Epithermal mineralization and intermediate volcanism in the Virginia City area, Nevada

Epithermal deposits in the Virginia City area of Storey, Washoe and Lyon Counties,
Nevada, were major sources of silver and gold. The area produced 8.25 Moz Au
and 192 Moz Ag, mostly from the Comstock Lode. Despite intense prior investigation,
including 49 K-Ar and fission-track age dates, age relationships between magmatism
and mineralization were imprecisely known. Our new geologic mapping and
40Ar/39Ar dating define several distinct Miocene magmatic and hydrothermal events
in the area. In addition, we propose revisions of the volcanic stratigraphy.
Middle Miocene calc-alkaline intermediate magmatism in the Virginia City area
occurred in at least four pulses. Andesite flows and flow breccias that mostly crop out
in the south part of the area represent the oldest pulse at 17.7–18.3 Ma. In the past,
these rocks were included in the lower part of the Alta Formation; we refer to them as
the Silver City andesites. Andesites erupted at 15.2–15.8 Ma in and west of the Comstock
district comprise the second pulse. We refer to these rocks, previously mapped as
the upper Alta Formation and the Kate Peak Formation, as the Virginia City volcanics.
The Davidson diorite was emplaced near the end of this pulse. The Virginia City
andesites are probably the remnants of a stratovolcano centered approximately on the
Comstock district, and the Davidson diorite may have been an extensive intrusion into
the core of this stratovolcano. Younger andesites that include extrusive and intrusive
rocks previously assigned to the Kate Peak Formation were emplaced at 14.2–14.9 Ma
in and north of the Comstock district. We have assigned these rocks to the Flowery
Peak magmatic suite. The latest pulse is represented by a 12.9-Ma dacite dome.
Hydrothermal alteration and mineralization in the area also occurred during several
mid-Miocene pulses. The oldest, sericitic alteration about 6 km west of the Comstock
Lode, has been dated at 18.1 Ma, overlapping with the eruption of the Silver
City andesites. Widespread high-sulfidation alteration at 15.3–15.5 Ma (alunite) is
approximately coeval with the Virginia City volcanics, which host it, and the Davidson
diorite, which cuts it. Approximately contemporary low-sulfidation mineralization
(15.5 Ma on adularia) occurred at Jumbo, about 5 km west of the Comstock
Lode. Dates on the contiguous Comstock and Silver City Lodes cluster tightly at
14.1 Ma (adularia); whereas dates from the Occidental and Flowery Lodes to the east
are 13.4 Ma (adularia). High-sulfidation alteration in the Geiger Grade area north of
Virginia City gave a relatively imprecise age of 13.5 Ma (alunite). During its waning stages, the 14.2- to 14.9-Ma Virginia City magmatic suite may
have provided heat and other components for the highly productive Comstock mineralization.
The extensive Davidson diorite intrusive activity, once considered a possible
source of Comstock hydrothermal components, took place more than 1 Ma before
Comstock mineralization. However, high-sulfidation hydrothermal activity may have
accompanied the Davidson intrusive episode.