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The source of fluids and their mechanism of metal precipitation in sediment-hosted, disseminated orogenic gold deposits are ambiguous. Pyrite texture, trace element, S, Pb, and He-Ar isotope compositions of sulfides and C-O isotope data of calcite from Chang'an orogenic gold deposit in the Ailaoshan orogenic belt, southwest (SW) China, were studied to provide a new genetic model for the sediment-hosted orogenic gold deposit, furthering knowledge of the source of fluids and their mechanism of metal precipitation. Orebodies at Chang'an are mainly hosted by Ordovician turbidite with a few in Oligocene syenite. Two stages of mineralization have been identified in the deposit: stage I disseminated quartz-arsenopyrite-pyrite and stage II veined quartz-calcite-polymetallic sulfides. Five generations of pyrite have been identified in turbidite: pre-ore syn-sedimentary pyrite, pyI-1, and pyI-2 in stage I, and pyII-1 and pyII-2 in stage II, and an unzoned pyrite population developed in syenite. PyI-1 commonly overgrows syn-sedimentary pyrite with irregular boundaries and contains arsenopyrite, galena, chalcopyrite, and electrum inclusions along the boundaries. PyI-1 is overgrown by thin and inclusion-free pyI-2, and crosscut by pyII-1, which is rimmed by pyII-2.The syn-sedimentary pyrite is distributed parallel to the sedimentary bedding and contains As (620.8 ppm), Pb (61.6 ppm), Ni (59.8 ppm), Mo (54.4 ppm), Co (23.4 ppm), and Cu (13.0 ppm) with low-Au content of 0.06 ppm. This pyrite has δ34S values of −18.1 to +30.4‰ and high-radiogenic Pb isotope ratios (average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of 19.05, 15.86, and 39.87, respectively). PyI-1 and coexisting arsenopyrite are enriched in invisible Au (up to 227.1 and 353.3 ppm, respectively), As, Ni, Cu, and Pb, while pyI-2 contain much lower trace element abundances relative to pyI-1 and arsenopyrite. Partial replacement of syn-sedimentary pyrite by pyI-1 plus arsenopyrite, galena, chalcopyrite, and electrum, and similar Pb isotope ratios between syn-sedimentary pyrite and pyI-1 indicate that reaction of external deep Au-rich fluids with syn-sedimentary pyrite is responsible for gold precipitation in stage I. PyI-1, arsenopyrite, and pyI-2 show a narrower δ34S range of −3.2 to 7.1‰ relative to syn-sedimentary pyrite, demonstrating that the fluid-pyrite interaction has homogenized the sulfur. The unzoned pyrite in syenite has similar mineral inclusions (arsenopyrite, galena, etc.), δ34S values (+0.6 to 6.3‰) and Pb isotope ratios to pyI-1, but much lower trace element abundances relative to pyI-1. It may be attributed to different reactions of similar fluids with different wall-rocks. PyII-1 and pyII-2 in stage II contain elevated As, Pb, Cu, Sb, Zn, and Ag with low mean Au content (3.3 ppm) and have δ34S ranges of −2.8 to +1.2‰ and −6.2 and −0.8‰, respectively. Galena in stage II has lower radiogenic Pb isotope ratios than stage I pyrites, indicative of a different Pb source or fluid evolution. The gases released from a mixture of pyII-1-pyII-2 have R/Ra of 0.38 to 0.98 and 40Ar*/4He of 0.50 to 1.34, falling between the fields of mantle-derived and crustal fluids. Late ore calcites have δ13CPDB of −8.7 to 2.7‰ and δ18OSMOW of 8.05 to 25.58‰, also plotting between sedimentary carbonate and mantle fields. These signatures indicate that ore fluids in stage II are base metal-rich fluids with a small amount of contribution from the mantle. Different ore assemblages, trace element composition and isotope data between stages I and II at Chang'an suggest that the deposit experienced an evolution from early Au-rich fluids to late base metal-rich ones. This study highlights that ore metals in sediment-hosted disseminated orogenic gold deposits may be sourced from both deep fluids and local wall-rock, and that fluid-rock interaction behaved as a key control on ore precipitation.

