Precipitation Of Pyrite Research Articles

Organic sulfur was integral to the Archean sulfur cycle

The chemistry of the Early Earth is widely inferred from the elemental and isotopic compositions of sulfidic sedimentary rocks, which are presumed to have formed globally through the reduction of seawater sulfate or locally from hydrothermally supplied sulfide. Here we argue that, in the anoxic Archean oceans, pyrite could form in the absence of ambient sulfate from organic sulfur contained within living cells. Sulfides could be produced through mineralization of reduced sulfur compounds or reduction of organic-sourced sulfite. Reactive transport modeling suggests that, for sulfate concentrations up to tens of micromolar, organic sulfur would have supported 20 to 100% of sedimentary pyrite precipitation and up to 75% of microbial sulfur reduction. The results offer an alternative explanation for the low range of δ34S in Archean sulfides, and raise a possibility that sulfate scarcity delayed the evolution of dissimilatory sulfate reduction until the initial ocean oxygenation around 2.7 Ga.

H2S sequestration traced by sulfur isotopes at Hellisheiði geothermal system, Iceland

Hydrogen sulfide emissions are one of the major environmental problems associated with high enthalpy geothermal energy utilization. Reykjavik Energy is currently undertaking a pilot project at Hellisheidi power plant, SW Iceland, where H2S is injected together with wastewater to be sequestered in the geothermal reservoir. Multiple sulfur isotope ratios (δ34S and Δ33S) of injection and reservoir fluids were measured as a potential tracer for subsurface sulfide sequestration processes. The values for δ34S and Δ33S in the injected H2S-fluid and wastewater and reservoir geothermal fluids at Hellisheidi were +10.6‰ and +0.047‰, −2.86‰ and −0.005‰ and −2.5 to +0.9‰ and −0.031 and −0.001‰, respectively. Our geochemical and isotope modeling reveals that upon sulfide mineralization the δ34S value of the fluids in the production wells are expected to become lower due to pyrite precipitation, while the value of Δ33S is expected to be conservative upon mineralization and degassing. It follows sulfur sequestration and mixing process can be monitored by the triple sulfur (32S, 33S, 34S) isotope systematics. The total H2S emissions at Hellisheidi are ˜16 kt per year. Given the size of the geothermal reservoir and the expected production time span of the power plant of about 100–300 years, H2S sequestration in the geothermal reservoir is expected to be a sustainable solution for the reduction of power plant emissions. Continuous measurements of multiple-sulfur isotope ratios of the reservoir as well as injection fluids will provide a valuable method to monitor sulfur sequestration processes in the geothermal reservoirs.

Reservoir Characterization and Modelling with Diagenetic Trends of carbonates of the Kawagarh Formation: A Section exposed in the Kala-Chitta Range, Pakistan

Present study is focused on the diagenetic studies and reservoir characterization of the Cretaceous KawagarhFormation exposed in the Gandab village, Kala-Chitta range, north-western Himalayan Fold-and-Thrust belt, Pakistan.The formation is composed of argillaceous limestone and dark grey marls. A total of thirty-three representativecarbonate rock samples were collected at equal intervals of three meters. Various diagenetic features includingcementation, micritization, pyrite precipitation, neomorphism, fracturing, sparitization and stylolitization were observedin the studied rocks which occur in the marine, meteoric and deep burial diagenetic environments respectively. Suchdiagenetic features control the reservoir quality of the rock unit. Porosity types include mostly vuggy and fracture,while minor stylolitic porosity were noted with quantity ranging from 2.66% to 3.88%. The carbonates of KawagarhFormation are highly fractured, but the filling of these fractures due to precipitation of calcite or micritic mud hasgreatly reduced its reservoir potential, while some unfilled fractures, stylolites and vuggs are the dominant factors thatenhance the reservoir potentiality of the Kawagarh Formation. However, the porosity values still do not mark the levelof reservoir rock. These diagenetic studies revealed very less chances for hydrocarbon accumulation as no significantporosity values have been observed and overall reservoir potential is characterized as poor.

