DECODING FINE-SCALE PB ISOTOPE DISEQUILIBRIUM BETWEEN GOLD AND VEIN GANGUE MINERALS IN SLEEPER EPITHERMAL DEPOSIT, NEVADA

In Sleeper Au-Ag epithermal deposit in northern Nevada gold often occurs as fine bands in veins composed mainly of gangue quartz and adularia. The Au-Ag (electrum) banding occurs in mm to cm scale and provides a microstratigraphic framework that can provide information for changes in chemistry and conditions as a function of time. The electrum in the bands is mainly as fine 3-dimentional dendritic textures embedded within the gangue. These fine dentritic textures make impossible to physically separate clean Au-Ag for chemical and isotope analyses. We attempted different chemical separation methods to isolate the gold ore from the silicate gangue. Clean electrum for chemical and isotope analyses can be obtained after removing the silicate fraction with double hydrofluoric acid attack followed by nitric acid cleaning. Pb from both the gold ore and gangue was separated with anion exchange resin and analyzed on MC-ICP-MS. The Pb isotope data show disequilibrium between the gold ore and the silicate fraction that cannot be explained with in-situ U and Th decay. The observed trend in Pb-Pb isotope space suggests distinct sources for gold and gangue minerals in the epithermal hydrothermal system. The silicate minerals show elevated ²⁰⁷Pb/²⁰⁴Pb indicating that the elements composing the gangue are most likely leached from local sedimentary or metamorphic rocks. In contrast, the gold ore shows lower ²⁰⁷Pb/²⁰⁴Pb indicating derivation from deeper, likely magmatic source. The observed Pb isotope disequilibrium between gangue and electrum occurs at mm to cm scale within the epithermal vein. This rapid change in the sources is suggestive of a dynamic interplay between two distinct components in the hydrothermal system. Meteoric water dominated hydrothermal fluid leaches silica and other gangue components from the local country rocks and precipitates gangue minerals. Discrete pulses of deep magmatic fluids bring Au-Ag to the otherwise barren epithermal system and precipitate the electrum bands. The electrum shows small, but discernible Pb isotopic compositions suggesting that each band may be a result of a discrete pulse of magmatic fluid.