Geochemical exploration using mercury vapor

Mercury is commonly present in trace amounts in base metal sulfides. The chemical characteristics of Hg permit its release from oxidizing sulfides and, being extremely volatile, it can migrate through considerable depths of overburden. Anomalous concentrations of Hg may often be detected in soil gas overlying a deeply buried sulfide source, providing sufficiently sensitive analytical techniques are available such as the new Au thin film Hg detector. Laboratory experiments simulating natural occurrences were used to evaluate the effects of changes in barometric pressure, temperature, and soil moisture on the migration and release of Hg from soil gas. Diurnal changes in barometric pressure affect the emission of Hg in soil gas so as to limit accurate sampling using a static collection system such as plastic hemispheres. By use of a soil pump a pressure gradient is artificially produced, overcoming short term variations in atmospheric pressure. Wet soil inhibits Hg migration in soil gas sufficiently to make accurate measurements difficult. A variety of base and precious metal deposits were tested and in all cases Hg anomalies were detected in soil gas. The most intense and reproducible anomalies occur over high-grade vein deposits, especially those containing Zn-Pb-Ag mineralization. Anomalies over porphyry Cu deposits are less intense and are often difficult to distinguish from normal background variation. Laboratory experiments and field studies indicate that Hg vapor in soil gas is a valuable geochemical prospecting tool.