt. % SO3 – likely a function of the elevated magmatic oxidation state in Utah and coupled increase in S solubility due to the transition from S2- to S6+ dominance at ~FMQ+0.5. We conclude the assimilation of reduced, carbonaceous sedimentary units in Nevada is a fundamental control on the Au-rich metallogeny of Nevada due to the modification of magmatic redox conditions to optimal levels (~FMQ) for the highest melt solubility and extraction efficiency of Au and reduced S into fluids.

We propose continental arcs emplaced into thick slope and basinal sedimentary sequences are a necessary component to form reduced, Au-rich, Cu-poor intrusion-related deposits. Due to the abundance of reduced S in magmatic-hydrothermal fluids and the buffering capacity of wallrock, these geologic settings are ideal for the transport of Au to low temperatures typical of Carlin-type gold deposits and should be targeted in regional exploration programs for new Carlin-type districts.

James Ingraffia on Tuesday, April 21 at 3:30 PM. Mike Ressel, advisor; M.S. thesis title: Thacker Pass Lithium Clay Deposit, McDermitt Caldera, Humboldt County, Nevada: Stratigraphy, Geochemistry, and Origin. Zoom defense, contact: Mike Ressel, mressel@unr.edu for meeting ID and password.

Gilberto Martinez on Monday, April 27 at 3:00 PM. Tommy Thompson and Mike Ressel, advisors; Ph.D. thesis title: Productive Versus Non-productive Porphyry Systems Surrounding the Peñasquito Diatreme-Porphyry System, Zacatecas, Mexico. Zoom defense, contact: Mike Ressel, mressel@unr.edu for meeting ID and password.

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