An occurrence of phlogopite-rich alteration in the Yerington District, Nevada

Texturally destructive phlogopite-rich alteration occurs as a narrow, curvilinear zone with a width of <0.5 to 3 m exposed discontinuously over approximately 1 km along strike in eastern exposures of the Yerington batholith, Yerington district, Nevada. The phlogopite is preferentially oriented and defines foliation, suggesting that the alteration zone has accommodated structural deformation. The mineral association consists of near-endmember phlogopite with local clusters of euhedral dravitic to oxy-dravitic tourmaline crystals. Tourmaline-hosted fluid inclusions are high-density aqueous inclusions that generally homogenize between 230 and 330 degrees C and contain approximately 7-20 wt.% NaCl equivalent. The alteration is hosted within the Luhr Hill porphyritic granite and, although timing of the alteration is unclear, it likely postdates and is unrelated to well-known porphyry systems in the district. The phlogopite-rich assemblages represent an unusual style of Mg-K-rich alteration of a granitoid without exposed adjacent ultramafic or dolomitic carbonate units. Phlogopite-rich alteration is potentially related to the circulation of moderately saline to hypersaline external fluids through the Luhr Hill porphyritic granite.