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 2 S/SO 2 . In such cases, the y 34 S 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 yD values of magmatichydrothermal alunite in high-sulfidation gold deposits because later fluids may effect a postdepositional retrograde hydrogenisotope exchange in the OH site of the alunite. This retrograde exchange also affects the reliability of the SO 4 -OH oxygenisotope 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 sulfurisotope and filling temperatures (315 to 730 8C) in quartz. The data from anhydrite and coexisting sulfides also record a This article is a U.S. government work, and is not subject to copyright in the United States. significant range in H 2 S/SO 4 2À and y 34 S P S among deposits and eve...

Mineralogical Distribution of Germanium, Gallium and Indium at the Mt Carlton High-Sulfidation Epithermal Deposit, NE Australia, and Comparison with Similar Deposits Worldwide

“…Stage 1A magmatic-hydrothermal alunite [55,56] comprises up to~300 µm long euhedral, platy crystals occurring as disseminations, as infill in vugs, and as replacements of pre-existing feldspar crystals ( Figures 4A and 5A). Stage 1B magmatic-steam alunite [55,56] occurs as coarse-grained, plumose and banded alunite in veins ( Figures 4B and 5B). Previous WDS analyses of both types of alunite showed that they belong to the alunite-natroalunite solid solution series [KAl 3 (SO 4 ) 2 (OH) 6 -NaAl 3 (SO 4 ) 2 (OH) 6 ] [30].…”

Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group

“…If a similar record were to exist on Mars, sulfur isotopic detection by the SAM QMS may detect it. Sulfur isotopic compositions of sulfate and reduced-sulfur minerals may also provide valuable information of environmental sulfur cycling by abiogenic hydrothermal (Rye, 2005;Greenwood et al, 2000) and atmospheric processes (Farquhar et al, 2000).…”

Large sulfur isotope fractionations in Martian sediments at Gale crater

“…16 . Times required to achieve 90% equilibrium between sulfate/sulfide for pH ~4-7 and total sulfur concentration of 0.01 mol S/kg H 2 O at several temperatures are annotated across top of frame 35,36 . The shaded region shows the approximate range of  34 S observed in Gale crater samples reported here.…”