The study evaluated the sources and controlling factors of the groundwater contaminants in an agroeconomic region of Lower Ganga Basin using principal component analysis (PCA), multivariable linear regressions (MLR), correlation analysis, and hierarchical cluster analysis, and evaluated the public health risks using the Latin Hypercube Sampling, goodness-of-fit statistics, Monte Carlo simulation and Sobol sensitivity analysis based on the 1000 samples collected in two sampling cycles (N = 1000). The study reveals that the dissolution of fluoride-bearing minerals and semi-arid climate regulate the fluoride concentrations (0.10–18.25 mg/L) in groundwater. Extensive application of inorganic nitrogenous fertilizers and livestock manure mainly contributed to elevated nitrate levels (up to 435.0 mg/L) in groundwater. The health risks analysis indicates that fluoride exposure is more prevalent in the residents of each age group than the nitrate and both contaminants exhibited higher non-carcinogenic health risks on the infant and child (minor) age groups compared to adolescents and adults. Based on the cokriging interpolation mapping, the minor residents of 17.88%–23.15% of the total area (4545.0 km2) are vulnerable to methemoglobinemia whereas the residents of all age-groups in 38.47%–44.45% of the total area are susceptible to mild to severe dental/skeletal fluorosis owing to consumption of untreated nitrate and fluoride enriched groundwater. The Sobol sensitivity indices revealed contaminant levels, groundwater intake rate and their collective effects are the most influential factors to pose potential health risks on the residents. Artificial recharge and rainwater harvesting practices should be adopted to improve the groundwater quality and the residents are advised to drink purified groundwater.

The significance of trace elements in initiating origins and driving evolution of life on Earth is indisputable. Trace element (TE) trends in the oceans through time broadly reflect their availability and allow speculation on all possible influences on early life. A comprehensive sedimentary pyrite–TE database, covering 3000 m.y. of the Precambrian, has improved our understanding of the sequence of bio-essential TE availability in the ocean. This study probed how changing availability (and scarcity) of critical TEs in the marine environment influenced early life. The pyrite-shale matrix TE sequence shows relatively elevated concentrations of Ni, Co, Cu, and Fe, Cr, respectively, in the Archean and Paleoproterozoic. Abundances of these elements in the Archean potentially facilitated their widespread utilization by prokaryotes. The Paleoproterozoic–Mesoproterozoic saw increases in Zn and Mo but a marked decline in Ni, Co, Cu, Se, and Fe. Our data suggest the evolution of the first complex cell in the Paleoproterozoic was probably triggered by this major change in TE composition of the oceans. A decline of elements prompted alternative utilization strategies by organisms as a response to TE deficits in the middle Proterozoic. An overall increase in a multitude of elements (Ni, Co, Cu, Cr, Se, V, Mo, and P) in the Neoproterozoic and Cambrian was highly advantageous to the various micro– and macro–life forms. Without questioning the importance of macronutrients and atmosphere-ocean redox state, multi-TE availability would have induced substantial heterogenous biological responses, owing to the effects of optimal, deficient, toxic, lethal, and survival levels of TEs on life.

