Age and Hydrothermal Alteration, Gold Road Vein, Oatman District, Northwestern Arizona, and Possible Differences in Epithermal Gold Deposits Along the Colorado River

Abstract

Oatman is a mid-Miocene adularia-sericite (low-sulfidation) epithermal Au system with banded, open-space veins and notably low Ag production and primary sulfide content. The purpose of this study is to characterize the host rocks, wall-rock alteration, and age of the system and then to infer causes for formation of contrasting types of epithermal deposits along the Colorado River Extensional Corridor (CREC). Hydrothermal fluids that alter wall rocks and precipitate metals in principle may be exsolved from magmas or be derived from diverse non-magmatic sources (e.g., dilute meteoric water, seawater, alkaline lakes, brines), subsequently heated and circulated. Geochemical modeling indicates that the salinity and redox conditions of non-magmatic fluids may control the type of deposit formed, perhaps dilute groundwaters adularia-sericite deposits versus brines for iron oxide copper-gold (IOCG) deposits. The Oatman district, including the active Gold Road and Moss mines, has produced >2 million oz Au with a Ag : Au mass ratio of 0.66. Previous studies indicate fluid temperatures of ~200-250° and salinities of 1.5 wt % NaCl eq. The Silver Creek caldera, the source of the regionally extensive Peach Spring Tuff (rhyolitic to trachytic, 76-66% SiO2, 18.78 ± 0.02 Ma), is located in the district a few kilometers west of the veins. Veins are hosted by lava flows and breccias of the Oatman Andesite and overlying Gold Road Dacite (18.6 ± 0.9 Ma). This study utilizes mapping, short-wave infrared (SWIR) spectroscopy, whole-rock geochemistry, and petrography of the Gold Road vein and adjacent wall rock from the present surface to depths of 500 m. Whole-rock geochemical and SWIR analyses affirm the presence of regional sulfide-poor propylitic alteration defined by presence of carbonates, epidote, and chlorite. Wall rocks exhibit local silicification and adularia within centimeters of veins, giving way laterally to the regional propylitic alteration, where illite + montmorillonite ± kaolinite replace feldspars. The contact between Oatman Andesite and Gold Road Dacite, obscured by alteration, can be identified from mineralogic and trace-element characteristics. Ore-related adularia formed at 17.23 Ma (40Ar/39Ar), ~1 m.y. after crystallization of the latest intra-caldera intrusions but only slightly younger than a small rhyolite dome several kilometers to the south at Boundary Cone, dated at 17.57 Ma (40Ar/39Ar on groundmass). The CREC was highly extended in the mid-Miocene, probably forming a topographic low, accumulating evaporites that were likely sources of brines that formed “detachment-style” gold deposits (e.g., Copperstone) and other deposits in the IOCG family. The Oatman district formed slightly earlier on the eastern shoulder of the CREC, likely forming a high-standing volcanic center, akin to the modern Jemez Mountains and Valles caldera of New Mexico that flank the Rio Grande rift. Oatman might be an ancient analog of the active geothermal system of the Jemez Mountains, which contains dilute epithermal fluids. Circulation of fluids in both modern and Miocene systems may have been driven by topography and nearby extension, with or without magmatic heat and volatiles.