Precipitation Of Pyrite Research Articles

Carbon–sulfur–calcium isotopic variability of lower Cambrian shale-hosted carbonate concretions: Insights into growth mechanisms and calcium cycling

Marine calcium cycling is closely linked with carbon cycling in the ocean, in which authigenic carbonates precipitated in sediments play a non-negligible role. However, calcium cycling during authigenic carbonate precipitation in organic-rich, shaly sediments in geological history remains underexplored. This study focuses on carbonate concretions (aggregates of authigenic carbonates) in the lower Cambrian Niutitang Formation, South China, to provide insights into calcium cycling during their growth. Sedimentological and mineralogical observations suggest that these concretions were formed through concentric growth by authigenic calcite and pyrite precipitation during the early diagenetic stage. Geochemical analyses reveal internal variations in “M-shaped” δ13Ccarb trends (from −11.9 ‰ to −4.4 ‰) and diverse δ34Spyr trends (from 4.7 ‰ to 14.0 ‰) along core-to-rim transects. These findings suggest formation through microbial sulfate reduction by organic matter in a shallow depth beneath the sediment–water interface. In contrast to the dynamic δ13Ccarb and δ34Spyr variations and multi-stage concentric growth, these carbonate concretions display nearly uniform δ44/40Cacarb values (from 0.80 ‰ to 1.03 ‰, average 0.96 ± 0.06 ‰, 1SD) and consistent internal trends, which are further attributed to strongly seawater-buffered porewater calcium geochemistry and small calcium isotope fractionation due to calcite precipitation at slow rates. This study confirms that early diagenetic carbonate concretions in the lower Cambrian Niutitang Formation are characterized by much heavier δ44/40Ca values compared to coeval shallow platform carbonates. In light of abundant authigenic carbonates observed in the lower Cambrian successions, their roles in calcium isotope mass balance in the early Cambrian ocean warrant further investigation in the future. Therefore, early diagenetic carbonate concretions in black shales could provide valuable insights into porewater and seawater calcium isotope signals, as well as early diagenetic and marine calcium cycling in geological history.

Pyritic stromatolites from the Paleoarchean Dresser Formation, Pilbara Craton: Resolving biogenicity and hydrothermally influenced ecosystem dynamics.

This study investigates the paleobiological significance of pyritic stromatolites from the 3.48 billion-year-old Dresser Formation, Pilbara Craton. By combining paleoenvironmental analyses with observations from well-preserved stromatolites in newly obtained drill cores, the research reveals stratiform and columnar to domal pyritic structures with wavy to wrinkly laminations and crest thickening, hosted within facies variably influenced by syn-depositional hydrothermal activity. The columnar and domal stromatolites occur in strata with clearly distinguishable primary depositional textures. Mineralogical variability and fine-scale interference textures between the microbialites and the enclosing sediment highlight interplays between microbial and depositional processes. The stromatolites consist of organomineralization - nanoporous pyrite and microspherulitic barite - hosting significant thermally mature organic matter (OM). This includes filamentous organic microstructures encased within nanoporous pyrite, resembling the extracellular polymeric substance (EPS) of microbes. These findings imply biogenicity and support the activity of microbial life in a volcano-sedimentary environment with hydrothermal activity and evaporative cycles. Coupled changes in stromatolite morphology and host facies suggest growth in diverse niches, from dynamic, hydrothermally influenced shallow-water environments to restricted brine pools strongly enriched in from seawater and hydrothermal activity. These observations, along with S stable isotope data indicating influence by S metabolisms, and accumulations of biologically significant metals and metalloids (Ni and As) within the microbialites, help constrain microbial processes. Columnar to domal stromatolites in dynamic, hydrothermally influenced shallow water deposits likely formed by microbial communities dominated by phototrophs. Stratiform pyritic structures within barite-rich strata may reflect the prevalence of chemotrophs near hydrothermal venting, where hydrothermal activity and microbial processes influenced barite precipitation. Rapid pyrite precipitation, a putative taphonomic process for preserving microbial remnants, is attributed to microbial sulfate reduction and reduced S sourced from hydrothermal activity. In conclusion, this research underscores the biogenicity of the Dresser stromatolites and advances our understanding of microbial ecosystems in Earth's early history.

Understanding the Fate of H2S Injected in Basalts by Means of Time‐Domain Induced Polarization Geophysical Logging

AbstractTo help meet emission standards, hydrogen sulfide (H2S) from geothermal production may be injected back into the subsurface, where basalt offers, in theory, the capacity to mineralize H2S into pyrite. Ensuring the viability of this pollution mitigation technology requires information on how much H2S is mineralized, at what rate and where. To date, monitoring efforts of field‐scale H2S reinjection have mostly occurred via mass balance calculations, typically capturing less than 5% of the injected fluid. While these studies, along with laboratory experiments and geochemical models, conclude effective H2S mineralization, their extrapolation to quantify mineralization and its persistence over time leads to considerable uncertainty. Here, a geophysical methodology, using time‐domain induced polarization (TDIP) logging in two of the injection wells (NN3 and NN4), is developed as a complementary tool to follow the fate of H2S re‐injected at Nesjavellir geothermal site (Iceland). Results show a strong chargeability increase at +40 days, interpreted as precipitation of up to 2 vol.% based on laboratory relationships. A uniform increase is observed along NN4, whereas it is localized below 450 m in NN3. Changes are more pronounced with larger electrode spacing, indicating that pyrite precipitation takes place away from the wells. Furthermore, a chargeability decrease is observed at later monitoring rounds in both wells, suggesting that pyrite is either passivated or re‐dissolved after precipitating. These results highlight that a sequence of overlapping reactive processes (pyrite precipitation, passivation, pore clogging and possibly pyrite re‐dissolution) results from H2S injection and that TDIP monitoring is sensitive to this sequence.

