APATITE FISSION-TRACK THERMAL ANOMALIES AND THE EVOLUTION OF SYNEXTENSIONAL HYDROTHERMAL FLOW ACCOMPANYING AU-MINERALIZATION ALONG THE NORTHERN CARLIN TREND, NEVADA

Au-deposits along the northern Carlin trend formed over ~42-37Ma. Apatite fission-track ages from pre-Cenozoic rocks in this area form two main clusters. An older, ~120-70Ma, cluster last cooled from >100°C during the Sevier orogeny. The younger cluster at ~60-20Ma reflect the resetting of fission-tracks by pulses in thermal energy from ~40-15Ma. Resetting was a product of hydrothermal flow concentrated around the northern Carlin trend and on the NE margin of a large subsurface, ~38Ma, intrusion complex at Welches Canyon south of the Carlin trend. The zone of thermal resetting encompasses, but extends beyond, mineralized areas and reaches its greatest width over the subsurface intrusion near Welches Canyon. Thermal resetting is more pervasively developed within Lower Plate carbonate dominated units than in the more siliciclastic Upper Plate rocks.

The thermal resetting of apatite fission-tracks and Au-deposition were contemporaneous with a period of ~40-38Ma crustal extension that is manifest as pervasive, NE trending, domino-style normal faults within Eocene units. In pre-Eocene bedrock, extension was largely accommodated by heterogeneous shear and tensional reactivation of older, variably oriented pre-Eocene structures. Extension enabled auriferous hydrothermal fluids from lower in the crust to flow upward along a dilating fracture mesh developed above large basement structures. Extension was initially concentrated along a limited number of structures. Fluid flow and Au-deposition was focused into these structures and along permeable carbonate horizons. With ongoing extension a greater number of pre-existing faults were reactivated, increasing transient crustal permeability. Large deeply circulating meteoric convection systems developed and swamped the earlier auriferous system. It was largely this stage of hydrothermal flow that reset apatite fission-tracks. The higher permeability of Lower Plate carbonate units produced a relatively pervasive pattern of resetting. In the less permeable Upper Plate rocks, fracture controlled fluid flow lead to a more heterogeneous pattern of thermal resetting. Magmatic thermal energy may have played a significant role in determining the scale of the meteoric dominated circulation systems responsible for resetting of apatite fission-tracks.