Pyrite is a common mineral in sedimentary rocks and is the major host for many chalcophile trace elements utilized as important tracers of the evolution of the ancient hydrosphere. Measurement of trace element composition of pyrite in sedimentary rocks is challenging due to fine-grain size and intergrowth with silicate matrix and other sulfide minerals. In this contribution, we describe a method for calculation of trace element composition of sedimentary pyrite from time-resolved LA-ICP-MS data. The method involves an analysis of both pyrite and pyrite-free sediment matrix, segmentation of LA-ICP-MS spectra, normalization to total, regression analysis of dependencies between the elements, and calculation of normalized composition of the mineral. Sulfur is chosen as an explanatory variable, relative to which all regressions are calculated. The S content value used for calculation of element concentrations from the regressions is calculated from the total, eliminating the need for independent constraints. The algorithm allows efficient measurement of concentrations of multiple chalcophile trace elements in pyrite in a wide range of samples, including quantification of detection limits and uncertainties while excluding operator bias. The data suggest that the main sources of uncertainties in pyrite composition are sample heterogeneity and counting statistics for elements of low abundance. The analysis of regression data of time-resolved LA-ICP-MS measurements could provide new insights into the geochemistry of the sedimentary rocks and minerals. It allows quantification of ratios of elements that do not have reference material available (such as Hg) and provides estimates on the content of non-sulfidic Fe in the silicate matrix. Regression analysis of the mixed LA-ICP-MS signal could be a powerful technique for deconvolution of phase compositions in complex multicomponent samples.

Redox sensitive trace elements in pyrite, including nodules, are increasingly used to infer the chemical conditions of ancient oceans - but considerable uncertainty remains regarding the mechanism and timing of nodule formation. Resolving these uncertainties is important because pyrite nodules must form in connection with the overlying water column, rather than during late diagenesis, to reflect the composition of the global ocean. Existing models for pyrite nodule formation have been specific to pyrite textures from individual sites, and we lack a unified model that can explain the compositional and textural diversity observed in nodules from different localities. In this study we examine ten pyrite nodules from several geological periods (Neoarchean to Carboniferous) using in situ LA-ICP-MS and SHRIMP-SI analyses. We present transects of spot analyses of trace elements (As, Ag, Cu, Co, Ni, Sb and Se) and S isotope ratios for each nodule. The pyrite nodules can be classified according to three main categories: those with (1) little to no trace element or isotopic zonation of the nodule from core to margin, (2) strong zonation from core to margin, and (3) minor zonation near the core but more significant zonation near the margin of the nodule. We further illustrate this zonation with a NanoSIMS element map from an additional pyrite nodule. These results are interpreted to indicate nodule formation along a spectrum between two end-member mechanisms. We suggest that the absence of trace element or isotopic zonation reflects nodule growth by a pathway that is analogous to the pervasive growth mechanism for carbonate nodules. This model involves the production of many nucleation sites that are evenly distributed within the volume that the nodule eventually occupies. Consequently, this mechanism results in a chemically homogenous nodule. Pyrites formed this way are suitable for paleoceanographic reconstruction. The other end-member mechanism is analogous to the concentric growth model for carbonate concretions. In this scenario, the core of the nodule forms first and is followed by the addition of concentric layers - each with a progressively different trace element content and δ34S signature as diagenesis progresses. Despite having limited utility for reconstructing ancient seawater, these late forming nodules may track the evolving availability of bioessential trace elements for the subsurface biosphere with important implications for global biogeochemical cycles. Spatial trends in trace elements and S isotopes thus speak to the mechanisms of pyrite nodule formation and provide a framework for evaluating nodule suitability for a range of paleoenvironmental studies.

Variations in atmosphere oxygen and ocean sulfate concentrations through time are regarded as important controls on the cycles of sediment-hosted and volcanic-hosted ore deposits. However, estimates of atmosphere oxygen in the Proterozoic have been frustrated by the lack of a direct measurement method and conflicting evidence from various proposed geochemical proxies. Studies in the 1970s to 1990s suggested a relatively oxygenated atmosphere (> 3 wt% O2) in the Proterozoic. However, since the late 1990s, new proxies and modelling have suggested very much lower levels of oxygen (

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&ndash, 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 &plusmn, 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 ծ34S as we move up stratigraphy with positive ծ34S values ranging from 5.9&permil, (lower) to 26.08&permil, (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.