Biological and paleoceanographic controls on the postglacial sulfur isotope records in Arctic shelf sediments

The Arctic Ocean has experienced environmental fluctuations during the Pleistocene glacial-interglacial cycles. Since the last deglaciation, the reinundation of the Bering Strait and regional transgression have led to dramatic changes in the western Arctic Ocean, affecting coastal landscapes, depositional processes, marine ecosystem, and, inherently, biogeochemical element cycles. Here, we investigate the records of sulfur, carbon, and iron cycles in the Chukchi Sea of the western Arctic, with a primary focus on microbial sulfate reduction and subsequent pyrite formation. Variations in the pyrite abundance and sulfur isotopic composition, derived from a 10-m sediment core, demonstrate that dynamic changes in the factors governing pyrite formation – organic electron donor, reactive iron, and sulfate – are closely linked to the regional environmental conditions. In the early Holocene sediment, pyrite precipitation was impeded by the lack of organic matter, even in the presence of abundant sulfate and reactive iron. In the overlying sediment, pyrite contents increased due to vigorous microbial sulfate reduction fueled by enhanced availability of organic substrate and additional methane from the sulfate-methane transition zone. In general, the increased supply of organic matter to the Chukchi Shelf sediments can be attributed to the resumed inflow of nutrient-rich Pacific Water through the flooded Bering Strait. The increase in pyrite content, however, does not correlate exactly with either the increase in TOC or the opening of the Bering Strait, both of which precede the increased pyrite formation. These lags reflect the balance between marine primary production and terrestrial carbon sources, as well as the balance between the Pacific Inflow and the Beaufort Gyre. While a comparable increase in pyrite and TOC contents during the Holocene deglaciation has also been reported in the Black and Baltic Sea sediments, their distinct sulfur isotope records, contingent upon the availability of sulfate and reactive iron, highlight that pyrite can serve as a valuable proxy for tracking past climate and environmental change, especially in barren sedimentary records such as those found in the Arctic Ocean.

Genetic association between carbonates and gold precipitation mechanisms in the Jinshan deposit, eastern Jiangnan orogen

Carbonates have been traditionally regarded as post-ore minerals in hydrothermal deposits, but they can also occur in pre- and syn-ore stages. However, the genetic relationships between multistage carbonates and gold precipitation mechanisms remain unclear. The Jinshan gold deposit, the largest gold deposit in the eastern Jiangnan orogen, is characterized by the occurrence of abundant hydrothermal carbonates. The nature of the carbonates, especially their association with gold precipitation, was examined in this study through petrographic and geochemical analyses as well as geochemical modeling. Field and petrographic work recognized four paragenesis stages in the Jinshan deposit: (1) quartz (Q1)-carbonate, (2) pyrite-quartz (Q2), (3) gold-polysulfide-quartz (Q3)-carbonate, and (4) chlorite-quartz (Q4)-carbonate. Carbonates associated with gold mineralization are mainly present in the first and third stages. Among them, the first-stage carbonates are pre-ore ankerites (Ank1), which are generally crosscut by syn-ore sulfides. Such carbonates mostly occur in altered host rocks that are characterized by bleaching. Mineralogical and geochemical analyses revealed that chlorite was consumed while ankerite was produced, and Fe contents of the host rocks remained generally unchanged during alteration. Consequently, ankerite (Ank1) is interpreted to have been generated by the interaction of CO2-bearing fluids and chlorite in the host rocks. Geochemical modeling revealed that both Fe-bearing chlorite and ankerite (Ank1) can lead to gold precipitation by triggering sulfidation, but ankerite has higher chemical reactivity and therefore is kinetically favorable for more efficient gold deposition. Syn-ore carbonates were mainly formed in the third stage, primarily consisting of ankerite (Ank2) with minor calcite, and they were coprecipitated with native gold, galena, sphalerite, and chalcopyrite. Two stages of carbonates shared similar δ13CVPDB values and Pb, Zn, and Cu contents but different Ca, Mg, Mn, and Sr contents, indicating they formed at different stages of the same hydrothermal event. Geochemical modeling demonstrated that pH increases, as indicated by the formation of syn-ore carbonates, together with the consumption of S due to the precipitation of pyrite and arsenopyrite in the second stage, resulted in the decrease of Au, Pb, Zn, and Cu solubility in aqueous fluids. This would have led to the coprecipitation of native gold with galena, sphalerite, and chalcopyrite, forming the general small-scale but commonly occurring native gold−polysulfide veins in the hydrothermal gold deposits. Consequently, both pre- and syn-ore carbonates in the Jinshan deposit were genetically associated with efficient Au-polymetallic deposition through distinct mechanisms. Pre-ore carbonates caused the large-scale bleaching of the host rocks, while syn-ore carbonates were closely related to native gold−polysulfides, and they can both be used as exploration indicators for the Jinshan deposit and other similar hydrothermal gold deposits worldwide.

