Chapter 18: Geology of Round Mountain, Nevada: A Giant Low-Sulfidation Epithermal Gold Deposit

The Round Mountain low-sulfidation epithermal Au deposit occurs within the rhyolitic tuff of Round Mountain (26.86 Ma) on the northeast side of an elliptical volcanic center that has morphology and volcanic facies suggesting it originated as a caldera. The hosting tuff comprises three pyroclastic flow and fall deposits (units T1 to T3). These are overlain successively by lacustrine sediments and volcaniclastic rocks. which may contain paleowater table levels formed at the time of ore formation and a 26.4 Ma postmineralization tuff unit. A linear vertical drop in the basement contact coincides with thick tuff fill and megabreccia, which is interpreted to follow the position of a WNW-trending ring fissure or vent wall that may have focused the locations of subsequent hydrothermal upflow zones. Orebodies are developed in strata-bound zones that are most extensive in poorly welded tuff, focused below overlying impermeable welded tuff in a WNW-trending, gently NW-plunging corridor above and mantling the SW-dipping paleoslope of basement rocks. Ore comprises disseminated pervasive adularia-quartz-pyrite ± illite alteration with electrum. The disseminated mineralization surrounds, and is most intensely developed in association with, a low-displacement extensional fault-vein network composed of conjugate NE- and SW-dipping faults and steeply dipping extensional veins. Vein orientations and kinematic indicators suggest ore formation occurred during localized NE-SW-directed extension that may have been related to late stages of volcanic subsidence, potentially in association with deep resurgent magmatism into ring fissures approximately 0.5 m.y. after deposition of the host tuff sequence.