Recent research has emphasized the potential relationships between supercontinent cycles, mountain building, nutrient flux, ocean–atmosphere chemistry and the origin of life. The composition of the Upper-Most Continental Crust (UMCC) also figures prominently in these relationships, and yet little detailed data on each component of this complex relationship has been available for assessment. Here we provide a new set of data on the trace element concentrations, including the Rare Earth Elements (REE), in the matrix of 52 marine black shale formations spread globally through the Archean and Proterozoic. The data support previous studies on the temporal geochemistry of shales, but with some important differences. Results indicate a change in provenance of the black shales (upper-most crustal composition), from more mafic in the Archean prior to 2700 Ma, to more felsic from 2700 to 2200 Ma, followed by a return to mafic compositions from 2200 to 1850 Ma. Around 1850 to 1800 Ma there is a rapid change to uniform felsic compositions, which remained for a billion years to 800 Ma. The shale matrix geochemistry supports the assertion that the average upper-most continental source rocks for the shales changed from a mix of felsic, mafic and ultramafic prior to 2700 Ma to more felsic after 1850 Ma, with an extended transition period between. The return to more mafic UMCC from 2200 to 1850 Ma is supported by the frequency of Large Igneous Provinces (LIPs) and banded iron formations, which suggest a peak in major mantle-connected plume events and associated Fe-rich hydrothermal activity over this period. Support for the change to felsic UMCC around 1850 Ma is provided by previous geological data which shows that felsic magmas, including, A-type granites and K–Th–U-rich granites intruded vast areas of the continental crust, peaking around 1850 Ma and declining to 1000 Ma. The implications of this change in UMCC are far reaching and may go some way to explain the distinct features of the Boring Billion (1800–800 Ma). Firstly, because mafic–ultramafic rocks contain significantly higher levels of the bio-essential nutrient elements (e.g. Fe, P, Ni, Cr, Co, Cu, Se, Mn, Zn) compared with felsic rocks, the flux of macro- and micro-nutrients to the ocean would have decreased significantly post 1850 Ma. This would have contributed to a drop in productivity and a drop in atmosphere O2 as suggested by the marine pyrite proxy. In addition, a change from mafic to felsic dominant composition of the UMCC post 1850 Ma, would have led to a decrease in the erosive flux of Ca and Mg to the ocean, affecting the oceanic carbonate equilibrium and likely contributing to a rise in atmosphere CO2. On this basis, we speculate that the commencement of the middle Proterozoic, commonly known as the Boring Billion period from 1800 to 800 Ma, marks the start of an extended time in Earth's evolution when the UMCC became dominated by felsic rocks, particularly K–U–Th–anorogenic granites. This led to a period of anomalously low concentrations of bio-essential trace elements, but elevated REE, U, Th, Pb, Tl, Rb/Al and K/Na in the oceans.

