Geochemical and geologic relations of gold and other elements at the Gold Acres open-pit mine, Lander County, Nevada

The distribution and association of gold and other elements at the Gold Acres open-pit mine was studied to determine the suites of elements introduced during mineralization and their bearing on the origin of this disseminated gold deposit. The mine lies on the east flank of the northern Shoshone Range in rocks of Ordovician, Silurian and Silurian(?), and Devonian age in the southernmost of three closely spaced belts defined by anomalous concentrations of many metals. These rocks occur in the upper plate of the Roberts Mountains thrust fault ofMississippian age. Rocks of Devonian and Mississippian age in the lower plate of the thrust crop out in a window adjacent to the mine; a few tabular blocks of rocks of lower-plate lithology now occur interleaved with typical upper-plate rocks in the mine workings.The thrust fault may lie at shallow depth beneath the workings. Contact metamorphism resulting from intrusion of a buried granitic pluton in Cretaceous time transformed some of the rocks of the mine to hornfels and tactite. U nmineralized samples collected from outcrops in the Cortez Range and from a drill core in upper-plate rocks 5.3 miles north of the open-pit mine provide a measure of background values for 31 elements; samples collected in an area of weak mineralization near Gold Acres were used to determine threshold concentrations. Median values used as a basis for comparing data on background and threshold concentrations with data from samples collected in the mine offer a means of establishing objective definitions of anomalies at the mine. Within the mine, gold, arsenic, boron, mercury, and tungsten occur in anomalous concentrations in mineralized fault zones and in carbonate rocks. These elements, concentrated in the lower part of the mine, have similar distribution patterns. Copper, arsenic, bismuth, cadmium, manganese, and zinc occur in anomalous concentrations in the southern part of the mine in hornfels, tactite, and metagreenstone. These elements are associated with iron, lead, and molybdenum, which do not occur in anomalous amounts. Fault zones contain anomalous concentrations of elements that occur with gold, anomalous concentrations of chromium, cobalt, lead, manganese, nickel, silver, titanium, vanadium, yttrium, zinc, and zirconium, and strong but not anomalous concentrations of iron. Geologic considerations and data on correlations indicate that the mine contains a gold suite consisting of gold, arsenic, boron, mercury, and tungsten; a base-metal suite containing copper, lead, zinc, arsenic, bismuth, cadmium, iron, manganese, and molybdenum; and an ironmanganese suite of elements consisting of iron, manganese, arsenic, boron, chromium, cobalt, copper, nickel, scandium, titanium, yttrium, and zirconium. Silver may occur in the gold and base-metal suites. Field relations of these suites of elements and data from fluid-inclusion studies indicate that mineralization began after contact metamorphism with the deposition of molybdenite in tactite at a temperature of 380°±50°C. Base metals were introduced during deposition of sphalerite, pyrite, and lesser amounts of other sulfide minerals at temperatures of 160°-235°C. Gold, arsenic, boron, mercury, and tungsten were deposited last, at 160°-l95°C. Mineralization probably occurred at a depth of about 5,000 feet during hydrothermal activity following emplacement of the Cretaceous pluton beneath the open-pit mine. The iron-manganese suite is interpreted to have formed by supergene processes after hypogene mineralization.