Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: integration of geological, stable-isotope, and fluid-inclusion evidence

Abstract

The Summitville Au–Ag–Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5±0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite–quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz–alunite, quartz–kaolinite, and clay. The acid-sulfate-altered rocks host subsequent covellite+enargite/luzonite+chalcopyrite mineralization accompanied by kaolinite, and later barite–base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 °C (90% between 190 and 310 °C), consistent with the range (200–250 °C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite–pyrite pair yielded a sulfur-isotope temperature of 390 °C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals. The δ 34S values of sulfides in magnetite (−2.3‰) and of sulfate in apatite (5.4‰) in unaltered quartz latite indicate that δ 34S ΣS was near 0‰. The δ 34S values of coexisting alteration alunite and pyrite are 18.2‰ to 24.5‰ and −8.1‰ to −2.2‰, respectively. Deep in the system, most of the change in δ 34S values occurs in the sulfates, indicating that the fluids were initially H 2S-dominant, their redox state buffered at depth by equilibration with igneous rocks. However, in the main alteration zone, most of the change in δ 34S values occurs in pyrite, indicating that the fluids moved off the rock buffer and became SO 4 2−-dominant as pyrite precipitated and SO 2 disproportionation produced the sulfuric acid requisite for acid leaching. The δ 34S values of the late-stage barite and sulfides indicate that the system returned to high H 2S/SO 4 2− ratios typical of the original rock-buffered fluid. The δD H2O of alunite parent fluids was near −45‰ and their δ 18O ranged from 7‰ to −1‰, depending on the degree of exchange in the alteration zone at low water–rock ratio, or mixing with unexchanged meteoric water. The low δD values of some alunite samples are interpreted to result from postdepositional exchange with later ore fluids. Fluid exsolved from the magma at depth had δD H2O and δ 18O H2O values near −70‰ and 10‰, respectively. During and following migration to the top of the magma chamber, the fluid underwent isotopic exchange with the partially crystallized magma and its solid and cooler, but still plastic, carapace just below the transition from a lithostatic to hydrostatic pressure regime. These evolved magmatic fluids had δD H2O and δ 18O H2O values close to −40‰ and 5‰, respectively, prior to release into the superjacent hydrostatically pressured fracture zone, wherein the fluids separated into a low-density phase and hypersaline brine. The low-density phase rose to the level of acid-sulfate alteration, where disproportionation of SO 2 commenced upon condensation of the vapor plume causing the acid-sulfate alteration. Hydrogen and oxygen isotopic compositions estimated from deep kaolinitic wallrock alteration and fluid inclusions in quartz stockwork veins indicate the brine mixed with highly exchanged meteoric water at depth and rose to the ore zone, producing the Cu–Au–Ag mineralization. The composition of inclusion fluids in a single sample of enargite also indicates mixing between ore fluids and unexchanged meteoric water at shallow depths. The sulfate in fluids responsible for precipitation of most barite samples was of magmatic origin, but the δ 34S of a single sample suggests mixing with sulfate derived from the atmospheric oxidation of H 2S in an overlying steam-heated zone.