A-40 AR-39, 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 dissem inated gold deposit presently known, both dimensionally and in terms o f contained metal. Ore occurs primarily as submicron-sized gold that i s disseminated in altered sedimentary rocks of the Lower Paleozoic Rob erts 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. Alt eration began with early decarbonatization and was followed by silicif ication and, finally, argillization. Phyllosilicate mineral zoning gra des from proximal kaolinite to kaolinite + sericite to unaltered rock. Based on geochronologic studies utilizing the K/Ar, Ar-40/Ar-39, and fission track techniques, we suggest that the age of gold mineralizati on is approximately 117 Ma. The premineralization Goldstrike stock was emplaced at about 158 Ma and a postmineralization sill was emplaced a t 39 Ma; these events place clear limits on the age of mineralization. Fine-grained sericite that is interpreted to have formed during hydro thermal events which also generated gold ore was dated by either the K /Ar method or a modification of the standard Ar-40/Ar-39 fusion techni que. Age determinations on several samples of fine-grained sericite fr om altered host rocks (both sedimentary and igneous) yield ages near 1 17 Ma. Younger dates are interpreted to be the result of either therma l disturbance of the K/Ar system or mixing of sericites of two ages. O ne sample of coarser grained sericite that was stepheated gave discord ant spectra. Fission track analyses of zircon and apatite from the pos tore sill clearly document the lack of significant thermal or hydrothe rmal activity younger than 39 Ma. Although less definitive, fission tr ack data on zircon from preore sedimentary and igneous hosts suggest t hat no widespread hydrothermal activity having temperatures above abou t 100-degrees-C has occurred since approximately 110 Ma. The 117 Ma ag e is consistent with one of the major magmatic pulses recognized in th e northern Great Basin. It is also similar to, within analytic uncerta inty, 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, co uld have been the driving force for the hydrothermal system at Post-Be tze. Published geochronologic data, though less well tied to mineraliz ation, suggest similar mid-Cretaceous ages for other sediment-hosted d isseminated gold deposits in Nevada. Based on these ages, we suggest t hat sediment-hosted disseminated gold deposits are not necessarily pro ducts of extensional environments and, in fact, appear to be associate d with compressional environments in Nevada during Cretaceous time. Th e alignment of sediment-hosted disseminated gold deposits (and other f eatures) 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, alt hough not all igneous episodes necessarily generated sediment-hosted d isseminated 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.