ALTERATION AND GEOCHEMICAL CHARACTERISTICS OF THE UPPER PARTS OF SELECTED PORPHYRY SYSTEMS, WESTERN NEVADA.

摘要

Six areas in western Nevada have characteristics that may be representative of the upper parts of porphyry copper systems. Slight erosion of the Miocene North Carson and Excelsior systems has exposed siliceous (cristobalite-opal), alunitic, argillic, illitic, and alsic-P (pyrophyllite + quartz (+OR-) diaspore) altered andesites. Most trace elements are depleted in the strong alteration assemblages with some enrichment of Mo and As. Somewhat deeper erosion of the ash-flow tuff-hosted Miocene Rhyolite Pass and the intermediate metavolcanic-hosted Miocene(?) Minnehaha Canyon systems has exposed alunitic, alsic-P, sericitic, and illitic alteration. Most trace elements are depleted but positive anomalies of Cu, Pb, Mo, and As are locally present. Deep erosion of the metavolcanic and plutonic-hosted Jurassic(?) Buckskin and Cretaceous Green Talc systems has exposed alsic-A (andalusite-corundum), alsic-P, sericitic, and potassic alteration. Positive anomalies of Cu, Mo, Pb, and Zn occur at Buckskin; Cu, Mo, and B at Green Talc.;Hypersaline (about 40 equiv. wt. % NaCl) boiling fluids with temperatures of 380 to 395(DEGREES)C were present in the potassic zones at Buckskin and Green Talc whereas the fluids at Rhyolite Pass were cooler (250 - 325(DEGREES)C) and less saline (about 10 equiv. wt. % NaCl).;A proposed model for alteration zonation in the upper parts of porphyry copper systems involves SO(,2), H(,2)S, CO(,2), HCl, and some alkali metallic chloride complexes partitioning into a magmatic vapor plume rising from the top of the crystallizing porphyry intrusion. During cooling and mixing with meteoric water, SO(,2) from the ascending and condensing magmatic vapor plume reacts with water to form sulfuric acid, which reacts with the wall rocks to form illitic, sericitic, alsic, and alunitic alteration zones migrating as reaction fronts laterally from major fluid channelways. Gradual collapse of the magmatic vapor plume during the solidification of the porphyry intrusion causes alteration assemblages to encroach downward towards the intrusion with higher a(,H)+/a(,K)+ alteration assemblages tending to replace lower a(,H)+/a(,K)+ assemblages. Hypogene mineralization is weakly developed in the upper portions of the system, since most metals are not partitioned into the magmatic vapor plume and because acid leaching tends to remove metals.;Upper parts of porphyry copper systems may share many alteration and geochemical similarities with, and may not be readily distinguishable from, the upper portions of simple meteoric water boiling systems, except where alsic-A or potassic alteration is exposed.