Timing of the Magmatic to Hydrothermal Transition and the Onset of Porphyry Mineralization Across the Yerington Porphyry System, Nevada, USA

Abstract Porphyry Cu(-Au-Mo) deposits form in the upper (~2–8 km) regions of large, long-lived magmatic-hydrothermal systems. Although extensively studied, the timescales over which sufficient volumes of mineralizing fluids are extracted from magma source regions and focused into narrow, shallow zones of mineralization remain poorly understood. Here, we provide constraints on the onset and duration of porphyry-style mineralization from detailed field, mineralogical, geochemical and geochronological studies of the classic Yerington porphyry district of Nevada, where Cenozoic extension and tilting has exposed an exceptional ~8-km top-to-bottom cross section through the porphyry system. From high-precision zircon U-Pb dating, the exposed Luhr Hill granite cupola of the Yerington batholith, and multiple generations of aplitic- and porphyry dikes, on which the porphyry-type deposits are centered, were emplaced within only ~100 k.y. Cu mineralization of miarolitic cavities and unidirectional solidification textures within the dikes, and in associated quartz veins, was late in the transition from magmatic to hydrothermal conditions, only occurring once fluids cooled below ~600°C. From Re-Os age determinations for vein molybdenite in the Bear and Ann Mason deposits, mineralization was coeval with, and much longer lived (>1 m.y.) than pluton and dike emplacement. We propose that magmatic-hydrothermal fluids exsolving early and deep (at porphyry-forming depths) were initially too hot to precipitate ore minerals. However, as these fluids ascended and cooled, mineralization could occur at shallower (epithermal) levels. At the depth of porphyry deposit formation, it was only as magmatism waned that the fluids could cool adequately to precipitate hypogene ore minerals. We suggest that future studies integrate microtextural studies with multifaceted geochronology to further characterize and model these processes in porphyry and similar types of magmatic-hydrothermal systems.