Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Abstract

Surface exposures in the Singatse Range at the Ann-Mason deposit yield a nearly complete vertical cross section, from 1 to 6 km in paleodepth, through one of three porphyry Cu deposits genetically tied to the Middle Jurassic Yerington batholith. Detailed field mapping of wall-rock alteration, veinlets, and sulfides, combined with petrographic, mineral composition, and fluid inclusion studies, has led to an assessment of temperatures, salinities, origins and flow paths of hydrothermal fluids through time and space. Ann-Mason is associated with a granite porphyry dike swarm emanating from a deep granite cupola eraplaced into earlier quartz monzodiorite. On the basis of crosscutting relations between different veins and age relative to porphyry dikes, hydrothermal alteration-mineralization can be divided into premain stage (endoskarn), main stage (propylitic, sodic-calcic, and potassic) and late stage (sodic, chloritic, and sericitic). Advanced argillic alteration in the nearby Buckskin Range represents the paleosurface environment of porphyry systems exposed in the Singatse Range.Pre-main-stage endoskarn (plagioclase + salite + or - garnet) in quartz monzodiorite is localized adjacent to metasedimentary wall rocks at paleodepths of 3 to >6 km. Main-stage propylitic alteration (albite + epidote + actinolite + chlorite) formed at paleodepths above 4 km; sodic-calcic alteration formed at greater paleodepths of 3.5 to >6 km along the granite-quartz monzodiorite contact and in the deep portion of the porphyry dike swarm. The most intense sodic-calcic alteration lacks sulfide and formed oligoclase-actinolite-sphene by addition of Na and leaching of K, Fe, and Cu. Potassic alteration, characterized by replacement of hornblende by biotite + or - K feldspar, formed in a vertical zone 4 km high and 1.2 km in diameter extending upward from the granite cupola along the axis of the dike swarm. The hypogene Cu orebody (495 Mt of 0.4 wt % Cu) lies within the area of most intense potassic alteration at a 2.5- to 4.0-km paleodepth, containing abundant hydrothermal biotite and quartz-K feldspar veins. Sulfide assemblages include chalcopyrite + or - bornite + or - molybdenite, and chalcopyrite > pyrite. Much of the Cu is found in later sulfide epidote veins.Late-stage alteration affected all porphyry dikes and formed a funnel-shaped zone, rooted in the Cu orebody at a palcodepth of 4 km. At a 1-km paleodepth, the late-stage alteration zone is 3 km wide. Late-stage alteration assemblages include early sodic and chloritic and late sericitic ones. Sodic alteration is zoned upward from albite-chlorite-vermiculite to albite-sericite-pyrite; albite-chlorite alteration leached Cu from the center of the ore zone, and both sodic assemblages contain albitized K feldspar. Sodic assemblages grade laterally into chloritic assemblages in which mafic minerals are replaced by chlorite + or - sericite. Sodic and chloritic assemblages are cut by pyrite veins with quartz-sericite-pyrite halos and quartz-tourmaline breccias at paleodepths above 2 km.Hydrothermal evolution can be modeled in terms of four fluids of different origins reacting with rocks by alkali exchange and H (super +) metasomatism. Phase equilibria and fluid inclusion studies constrain temperatures and salinities of hydrothermal fluids to the following values: (1) garnet endoskarn fluids at >500 degrees C, low salinity, X (sub CO 2 ) 400 degrees C, 31 to 41 wt percent NaCl equiv; (3) potassic, ore-forming fluids at 700 degrees to <400 degrees C, 32 to 62 wt percent NaCl equiv; and (4) late-stage fluids at 240 degrees to 200 degrees C, 2 to 13 wt percent NaCl equiv. Endoskarn mostly predated porphyry dike eraplacement and resuited from local influx of Ca-bearing fluids from carbonate wall rocks; these fluids are not linked to the ore-forming process. Sodic-calcic alteration was due to prograding, saline, nonmagmatic fluids (deep formation waters?) that flowed from the wall-rock contact laterally 3 km into the batholith at depths of 4 to 6 km. As they heated, these fluids altered K feldspar to oligoclase and leached Fe and Cu from approximately 10 km 3 of quartz monzodiorite. In the dike swarm, these fluids mixed with upwelling, cooling, saline fluids of magmatic origin and may have contributed some K, Fe, and up to 30 wt percent of the Cu in the ore zone. Magmatic and nonmagmatic, main-stage hydrothermal fluids flowed in convective cells coincident with emplacement of multiple dikes; maximum thermal gradients, fluid fluxes, and metasomatism occurred in the orebody at 500 degrees to 350 degrees C. Dilute, late-stage fluids of nonmagmatic (shallow meteoric?) origin initially exchanged sodium for potassium and leached copper along warming paths within the ore zone, but as they cooled and acidified, they caused widespread sericitic alteration near the paleosurface. The zone of maximum fluid flux moved progressively upward and major metasomatism ceased at about 200 degrees C.The volume and intensity of Na metasomatism that characterizes the Ann-Mason deposit is recognized in only a few other porphyry Cu districts; the processes related to Na metasomatism are unlikely to be necessary for the formation of such deposits. Na metasomatism does occur in numerous (nonporphyry-related) mineralized and barren plutonic-volcanic terranes around the world and is the expected result of heating (e.g., prograde) saline fluids which circulate near igneous heat sources. Where such fluids are involved in porphyry Cu systems, they may both add components to orthomagmatic ores and redistribute components from earlier concentrations.