Reservoir Characterization and Modelling with Diagenetic Trends of carbonates of the Kawagarh Formation: A Section exposed in the Kala-Chitta Range, Pakistan

Present study is focused on the diagenetic studies and reservoir characterization of the Cretaceous KawagarhFormation exposed in the Gandab village, Kala-Chitta range, north-western Himalayan Fold-and-Thrust belt, Pakistan.The formation is composed of argillaceous limestone and dark grey marls. A total of thirty-three representativecarbonate rock samples were collected at equal intervals of three meters. Various diagenetic features includingcementation, micritization, pyrite precipitation, neomorphism, fracturing, sparitization and stylolitization were observedin the studied rocks which occur in the marine, meteoric and deep burial diagenetic environments respectively. Suchdiagenetic features control the reservoir quality of the rock unit. Porosity types include mostly vuggy and fracture,while minor stylolitic porosity were noted with quantity ranging from 2.66% to 3.88%. The carbonates of KawagarhFormation are highly fractured, but the filling of these fractures due to precipitation of calcite or micritic mud hasgreatly reduced its reservoir potential, while some unfilled fractures, stylolites and vuggs are the dominant factors thatenhance the reservoir potentiality of the Kawagarh Formation. However, the porosity values still do not mark the levelof reservoir rock. These diagenetic studies revealed very less chances for hydrocarbon accumulation as no significantporosity values have been observed and overall reservoir potential is characterized as poor.

Fault-fluid interaction in porphyry copper hydrothermal systems: Faulted veins in radomiro Tomic, northern Chile

The relationship between hydrothermal fluids and fault development is studied through a petrographic analysis of faulted veins in an open-pit copper mine, Radomiro Tomic (RT), northern Chile. Brittle deformation in RT was initiated with the formation of veins, disrupting low-strain crystal-plastic deformation. Following cooling, shear fractures propagated along these veins with associated fluid influx, leading to phyllic alteration. Hydrolysis converted all plagioclase to kaolinite within the damage zones and all orthoclase to illite within fault cores and the less deformed rocks surrounded by faults. In turn, precipitation of quartz and pyrite locally cemented gouge in the fault cores. Concentration of clays around faults and veins weakened the rocks and promoted shear strain along them. These processes are expected to occur in any porphyry-related hydrothermal system and can combine during the development of the system. Although cementation tends to strengthen fault cores, the overall strength of the faults is expected to be reduced by hydrolysis of plagioclase, reducing the friction of the faults and their capacity to accumulate stress. These findings may have strong implications for understanding the evolution of ore deposit, the rheology of the crust, earthquake generation, and natural hazard assessment.

Coupled trace element and SIMS sulfur isotope geochemistry of sedimentary pyrite: Implications on pyrite growth of Caixiashan Pb–Zn deposit

