The geology and evolution of the Hot Spring gold deposit at Cherry Hill, California.

摘要

At Cherry Hill, California, gold-precipitating hot springs are associated with an economically significant body of gold mineralization. The deposit is about 0.56 Ma old and has a simple mineralogy: gold - quartz - adularia - carbonate - iron sulfide microveinlets dominate the ore. The paragenetic sequences consists of twelve stages: early adularia (I), an alternating sequence of silica and carbonates (II-XI) and late sulfate deposition (XII). Gold deposition occurred during stages V-VI and IX. Early adularia - sericite alteration in the central part of the deposit was followed by the development of anomalies of gold, silver, mercury, antimony, thallium and arsenic to the northwest and southeast as well as the central zone. Most recently, strong argillic alteration has developed to the northwest of the central zone. Gold anomalies correlate broadly with those of silver, mercury, antimony, thallium and arsenic.; Early in the history of the deposit temperatures were relatively high (ca 185{dollar}spcirc{dollar}C), salinities varied widely (8-14 wt % NaCl eq.), and there was boiling. As the system evolved, temperatures and salinities dropped to the values of the present springs (about 55{dollar}spcirc{dollar}C and 2.8 wt % tds). Oxidation potential was kept relatively low by abundant hydrocarbons, and fo{dollar}sb2{dollar} dropped over time largely in response to decreasing temperature. Sulfur fugacity fluctuated between pyrrhotite-stable and pyrite {dollar}pm{dollar} cinnabar-stable. Fluid pH remained near neutral and was probably buffered mostly by mudstone. {dollar}delta{dollar}18{dollar}sb{lcub}rm O{rcub}{dollar} values of veinlet minerals suggest that the fluids were generally close to equilibrium with mudstone. Gold solubility was controlled mainly by changes fs{dollar}sb2{dollar} and {dollar}Sigma{dollar}S concentration rather than by variations in pH, fo{dollar}sb2{dollar}, temperature or salinity.; Solid bitumen and/or primary fluid inclusions with a vapor bubble and variable proportions of oil and water occur in Stages I-II and VII-XI. Stages during which gold was deposited sometimes trapped bitumen and/or oil (IX) and sometimes not (V and VI). Most of the oils have no n-alkanes and only traces of hydrocarbons in the range C{dollar}sb1{dollar}-C{dollar}sb{lcub}14{rcub}{dollar}. Gold contents of the bitumen range from 0.3-37 ppm. The gold in the bitumen is probably the result of encapsulation of preexisting gold by hydrocarbons, entrainment of fine gold particles by petroleum or the scavenging of gold from later hydrothermal solutions.