Abstract

Although some recent gold exploration efforts have been directed at carbonaceous sediments as hosts for gold deposits, extrapolation of that same exploration philosophy to include the possibility of coal beds as favorable host horizons has been generally ignored. Coal, sub-coal, and carbon-rich sediments are ideal concentrating units for gold in hydrothermal and diagenetic systems. Coal is typically the most permeable of carbon-rich sediments due to its inherent brittleness, which leads to high fracture density. Coal is also typically enclosed by packages of relatively impermeable sediments, which can serve to channel potentially mineralizing solutions into coal seams. Coal concentrates metals both by the carbon and hydrocarbons acting as chemical reductants and by the elemental carbon acting as a molecular lattice trap. The entrained gold is finely dispersed in micrometer-sized particles of metal in the coal. Gold concentrations of a few ppm are known to occur naturally in some coal beds. Extreme concentrations of gold (>200 ounces per ton) are commonly loaded onto carbon in conventional gold mill circuits. Regional compilations of coal occurrences coincident with occurrences of gold and other epithermal-suite metals could help provide the framework for an exploration model.