2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 5
Presentation Time: 2:30 PM


CASTOR, Stephen B.1, GARSIDE, Larry J.2, HENRY, Christopher D.2, HUDSON, Donald M.3, MCINTOSH, William C.4 and VIKRE, Peter G.5, (1)Nevada Bureau of Mines and Geology, Univ of Nevada, mail stop 178, Reno, NV 89557, (2)Nevada Bureau of Mines and Geology, University of Nevada, mail stop 178, Reno, NV 89557, (3)1540 Van Petten St, Reno, NV 89503, (4)New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM 87801, (5)U.S. Geol Survey, mail stop 175, University of Nevada, Reno, NV 89557, scastor@unr.edu

The historic Comstock district produced 8.25 Moz Au and 192 Moz Ag, mostly from the Comstock lode. Despite prior investigation, including 49 K-Ar and fission-track ages, 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. Although previously defined stratigraphic units are diachronous, we retain their formation names.

Magmatism in the Comstock district occurred in at least three pulses. Lower Alta Fm andesites erupted in and around the Comstock at ~18 Ma represent the oldest Tertiary magmatism. Upper Alta andesites erupted at ~15.5 Ma in and west of the district; and the Davidson diorite, largest intrusion in the district, was emplaced during this pulse. Younger andesites of the Kate Peak Fm were emplaced at ~14.5 Ma in and north of the district. We have not yet dated stratigraphically higher units north of the district, but ~12 Ma andesitic-dacitic volcanic rocks occur ~10 km northeast.

Three adularia dates on the NNE Comstock vein and one from the NNW Silver City vein cluster tightly at 14.0 Ma. Adularia dates from the Occidental and Flowery veins, which parallel the Comstock vein to the east, are 13.3 Ma. Comstock mineralization postdated all igneous activity that we have dated by 40Ar/39Ar methods. The Davidson diorite was emplaced at least 1 Ma before Comstock mineralization. Minor mineralization occurred at ~18 Ma in the Jumbo district to the west where alteration was coeval with and hosted by lower Alta rocks, implying a genetic relationship.

Unit/Deposit (number of dates)

Dated Mineral(s)

Age/Age Range (Ma)

Occidental & Flowery veins (2)


13.30±.04; 13.28±.05

Comstock & Silver City veins (4)


13.97±.04 to 14.08±.06

Kate Peak Fm (4)

Biotite & Hornblende

14.30±0.10 to 14.66±0.11

Davidson diorite (1)



upper Alta Fm (3)


15.13±0.10 to 15.68±0.16

Jumbo alteration (1)



lower Alta Fm (5)


17.58±0.11 to 18.20±0.16