Metallogenesis of the Yerington Batholith, Nevada
John H. Dilles, John M. Proffett, 2000. "Metallogenesis of the Yerington Batholith, Nevada", Part I. Contrasting Styles of Intrusion-Associated Hydrothermal Systems: Part II. Geology & Gold Deposits of the Getchell Region, John H. Dilles, Mark D. Barton, David A. Johnson, John M. Proffett, Marco T. Einaudi, Elizabeth Jones Crafford
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The geometry of the Middle Jurassic Yerington Batholith has been reconstructed by removing the effects of Ceno-zoic normal faulting, which has exposed a cross section of the batholith from less than 1 to more than 6 kilometers paleodepth. The batholith is a composite pluton approximately 15 kilometers in diameter and extends at least 6 and possibly 8 to 9 kilometers in vertical dimension. Total volume of the batholith exceeds 1,000 cubic kilometers. It was emplaced into a Triassic-Jurassic volcanic and sedimentary rock sequence by bulk assimilation and ductile deformation of wallrocks. The roof is at approximately 1 kilometer depth and is formed by cogenetic volcanic sequences. The upper mineralized portion of the batholith and its roof are preserved because the batholith has dropped down more than 2.5 kilometers along steeply dipping faults.
Porphyry copper and copper skarn mineralization are spatially and temporally associated with emplacement of granite porphyry dikes that are cogenetic with and grade downward into the Luhr Hill Granite. This youngest phase of the batholith is estimated to be about 65 cubic kilometers in volume and was emplaced into the center of the batholith, largely at depths of 5 to 9 (?) kilometers. The Luhr Hill Granite has low copper content (10 ppm) and copper-zinc ratio (0.25) relative to the early and voluminous McLeod Hill Quartz Monzodiorite phase of the batholith (60 ppm copper and copper-zinc ratio of 1). Zinc decreases with differentiation and increasing silica content in the batholith and thus behaves compatibly, whereas copper content does not vary significantly with differentiation except for its sharp decrease in the Luhr Hill Granite. Whole rock chemical variations are consistent with low contents of copper (less than 150 ppm) and significant contents of zinc (about 350-800 ppm) in biotite, one of the early crystallizing and fractionating phases. Application of the theoretical model of Cline and Bodnar (1991) for crystallization of granite at 2 kilobars pressure indicates that hypersaline magmatic ore fluids would have separated late during crystallization and extracted most copper but less than 25 percent of zinc from the magma; zinc would have been sequestered in earlier-crystallized biotite. The fluids from the Luhr Hill Granite apparently migrated from 5 to 9 kilometers depth upward into granite cupolas at 4 to 5 kilometers depth, where they caused hydrofracturing leading to emplacement of granite porphyry dikes along which fluids continued to move upward and outward from the cupolas.
The dominance of copper sulfide and lack of zinc sulfide in the Yerington District is consistent with mineralization caused by magmatic ore fluids rich in copper and sulfur but poor in zinc. Metal zoning from inner porphyry copper with or without molybdenum to intermediate skarn copper to outer replacement/skarn copper-iron and vein copper-gold is generally consistent with declining temperature of magmatic hydrothermal fluids, but magnetite-rich iron-replacement ores poor in sulfide may be derived in part from non-magmatic fluids that stripped iron during sodic-calcic alteration of the batholith. Exploration criteria for porphyry copper deposits following the Yerington model should focus on shallowly’ emplaced batholiths with a late and relatively deep granite phase depleted in copper and having a low copper-zinc ratio.