Geology and Geochemistry of the Golden Butte Mine- A Small Carlin-Type Gold Deposit in Eastern Nevada

The Golden Butte sediment-hosted disseminated (Carlin-type) gold deposit is located on the western flank of the Cherry Creek Range in east central Nevada. The ore body contains nearly 3 million tons of gold ore with an average grade of 0.031 ounces per ton. The main host of the gold ore is the Mississippian Chainman Shale, a carbonaceous, silty claystone to siltstone. A low-angle (normal?) fault has superimposed Chainman Shale over unmineralized Devonian Simonson Dolomite. A second low-angle fault has placed the Simonson Dolomite over Chainman Shale. Two high-angle, basin-and-range-style faults cut the low-angle faults and served as conduits for ascending hydrothermal fluids. Solution by hydrothermal fluids and repeated movement on both sets of faults before and during mineralization created highly permeable pathways and many bodies ofbreccia. Hydrothermal alteration of the Chainman Shale included removal of carbonate, silicification (jasperoid), argillization, and dolomitization. Gold mineralization is found in all alteration types, but jasperoid is the most important host for ore. Jasperoid is commonly brecciated, and vugsand veins indicate multiple stages of silicification. Minerals associated with the jasperoid include coarsely crystalline quartz, stibnite, pyrite, and local cinnabar and realgar. Oxidized zones and veins cut across other alteration types and include lateformed stibiconite, barite, calcite, limonite, and hematite. Alteration in the Simonson Dolomite is less extensive and includes jasperoid, argillic, sanded dolomite, and late oxidized types. Major-element analyses show the jasperoid at Golden Butte is similar to other jasperoids related to Carlin-type deposits. Isocon diagrams reveal that Au, Ag, As, Sb, Hg, TI, Sc, and Th are significantly enriched in the alteration products. Volume increases (locally as much as 400%) during alteration, especially for the ore-hosting jasperoid, where up to 500% Si02 was introduced, were important. The change in volume occurred by filling of open spaces in a porous, repeatedly brecciated host. Principal component analysis of the geochemical data partitioned most of the pathfinder elements in a factor separate from Au and Ag in all alteration types. This suggests that precious-metal mineralization may have occurred separate in space and/or time from the event that deposited most of the pathfinder elements or that there were differing solubilities of the elements in the hydrothermal solutions. For jasperoid samples, another factor may be related to residual immobile elements and argillic components. Fluid inclusions suitable for microthermometry are rare. However, inclusions found in jasperoid and vein quartz, vein barite, and late calcite are uniformly liquid-rich and C02-poor; no evidence for bOiling of the hydrothermal fluids was found. Homogenization temperatures of inclusions in jasperoid quartz ranged between 180 and 295°C, with salinities between 4.5 and 6 equiv wt percent NaCl. Inclusions in quartz veins had homogenization temperatures from 135 to 200°C and salinities of 4.5 to 6 equiv wt percent NaCl. Inclusions in late barite and calcite veins had low salinities « 2 equiv wt% NaCI). Accurate homogenization temperatures could not be obtained from inclusions in barite due to stretching of the fluid inclusions.