4:00 Wednesday October 30, 1996 as part of The Magmatic-Hydrothermal-Epithermal Transition and Associated Alteration and Mineralization Theme session, vol. 28, no. 7, p. A-403.

REACTION PATHS OF HIGH-SULFIDATION STATE COPPER-GOLD ORE FLUIDS IN CARBONATE ROCKS - A CASE STUDY AT THE SUPERIOR PORPHYRY-RELATED DEPOSIT, ARIZONA

FRIEHAUF, Kurt. C., Dept. of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115, kurt@pangea.Stanford.edu.

High-sulfidation acidic Cu-Au ore fluids in porphyry copper systems responsible for high-grade veins and advanced argillic alteration in granitic (e.g., Butte, Chuquicamata) and volcanic (e.g., Lepanto, Bor) rocks react with carbonate wall-rocks to form massive sulfide-iron oxide quartz lacking calc-silicates (e.g., Bisbee, Superior). The spatial zonation of ore types within these replacement bodies represents a time-integrated record of the progressive stages of neutralization of high-sulfidation state acidic ore fluids by carbonate wall rocks.

Massive replacement bodies in dolostone of the C-bed orebody at Superior contain siliceous, sulfide-rich cores and hematite margins. The sulfide cores are zoned outward from (1) central bornite-pyrite white mica chalcocite (with local shale altered to zunyite-kaolinite-quartz), (2) intermediate bornite-chalcopyrite-quartz-sericite chlorite chalcocite, and (3) outer pyrite-chalcopyrite-quartz-chlorite. The outer sulfide zone is in sharp contact with marginal hematite-quartz pyrite chalcopyrite which lacks aluminum-bearing phases (with local shale containing quartz-illite-chlorite). Porphyry dikes within the marginal hematite zone are altered to quartz-sericite-pyrite-chlorite. Outside the replacement bodies, quartz-dolomite hematite sphalerite veinlets cut dolostone. Ores at Superior formed by intial replacement of carbonate by hematite-quartz which armored fresh ore fluid from carbonate and allowed it to deposit Cu-Fe sulfides at higher acidities and sulfidation states. Replacement of hematite by sulfides neutralized and oxidized the solution and consumed sulfur, forming a residual fluid that deposited hematite at the margins of the orebody. Depletion of sulfur in solution led to cyclic saturation/undersaturation with respect to chalcopyrite, forming rhythmic mm-scale hematite / chalcopyrite layers parallel to the massive sulfide / hematite contact.