Combined Effect of Organic Carbon and Arsenic on the Formation of Sediment-Hosted Gold Deposits: A Case Study of the Shahuindo Epithermal Deposit, Peru

Abstract Sediment-hosted gold deposits represent a significant portion of the world’s gold resources. They are characterized by the ubiquitous presence of organic carbon (Corg; or its metamorphosed product, graphite) and the systematic occurrence of invisible gold-bearing arsenian pyrite. Yet the role played by these features on ore formation and the distribution of gold remains a long-standing debate. Here, we attempt to clarify this question via an integrated structural, mineralogical, geochemical, and modeling study of the Shahuindo deposit in northern Peru, representative of an epithermal gold deposit contained in a sedimentary basin. The Shahuindo deposit is hosted within Lower Cretaceous fluvio-deltaic carbon-bearing sandstone, siltstone, and black shale of the Marañón fold-and-thrust belt, where intrusions of Miocene age are also exposed. The emplacement of the auriferous orebodies is constrained by structural (thrust faults, transverse faults) as well as lithological (intrusion contacts, permeable layers, anticlinal hinge in sandstone) features. The defined gold reserves (59 tons; t) are located in the supergene zone in the form of native gold grains. However, a primary mineralization, underneath the oxidized zone, occurs in the form of invisible gold in arsenian pyrite and arsenopyrite. Here, four subsequent pyrite generations were identified—namely, pyI, pyII, pyIII, and pyIV. PyI has mean Au concentrations of 0.3 ppm, contains arsenic that is not detectable, and is enriched in V, Co, Ni, Zn, Ag, and Pb compared to the other pyrite generations. This trace element distribution suggests a diagenetic origin in an anoxic to euxinic sedimentary basin for pyI. Pyrite II and pyIV have comparable mean Au (1.1 and 0.7 ppm, respectively) and As (2.4 and 2.9 wt %, respectively) concentrations and precipitated under conditions evolving from lower (pyrrhotite, chalcopyrite, sphalerite) to higher (enargite, digenite, chalcocite) sulfidation, respectively. The pyIII generation is the major gold event in the primary mineralization, with pyrite reaching 110 ppm Au (mean ~7 ppm) and 5.6 wt % As (mean ~1.8 wt %), while coeval arsenopyrite attains 460 ppm Au. Pyrite III is also enriched in other trace elements such as Se, Ge, Mo, In, Ga, and Bi compared to the other pyrite generations, which is indicative of a magmatic source. Bulk analyses of the surrounding unmineralized rocks show only parts per billion levels of Au and less than 25 ppm As. These data, combined with mass balance considerations, demonstrate that the sedimentary rocks could not be the sole source of gold, as they could only contribute a minor portion of arsenic and sulfur (and iron) to the deposit. Conversely, fluids exsolved from a pluton crystallizing at depth likely provided the great part of the gold endowment. Equilibrium thermodynamics simulations, using geochemical constraints established in this study, demonstrate that interaction between Au-As-S-Fe–bearing fluids and organic carbon-bearing rocks strongly enhanced the fluid ability to transport gold by maximizing its solubility as AuI hydrosulfide complexes via a combined increase of pH and aqueous sulfide concentration. This finding challenges the traditional qualitative view of organic matter acting exclusively as a reducing agent for AuI that should promote gold deposition in its native state (Au0) rather than enhance its solubility in the fluid. Our results have significant implications for the exploration of carbonaceous sedimentary environments. Such settings may provide a very effective mechanism for focusing gold transport. Subsequent scavenging of AuI from solution in a chemically bound form is promoted by the precipitation of arsenian pyrite in permeable structural and lithologic traps, bound by more impermeable units, similar to what occurs in petroleum systems. Our integrated study underlines the important potential of sedimentary Corg-bearing rocks in the formation and distribution of gold and associated metal resources.

Petrophysical rock typing based on the digenetic effect of the different microfacies types of Abu Madi clastic reservoir in Faraskur Gas Field, onshore Nile Delta, Egypt

The sandstone of Abu Madi clastic reservoir constitutes the prime gas-producing reservoir in the Nile Delta region. So, the present research aims to delineate its microfacies and dominant diagenetic features and their effect on the petrophysical characteristics of the Abu Madi reservoir in the Faraskur Gas Field. This could be achieved by integrating the petrographical studies and the conventional data to divide the reservoir into some reservoir rock types (RRTs) to delineate the impact of the mineral composition and diagenetic features on their reservoir quality and hydraulic flow zones. Petrographically, most samples are immature and described as angular to subrounded, poorly sorted, and slightly cemented. Five clastic microfacies types are recorded in the Abu Madi Formation: (1) quartz arenite, (2) feldspathic quartz arenite, (3) sub-feldspathic quartz arenite, (4) quartz wacke, and (5) sub-feldspathic quartz wacke. The most important post-depositional processes affecting the reservoir quality are the precipitation of pyrite and microsparite, as well as the feldspars alteration into authigenic dispersed clays represented by kaolinite and smectite/illite mixed layers. The dominant pore types are the primary intergranular, with some subsidiary fractures and dissolution pores. To check the reservoir quality of the Abu Madi clastics, the NPI (normalized porosity index), FZI (flow zone indicator), RQI (reservoir quality index), and λk (permeability anisotropy) were estimated. Petrophysically, the studied samples are summed up into four RRTs, with the RRT4 group has the lowest ∅ and k values (av. ∅ = 16.2%, av. kH = 6.87 md) and the lowest reservoir quality parameters (av. FZI = 0.89 μm, av. DRT = 10.4, av. RQI = 0.18 μm, av. R35 = 1.26 μm). On the contrary, the RRT1 group has the best petrophysical and reservoir parameters (av. ∅ = 25.9%, av. kH = 2695 md, av. DRT = 14.9, av. RQI = 3.12 μm, av. FZI = 8.85 μm, av. R35 = 32.7 μm). The permeability anisotropy of the RRT2 and RRT3 is the highest in Faraskur Field (av. λk = 2.4 and 2.62, respectively). This study is applicable to other extensions of the Abu Madi reservoirs in the Nile Delta basin to help in predicting their reservoir quality, petrophysical properties, and their hydrocarbon potentiality.

