Geochemistry, mineralogy, and genesis of the Copperstone gold deposit, La Paz County, Arizona.

Abstract

Copperstone is a mid-Tertiary "detachment gold" deposit that is herein classified as a new subtype of epithermal deposit. Copperstone is similar to many active geothermal systems in terms of host lithology, temperature, and related siliceous alteration mineralogy, except in the hypogene oxide assemblages, the high salinity, and the lack of sulfides and the pathfinder toxic elements. The host rock is mainly Jurassic quartz latite porphyry and Tertiary sedimentary breccias. Early stages of alteration were K-metasomatic and propylitic, and alteration that accompanied gold mineralization was mainly chloritization and silicification. Mineralization is structurally controlled and is restricted along the Copperstone Listric Fault with minor mineralization along high-angle NE and NW faults, as with many other deposits in west-central Arizona. The gold stage of mineralization was superimposed on previously K-metasomatized rocks. Hypogene mineralization can be divided into 3 paragenetic stages: early amethyst-quartz-Fe-rich-chlorite-specularite-hematite-Au° of stage C; late-stage fine-grained euhedral quartz-adularia-chrysocolla ± malachite ± magnetite ± chalcopyrite-pink fluorite-barite-ankerite-calcite-Au° of stage D; and barren quartz-pale green fluorite-barite-calcite-hematite of stage E. Gold occurs as free particles or is encapsulated in amethyst and late fine-grained euhedral quartz. The time of mineralization is Miocene or younger and the depth of mineralization was 1 Km. Gold mineralization was related to boiling such that a hot spring system did operate at Copperstone. Copperstone is a hydrothermal system created during the last stages of detachment faulting, with mineralization controlled by boiling, and "second boiling" was the principal trigger of Copperstone gold deposition in an environment of falling temperatures and pressures on chloride-rich brine fluids. Variation in δ³⁴S indicates that two different fluids are involved in this system, and that a deeper, "lighter" hydrothermal fluid mixed with a "heavier" basin brine. δ¹⁸O values from carbonates confirm the extent of isotopic exchange with an aqueous reservoir and indicates that oxidation accompanied gold deposition.