EES acknowledges financial support from the School of Earth & Environmental Sciences, St Andrews. DDG acknowledges the support of NSERC for his Discovery grant (grant number 04834). Ore deposits found in Proterozoic marine sedimentary basins supply much of the world's zinc. Many of the deposits formed contemporaneously with their host sediments when saline brines circulating from deeper in the basin reached the sea floor. Textural, geochemical and isotopic features of these SEDEX (‘sedimentary-exhalative’) deposits and their host sediments indicate that biologically active seeps, vents and brine pools were a feature of many ore-forming systems. In mineralised pockets of mid-Proterozoic basins, these ‘microbial oases’ were productive areas in an otherwise low productivity, anoxic, deep marine realm. Here we hypothesize that these metal-rich brines which circulated through organic matter-rich substrate also carried high levels of fixed nitrogen and stimulated distinct ecosystems at sites of mineralisation, or enhanced productivity more broadly in the basin. We tested this hypothesis with organic carbon and nitrogen analyses of samples of carbonaceous siltstone and shale from the 1.64 Ga Barney Creek Formation of northern Australia. The Barney Creek Formation hosts several SEDEX Zn systems, including one of the world's largest deposits at McArthur River Mine (the HYC deposit). Samples come from the mineralised edge of HYC and from correlated strata in drill cores at varying distances (1-60 km) from the deposit. The data reveal lower ratios of total organic carbon (TOC) to total nitrogen (TN) closer to the ore body. Strong correlations (r2>0.7) between TOC and TN and the absence of excess N in the samples suggest that most N was buried as bound to organic matter. Bioavailable N was thus probably more abundant closer to HYC, consistent with fixed nitrogen input by hydrothermal fluids. If correct, our data may suggest that such a hydrothermal nitrogen point source enabled microbes to develop lower C:N ratios in their biomass. A hydrothermal nitrogen source is also supported by a gradient in δ15N values from = +4‰ proximal to the vent to +7.5‰ in distal sites, which may point towards recycling of ammonium from the underlying Wollogorang Formation (1.73 Ga). This unit has previously been identified as a source of over-mature hydrocarbons to the ore-forming fluid. We speculate that, during the mid-Proterozoic, fixed nitrogen carried by SEDEX hydrothermal brines may have locally offset the lack of aerobic nutrient remineralization that characterized most of the anoxic Precambrian deep ocean and thus stimulated biological productivity in areas where the brines reached the sea floor, and, possibly, more broadly as spent brines mixed into the water column. Postprint

Sedimentary pyrites in black shales contain abundant trace elements that provide information on the chemistry of the seawater at the time of sedimentation. This study focuses on the Barney Creek Formation (~ 1640 Ma) in the McArthur Basin in the Northern Territory of Australia, which is host to one of the world's largest SEDEX Zn-Pb-Ag deposits, and several smaller deposits. Fine-grained sedimentary pyrite has been sampled from three drill holes through the Barney Creek Formation at various distances from SEDEX mineralisation. Samples were selected through the stratigraphy of each hole and analysed by LA-ICPMS for a suite of 14 trace elements. The data show that sedimentary pyrite at the base of the Barney Creek Formation, closest (within 1 km) to SEDEX mineralisation, is strongly enriched in Zn and Tl by one to two orders of magnitude compared to the global average for sedimentary pyrite. In contrast sedimentary pyrite from the hole furthest from SEDEX mineralisation (~ 60 km) contains mean Zn and Tl values equal to, or less than, the global average. Based on the three drill hole pyrite data sets it is concluded that trace elements that are contributed to the basin during hydrothermal exhalation, and adsorbed into contemporaneous sedimentary pyrite, are principally Zn, Tl, Cu, Pb, Ag and As. In contrast, trace elements that are adsorbed into sedimentary pyrite from background seawater are principally Mo, Ni, Co, Se and As. These differences have enabled the development of a SEDEX fertility diagram for sedimentary basins, based on the composition of sedimentary pyrite, that distinguish high Zn, but barren shales, from high zinc SEDEX-related shales. In parallel with the increase in Zn and Tl in sedimentary pyrite approaching mineralisation there is a decrease in Ni, Co and Mo. This means that the ratios Zn/Ni and Tl/Co are particularly good pyrite vectors to SEDEX mineralisation in the McArthur Basin, varying over 4 to 6 orders of magnitude from barren shales to mineralised shales. It is speculated that the reason for the reverse relationship between Ni, Co and Zn, Tl may be caused by hydrothermal exhalations into the water column that effect the ion-exchange pyrite surface complexation processes that alter the uptake of these elements into sedimentary pyrite. Another important conclusion of this study is that hydrothermal exhalations into a sedimentary basin may affect the redox sensitive trace element chemistry of sedimentary pyrite and therefore the trace element chemistry of pyritic black shales. Nickel, Co and Mo all decrease in proximity to hydrothermal vents that form SEDEX deposits, whereas Zn, Tl and Pb increase. Selenium and bismuth are the only redox sensitive trace elements that appear to be unaffected by hydrothermal activity in the McArthur Basin. This has implications on how trace element concentrations of black shales and pyrite are used to reflect past global ocean chemistry.

