The Hilltop gold deposit is 29 km southeast of Battle Mountain,
Lander County, Nevada, in the northern Shoshone Range. The
deposit is estimated to contain nearly two million ounces of gold at
an average grade of 0.028 opt. The main host of the gold ore is chert
and argillite with minor siltstone and quartzite of the Ordovician
Valmy Formation, within the upper plate of the Roberts Mountains
allochthon. Minor to moderate amounts of Tertiary intrusive stocks
and sills are also present. Two sub-parallel west-dipping faults
served as conduits for mineralization. Thrust movement followed by
Basin-and-Range style extension along both faults before and during
mineralization created a highly permeable megabreccia (termed the
main zone) which received later igneous intrusions and hydrothermal
fluid deposition. A discordant quartz breccia pipe of uncertain
paragenetic position hosts both main zone and porphyry-related(?)
Initial weak porphyry-style Cu-Mo-Au mineralization
throughout the Hilltop area is probably associated with a 41.2±0.5
Ma west-northwest trend of diorite-granodiorite intrusions. The porphyry
mineralization is localized around the periphery of the intrusions.
Precious metal mineralization within the main zone followed
the porphyry event and consists of at least six temporally distinct but
spatially overlapping events. Main zone mineralization exhibits
characteristics of both high- and low- sulfidation epithermal systems.
Gold is associated with silica and arsenopyrite in three of six
main zone mineralization events and shows no affinity for a particular
host rock type.
Hydrothermal alteration of the Valmy Formation is predominantly
bleaching (removal of organic carbon) and recrystallization/
silicification resulting from the emplacement of Tertiary
igneous intrusions. Tertiary igneous units exhibit typical phyllic
alteration with local argillic alteration.