40Ar/39Ar, K/Ar, and Fission Track Geochronology of Sediment-Hosted Disseminated Gold Deposits at Post-Betze, Carlin Trend, Northeastern Nevada

The Post-Betze deposit of Nevada is the largest sediment-hosted disseminated gold deposit presently known, both dimensionally and in terms of contained metal. Ore occurs primarily as submicron-sized gold that is disseminated in altered sedimentary rocks of the Lower Paleozoic Roberts Mountains Formation. However, significant portions of the ore are present in altered monzonite of the Goldstrike stock. Alteration and mineralization were controlled by both structure and stratigraphy. Alteration began with early decarbonatization and was followed by silicification and, finally, argillization. Phyllosilicate mineral zoning grades from proximal kaolinite to kaolinite + sericite to unaltered rock.

Based on geochronologic studies utilizing the K/Ar, 40Ar/39Ar, and fission track techniques, we suggest that the age of gold mineralization is approximately 117 Ma. The premineraliza-tion Goldstrike stock was emplaced at about 158 Ma and a postmineralization sill was em-placed at 39 Ma; these events place clear limits on the age of mineralization. Fine-grained sericite that is interpreted to have formed during hydrothermal events which also generated gold ore was dated by either the K/Ar method or a modification of the standard 40Ar/39Ar fusion technique. Age determinations on several samples of fine-grained sericite from altered host rocks (both sedimentary and igneous) yield ages near 117 Ma. Younger dates are interpreted to be the result of either thermal disturbance of the K/Ar system or mixing of sericites of two ages. One sample of coarser grained sericite that was stepheated gave discordant spectra.

Fission track analyses of zircon and apatite from the postore sill clearly document the lack of significant thermal or hydrothermal activity younger than 39 Ma. Although less definitive, fission track data on zircon from preore sedimentary and igneous hosts suggest that no widespread hydrothermal activity having temperatures above about 100°C has occurred since approximately 110 Ma.

The 117 Ma age is consistent with one of the major magmatic pulses recognized in the northern Great Basin. It is also similar to, within analytic uncertainty, a K/Ar date of 109 Ma from the Welches Canyon stock (20 km south of Post-Betze). An intrusion of similar age, unexposed at present, could have been the driving force for the hydrothermal system at Post-Betze.

Published geochronologic data, though less well tied to mineralization, suggest similar mid-Cretaceous ages for other sediment-hosted disseminated gold deposits in Nevada. Based on these ages, we suggest that sediment-hosted disseminated gold deposits are not necessarily products of extensional environments and, in fact, appear to be associated with compres-sional environments in Nevada during Cretaceous time. The alignment of sediment-hosted disseminated gold deposits (and other features) in the Basin and Range province of the western United States suggests that zones of long-term crustal weakness may have controlled igneous intrusion and consequent hydrothermal gold mineralization, although not all igneous episodes necessarily generated sediment-hosted disseminated gold ore deposits. The strong association between sediment-hosted disseminated gold deposits, the Precambrian craton, and these deep-seated structures also suggests the possibility of a deep source for gold and other components of these systems.