Revisiting hydrocarbon phase mobilization of Au in the Au–Hg McLaughlin Mine, Geysers/Clear Lake area, California

Abstract

Carbonaceous material (CM)-enriched silica is co-located with gold (Au) mineralization at the Au–Hg McLaughlin deposit, Geysers/Clear Lake area, California, U.S.A. The co-location suggests that hydrocarbons (HC) may be involved in mineralization and metal concentration processes, but little is known about the role of HC in the formation of ore deposits. Previous studies noted liquid oil inclusions in samples from the McLaughlin deposit, and proposed that the HC were liquid at the time of ore deposition. Hydrocarbon materials in the McLaughlin deposit occur as solid and liquid. Textural evidence suggests that hydrocarbon-rich and aqueous, silica-rich fluids were present simultaneously, as well as separately in alternating pulses. Synchrotron X-ray fluorescence microscopy of microscopic silica-free carbonaceous material reveals that the CM contains abundant ore metals e.g., Au, Ag, Hg, and Pb. The CM could have become metal-enriched by scavenging metals from other ore fluids, or it could have transported metals when the CM was still liquid, with subsequent in-situ degradation due to hydrothermal heat. Gold concentrations of up to 18 ppm were measured via acid digestion of solid and liquid HC material and subsequent inductively coupled plasma–mass spectrometry (ICP–MS) analyses. Hydrocarbon material with liquid to medium viscous properties bearing 10.8 ppm Au provides evidence that Au in liquid HC in the McLaughlin Au-Hg deposit is still mobile and that remobilization and/or transport of metals to the deposit by HC liquids cannot be ruled out.