THE SEARCHLIGHT MINING DISTRICT: LINKING LOW SULFIDATION EPITHERMAL MINERALIZATION WITH AN UNDERLYING GRANITIC PLUTON USING S AND PB ISOTOPES

The Searchlight Mining District is a relatively small but high-grade low sulfidation epithermal district, located within the Colorado River Extensional Corridor (CREC). The CREC is characterized by a high degree of extension during the Middle Miocene, where low angle extensional faults tilted the rocks as much as 90 degrees to the west exposing a 13 km cross section. This exposure provides a rare opportunity to examine in detail the link between a low sulfidation epithermal system and the underlying magma chamber. Contact relationships and previous ⁴⁰Ar/³⁹Ar and ²³⁸U/²⁰⁶Pb dates from others indicate that the pluton and the mineralization are coeval, and formed just prior to the onset of major east-west extension.

Here we present new lead and sulfur isotope data on galena, chalcopyrite, and pyrite from the mineralized veins. Two hand-picked mineral separates of galena were analyzed for Pb isotopes using TIMS. Analyzed samples are from the Quartette mine, the major deposit in the district, and the Big Casino mine, located about 5 km east of the town of Searchlight. The respective results are ²⁰⁶Pb/²⁰⁴Pb (18.00 and 18.01), ²⁰⁷Pb/²⁰⁴Pb (15.55 and 15.57), ²⁰⁸Pb/²⁰⁴Pb (38.77 and 38.83). These results compare well with the Pb isotope values for three samples from the Searchlight pluton (Bachl et al., 2001) [²⁰⁶Pb/²⁰⁴Pb (18.09 to 18.17), ²⁰⁷Pb/²⁰⁴Pb (15.56 to 15.59), ²⁰⁸Pb/²⁰⁴Pb (38.80 to 38.91)].The similarity between Pb isotopes from the Searchlight pluton and the ore deposits suggests that metals were sourced from the pluton and that there was little or no interaction with the prominent Proterozoic gneiss. Sulfur isotopes of the same galena samples have δ³⁴S values of -0.7 and -3.7 ‰, respectively. A hand-picked chalcopyrite separate from the Big Casino mine sample yielded a δ³⁴S value of -0.5 ‰. A hand picked pyrite from trachy-dacite porphyry that was moderately to strongly altered to adularia and chlorite, and which is located west of the Searchlight M&M mine in the northern part of the district yielded a δ³⁴S of -1.7 ‰. The results of the S isotopes are compatible with a magmatic sulfur origin. These results, coupled with system geometry, provide compelling evidence that at least some metals and sulfur were sourced from the pluton.