This area was first explored for gold in the 1980s (Wendell, 1985; Casaceli and others, 1986). More recently, hot water was reported during exploration drilling at the McGinness Hills gold property, located at the southern end of Grass Valley, Lander County. The name for the hills is taken from a triangulation station in the north part of a group of low hills just east of Nevada Highway 306 (formerly highway 21) about 17 km north of its junction with U.S Highway 50 near Bob Scotts Summit. The drill holes with hot water are located in SE¼ NW¼ Sec. 15, T20N, R45E (Barton Spring 7.5-minute topographic map). Drilling of 300-meter-deep exploration holes through the sinter cap (Fig. 4, Casaceli and others, 1986) by Newcrest Resources, Inc. in 2004 intercepted near boiling waters (up to 88°C) with some geysering action observed in one hole. Recognizing the significance of the discovery for geothermal exploration, Newcrest geologists had samples of artesian hot water collected from two drill holes, which yielded quartz geothermometer (no steam loss, Fournier, 1977, 1981) temperatures of 151° and 193°C and K-Na-Ca-Mg geothermometer temperatures (Fournier and Potter, 1979; Fournier and Truesdell, 1973) of 209° and 214°C (Coolbaugh and others., 2006, see Chemistry below).
There are two ages of hydrothermal alteration at McGinness Hills, a high-sulfidation system dated at about 35 Ma and a low-sulfidation (quartz-adularia or hot-springs) system dated by K-Ar methods at 3.2 and 2.2 (±0.4) Ma (Casaceli and others, 1986). The Late Pliocene mineralization is associated with opaline and chalcedonic sinter as well as quartz-adularia veins.
The sinter, which covers an area of about 0.5 square kilometers, is located just east of a concealed northerly-striking fault that may have considerable throw. That fault is probably part of the fault system that controls the east flank of the Toiyabe Range (Casaceli and others, 1986). Fault scarps of similar strike cut Quaternary fan deposits about 2.5 km to the north (Stewart and McKee, 1968), and somewhat farther north, there is evidence for at least one faulting event that probably is no older than late Pleistocene (Lidke, 2000).