INDICATOR MINERALOGY AND POSSIBLE HIGHLY-OXIDIZED STATE ORE FLUIDS AT THE PERIPHERY OF THE ROBINSON PORPHYRY COPPER-GOLD SYSTEM, NEVADA

In “porphyry” copper-gold deposits (bulk-mineable chalcopyrite, abundant pyrite) localized in calc-alkaline plutons, recent work has focused on the high temperature chemistry of fluids within ore bearing zones. The character of the lower-temperature outer fringes of one such system, where peripheral lower temperature alteration grades outward into visibly-unaltered host rocks was studied in the Robinson District, Nevada. There, irregular porphyry sills and small barren plutons with propylitic alteration crop out to the north and east of the Ruth and Kimbley porphyry ore bodies. They were injected in late Cretaceous time into Ely Group (Pennsylvanian) marine calcareous sediments.

Shallow underground workings exploring small lead-zinc-gold bearing zones associated with irregular jasperoids and fractures provide a cross section of these weakly altered porphyritic rocks and hydrothermal alteration near their contact with overlying/adjacent Ely limestones (NOTE??-use latest usage IN district; elsewhere Ely Group)) These host rocks, in the vicinity of the intrusions exhibit beds a few m thick, some rich and others poor in chert and/or detrital quartz. Though visually undistinguishable, Mg and dolomite content of these strata, as determined by chemical and XRD analysis, vary considerably. Close to the barren plutonic rocks, the magnesian beds are bleached in places. Above the prominent Lane Valley sill, of unknown thickness, dolomite and white (Mg) saponite are notable. XRD and thin section show significant fine grained colorless diopside near some contacts. In areas of dark carbonate host, this mineral is scarcely visible in outcrop. In places, chert nodules show widespread removal of silica, which apparently was redeposited in nearby brown to gray dense pyritic jasperoid bodies.

Recent studies have discussed primary willemite deposits formed from saline, high-temperature fluids (Australia, Namibia). We calculated equilibrium conditions between the minerals present at the distal edge of the Robinson porphyry system and geologically reasonable fluids at temperatures probable for propylitic alteration assemblages. This work evaluates the possibility that fluids capable of precipitating primary willemite could also generate the observed alteration and mineralization near sill boundaries at Robinson.