| Abstract |
The Midas geothermal system is blind to the surface, and it was discovered in the course of drilling and deepening of the Ken Snyder mine in 2006. The mine is situated in an uplifted block, north of the Midas Trough, and gold-silver ore is produced from epithermal quartz veins, which are hosted by an alternating sequence of hydrothermally altered basalt and rhyolite. Hot water discharges (50-85°C) were encountered around 4550 ft asl, coinciding with the water table, which occurs 1000 ft below the surface. The modern thermal anomaly defined by a 50°C isotherm appears to extend for about 5 km2, but its continuity and extent are poorly known. Anomalous heat flow is estimated to be ~1.5 MW. Chemical and isotope analyses indicate thermal waters are dominated by aqueous bicarbonate and derive from old meteoric water. They also indicate that thermal waters have a common origin and that they have been affected by deep circulation and hydrothermal water rock interaction. Equilibration temperatures based on the Na/K, K/Mg and quartz-silica geothermometers range from 120 to 180°C. Hydrothermal alteration formed with the precious metal mineralization in the mid-Miocene, and a modern alteration overprint has yet to be detected. The Midas thermal water compositions are similar to Beowawe and Tuscarora, which suggest that hydrothermal fluids derive from deep concealed Paleozoic carbonate aquifers. The basin bounding faults that localize near surface fluid flow in these systems possibly act as leakage points to regionally extensive reservoirs in hot sedimentary basins in northern Nevada. |