Trace element concentrations in marine pyrite, measured by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), have the potential to open a new window into deep-time ocean chemistry, atmosphere oxygenation, and genesis of basin-hosted ore deposits. Only early-formed syngenetic and early diagenetic marine pyrite preserves the trace element chemistry of the past oceans, as late diagenetic and metamorphic recrystallization of pyrite changes the trace element budget. A database of over 5,000 marine pyrite trace element analyses by LA-ICP-MS has enabled the development of deep-time proxies for nutrient supply, productivity, ocean pH, and atmosphere oxygenation. These proxies suggest that the Archean ocean was enriched in Fe, Ni, Co, As, Au, and Hg compared with modern oceans, probably related to composition of erosive flux from the continents and active seafloor hydrothermal activity. This was also a time for major iron, gold, and nickel ore formation in sedimentary and greenstone settings. In the Paleoproterozoic, there was a decrease in Ni, Co, As, and Au, replaced by increasing Cu, Zn, and ![Formula][1] in the oceans and O2 in the atmosphere. The first appearance of red beds and evaporites is a response to the rise in O2 and ![Formula][2] , and provided the conditions necessary for sediment-hosted Cu and stratiform Pb-Zn-Ag sedimentary exhalative (SEDEX) deposits. Through 1700 to 1500 Ma, phosphorous, gold, and most other nutrient trace elements dropped to a minimum in the ocean, possibly related to tectonic stasis and changes in atmosphere O2 and/or ocean pH. Sediment-hosted Au, orogenic Au, and volcanic-hosted massive sulfide (VHMS) deposits are virtually absent from this period, whereas mineral systems that required relatively oxidized ore fluids, such as SEDEX Zn-Pb, iron oxide copper-gold (IOCG), and unconformity uranium became more abundant, due to these changed conditions. All redox-sensitive and nutrient trace elements rose dramatically in concentration at the Proterozoic-Phanerozoic boundary and peaked in the mid- to Late Cambrian oceans, accompanied by black shale deposition enriched in Mo, Se, Ni, Ag ± Au, and platinum group elements. Cyclic variation in nutrient trace elements increased in frequency through the Phanerozoic on a wavelength of 50 to 100 m.y., compared with 500 to 1,000 m.y. in the Proterozoic. The more frequent Phanerozoic cycles relate to repeated episodes of continent collision, mountain building, and increased erosive flux of trace elements into the oceans. Ore deposit cycles in the Phanerozoic of SEDEX Zn-Pb, orogenic sediment-hosted Au, and VHMS have a time frame similar to the tectonic and seawater chemistry cycles. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif

The trace element content of sedimentary pyrite in black shales of varying ages has recently been used to construct secular trends of trace element variation in the ocean. The approach also has potential to be used as a proxy for estimating evolution of the redox state of the ocean/atmosphere system through time. Here, we apply a combination of whole-rock chemostratigraphy and laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) analyses of marine pyrite to the carbonaceous mudstones of the Mesoproterozoic (∼1400 Ma) Velkerri Formation, and underlying Corcoran Formation, Roper Group, McArthur Basin to interpret and compare basin water conditions and basinal trace element chemistry at the time of sedimentation. Our results suggest that the black shales of the Velkerri Formation deposited under different geochemical conditions in comparison to the underlying Corcoran Formation. This proportionate difference is manifested in the form of high total organic carbon (TOC) contents coupled with an increase of trace elements such as P, Mo, Cd, Se, Ni, Se/Co, Ni/Co in the mudstones of Upper Velkerri Formation in comparison to Lower Velkerri and underlying Corcoran Formation. Cobalt on the other hand, exhibits an opposite trend compared to other redox sensitive trace elements (Mo, Se) due to its unique redox chemistry, particularly the cationic nature of its soluble species making Se/Co a useful proxy for redox conditions of the atmosphere. This progressive increment in trace elements (P, Mo, Cd, Se, Ni, Se/Co, Ni/Co) up stratigraphy from Corcoran Formation to Lower Velkerri Formation to Upper Velkerri is herein attributed to an increase in nutrient trace element supply into the marine reservoir, possibly in response to tectonic activity and an increase in oxygen in the atmosphere. This increased nutrient supply subsequently promoted an increase in primary productivity as indicated by high TOC contents in the Upper Velkerri Formation. This positive change in nutrient supply also coincides with an increased supply of sulphate to the ocean (modelled paleo-seawater [SO42−]) as evidenced by previous sulphur isotope studies and development of morphological complexity in eukaryotes based on paleontological observations of previous workers. The totality of evidence from this study and previous studies, suggest that there was a possible oxygenation event around ∼1400 Ma. This is in contrast with the general notion that Mesoproterozoic oxygen levels were low and devoid of significant fluctuations.