A transition from porphyry Mo to vein-style Zn-Pb mineralization: Insights from hydrothermal fluid evolution of the Chalukou deposit, NE China

To reveal the hydrothermal evolution from porphyry Mo to vein-style Zn-Pb mineralization, the Chalukou deposit in NE China with these two typical mineralization overprinted is selected for detailed research. From early to late hydrothermal veins, the quartz cathodoluminescence intensities change from bright to dark and the textures also evolve. The Ti concentrations of quartz in these veins change from high to moderate and then to low values. The fluid inclusion petrography, microthermometry and stable isotope analyses indicate intermediate-density magma-derived fluids (δ18Owater = +4.7 to + 6.2‰, δD = −132 to −136‰) exsolved at high temperatures (560° to 600 °C) with low to moderate salinity (2.4 to 8.0 wt% NaCl equiv) containing minor CO2 (<5.7 mol %). Early minor molybdenite occurred under lithostatic conditions at high temperatures (540° to 580 °C) during fluids pH change and early magnetite precipitation. Later main molybdenite precipitation at moderate temperatures (350° to 430 °C), result from fluid cooling and involving of meteoric water under the transition from lithostatic to hydrostatic conditions. The precipitation of pyrite in transitional veins at moderate temperatures (330° to 420 °C), and the late sphalerite-galena from a lower-temperature fluid (180° to 300 °C) is consistent with the cooling of magmatic fluid, and gradually dilution with meteoric water under hydrostatic conditions. It is deduced the transition from porphyry Mo to vein-style Zn-Pb mineralization is likely driven by progressive decrease in temperature of the magmatic fluids and the increasing influx of meteoric water.

An emulation-based approach for interrogating reactive transport models

Abstract. We present an emulation-based approach to understand the interactions among different chemical and biological processes modelled in environmental reactive transport models (RTMs) and explore how the parameterisation of these processes influences the results of multi-component RTMs. We utilise a previously published RTM consisting of 20 primary species, 20 secondary complexes, 17 mineral reactions, and 2 biologically mediated reactions; this RTM describes bio-stimulation using sediment from a contaminated aquifer. We choose a subset of the input parameters to vary over a range of values. The result is the construction of a new dataset that describes the model behaviour over a range of environmental conditions. Using this dataset to train a statistical model creates an emulator of the underlying RTM. This is a condensed representation of the original RTM that facilitates rapid exploration of a broad range of environmental conditions and sensitivities. As an illustration of this approach, we use the emulator to explore how varying the boundary conditions in the RTM describing the aquifer impacts the rates and volumes of mineral precipitation. A key result of this work is the recognition of an unanticipated dependency of pyrite precipitation on pCO2 in the injection fluid due to the stoichiometry of the microbially mediated sulfate reduction reaction. This complex relationship was made apparent by the emulator, while the underlying RTM was not specifically constructed to create such a feedback. We argue that this emulation approach to sensitivity analysis for RTMs may be useful in discovering such new coupled sensitives in geochemical systems and for designing experiments to optimise environmental remediation. Finally, we demonstrate that this approach can maximise specific mineral precipitation or dissolution reactions by using the emulator to find local maxima, which can be widely applied in environmental systems.

Molybdenum isotope heterogeneity of metal sulfides from magmatic hydrothermal systems

