GREISEN ALTERATION IN THE ROOT ZONES OF PORPHYRY COPPER SYSTEMS AT YERINGTON, NEVADA

Petrographic, fluid inclusion, and field studies at Luhr Hill in Yerington, Nevada, show that coarse-grained white mica alteration formed at deep levels and late in the magmatic-hydrothermal history of the Yerington batholith. This white mica alteration, or greisen, has not previously been described in the Yerington district.

There are three dominant styles of alteration at Luhr Hill: Na-Ca alteration characterized by actinolite-plagioclase-epidote, pervasive phlogopite-tourmaline-Kfeldspar replacement, and greisen alteration. Greisen at Luhr Hill forms discrete veins (quartz-white mica ± pyrite) with white mica-quartz-pyrite-hematite alteration haloes. Field relations show that greisen formed late in the development of the magmatic-hydrothermal system, postdating aplite and pegmatite dikes and the majority of Na-Ca alteration in the area but crosscut by a few Na-Ca veins. Greisen veins strike N-S with steep dips, and fresh rocks occur both to the west and east, thus there is no evidence for fluids having been fed from below or feeding alteration at higher levels, where the porphyry copper deposits formed. Greisen veins restore to subhorizontal during the Jurassic and suggest local exsolution of overpressured magmatic-hydrothermal fluids. Fluid inclusions in the discrete greisen veins are high-density, two-phase liquid-vapor inclusions that contain no observed daughter minerals, contain ~1 mol% CO₂, with 2-8 wt% NaCl equiv., homogenize between 150-200 °C, and yield trapping temperatures between 250-300 °C at 6-7 km paleodepth.

The root zones of porphyry copper systems are rarely exposed, but recent work by Seedorff et al. (2008) documented greisen alteration in the deep levels of four porphyry copper in Arizona (Sierrita, Ray, Globe-Miami, and Kelvin-Riverside) and noted the lack of reported greisen in the Yerington district. This study verifies the presence of deep greisen in the Yerington district and documents low-temperature, low- to moderate-salinity fluid inclusions within these veins.