The silver-gold mines along the Comstock Lode were known for their extremely hot, difficult working conditions (Lord, 1883, p. 389-406); the miners commonly worked in temperatures of 38-51°C. Church (1879, p. 289) considered the Comstock mines "to be the hottest in the world." Smith (1943, p. 245) stated that "no other mines in the world have encountered such heat and such floods of scalding water."
Because of variations in ventilation, air temperatures in the workings varied considerably over short distances and are difficult to interpret. Rock temperatures also were modified by ventilation and water removal, thus temperatures taken in drill holes or immediately after a rock face was exposed are more useful. As Locke (1912) put it, the "temperatures are deranged by the presence of the mine workings which make possible the presence of the observer."
In a general way rock temperatures in these mines 1.94C for every 30.5 m of depth (Becker, 1882, p. 230; figure). This gradient persisted for some distance away from the Lode, but water temperatures taken at the face of the Sutro drainage tunnel while it was being driven showed that temperatures rose rapidly as the Lode was approached, even though the depth of the tunnel below the surface remained relatively constant ( figure). Water presently flowing from the portal of the tunnel in Lyon County is 28°C (Glancy and Katzer, 1975).
The highest rock temperature recorded was 75°C from a dry drill hole on the 900 meter level of the Yellow Jacket Mine (diary, Superintendent Thomas G. Taylor). Mr. Cosgrove, foreman of the Yellow Jacket measured rock temperatures of 60 and 58°C on the 670-m level. Temperatures of about 54°C were recorded at numerous spots at depths of 580 to 610 m in the Ophir, Chollar, Potosi, Crown Point, and other mines. All these temperatures were measured in drill holes immediately after a hole was finished. The rock surface temperatures of workings in the same area were 51°C or less.
The highest temperature of any considerable quantity of water was recorded during the flooding of the 900-m level of the New Yellow Jacket shaft in November, 1880 (Becker, 1882, p. 230); 77°C water under considerable pressure was struck in a drill hole at a depth of 939 m in the bottom of the shaft and soon flooded the mine. On February 13, 1882, a flood of 69°C water from the 853 m level of the Exchequer Mine again drowned the pumps in the New Yellow Jacket shaft; all the mines in the vicinity were flooded, the water rising to the level of the Sutro drainage tunnel (annual report, Superintendent Thomas G. Taylor, July 1, 1882). A small flow of water in the cast crosscut on the 600-m level of the Crown Mine had a temperature of 69°C (Church, 1879, p. 291). The body of water that flooded the Savage and Hale, and Norcross Mines in 1877(?) still had a temperature of 68°C two years later (Church, 1879, p. 291) even though over a million metric tons of water had been removed.
As would be expected, the circulation of water was eccentric. Numerous clay seams sealed off the flow. Cutting such a clay seam frequently released dammed-up bodies of water which flooded the workings. The seams also appear to have greatly inhibited the upward convective flow of the hot water; there were no hot springs along the Lode's surface croppings, and the water encountered in the upper workings was cold, suggesting that the upward flow of hot water was feeble compared to the downward percolation of meteoritic water. The "perched," imprisoned nature of much of the water encountered is illustrated by the fact that once the water level was lowered below the Sutro Tunnel, the water never rose to that level again (as long as the workings remained open to observation), even after pumping had stopped. The Comstock Lode obviously did not provide as easy a passageway for the upward flow of hot water as one might expect.
The restricted flow strongly suggests that the volcanic and intrusive wall rocks were the source of the heat, rather than the heat being introduced from some more distant source by hot-water flow. Exothermic reactions involving vein materials have been proposed as a possible source of the heat, but the low acidity and relatively unmineralized condition of the water, and rarity of exothermic reaction products in the vein material suggest that little heat has been generated by this mechanism.