The Carlin gold deposit, largest of the epithermal disseminated replacement-type gold deposits discovered to date in the United States, formed as a result of hydrothermal processes associated with a shallow-seated late Tertiary igneous event. The orebodies formed by the replacement of carbonate minerals, principally calcite, in thin-bedded argillaceous arenaceous dolomitic beds favorable for mineralization within the upper 245q-m of the Roberts Mountains Formation. Early hydrothermal fluids dissolved calcite and deposited quartz. Fluids during the main hydrothermal stage introduced Si, A1, K, Ba, Fe, S, and organic materials, plus Au, As, Sb, Hg, and T1; quartz and pyrite were deposited, potassium clays formed, and more calcite was dissolved. Sulfides and sulfosalts containing As, Sb, Hg, and T1, and base metal sulfides of Pb, Zn, and Cu probably formed later in the paragenesis. The main stage of ore deposition was terminated with the deposition of barite veins and the onset of boiling. The fluids lost H20, CO2, H2S, and other components, leading to the production of HeO4 in the upper levels of the deposit and to subsequent intense acid leaching and oxidation of rocks and ore near the surface. Within this zone, calcite and large amounts of dolomite were removed, sulfides and organic compounds oxidized, kaolinire and anhydrite formed, and silica was added. After the hydrothermal event, the upper part of the deposit underwent weak oxidation by cooler ground water. Fluid inclusion evidence indicates that main-stage mineralization temperatures were 175 ø to 200øC. During later stage acid leaching and vein formation, when boiling was widespread in the hydrothermal fluids, temperatures may have reached as high as 275 ø to 300øC. The salinity of the fluids increased markedly from about 3 ñ 1 equivalent weight percent NaC1 during the main stage to as much as 17.4 percent during the later boiling. The salinity as well as the temperatures of fluids during late-stage supergene oxidation were quite low. Hydrogen and oxygen isotope data indicate that the hydrothermal fluids were highly exchanged meteoric waters of 8D-140 to-160 per mil and that 8x80 values increased with boiling from about 3 ñ 3 to over 10 per mil. Along some fracture zones the hydrothermal fluids mixed with unexchanged surface water. The 8xso values of different forms of silica (sedimentary chert, jasperold, and quartz veinlets) are distinctive, as are different generations of calcite, and may be used as an aid to sort out obscure paragenetic features. Calcite in the favorable facies has undergone extensive recrystalliza-* Present Address: Radtke and Associates,
