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.
Key Words:
high-sulfidation, carbonate, porphyry, sulfide, replacement
