ANALOGS OF EPITHERMAL GOLD-SILVER DEPOSITION IN GEOTHERMAL WELL SCALES

ACTIVE geothermal fields are generally recognized1-5 as modern analogues of the systems responsible for the formation of epithermal gold-silver ore deposits. We have investigated this analogy by studying scales precipitated at various depths in a geothermal well containing boiling fluids. Our results, presented here, show that concentrations and distributions of gold, silver, copper, lead and zinc in well M-53 at Cerro Prieto, Mexico, are comparable to those found in economic epithermal vein deposits, and that the highest concentrations of metals occur at or near the fluid production horizon. The onset of vapour separation or flashing at these depths in the well indicates that boiling caused sequential base and precious metal saturation through the loss of CO2 and H2S, respectively, to the vapour phase. These phenomena provide direct evidence that boiling of chloride-rich hydrothermal solutions may be responsible for the deposition and zonation of gold, silver, copper, lead and zinc in epithermal vein deposits. Geothermal well scales can thus provide the opportunity to study epithermal ore genesis in a wide variety of natural environments. © 1990 Nature Publishing Group.