Oxygen concentration in the Precambrian atmosphere-ocean system is currently estimated from a series of geochemical proxies that depend on the concentration of redox-sensitive trace elements or their isotopic ratios measured in organic-bearing marine shales. This research has indicated that atmospheric oxygen content throughout the Precambrian was very low, Atmospheric O 2 % = 30 × P / 1 + P Where P = 10 0.89 × log Se/Co + 0.07 , r 2 ; r 2 = 0.85 The oxygen curve revealed by this relationship suggests a broad first-order trend of increasing oxygen of ~0.25 to 27% from 3500 to 1850 Ma, followed by a general decrease to around ~1.0% at ~1000 Ma, with a spike around 1400 Ma. Oxygen then rose gradually through the Ediacaran to reach a maximum of 20 to 29% in the early Cambrian. Detailed analysis of sedimentary pyrite in sixteen separate black shale formations, spaced throughout the Precambrian, demonstrates consistent results using the marine pyrite proxy, with standard deviations of the predicted oxygen for each formation of between 0.4 and 7%. In seven formations, we show that each formation demonstrates an increase in oxygen passing up stratigraphy from the base of the black shale facies toward the top.

The ubiquitous iron sulphide, pyrite, occurs in trace amounts in rocks or may form massive pyritic ore bodies with all perceivable gradations in between. Very often it shares the deformational and metamorphic history of its host rocks. Textural characteristics of pyrite, and its behavior in natural ores and in experimental conditions under varying temperature and pressure, have therefore been studied by different workers from time to time. An overview of these studies shows that there is a mismatch between the experimentally achieved deformation mechanisms at different temperature and pressure, and the observed brittle or ductile behaviour of pyrite in naturally deformed sulphide bodies. An attempt is made here to analyze the deformation behaviour of pyrite under different temperature-pressure conditions, by studying pyritic ores in three sediment-hosted Pb–Zn sulphide deposits of Rajasthan (Balaria-Zawar, Rajpura-Dariba and Rampura-Agucha), occurring in broadly similar geological settings, but deformed and metamorphosed at different grades (upper greenschist, middle amphibolite and upper amphibolite to granulite facies respectively). Observations of hand specimens and optical microscopy of pyritic ores from Rajasthan have shown that the mineral behaved in a macroscopically ductile manner—not only in the form of mesoscopic and microscopic folding of layers, but also by distortions, bending and stretching of individual grains. In general, pyrite plasticity increases with temperature as revealed by more definitive evidences of plastic deformation in higher metamorphic grade deposits (e.g. Rajpura-Dariba and Rampura-Agucha) than in lower grade Balaria ores from the Zawar ore district. However, the Balaria ore, characterized by the coexistence of framboidal (sedimentary-diagenetic) and idiomorphic (metamorphic) pyrite, is more intensely folded. Higher grade ores may, on the other hand, induce more grain growth and thereby are likely to lose the evidence of plastic deformation through polygonization and grain coarsening. This may be one reason behind the apparent scarcity of plastic deformation textures observed in pyrite from naturally deformed and metamorphosed sulphide ore deposits.