The sensitivity of molybdenum (Mo) stable isotopes to redox changes enables innovative geochemical means of tracing the evolution of medium- to high-temperature magmatic hydrothermal processes. However, the Mo isotope geochemical behaviors and fractionation mechanisms in magmatic-hydrothermal systems are still unclear. This study performed high-precision Mo isotope research on well-characterized hydrothermal metal sulfides (pyrite, chalcopyrite, molybdenite and pyrrhotite) from different metallogenic stages and the ore-forming porphyries and surrounding rocks from the giant Pulang porphyry copper deposit in Yunnan, Southwest China, to better understand Mo isotope behaviors in magmatic hydrothermal systems. The results show that compared to the high-temperature magmatic system with homogeneous Mo concentrations (1.44 ppm to 11.3 ppm) and δ98Mo values (−0.41‰ ± 0.05 to −0.08‰ ± 0.03), medium- to high-temperature hydrothermal stage molybdenite displays slightly lighter δ98Mo values (−0.90‰ ± 0.04 to 0.08‰ ± 0.04), and pyrite, chalcopyrite and pyrrhotite, which have low Mo concentrations (0.13 ppm to 28.5 ppm), feature significant δ98Mo variations from −0.34 ± 0.04‰ to 3.01 ± 0.03‰. The absence of fractionated but low δ98Mo values in the magmatic system, combined with the lack of any correlation between the δ98Mo and geochemical index values (e.g., Co/Ni value) of the metal sulfides, implies that the Mo isotope signatures of sulfides in the hydrothermal stage were not inherited from component signatures of the magmatic phase or contamination of surrounding rocks. Our study proposes that heterogeneous partitioning and transportation of heavy/light Mo isotopes into progressively precipitated metal sulfides as a result of changes in the redox-driven metallogenic setting and differential geochemical behaviors of Mo species contribute to the observed δ98Mo variations and that the heavier and strongly variable Mo isotopic signatures correspond to the dominant metallogenic stage of the deposit. Such fractionation may be explained by the Rayleigh fractionation model, whereby 95Mo is preferentially partitioned into isotopically light molybdenite, and the subsequent precipitation of pyrite, chalcopyrite and pyrrhotite from the residual 98Mo-rich ore-forming fluids manifests heavier isotopic compositions. We therefore highlight that the Mo isotopes of metal sulfides provide valuable insights into the precipitation of metal-rich fluids, the identification of favorable mineralization zones, and the ability to trace the evolution of sophisticated magmatic-hydrothermal systems.

The origin, nature and fluid characteristics of the hydrothermal veining crosscutting the Yzerfontein gabbro-diorite, Cape Granite Suite, South Africa

Abstract The mafic to felsic rocks at Yzerfontein form part of a composite 535 Ma post-orogenic pluton belonging to the Cape Granite Suite (CGS) of the Pan-African Saldania Belt of southwestern Africa. The pluton ranges from olivine gabbros (mostly at the northern end) to quartz monzonites, with monzonites the most abundant rock type. Minor felsic rocks in the form of quartz microsyenites and monzogranite occur more toward the southern end. The comparatively unaltered gabbro/monzonite has pyroxene δ18O values of 6.4‰, consistent with a mildly crustal-contaminated mantle-derived magma. Published εNd values of -1.9 to -0.3 similarly reflect crustal contamination of these mafic to intermediate rocks. The pluton is crosscut by extensive hydrothermal veining. Early quartz veining was followed by the precipitation of pyrite, calcite, tourmaline, jasper, epidote and a final stage involving the formation of barren quartz veins. The veins contain low-salinity (3 to 11 wt.% NaCl eq.), three phase (liquid + vapour ± opaques) fluid inclusions with total homogenisation temperatures of 125 to 320°C. The δ18O values of the quartz veins range from 9.8 to 14.7‰ and the fluid inclusion δD values range from -9 to -17‰. These ranges, together with the low salinities, are consistent with a meteoric origin for the vein fluids. Bulk-rock δ18O values range from 7.3 to 13.3‰ consistent with interaction of near surface fluids at temperatures of &amp;lt;300°C with the magmatic rocks. Open-space filling textures in quartz veins suggest vein formation in a hydrostatic regime. These, together with fluid inclusion isochores, suggest temperature-pressure conditions of no more than 170 to 300°C at ~0.5 kbar and no more than 1.6 kbar. The mineralised veins represent a shallow, poorly developed and possibly low-grade epithermal system. The mineralisation is likely related to drawdown of meteoric fluids during sub-solidus cooling of the dominantly intermediate pluton with little to no magmatic fluid influence. The mineralisation is poorly developed, possibly due to a lack of mixing between different fluid types. The various magmatic rocks, varying from mafic through dominantly intermediate to minor felsic, represents a minor mantle-derived mafic component of the Cape Granite Suite and likely related to the heat source that gave rise to voluminous crustal melting that generated the granitic magmas of the majority of the CGS.

Fluid-rock interaction modeling to constrain Au enrichment: Implication for the giant Jiaodong Au mineralization, eastern North China Craton

The devolatilization model of the metasomatized lithospheric mantle without pre-enriched gold has been proposed to account for the giant gold mineralization. An excellent example is the world-class Jiaodong gold province with >5000 tonnes Au resources in the eastern North China Craton. The auriferous fluid transport and gold enrichment during wallrock alterations are two vital processes to determine the giant gold mineralization formation in this province. However, the effects of the fluid-rock interaction with alterations on the auriferous fluid transport and gold enrichment still keep poor understanding, which leads the above model to be imperfect. The giant Jiaojia goldfield in this province recorded a wallrock alteration evolution from K-feldspar alteration to pyrite-sericite-quartz alteration, and some parts of the latter can become gold orebodies when the gold grade is >1 ppm. This study conducts thermodynamic fluid-rock interaction modeling to reveal auriferous fluid transport and coupled relationship between gold enrichment and alteration mineral assemblage based on the alteration-mineralization and ore fluid characteristics of the goldfield. The modeling of fluid-rock interaction with cooling indicates the transformation of Au-Cl complexes to Au-S complexes combined with the total sulfur concentration decrease by pyrite precipitation when cooling from ∼460 °C can trigger the dispersive gold precipitation, which should hinder the gold long-range transport to lower ambient temperature. The high oxygen fugacity at >400 °C can enhance Au-Cl complexes stability, and the low pH can maintain high total sulfur concentration in the auriferous fluid, both of which facilitate the long-range gold transport to a lower-temperature environment. The auriferous fluid would acquire higher pH by the buffering of feldspars or sericite, which was beneficial for the high-efficiency precipitations of pyrite and gold. The ankerite-siderite assemblage without pyrophyllite in the pyrite-sericite-quartz alteration zone indicates that a cumulative fluid to rock mass ratio (f/r) of 3.8–4.8 should be needed for the transformation from K-feldspar alteration to pyrite-sericite-quartz alteration according to the fluid-rock interaction modeling at 300 °C and 2000 bar. In the case of auriferous fluids with ≤200 ppb Au concentration, the single fluid-rock interaction can only elevate the gold grade to ≤0.69–0.87 ppm in the pyrite-sericite-quartz alteration zone at f/r 3.8–4.8. Therefore, the fracture-induced fluid flow coupled with fluid-rock interaction is proposed to the prerequisite to elevate the gold grade to >1 ppm in the pyrite-sericite-quartz alteration zone. The metasomatized lithospheric mantle volume for the required ore fluid and Au in the Jiaodong province is estimated according to the modeling results and alteration-mineralization characteristics, which provides a link between the mantle without abnormal Au enrichment and the alteration-mineralization processes.

