A PETROGRAPHIC, GEOCHEMICAL AND STABLE ISOTOPE STUDY OF THE UNITED VERDE OREBODY AND ITS ASSOCIATED ALTERATION, JEROME, ARIZONA

Abstract

The United Verde orebody, a Proterozoic volcanogenic massive sulfide deposit, is hosted by the Cleopatra Formation. The Cleopatra Formation is subdivided into two distinct members, the Upper and Lower, on the basis of alteration facies, whole rock geochemistry and the chemistry of alteration minerals. The Lower member was deposited prior to ore deposition and consists of five major alteration facies. Facies Bl, the most distant from the orebody represents the recharge area for the ore-forming fluid. Facies B2 surrounds the major discharge area or the chlorite pipe. These three facies contain chlorite and quartz as alteration minerals in variable amounts. Two facies, 81 and S2, contain quartz and sericite as alteration minerals. Mass balance calculations show progressive removal of Na and Ca, and addition of MgO and FeO* from the area of recharge (facies Bl) to facies B2 to the chlorite pipe. Whole rock δ¹⁸O values are high relative to least altered Cleopatra Formation in the recharge area and low in the discharge zone. Mineralogy and geochemistry of samples from the Upper member indicate that it was deposited following ore deposition and interacted with fluids rich in silica and iron. The hydrothermal fluid, which is interpreted to have been seawater, evolved to a high temperature slightly acidic, reduced fluid during water-rock interaction(log a₀₂ = -33 to -41; log a(H2S) = -2.6 to -5.0). The fluid δ¹⁸O and δ¹³C₀₂ values increased. Calculated δ¹³C₀₂ and δ¹⁸O fluid values, and sphalerite and chlorite chemistry imply that mixing of the hydrothermal fluid with seawater occurred in the orebody. the upper The levels of the chlorite pipe and in limited range in δ³⁴S values of sulfides is consistent with the derivation of the oreforming fluids from the reduced basal layer of a stratified basin. The study area represents only a small part of the United Verde circulation cell. Increased δ¹⁸O values of the fluid, and the need for a source of Mg, Fe and other metals suggest that the fluids may have circulated into the Shea Basalt.