Fluid Inclusion and H–O–S–Pb Isotope Geochemistry of the Yuka Orogenic Gold Deposit, Northern Qaidam, China
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The Yuka gold deposit, located in the western part of northern Qaidam, contains Au orebodies hosted in early Paleozoic metamorphic basic volcaniclastic rocks. The Yuka mineralization can be divided into three stages: early quartz-pyrite (stage-I), middle quartz-gold-polymetallic sulfide (stage-II), and late quartz-carbonate (stage-III). Gold deposition is primarily contained within stage-II. Three types of fluid inclusions were identified in the vein mineral assemblages using petrography and laser Raman spectroscopy: H 2 O-CO 2 -NaCl (C-type), H 2 O-NaCl (W-type), and pure CO 2 (PC-type). Stage-I fluids record medium temperatures (205.2°C to 285.5°C) and H 2 O-CO 2 -NaCl±CH 4 fluids with variable salinities (0.6–8.5 wt.% NaCl equiv.). Stage-II fluids evolved towards a more H 2 O-rich composition within a H 2 O-CO 2 -NaCl±CH 4 hydrothermal system at medium temperatures (193.1°C to 271.1°C), with variable salinities (0.4–11.7 wt.% NaCl equiv.). Stage-III fluids are almost pure H 2 O and characterized by low temperatures (188.1°C to 248.5°C) and salinities (0.4–16.1 wt.% NaCl equiv.). These data indicate that ore-forming fluids are characterized by low to medium homogenization temperatures and low salinity and are evolved from a CO 2 -rich metamorphogenic fluid to a CO 2 -poor fluid due to inputs of meteoric waters, which is similar to orogenic-type gold deposits. The average Keywords:
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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.
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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.
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