Geology and stable isotope studies of the Carlin gold deposit, Nevada

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 245+ m of the Roberts Mountains Formation. Early hydrothermal fluids dissolved calcite and deposited quartz. Fluids during the main hydrothermal stage introduced Si, Al, K, Ba, Fe, S, and organic materials, plus Au, As, Sb, Hg, and Tl; quartz and pyrite were deposited, potassium clays formed, and more calcite was dissolved. Sulfides and sulfosalts containing As, Sb, Hg, and Tl, 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, C02, H2S, and other components, leading to the production of H2S04 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, kaolinite 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° to 200°C. During later stage acid leaching and vein formation, when boiling was widespread in the hydrothermal fluids, temperatures may have reached as high as 275° to 300°C. The salinity of the fluids increased markedly from about 3 ± 1 equivalent weight percent NaCl 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 8lsO values increased with boiling from about 3 ± 3 to over 10 per mil. Along some fracture zones the hydro-thermal fluids mixed with unexchanged surface water. The 8180 values of different forms of silica (sedimentary chert, jasperoid, and quartz veinlets) are distinctive, as are different generations of calcite, and may be used as an aid to sort out obscure para-genetic features.    Calcite in the favorable facies has undergone extensive recrystallization and isotope exchange near feeder faults.   Dolomite in the favorable facies and both calcite and dolomite in unfavorable facies show little recrystallization and exchange.

The S34S values of 4.2 to 16.1 per mil for hydrothermal pyrite compare well with values for diagenetic pyrite from the Roberts Mountains Formation and suggest that most of the hydrothermal sulfide sulfur was of sedimentary origin and was derived from the lower Paleozoic section. The 834S values for vein barite range from 27.9 to 31.7 per mil. The sulfate sulfur may have been derived from the same source as the sulfide sulfur by equilibrium distribution of sulfur species in the hydrothermal fluid or from disseminated sedimentary barite. The sulfide-sulfate sulfur isotope temperatures are in excellent agreement with fluid inclusion filling temperatures in the late barite veins.