Geology of the massive sulfide deposits at Jerome, Arizona; A reinterpretation
The massive sulfide deposits at Jerome, consisting largely of pyrite, chalcopyrite, and sphalerite, are concordant stratabound lenses in massive quartz-bearing crystal tuffs of Precambrian age and overlying bedded tuffaceous rocks. The crystal tuffs were eraplaced as submarine pyroclastic flows. The evidence permits the interpretation that the massive sulfide lenses are essentially syngenetic and related to hydrothermal brines that discharged into a submarine basin. The host rocks and sulfide lenses were folded and metamorphosed. Two periods of folding can be recognized; during the second period, the older folds were deformed along vertical axes, and some of the chalcopyrite in the major lens migrated downward to form shoots of intersecting veins in the crystal tuff and chloritized tuff (black schist). Fluid inclusions in volcaniclastic and vein quartz appear to be related to deformation and metamorphism and provide no direct evidence on the nature of the primary fluids that presumably introduced the ore metals to the volcanic rocks accumulating on the sea floor. Liquid CO2-bearing inclusions are restricted to rocks and veins near the United Verde deposit and associated chalcopyrite ore shoots. Fluids at that time were CO2-rich, with temperatures of approximately 235 ø C and confining pressures of about 700 bars or less.