Abstract

Alteration and lithologic characteristics suggest that Red Mountain, Arizona, represents the upper level lithocap region of a porphyry copper system, and this interpretation is reinforced by recent disclosure of deeply buried porphyry copper-type mineralization at this locality. Heating/freezing tests on 1,131 fluid inclusions in sulfide-bearing quartz veins from drill core samples and surface outcrops indicate that two fluids, distinctly different in their physicochemical properties, attended alteration-mineralization at Red Mountain. The early fluids had salinities of 10 + or - 5 equiv. wt percent NaCl and boiled at approximately 375 degrees -425 degrees C. Based on these inclusions, a geothermal gradient of approximately -25 degrees C/km and pressures of 275 + or - 75 bars prevailed during this early stage of vein filling. With passing time, boiling ceased and the fluids cooled to approximately 300 degrees C. A later fluid with temperatures of approximately 375 degrees -250 degrees C and salinities of approximately 30-50 equiv. wt percent NaCl was responsible for deposition of chalcopyrite. Atomic K/Na ratios of these later hydrothermai fluids, as determined from sylvite and halite dissolution temperatures, ranged from 0.52 to 1.76. The time-temperature-salinity relationships observed from fluid inclusion studies at Red Mountain are consistent with those predicted for similar geologic environments by numerical simulation and suggest that the earliest vein-filling fluids at Red Mountain had sources in the nearby volcanic wall rock and that later, higher salinity fluids originated in more distant wall rocks or in an as yet unseen intrusion at depth.

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