Magmatic rare earth element mineralization and secondary fluid-assisted redistribution in the Kamthai carbonatite complex, NW India

Calcite carbonatites from the Kamthai alkaline complex in western India are highly enriched in Sr and the rare earth elements (REE) owing to their formation from highly differentiated carbonatitic magma. Pegmatitic calcite carbonatites preserve rare magmatic intergrowth of rod/pod-like carbocernaite [(Ca, Na)(Sr, REE, Ba)(CO3)2] and REE + Sr-rich calcite. This intergrowth is interpreted to be the result of rapid simultaneous crystallization of the phases from a brine-rich carbonatitic magma. Primary carbocernaite, calcite, and britholite [(LREE, Ca)5(SiO4, PO4)3(OH, F)] sequestered Sr and REE from the brine-rich melt and constitute the major sources of these elements during hydrothermal redistribution. Replacement textures suggest fluid-assisted, post-magmatic hydrothermal REE redistribution through dissolution-reprecipitation process that concentrated the REE into secondary REE-(fluor/hydroxyl)carbonates, cerianite (ideally Ce4+O2), ancylite [(Sr, Ca)LREE(CO3)2(OH)∙(H2O)], and monazite. Hydrothermal alteration of magmatic calcite leached out the LREE, whereas in case of carbocernaite and britholite, an increase in their REE contents is noticed, followed by pseudomorphic replacement by REE-(fluor/hydroxyl)carbonate minerals.The carbonatite-derived hydrothermal fluids initially transported the REE as sulfate and later as hydroxyl‑carbonate complexes. The ligand transition was a consequence of the precipitation of baryte, celestine, and pyrite, which led to a reduction in fluid sulfur content and increase in its pH due to interaction with carbonates. Precipitation of secondary REE minerals was triggered by decrease in ore fluid temperature due to interaction with relatively cooler, oxidized fluid and interaction with carbonates. Carbocernaite, ancylite, cerianite, and Ce-bearing REE-(fluor)carbonates constitute early precipitated secondary REE minerals along with baryte, celestine, and pyrite as by-product phases, whereas both Ce- and La-rich, hydroxyl-dominated members of bastnäsite, parisite, and synchysite, along with monazite were the dominant REE minerals which precipitated during the advanced hydrothermal stage, with siderite as a by-product.

Disseminated gold mineralization under varied redox conditions: Constraints from microscopic observation, geochemistry, and thermodynamic modeling on fluid-rock interactions in lamprophyres, Zhenyuan gold deposit

The controls of fluid redox states on disseminated gold mineralization remain ambiguous. To address this, this study compared the hydrothermal alteration and auriferous pyrite geochemistry in lamprophyres, Zhenyuan gold deposit, which were infiltrated by fluids with varied redox states. Vein-A, vein-B, and vein-C were distinguished and they contain different sulfur-bearing minerals, i.e. pyrite, pyrite and barite, and barite, respectively. This suggests the gradual oxidzing formation conditions from vein-A to vein-C. Alteration halos enveloping vein-A and vein-B develop disseminated pyrite. In the vein-A halos, alteration minerals transit from pyrite to Fe‑carbonates outwards, which was well replicated by the thermodynamic simulation of the successive reaction of H2O-CO2-H2S-Au-bearing fluid with lamprophyres. The precipitation of pyrite grains in vein-B halos is farther from vein-B, suggesting progressive fluid-rock interaction (FRI) first induced the reduction of oxidized S followed by the pyrite precipitation. Additionally, δ34S values of pyrite are more scattered in vein-B halos (−8.0 to 4.7‰) than in vein-B, vein-A, and vein-A halos, implying the sulfate-sulfide disequilibrium. Pyrite in vein-B halos displays higher As concentrations than that in vein-A halos due to the strong redox dependence of As partition from fluids to pyrite. Au and Au/As of halo-hosted pyrite grains decrease outwards, and pyrite in vein-B halos has low Au and Au/As (below 93.93 ppm and 0.005, respectively) compared to that in vein-A halos (below 1597.08 ppm and 0.031, respectively). It is thus suggested that Au concentrations in ore fluids decreased during outwards FRI, and the lack of pyrite grains in proximal alteration zones results in lower gold concentration in pyrite under relatively oxidized conditions. This study reveals that the fluid redox conditions can affect As concentrations and spatial distribution of pyrite in alteration halos, which directly controls invisible gold precipitation.

