Pyrite Textures, Trace Elements and Sulfur Isotope Chemistry of Bijaigarh Shales, Vindhyan Basin, India and Their Implications

Abstract

<jats:p>The Vindhyan Basin in central India preserves a thick (~5 km) sequence of sedimentary and lesser volcanic rocks that provide a valuable archive of a part of the Proterozoic (~1800–900 Ma) in India. Here, we present an analysis of key sedimentary pyrite textures and their trace element and sulfur isotope compositions in the Bijaigarh Shale (1210 ± 52 Ma) in the Vindhyan Supergroup, using reflected light microscopy, LA-ICP-MS and SHRIMP-SI, respectively. A variety of sedimentary pyrite textures (fine-grained disseminated to aggregates, framboids, lags, and possibly microbial pyrite textures) are observed reflecting quiet and strongly anoxic water column conditions punctuated by occasional high-energy events (storm incursions). Key redox sensitive or sensitive to oxidative weathering trace elements (Co, Ni, Zn, Mo, Se) and ratios of (Se/Co, Mo/Co, Zn/Co) measured in sedimentary pyrites from the Bijaigarh Shale are used to infer atmospheric redox conditions during its deposition. Most trace elements are depleted relative to Proterozoic mean values. Sulfur isotope compositions of pyrite, measured using SHRIMP-SI, show an increase in δ34S as we move up stratigraphy with positive δ34S values ranging from 5.9‰ (lower) to 26.08‰ (upper). We propose limited sulphate supply caused the pyrites to incorporate the heavier isotope. Overall, we interpret these low trace element signatures and heavy sulfur isotope compositions to indicate relatively suppressed oxidative weathering on land during the deposition of the Bijaigarh Shale.</jats:p>

Primary source of placer gold in the Luk Ulo Metamorphic Complex, Central Java, Indonesia

“…A signature of bacterial reduction, framboidal pyrite (Figure 5b), is also compatible with seafloor processes. This feature has previously been reported in many sedimentary pyrites from several deep-sea sedimentary units (e.g., Mukherjee et al, 2020;Steadman et al, 2015) and massive sulfide ores rich in framboidal pyrite from Kuroko-type massive sulfide ore deposits in Japan (e.g., Ishizuka & Imai, 1998;Komuro & Sasaki, 1985). Thus, the sulfur of the seafloor basalt-hosted massive sulfide mineralization was derived from magmatic source and/or extracted from basaltic rocks with a small influence of bacterial activity in the seafloor environment.…”

“…As earlier studies suggested, sulfur is mostly liberated from sedimentary pyrite during prograde metamorphism when pyrite-pyrrhotite transformation occurs (e.g., Large et al, 2011;Pitcairn et al, 2010). High concentrations of trace elements such as As, Sb, Pb, and Cu of the hydrothermal pyrite are also similar to the composition of common sedimentary pyrite elsewhere (e.g., Mukherjee et al, 2020;Steadman & Large, 2016). Near-zero δ 34 S value of pyrite formed by the metamorphic rocks-hosted vein type mineralization could possibly be a result of mixing of sulfur between negative and strongly positive δ 34 S values during the remobilization of sulfur by metamorphic or hydrothermal processes (e.g., Hou et al, 2016).…”

Editorial for Special Issue “Pyrite Varieties and LA-ICP-MS Geochemistry in Ore Genesis and Exploration”

“…The fourth paper in the Special Issue explains sedimentary pyrite textures, their trace element and sulfur isotope ratios in pyrite varieties to understand paleo-redox conditions during the deposition of the Mesoproterozoic Bijaigarh Shale [4]. The combination of reflected light microscopy, LA-ICP-MS and SHRIMP-SI is involved in this study.…”

“…However, the capabilities of the present-day LA-ICP-MS method in terms of high sensitivity and using mapping of trace elements continues to provide new insights into how these ore-forming processes take place, and in what order, during formation of gold [1], massive sulfide [2,3] and other deposits. This method yields plentiful trace element data on pyrite varieties in oil shales, which could be useful to develop inorganic-organic models of oil formation and predict the metal potential of adjacent pyrite mineralization [4]. Research on trace elements in pyrite of black shales in different basins should be considered as a new step useful for the recognition of sedimentation conditions and in the reconstruction of the geochemical evolution of oceans [5].…”