Secondary precious metal enrichment by steam-heated fluids in the Crofoot-Lewis hot spring gold-silver deposit and relation to paleoclimate

The Crofoot-Lewis deposit is an adularia-sericite-type (low-sulfidation) epithermal Au-Ag deposit, whose well-preserved paleosurface includes abundant opaline sinters, widespread and intense silicification, bedded hydrothermal eruption breccias, and a large zone of acid sulfate alteration. Radiogenic isotope ages indicate that the system was relatively long-lived, with hydrothermal activity starting around 4 Ma and extending, at least intermittently, for the next 3 m.y.Field evidence indicates that the surficial zone of acid sulfate alteration formed in a steam-heated environment within an active geothermal system. A drop in the water table enabled descending acid sulfate waters to leach Au and Ag from zones of low-grade disseminated mineralization, resulting in the redistribution and concentration of Au and Ag into ore-grade concentrations. These zones of secondary Au-Ag enrichment are associated with opal + alunite + kaolinite + montmorillonite + or - hematite and were deposited in open space fractures at, and within a few tens of meters below, the paleowater table.The stable isotope systematics of alunite and kaolinite in the steam-heated environment are relatively complex, due to variations in the residence time of aqueous SO ₄ that formed from the oxidation of H ₂ S prior to precipitation of alunite, and the susceptibility of fine-grained kaolinites to hydrogen isotope exchange with later waters. Most of the alunites are enriched in ³⁴ S relative to early sulfide minerals, reflecting partial S isotope exchange between aqueous SO ₄ and H ₂ S. About half of the alunites give reasonable calculated delta ¹⁸ O (sub SO ₄ -OH) temperatures for a steam-heated environment indicating O isotope equilibrium between aqueous SO ₄ and water. The delta D (sub H ₂ O) values of the hydrothermal fluids varied by almost 60 per rail over the life of the meteoric water-dominated system, suggesting significant climate changes.Mineralization is believed to have resulted from large-scale convection of meteoric water controlled largely by basin and range fractures and a high geothermal gradient with H ₂ S for Au complexing derived from organic matter in basin sediments. A wet climate resulted in the formation of a large inland lake which provided abundant recharge water for the hydrothermal system. A fluctuating water table controlled by changing climatic conditions enabled steam-heated acid sulfate fluids to overprint lower grade mineralization resulting in ore-grade precious metal enrichment.