RESTORATION OF THE EOCENE LANDSCAPE IN THE CARLIN-JERRITT CANYON MINING DISTRICT: CONSTRAINING DEPTH OF MINERALISATION FOR CARLIN-TYPE AU-DEPOSITS USING LOW-TEMPERATURE APATITE THERMOCHRONOLOGY

Carlin-type Au-deposits in the Carlin-Jerritt Canyon region of NE Nevada formed between ~42 and 37Ma. The depth at which the deposits developed has been variably estimated at >2-6km (fluid inclusion data) to <1km (devitrification textures in dikes). Low-temperature apatite thermochronology (fission-track and (U-Th)/He) provides a means to independently determine the denudation history of the Carlin-Jerritt Canyon region and estimate the depth at which the deposits formed.

One hundred and thirteen samples collected along Miocene horst blocks from the central Carlin trend to Jerritt Canyon yielded a range of Cretaceous to Miocene apatite fission-track (AFT) ages. Samples from areas lacking geological and geophysical evidence of Eocene magmatism or mineralisation mainly have Cretaceous fission-track and (U-Th)/He ages (~140-60Ma). Thermal modelling of the fission-track and (U-Th)/He data suggests these samples had cooled to 30-60°C at rates of 1-9°C/my by the end of the Cretaceous (~60Ma). This episode of cooling probably reflects denudation during the Sevier orogeny. Subsequent thermal histories are constrained by geological evidence that suggests, except for a period of Eocene sedimentation and volcanism from ~42-37Ma, rocks in the Carlin-Jerritt Canyon region have been undergoing continuous exhumation since at least the Cretaceous. Modelled cooling rates decrease from the end of the Cretaceous and by the middle Eocene were <0.5-1.3°C/my, suggesting relatively low rates of denudation with little relief development.

Restoration of the Eocene landscape at the time of Au-mineralization using the thermal modelling of AFT data and assuming a geothermal gradient of 30°C/my, suggest that, in areas adjacent to mineralisation in the northern Carlin trend, the current erosion surface lies ~500-1500m below that developed at 42Ma. Given the observed vertical distribution of mineralisation, this restoration suggests Au-deposits in the north Carlin trend formed over a depth range of 800-2200m below the Eocene surface. Such depths are not inconsistent with minimum trapping pressures estimated from main ore-stage fluid inclusions if the fluids had <2mol% CO2 and/or there were minor levels of fluid overpressuring.