THE SEARCHLIGHT MINING DISTRICT, NEVADA: LINKING LOW SULFIDATION EPITHERMAL MINERALIZATION WITH AN UNDERLYING GRANITIC PLUTON

The Searchlight Mining District is a relatively small (250,000 oz Au) high-grade epithermal gold district located in the Colorado River Extensional Corridor, which has undergone a high degree of extension during the Middle Miocene. Low angle extensional faults have tilted the district as much as 90 degrees to the west exposing a near complete cross section (13 km depth) from post-ore volcanic rocks on the west, through older volcanic rocks cut by mineralized veins, and into underlying plutonic rocks to the east. This extraordinary exposure provides a rare opportunity to examine in detail the link between a low sulfidation epithermal system and the underlying magma chamber. ⁴⁰Ar/³⁹Ar dates on early, unmineralized hydrothermal alunite and on adularia associated with Au and Ag, and ²³⁸U/²⁰⁶Pb dates on zircons from the pluton and volcanic host rocks demonstrate that the mineralization and pluton are coeval.

Early hydrothermal activity produced S-rich alunite alteration. Later mineralization consists of S-poor Au-bearing veins with minor Cu, Ag, Pb, and Zn distributed above and cross-cutting the pluton roof. Veins are hosted predominantly in Middle Miocene plagioclase-phyric trachydacite porphyry, trachyandesite, volcanic breccia, and quartz monzonite. The deep part of the epithermal system is characterized by high base metals, and minerals include quartz, specular hematite, chalcopyrite, galena, sphalerite, pyrite, and native Au. Fluid inclusions in primary, secondary and unknown fluid inclusion assemblages homogenized at 290 to 320° C and have salinities ranging from 3.4 - 0.9 wt % NaCl eq. In contrast, the shallow part of the system is characterized by a low base metal content and the mineralogy consists of quartz, adularia, electrum, acanthite, native Ag, and nil to low base metal minerals. Fluid inclusions in primary, secondary and unknown fluid inclusion assemblages homogenized at 190 to 220° C and salinities range from 0.5 - 0 wt % NaCl eq.

Results and system geometry are consistent with metals and hydrothermal fluids being sourced from the pluton. Alunite alteration and vein mineralization resulted from distinctly different magmatic-hydrothermal events. The deep and shallow veins may have resulted from fluids generated by discrete events and/or fluid evolution through mixing with meteoric waters.