A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems

Abstract

The stable-isotope geochemistry of sulfate minerals that form principally in I-type igneous rocks and in the various related hydrothermal systems that develop from their magmas and evolved fluids is reviewed with respect to the degree of approach to isotope equilibrium between minerals and their parental aqueous species. Examples illustrate classical stable-isotope systematics and principles of interpretation in terms of fundamental processes that occur in these systems to produce (1) sulfate in igneous apatite, (2) igneous anhydrite, (3) anhydrite in porphyry-type deposits, (4) magmatic-hydrothermal alunite and closely related barites in high-sulfidation mineral deposits, (5) coarse-banded alunite in magmatic-steam systems, (6) alunite and jarosite in steam-heated systems, (7) barite in low-sulfidation systems, (8) all of the above minerals, as well as soluble Al and Fe hydroxysulfates, in the shallow levels and surface of active stratovolcanoes. Although exceptions are easily recognized, frequently, the sulfur in these systems is derived from magmas that evolve fluids with high H 2S/SO 2. In such cases, the δ 34S values of the igneous and hydrothermal sulfides vary much less than those of sulfate minerals that precipitate from magmas and from their evolved fluids as they interact with igneous host rocks, meteoric water, oxygen in the atmosphere, and bacteria in surface waters. Hydrogen isotopic equilibrium between alunite and water and jarosite and water is always initially attained, thus permitting reconstruction of fluid history and paleoclimates. However, complications may arise in interpretation of δD values of magmatic-hydrothermal alunite in high-sulfidation gold deposits because later fluids may effect a postdepositional retrograde hydrogen–isotope exchange in the OH site of the alunite. This retrograde exchange also affects the reliability of the SO 4–OH oxygen–isotope fractionations in alunite for use as a geothermometer in this environment. In contrast, retrograde exchange with later fluids is not significant in the lower temperature steam-heated environment, for which SO 4–OH oxygen–isotope fractionations in alunite and jarosite can be an excellent geothermometer. Sulfur isotopic disequilibrium between coexisting (but noncontemporaneous) igneous anhydrite and sulfide may occur because of loss of fluid, assimilation of country-rock sulfur during crystallization of these minerals from a magma, disequilibrium effects related to reactions between sulfur species during fluid exsolution from magma, or because of retrograde isotope exchange in the sulfides. Anhydrite and coexisting sulfide from porphyry deposits commonly closely approach sulfur–isotope equilibrium, as is indicated by the general agreement of sulfur–isotope and filling temperatures (315 to 730 °C) in quartz. The data from anhydrite and coexisting sulfides also record a significant range in H 2S/SO 4 2− and δ 34S ΣS among deposits and even during the course of mineralization at a single deposit. Sulfur isotopic disequilibrium among aqueous sulfur species may occur in any hydrothermal environment except the relatively high-temperature (200–400 °C) low-pH (<3) magmatic-hydrothermal environment, in which SO 4 2− forms along with H 2S from the disproportionation of SO 2 during the condensation of a magmatic vapor plume. Magmatic–steam alunite forms from expanding SO 2-rich magmatic vapors that rise so rapidly that sulfur isotopic exchange between SO 4 2− and H 2S does not occur during disproportionation of SO 2. Such alunite has δ 34S values similar to that of the bulk sulfur in the magma. Residence times of SO 4 2− in steam-heated systems, although seldom long enough to permit more than partial sulfur–isotope exchange with streaming H 2S, often are long enough to permit oxygen–isotope equilibrium with water. In active stratovolcanoes, aqueous sulfate derived from the disproportionation of SO 2 and the oxidation of H 2S can also mix with that derived from the oxidation of pyrite near the surface. In the near-neutral low-sulfidation system at Creede, CO, isotopic exchange among hydrothermal aqueous species was so slow that sulfur and even oxygen isotopic signatures derived from bacteriogenic and thermochemical reduction of sulfate in moat sediments are observed in hydrothermal barite.