Regional variation of evaporite diagenesis in Xingouzui sandstones in Jianghan Basin, China: Implications for the evolution and prediction of saline reservoirs’ quality

Diagenetic processes and associated pore evolution are essential factors that control the development and distribution of porosity and permeability in saline reservoirs. The goal of this study is to systematically describe the comparative diagenetic processes, porosity evolution, and diagenetic facies of sandstone reservoirs in the Eocene Lower Xingouzui Formation (LXF) in two depressions in the Jianghan Basin, China. To achieve these goals, an integrated approach of petrographic, mineralogical, geochemical, and petrophysical analyses such as thin-section observations, X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP) was applied to sandstone samples obtained from the Qianjiang and Jiangling sags. Diagenetic studies showed that the LXF samples had experienced several phases of diagenesis, including compaction, non-ferroan carbonate/sulfate cementation, feldspar and carbonate dissolution, quartz overgrowth cementation, ferroan carbonate and sulfate cementation, cubic pyrite precipitation, and limited gypsum dissolution. Although the LXF samples from both the Qianjiang and Jiangling sags experienced a noticeable porosity reduction induced by the compaction, the differences in diagenesis between these two sags were primarily due to complex and diverse cementation types. For instance, both early carbonate and sulfate cementation were recognized in Qianjiang samples, while only early carbonate cements were present in Jiangling samples. Based on a quantitative evaluation of the relative importance of diagenetic processes, diagenetic facies were defined in the LXF sandstones. Additionally, the pore evolution study indicated that two (upper and lower) zones of abnormally high porosity and permeability (AHPP) were developed in both sags. In particular, the upper AHPP zones occurred in sandstones in the Qianjiang and Jiangling depressions at depths of 1300 and 1800 m, respectively, while the lower AHPP zones in both depressions were located at a depth of 3300 m. The differences in the development mechanisms between the upper and lower AHPP zones were determined. This work provides an overview of the evolution and evaluation of sulfate- and carbonate-bearing sandstone reservoirs, such as the LXF sandstone, which have experienced strong diagenetic modifications during their geological histories.

Contribution of carbonaceous strata to intrusion–related Au mineralization: Evidence from trace elements and S–Pb isotopes in pyrite from the Qukulekedong Au–Sb deposit, East Kunlun, NW China

The large intrusion–related Qukulekedong Au–Sb deposit was recently discovered in the East Kunlun Orogenic Belt (EKOB). Mineralization occurs in the Late Triassic granitoid intrusion and its surrounding carbonaceous siltstone. Pyrite is an important Au-rich mineral in the deposit, and can be classified into three types: Py1, Py2 and Py3. The lattermost Py3 represents the main metallogenic episode. In–situ trace elements and S–Pb isotopes of pyrites indicate that the Py3 has a relatively high Au content (mean 7.84 ppm), high Co/Ni ratios (mean 4.93), with δ34S values around 0‰ (-2.03 to 1.69‰) and matched Pb isotopic ratios to the intrusions, suggesting that Au and other ore-forming material were most likely derived from a magmatic source. Carbonaceous siltstone contains organic carbon up to 3.39 wt%. According to the evidence of paragenetic minerals, texture, and S isotopes, the carbonaceous material may have played a key role in the precipitation of Au and pyrite in the carbonaceous strata. The Au–As decoupling of pyrite in intrusions may be attributed to the Au precipitation and crystallization of arsenopyrite, which was caused by the decreasing of oxygen fugacity. Overall, the new understanding highlights the importance of carbonaceous strata in intrusion–related Au ore-forming processes, the combination of carbonaceous strata and Late Triassic intrusions provides a new exploration direction for Au prospecting in the EKOB and similar settings.

Automated quantitative mineralogy analysis reveals characteristics of Co occurrence in the Jinchang porphyry deposit, NE China

A critical and strategic metal, demand for cobalt (Co) has increased in recent years given its widespread use in new energy batteries. This has led to the need to search for new Co sources. Porphyry deposits have provided large quantities of Cu, Mo, Au and other critical elements such as Re, Se and Te. However, there is a paucity of research on the occurrence, enrichment mechanisms, and metallogenic potential of Co in porphyry deposits. The Palaeo-Pacific slab subduction related Jinchang Au deposit (∼80 t @ 8 g/t Au; Heilongjiang province, NE China) has recently been recognized as a cobalt-rich porphyry Au deposit. We conducted MapsMin (automated mineralogy analysis) on two bulk samples (10-c and Z1-13) from the J0 orebody of the Jinchang deposit to accurately determine the elemental content and Co distribution, and to definitively identify the Co minerals. MapsMin analysis revealed Co contents of 0.27% and 0.07% for samples 10-c and Z1-13, respectively. In addition, we conducted whole-rock geochemistry on ten powdered samples with inductively coupled plasma-atomic emission spectroscopy/mass spectrometry. The results showed Co ranges from 0.04 wt% to 0.29 wt% (average 0.1 wt%). We compared the results of MapsMin on bulk samples with inductively coupled plasma-atomic emission spectroscopy/mass spectrometry on powders, and confirmed the accuracy of the MapsMin analysis. The relative difference in the content of the main ore-forming elements Cu, Fe, Ti, and Co in the samples is less than 30%. MapsMin identified two Co minerals (siegenite and cobaltite) and revealed that there is an isomorphic Co-rich zone in pyrite rims. The good negative correlation between Co, (Co + Cu + Ni) and Fe in pyrite indicates that Co can enter the pyrite lattice through the substitution Co2++ (Cu2++Ni2+)↔ Fe2+. Crystal size distribution data shows that siegenite in 10-c was formed under two different conditions and petrographic evidence indicates that siegenite was the first to precipitate with chalcopyrite and pyrite in the Jinchang deposit. The formation of pyrite spans the entire mineralization process. During late-formed pyrite precipitation, Co-rich hydrothermal fluids were reinjected, causing a second precipitation of siegenite. High levels of Co entered the rim of pyrite, forming a Co-rich zone. Finally, a decrease of fO2 led to the precipitation of cobaltite. Mapsmin can provide a fast, accurate and reliable mineralogy data which can be used to constrain the contents and occurrences of cobalt and other metals, such as nickel, copper, gold, rare earth elements, etc.

