Geology and geochemistry of jasperoids from the Gold Bar district, Nevada

Gold Bar is one of several Carlin-type gold mining districts located in the Battle Mountain-Eureka trend, Nevada. It is composed of one main deposit, Gold Bar; five satellite deposits; and four resources that contain 1.6 Moz (50 t) of gold. All of the deposits and resources occur at the intersection of north-northwest- and northeast-trending high-angle faults in slope facies limestones of the Devonian Nevada Group exposed in windows through Ordovician basin facies siliciclastic rocks of the Roberts Mountains allochthon. Igneous intrusions and magnetic anomalies are notably absent. The Gold Bar district contains a variety of discordant and stratabound jasperoid bodies, especially along the Wall Fault zone, that were mapped and studied in some detail to identify the attributes of those most closely associated with gold ore and to constrain genetic models. Four types of jasperoids, J(0), J(1), J(2), and J(3), were distinguished on the basis of their geologic and structural settings and appearance. Field relations suggest that J(0) formed during an earl event. Petrographic observations, geochemistry, and delta O-18 values of quartz suggest it was overprinted by the hydrothermal event that produced ore-related J(1), J(2), and J(3) jasperoids and associated gold deposits. The greater amount of siliciclastic detritus present in Jo jasperoids caused them to have higher 6180 values than J(1,2,3) jasperoids hosted in underlying limestones. Ore-related jasperoids are composed of main-ore-stage replacements and late-ore-stage open-space filling quartz with variable geochemistry and an enormous range of delta O-18 values (24.5 and -3.7 parts per thousand). Jasperoids hosted in limestones with the most anomalous Au, A, Hg, +/-(As, Sb, T1) concentrations and the highest delta O-18 values are associated with the largest deposits. The 28 parts per thousand range of jasperoid delta O-18 values is best explained by mixing between an O-18-enriched fluid and an O-18-depleted fluid. The positive correlation between the sizes of gold deposits and the delta O-18 composition of jasperoids indicates that gold was introduced by the O-18-enriched fluid. The lowest calculated 6180 value for water in equilibrium with late-ore-stage quartz at 200 degrees C (-15 parts per thousand) and the measured delta D value of fluid inclusion water extracted from late-ore-stage orpiment and realgar (-116 parts per thousand) indicate that the O-18-depleted fluid was composed of relatively unexchanged meteoric water. The source of the O-18-enriched ore fluid is not constrained. The delta S-34 values of late-ore-stage realgar, orpiment, and stibnite (5.7-15.5 parts per thousand) and barite (31.5-40.9 parts per thousand) suggest that H2S and sulfate were derived from sedimentary sources. Likewise, the 613 C and 6180 values of late-stage calcite (-4.8 to 1.5 parts per thousand and 11.5 to 17.4 parts per thousand, respectively) suggest that CO2 was derived from marine limestones. Based on these data and the apparent absence of any Eocene intrusions in the district, Gold Bar may be the product of a nonmagmatic hydrothermal system.