Genesis of the Hatu gold deposit: Constrained by in situ S Pb isotope geochemistry of pyrite
2
Citation
69
Reference
10
Related Paper
Citation Trend
Keywords:
δ34S
Siltstone
δ34S
Sulfur Cycle
Sedimentary organic matter
Sulfide Minerals
Cite
Citations (80)
δ34S
Authigenic
Cold seep
Cite
Citations (61)
The Paleoproterozoic metasedimentary rocks of the Zaonega Formation (Onega Basin, NW Russia) are important archives of inferred global environmental change following the initial oxygenation of the Earth's atmosphere and oceans. However, the geochemical signals preserved in these exceptionally organic-and pyrite-rich metasedimentary rocks and their environmental meaning remain contested. In particular, the Zaonega Formation's unusually high pyrite sulfur isotope ratios (δ34S) have been explained by either global or local forcings acting on sulfur cycling processes. We tested former interpretations of the Zaonega Formation's sedimentary pyrite record by integrating bulk and micro-scale δ34S analysis to discriminate the isotopic signatures of different generations of pyrite and determine the underlying mechanisms contributing to δ34S variability. We show that the prolonged genesis of pyrite occurred via multiple stages and included precipitation from early diagenetic fluids, organic matter pyritization, and late-stage alteration fluids. Our results demonstrate that early-stage pyrite typically carries more variable and lower δ34S values than late-stage pyrite. Although the early pyrite captures pore water S isotope signatures least evolved from the seawater, their contribution to the bulk δ34S results can be dwarfed by the greater volume of late-stage coarse pyrite. Consequently, determining the sequence of pyrite precipitation and δ34S characteristics of individual generations in any given sample are fundamental to interpreting bulk δ34S records. Our micro-scale results suggest that previous estimates based on bulk pyrite data (ca. 6 to 18‰) should not be related to the original seawater sulfate's isotopic composition. These results demonstrate that a thorough understanding of the geological context and mechanisms associated with S-cycling, and pyrite formation is necessary to interpret bulk δ34S records accurately.
δ34S
Cite
Citations (9)
Authigenic
δ34S
Cite
Citations (168)
Uranium ore
Cite
Citations (1)
δ34S
Cite
Citations (30)
Abstract Sedimentary pyrite formation links the global biogeochemical cycles of carbon, sulfur, and iron, which, in turn, modulate the redox state of the planet's surficial environment over geological time scales. Accordingly, the sulfur isotopic composition (δ34S) of pyrite has been widely employed as a geochemical tool to probe the evolution of ocean chemistry. Characteristics of the depositional environment and post-depositional processes, however, can modify the δ34S signal that is captured in sedimentary pyrite and ultimately preserved in the geological record. Exploring sulfur and iron diagenesis within the Bornholm Basin, Baltic Sea, we find that higher sedimentation rates limit the near-surface sulfidization of reactive iron, facilitating its burial and hence the subsurface availability of reactive iron for continued and progressively more 34S-enriched sediment-hosted pyrite formation (δ34S ≈ −5‰). Using a diagenetic model, we show that the amount of pyrite formed at the sediment-water interface has increased over the past few centuries in response to expansion of water-column hypoxia, which also impacts the sulfur isotopic signature of pyrite at depth. This contribution highlights the critical role of reactive iron in pyrite formation and questions to what degree pyrite δ34S values truly reflect past global ocean chemistry and biogeochemical processes. This work strengthens our ability to extract local paleoenvironmental information from pyrite δ34S signatures.
δ34S
Biogeochemical Cycle
Sedimentation
Cite
Citations (35)
δ34S
Cite
Citations (25)
δ34S
Siltstone
Cite
Citations (2)
The Howard's Pass district of sedimentary exhalative (SEDEX) Zn-Pb deposits is located in Yukon Territory and comprises 14 Zn-Pb deposits that contain an estimated 400.7 Mt of sulfide mineralization grading 4.5 % Zn and 1.5 % Pb. Mineralization is hosted in carbonaceous and calcareous and, to a lesser extent, siliceous mudstones. Pyrite is a minor but ubiquitous mineral in the host rocks stratigraphically above, within, and below mineralization. Petrographic analyses reveal that pyrite has a complex and protracted growth history, preserving multiple generations of pyrite within single grains. Sulfur isotope analysis of paragenetically complex pyrite by secondary ion mass spectrometry (SIMS) reveals that sulfur isotope compositions vary with textural zonation. Within the Zn-Pb deposits, framboidal pyrite is the earliest pyrite generation recognized, and this exclusively has negative δ34S values (mean = −16.6 ± 4.1 ‰; n = 55), whereas paragenetically later pyrite and galena possess positive δ34S values (mean = 29.1 ± 7.5 and 22.4 ± 3.0 ‰, n = 13 and 13, respectively). Previous studies found that sphalerite and galena mineral separates have exclusively positive δ34S values (mean = 16.8 ± 3.3 and 12.7 ± 2.8 ‰, respectively; Goodfellow and Jonasson 1986). These distinct sulfur isotope values are interpreted to reflect varying contributions of bacterially reduced seawater sulfate (negative; framboidal pyrite) and thermochemically reduced seawater sulfate and/or hydrothermal sulfate (positive; galena, sphalerite, later forms of pyrite). Textural evidence indicates that framboidal pyrite predates galena and sphalerite deposition. Collectively, the in situ and bulk sulfur isotope data are much more complex than δ34S values permitted by prevailing genetic models that invoke only biogenically reduced sulfur and coeval deposition of galena, sphalerite, and framboidal pyrite within a euxinic water column, and we present several lines of evidence that argue against this model. Indeed, the new data indicate that much of the base metal sulfide mineralization was emplaced below the sediment-water interface within sulfidic muds under reducing conditions during early diagenesis. Furthermore, thermochemical sulfate reduction provided most of the reduced sulfur within the Zn-Pb deposits.
δ34S
Sulfide Minerals
Cite
Citations (57)