Proterozoic sedimentary exhalative (SEDEX) deposits and links to evolving global ocean chemistry

Abstract

Sedimentary exhalative (SEDEX) Zn-Pb-sulfi de mineralization fi rst occurred on a large scale during the late Paleoproterozoic. Metal sulfi des in most Proterozoic deposits have yielded broad ranges of predominantly positive δ 34 S values traditionally attributed to bacterial sulfate reduction. Heavy isotopic signatures are often ascribed to fractionation within closed or partly closed local reservoirs isolated from the global ocean by rifting before, during, and after the formation of Rodinia. Although such conditions likely played a central role, we argue here that the fi rst appearance of signifi cant SEDEX mineralization during the Proterozoic and the isotopic properties of those deposits are also strongly coupled to temporal evolution of the amount of sulfate in seawater.The ubiquity of 34 S-enriched sulfi de in ore bodies and shales and the widespread stratigraphic patterns of rapid δ 34 S variability expressed in both sulfate and sulfi de data are among the principal evidence for global seawater sulfate that was increasing during the Proterozoic but remained substantially lower than today. Because sulfate is produced mostly through weathering of the continents in the presence of oxygen, low Proterozoic concentrations imply that levels of atmospheric oxygen fell between the abundances of the Phanerozoic and the defi ciencies of the Archean, which are also indicated by the Precambrian sulfur isotope record. Given the limited availability of atmospheric oxygen, deep-water anoxia may have persisted well into the Proterozoic in the presence of a growing sulfate reservoir, which promoted prevalent euxinia. Collectively, these observations suggest that the mid-Proterozoic maximum in SEDEX mineralization and the absence of Archean deposits refl ect a critical threshold in the accumulation of oceanic sulfate and thus sulfi de within anoxic bottom waters and pore fl uids-conditions that favored both the production and preservation of sulfi de mineralization at or just below the seafl oor. Consistent with these evolving global conditions, the appearance of voluminous SEDEX mineralization ca. 1800 Ma coincides generally with the disappearance of banded iron formations-marking the transition from an early iron-dominated ocean to one more strongly infl uenced by sulfi de availability. 170 T.W. Lyons et al.

Trace sulfate in mid-Proterozoic carbonates and the sulfur isotope record of biospheric evolution

“…In addition to limited sulfate availability, calcium in the ocean may have been drawn down by abundant calcium carbonate precipitation under high degrees of supersaturation (Grotzinger, 1989;Grotzinger and Kasting, 1993), further elevating the degree of evaporation required to achieve gypsum saturation (Kah et al, 2001. Limited sulfate availability might also be expressed in the preponderance of 34 S-enriched mid-Proterozoic iron sulfides (Lambert and Donnelly, 1991;Bottomley et al, 1992;Logan et al, 1995;Canfield, 1998;Canfield and Raiswell, 1999;Gorjan et al, 2000;Shen et al, 2002Shen et al, , 2003Strauss, 2002;Lyons et al, 2005). Furthermore, given that sulfate is argued to inhibit dolomite formation (Baker and Kastner, 1981), the great abundance of Precambrian dolomite might reflect limited sulfate availability in the ocean.…”

“…Also, the observation that ␦ 34 S values both increase and decrease in the sulfate and sulfide data suggests that the common thread is not simply progressive isotopic evolution through bacterial sulfate reduction and pyrite burial within individual restricted marine rift basins (Lyons et al, 2005). ␦ 34 S values in such settings would consistently increase up-section.…”

“…The comparatively rapid isotopic variability that characterizes the bacteriogenic pyrite of the Belt Supergroup, and the pyrrhotite that derives from its metamorphism, is a style of ␦ 34 S variability now recognized in many Proterozoic sulfides (e.g., Ross et al, 1995;Strauss, 1997Strauss, , 2002, including sedimentary exhalative (SEDEX) sulfide deposits (Carr and Smith, 1977;Smith et al, 1978;Lyons et al, 2005). The general coherence of the isotopic trends among a number of mid-Proterozoic sections of widely different geographic position, geologic age, and depositional setting, and recorded in both sulfate and sulfide data, suggests a style of isotope behavior that is more likely a product of global oceanic trends than local processes.…”

Sulfur isotope biogeochemistry of the Proterozoic McArthur Basin

“…The lower division of the McArthur Basin (Nathan and McArthur Groups) hosts world class sedimentary exhalative, or SEDEX mineral deposits (Large et al, 2001;Lyons et al, 2006), in addition to stromatolitic and evaporitic carbonates. The upper division (Roper Group) is dominated by well-sorted quartzarenites and interbedded shales (Jackson et al, 1987;Crick et al, 1988).…”

“…Sulfur isotope studies (Canfield and Teske, 1996;Canfield, 1998;Lyons et al, 2000Lyons et al, , 2006Shen et al, 2002Shen et al, , 2003Kah et al, 2004;Poulton et al, 2004;Gellatly and Lyons, 2005) provide insight into Proterozoic geochemical cycles, as information regarding biological activity and ocean chemistry is preserved in these chemical records. Generally, questions of Precambrian geobiological interest revolve around (1) levels of seawater sulfate [as sulfate may relate to both atmospheric oxygen levels via oxidative weathering and biological activity], (2) pinpointing the presence or absence of different sulfur-utilizing microorganisms [as microbes provide insight into local environmental redox cycling], and (3) the possibility and implications of a longlived, pervasive sulfidic deep-ocean (related to the overall Earth surface redox state).…”