During sedimentation and diagenesis of carbonaceous shales in marine continental margin settings, Au is adsorbed from seawater and organic matter and becomes incorporated into sedimentary pyrite. LA-ICPMS analysis of over 4000 sedimentary pyrite grains in 308 samples from 33 locations around the world, grouped over 123 determined ages, has enabled us to track, in a first order sense, the Au content of the ocean over the last 3.5 billion years. Gold was enriched in the Meso- and Neoarchean oceans, several times above present values, then dropped by an order of magnitude from the first Great Oxidation Event (GOE1) through the Paleoproterozoic to reach a minimum value around 1600 Ma. Gold content of the oceans then rose, with perturbations, through the Meso- and Neoproterozoic, showing a steady rise at the end of the Proterozoic (800 to 520 Ma), which most likely represents the effects of the second Great Oxidation Event (GOE2). Gold in the oceans was at a maximum at 520 Ma, when oxygen in the oceans rose to match current maximum values. In the Archean and Proterozoic, the Au content of seawater correlates with the time distribution of high-Mg greenstone belts, black shales and banded iron formations, suggesting that increases in atmospheric oxygen and marine bio-productivity, combined with the higher background of Au in komatiitic and Mg-rich basalts were the first order causes of the pattern of Au enrichment in seawater. We suggest the lack of major Au deposits from 1800 to 800 Ma, is explained by the low levels of Au in the oceans during this period.

We present δ 13 C and δ 18 O records of planktonic ( Globigerinoides ruber ) and benthic foraminifera ( Cibicidoides spp.) from two deep sea sediment cores (SK157-15 and SK157-16) from the southern Bay of Bengal. The chronology of these gravity cores was established using eleven AMS radiocarbon ages from mixed species of planktonic foraminifera ( Globigerinoides ruber and Globigerinoides sacculifer ) and oxygen isotope stratigraphy. Large variations in δ 13 C and δ 18 O of planktonic and benthic foraminifera in these cores are suggestive of significant changes in surface and deep water masses for the ∼65 ka BP. The δ 18 O values in planktonic foraminifera in both these cores are significantly higher relative to sediment cores from the western Bay of Bengal and the Andaman Sea because of the reduced influence of riverine fresh water fluxes. Benthic δ 13 C values in cores SK157-15 and SK157-16 exhibit significant variations in source water characteristics during the Holocene and the last glaciation. Large decrease in δ 13 C values of bottom dwelling benthic foraminifera ( Cibicidoides spp.) during the Last Glacial Maximum (LGM) and ∼50–65 ka BP suggest drastic reduction in North Atlantic Deep Water (NADW) fluxes into the southern Bay of Bengal and concurrent increase of deep water formed in the Southern Ocean (SODW) with significantly low δ 13 C values.

Aerial photographs and LANDSAT images have been used in conjunction with existing topographical and geological information to prepare a land unit map of the Piedmont plain of the Himalaya between the Tista and Jaldhaka Rivers, in parts of Darjeeling Jaiguri districts, West Bengal. Such study was carried out to analyse the Piedmont Fan deposits, fluvial processes, effect of Quaternary tectonics on the morphogenesis of terraces (land units), morphodynamic processes operating on the units, and present land utilisation pattern. Geomorphologically, the area encompasses a multi-staged, fluviogenetic compound land units of five distinct Fan-cut and fill terraces originated due to an intensive interaction of endogenic (related to Himalayan upheaval), masogenic (relared to reactivation of lineaments) and exogenic (surfacial morphodynamic processes including mass wasting) factors during the Quaternary period. Widespread change of Paleo-environmental conditions along with tectonic disposition during the Quaternary period had the major influence on the Piedmont morphogenesis. The area probably underwent at least five phases of tectonic rejuvenation in a cycle of anagenetic revival of this land system.