Application of inverse modelling for prediction of the surface water chemistry of the ekulu river in enugu, southeastern nigeria

Surface water chemistry evolution of Ekulu River was conducted by inverse geochemical modelling using the PHREEQC computer program. In this study, samples were collected from the Ekulu river at 100m intervals, and were taken to the laboratory for major anions and cations, heavy metal and bacterial content analyses. Inverse geochemical modelling results reveal that the river evolved as a result of dissolution of siderite, quartz, gypsum, galena, sphalerite, pyrolusite and precipitation of goethite and pyrite. The Piper diagram indicates Calcium-Chloride water type, with high Ca2+ + Mg+ and Cl- + SO42 contents, which is typical of water originated from gypsum dissolution and mine drainage. The results of the analyses show that the water cannot be recommended for drinking because the E. coli and coliform contents are higher than the permissible limits. The water chemistry of Ekulu river is as a result of secondary minerals dissolution and mine drainage from Onyeama coal mine whereas the microbial content resulted from pollution from recent faeces.&#x0D; &#x0D;

Biogeochemistry of low‐ and high‐centered ice‐wedge polygons in wetlands in Svalbard

AbstractArctic wetlands are a globally significant store of soil organic carbon. They are often characterized by ice‐wedge polygons, which are diagnostic of lowland permafrost, and which greatly influence wetland hydrology and biogeochemistry during summer. The degradation of ice‐wedge polygons, which can occur in response to climate change or local disturbance, has poorly understood consequences for biogeochemical processes. We therefore used geochemical analyses from the active layer and top permafrost to identify and compare the dominant biogeochemical processes in high‐centered (degraded) and low‐centered (pristine) polygons situated in the raised beach sediments and valley‐infill sediments of Adventdalen, Central Svalbard. We found similar organic‐rich sediments in both cases (up to 38 dry wt.%), but while low‐centered polygons were water‐saturated, their high‐centered counterparts had a relatively dry active layer. Consequently, low‐centered polygons showed evidence of iron and sulfate reduction leading to the precipitation of pyrite and siderite, whilst the high‐centered polygons demonstrated more oxidizing conditions, with decreased iron oxidation and low preservation of iron and sulfate reduction products in the sediments. This study thus demonstrates the profound effect of ice‐wedge polygon degradation on the redox chemistry of the host sediment and porewater, namely more oxidizing conditions, a decrease in iron reduction, and a decrease in the preservation of iron and sulfate reduction products.

Coupling and decoupling of Au and As in pyrite from Carlin-type Au deposits, southwest China

The low-temperature geochemistry of gold (Au) and arsenic (As) in pyrite from Carlin-type Au deposits is significant for understanding the enrichment mechanisms of Au in pyrite, but remains poorly understood due to the complex processes that lead to the precipitation of micron-scale pyrite grains. This paper concerns the geochemical behaviors of Au and As during the precipitation of pyrite from six Carlin-type Au deposits (the Nibao, Shuiyindong, Banqi, Jinya, Yata, and Zimudang deposits), southwest China. These deposits are located in different locations and ore-bearing horizons of the Youjiang Basin. Arsenian pyrite and/or arsenopyrite are the dominant Au-bearing minerals. Gold-bearing pyrite grains are typically zoned (core, intermediate, and rim), with multiple, oscillatory banded sub-zones. In situ analysis demonstrates that invisible Au exists mainly as cationic Au+ in the growth zones of pyrite. Gold and As are positively correlated (i.e., coupled) in the Nibao and Shuiyindong deposits, negatively correlated in the Banqi, Jinya, and Yata deposits, and poorly correlated in the Zimudang deposit. Arsenic and S substitution in pyrite and the presence of a reduced fluid promoted the incorporation of Au, resulting in the coupling of Au and As. The Au precipitation was affected by changes in the physicochemical conditions of the hydrothermal systems, independent of the As concentration, resulting in decoupling of Au and As. As such, invisible Au precipitation is controlled by the extrinsic physicochemical conditions of the ore-forming fluids and microscopic reactions at the fluid–mineral interface, and also the episodic incursion of Au-rich hydrothermal fluids. High As contents in pyrite favor Au incorporation, but are not solely responsible for Au enrichment. Gold and As contents in pyrite in Carlin-type Au deposits may be coupled or decoupled.