The oldest rocks near Steamboat Springs consist of metamorphosed volcanic and sedimentary rocks of unknown age intruded by granodiorite that is probably related to the Sierra Nevada batholith. The younger rocks are largely volcanic, consisting of Tertiary andesite, late Tertiary or early Pleistocene basalt and Pleistocene or possibly Recent pumiceous rhyolite. The valleys are filled with alluvium, except a small area consisting of siliceous sinter deposited by the hot springs. The springs arc on the northeast edge of the Steamboat Hills, which lie in a structural trough about six miles wide. The springs appear to be structurally related to recent faults and genetically related to the magmatic source of the recent volcanic domes of pumiceous rhyolite.
Many of the springs are at or near the boiling point for this altitude; the geothermal gradient is very high, commonly exceeding one degree F per foot of depth within the explored range. Water analyses indicate a parent saline water diluted to varying degrees by water of low concentration; saline magmatic water is believed to mix with meteoric water at some unknown depth below the surface. Heavy rains have marked superficial effects on the discharge, salinity, and temperature. The discharge responds to changes in atmospheric pressure, a relationship previously found only in wells piercing artesian aquifers, but artesian conditions are either obscured or are absent.
Associated gases are principally CO2, H2S, nitrogen, and minor amounts of oxygen, hydrogen, and argon. High‐temperature vapors also contain mercury. Cinnabar, stibnite, pyrite, and other sulfides have crystallized at depths of five to 50 feet below the surface in sinter and gravels reworked by hot water after burial under new sinter. Some siliceous muds deposited from the springs contain gold and antimony equivalent to commercial ores, and also considerable silver, mercury, copper, and arsenic. The muds are believed to precipitate largely from the thermal water at shallow depth, and rise in mechanical suspension, most abundantly during stages of high discharge; some precipitation may continue at the surface.
The acidity or alkalinity of the waters appears to be significantly related to the temperature of the springs, with the most alkaline waters tending to occur in the highest temperature springs and wells. This may be a very significant factor in the chemistry of ore‐bearing solutions.
A conspicuous acid type of rock alteration is taking place near the surface at and above water level; it is believed to be largely due to sulfuric acid formed by oxidation of H2S. Presumably a different type of alteration is taking place below water level, where rocks are In contact with near‐neutral and alkaline water.