UNR DISSERTATION DEFENSE: Ajeet Milliard and Justin Milliard

Ajeet Milliard—Tuesday, April 7 at 12 PM
Justin Milliard—Thursday, April 9 at 11 AM

Ph.D. Dissertation Defense by Ajeet Milliard:
Temporal and Spatial Constraints on a Sedimentary Rock-hosted Gold Deposit: Pequop Mountains, Nevada

Advisor: Dr. Mike Ressel (NBMG)
Time: Tuesday, April 7, 2020 at 12:00 PM PST via Zoom
Please contact Mike Ressel for meeting password: mressel@unr.edu

The Pequop Mountains of northeast Nevada contain the Long Canyon and West Pequop sedimentary rock-hosted gold deposits (SRGD). Gold is fine-grained and disseminated in metamorphosed Lower Paleozoic, silty carbonates and breccias. Pequop SRGD occur 130 km east of other, major gold trends in Nevada which are known to have formed during the Eocene (42 to 34 Ma), at depths of ~1 to 3 km, and in rocks unaffected by regional metamorphism. In contrast, Pequop gold deposits occur in variably metamorphosed rocks exhumed from greater depth, yet are likely coeval or slightly younger than known trends to the west.

Geochronology and thermochronometry on zircon, apatite and illite from igneous and sedimentary rocks place temporal and spatial constraints on Au mineralization in the Pequops. U-Pb ages from widespread hydrothermally altered intrusions are Jurassic and Eocene, consistent with regional magmatism. Apatite fission-track and (U-Th)/He ages indicate cooling progressed from NE to SW from ~51 to 3.5 Ma, respectively. Illite 40Ar/39Ar analyses of mineralized rocks with relatively high concentrations (≤12 ppm) resulted in apparent ages ranging from ~105.9 to 33.3 Ma. The range in ages may reflect a relatively low-temperature fluid event associated with SRGD and the variable effects such fluids had on earlier-formed illite. Illite from highly Au-mineralized Paleozoic sedimentary breccia yielded an age of 33.3+7.7 Ma, interpreted to approximate the age of SRGD mineralization at Long Canyon.

Based on thermochronologic and geologic constraints, and assuming a geothermal gradient of 25°C/km, Pequop SRGD formed at paleo-depths of <1.5 km at Long Canyon and <4 km at West Pequop. Considering all data, hydrothermal activity associated with Au mineralization in the Pequops is constrained between 38 and 32 Ma and depths of <1.5 to 4 km, similar to other Au districts in Nevada. These data imply that significant exhumation of metamorphosed rocks occurred prior to a late Eocene- Early Oligocene Au mineralization event that was associated with crustal extension.

Ph.D. Dissertation Defense by Justin B. Milliard:
Genetic Links Between Extensional Tectonics and the Formation of Low Sulfidation Epithermal Au-Ag Deposits

Advisor: Dr. John L. Muntean, NBMG
Time: Thursday, April 9, 2020 at 11:00 AM PST via ZOOM
Please contact Justin before April 9 for meeting password: jmilliard@nevada.unr.edu

Low sulfidation epithermal deposits form the highest-grade gold-silver deposits in the world, containing ~60% of bonanza vein systems (>1 oz/ton Au). The grade comes from individual bonanza bands that often contain >50 vol% precious metals. These individual bands make up only a small proportion of the total vein (<10%). This feature, unique among deposit types, suggests that brief, episodic events funnel fluids carrying elevated concentrations of metals and deposit them efficiently in the shallow crust. For reference, the average crustal abundance of Au is four ppb, and the rocks that make up a low sulfidation epithermal vein are 100’s to 1000’s ppm Au.

That is one hand sample not to be tossed into a rock garden!

The geologic processes enabling this extremely efficient distillation, transportation, and deposition process of gold are poorly understood. Further, the global occurrence of these deposits suggests that a common set of processes enables deposit formation regardless of different tectonic setting. This project identifies the links between the geologic processes of tectonism, magmatism, and hydrothermal activity that influenced when and where low sulfidation epithermal deposits formed in the northern Nevada rift. By establishing the genetic links between processes in the northern Nevada rift, we not only gain a better understanding of deposit controls here, but we also glean more about the processes controlling deposit formation globally.

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