CENOZOIC STRUCTURAL AND THERMAL HISTORY OF PART OF THE WESTERN MARGIN OF THE HUMBOLDT RANGE, NEVADA: PRELIMINARY INTERPRETATIONS

The western margin of the Humboldt Range, Nevada, contains noteworthy young epithermal gold deposits and modern active geothermal systems. To determine if exposed faults have served as conduits for geothermal fluids, structural mapping has been done between Florida and Sacramento Canyons, and d18O values and apatite fission-track (AFT) ages have been obtained from Triassic siliciclastic and carbonate rocks within and adjacent to selected faults. Background d18O values of limestones range between +25 and +30‰, while in places along the Humboldt City and Buffalo Canyon faults, values as low as +10‰ reflect isotopic exchange with circulating meteoric hydrothermal fluids. AFT ages are interpreted to yield thermal history information at a regional scale on uplift along rangefront faults as the Humboldt Range was rising. In addition, some AFT data indicate localized hydrothermal fluid flow events of various ages. Our oldest AFT ages from along the rangefront range from about 46 to 21 Ma, possibly recording Early Miocene and older unroofing. A number of samples yield ages between 14.5 and 10.1 Ma, which may signal uplift and cooling of footwalls of the range-bounding faults. In El Dorado Canyon, 2 km east of the currently active Rye Patch geothermal system, samples adjacent to the faults yield ages of 3.3+0.8, 0.0+1.7, and 1.0+0.6 Ma. Carbonate samples from these same sites also show low d18O values. We interpret these young AFT ages to reflect resetting by geothermal fluids, and suggest that fault-controlled geothermal systems in this area have been active for up to 3 Ma, and possibly longer. More detailed sampling currently underway may yield insights as to how geothermal systems have migrated along the rangefront faults during uplift of the range.