Application of fluid inclusion and rock-gas analysis in mineral exploration

Abstract

Gases are known to form haloes around presently active geothermal systems and might be useful in exploration for older, extinct hydrothermal systems. Gases from such extinct hydrothermal systems can be liberated for analysis from fluid inclusions in hydrothermal minerals by thermal decrepitation or they can be desorbed from alteration mineral surfaces by heating. The most abundant of these gases, besides H2O, are usually CO2, CH4, CO and N2. We have used a gas chromatograph to analyze these gases in fluid inclusions in jasperoid around the Pueblo Viejo gold-silver deposit, in vein minerals from the Creede silver-lead-zinc deposit, and from clays in the alteration cap overlying veins at Creede to test for gas haloes useful in exploration. At Pueblo Viejo CO2 abundances in the jasperoid range from less than 1 mole percent (with respect to the system CH4-CO2-CO-N2-H2O) in the ore zone to as much as 6 mole percent in surrounding, barren jasperoid. Fluid inclusion analyses at Creede suggest that a drop in the CO2 content of the fluid may relate to ore deposition and clay directly above veins has large amounts of adsorbed CO2. These results suggest that primary gas abundances exhibit patterns in and around hydrothermal ore deposits that can be used in mineral exploration.