Colloform pyrite with core-rim texture is commonly deposited in carbonate platforms associated with the sulfide ores such as the Caixiashan Pb–Zn deposit. However, the genesis of colloform pyrite in Pb–Zn deposits, its growth controls and their geological implication are insufficiently understood. Integration of in-situ trace element and SIMS sulfur isotopes has revealed geochemical variations among these pyrite layers. These colloform pyrite occur as residual phases of core-rim aggregates, the cores are made up of very fine-grained anhedral pyrite particles, with some rims being made up of fine-grained and poorly-crystallized pyrite, while the other rims were featured with euhedral cubic pyrite, which are cemented by fine-grained calcite and/or dolomite with minor quartz. Sulfur isotope analysis shows that some well-preserved rims have negative δ34S values (–28.12‰ to –0.49‰), whereas most of the cores and rims have positive δ34S values (>0 to +44.28‰; peak at +14.91‰). Integrating with the methane and sulfate were observed in previous fluid inclusion study, we suggest that the 34S depleted rims were initially formed by bacteria sulfate reduction (BSR), whereas the positive δ34S values were resulted from the sulfate reduction driven by anaerobic methane oxidation (AOM). The well-developed authigenic pyrite and calcite may also support the reaction of AOM. Combined with petrographic observations, trace element composition of the colloform pyrite reveals the incorporation and precipitation behavior of those high abundance elements in the pyrite: Pb and Zn were present as mineral inclusion and likely precipitated before Fe, as supported by the time-resolved Pb–Zn signal spikes in most of the analyzed pyrite grains. Other metals, such as Hg, Co and Ni, may have migrated as chloride complexes and entered the pyrite lattice. Arsenic and Sb, generally influenced by complex-forming reactions rather than substitution ones, could also enter the pyrite lattice, or slightly predate the precipitation of colloform pyrite as mineral inclusions, which are controlled by their hydrolysis constant in the ore fluids. The colloform pyrite may have grown inward from the rims. The successive BSR reaction process would enrich H232S in the overlying water column but reduce the metal content, the nucleation of these pyrite rims was featured by strongly negative sulfur isotopes. The following AOM process should be activated by deformation like the turbidity sediment of the mudstone as the sulfide deposition are associated with fault activities that caused the emission of methane migration upward and simultaneously replenishing the metal in the column. The higher AOM reaction rate and the higher metal supply (not only Fe, but with minor other metals such as Pb and Zn) caused by sediment movement enhanced the metal concentration within the pyrite lattice.

Diagenesis and iron paleo-redox proxies: New perspectives from magnetic and iron speciation analyses in the Santa Barbara Basin

Sedimentary redox proxies are usually employed to reconstruct the paleo-redox conditions of bottom water environments, assuming that porewater and bottom water dissolved oxygen concentrations are similar. Using a combination of geochemical and magnetic techniques, we investigate the relationship between iron speciation and mineralogy in recent (~1760–2009 CE) sediments retrieved from the Santa Barbara Basin (SBB) – a modern silled basin with low-oxygen (dissolved O2 < 10 μmol/kg) and sporadically anoxic (no O2 detected) bottom waters. Magnetic analyses reveal that biogenic magnetite is preserved in SBB sulfidic porewaters on at least decadal to centennial time scales. Highly-reactive Fe (oxyhydr)oxides remain preserved despite observed sulfidic porewaters, indicating incomplete pyrite conversion, and producing low Fepy [pyrite Fe]/FeHR ratios. We attribute this observation to restricted porewater reaction kinetics under high sedimentation rates. Our results also reveal non-steady state diagenesis caused by instantaneous depositional events (e.g., turbidites and flood layers). The most reducing water column suggested by Fe speciation coincided with the Macoma layer, where in situ colonization of hypoxia-intolerant bivalve shells argues for the most oxygenated bottom water in the 250 year record. A turbidite potentially introduced fresh unsulfidized FeHR that buffered upward-diffusing sulfide from underlying sediments. Subsequent pyrite precipitation following re-establishment of sulfidic porewaters could have facilitated a “false positive” interpretation. In comparison, redox-sensitive metal enrichments (MoEF, UEF, and ReEF) were not obscured by post-depositional diagenesis and appear to accurately record redox geochemistry at the sediment-water interface.

Petrography and geochemistry of tourmaline breccia in the Longtoushan Au deposit, South China: genesis and its exploration significance

The Longtoushan Au deposit, located in the Qin-Hang Metallogenic Belt, South China, is a breccia-hosted epithermal Au deposit. It is characterized by the distinctive occurrence of magmatic-hydrothermal tourmaline-cemented breccia. Tourmaline-group minerals have an extensive range of chemical composition that could reflect their host environments, including some of economic interest. To investigate the potential of tourmaline as a geochemical indicator for ore exploration, chemical and boron isotope compositions of tourmaline from the Longtoushan Au deposit have been determined. Tourmaline mainly occurs as orbicular aggregates, clusters and veinlets in rhyolite porphyries, granite porphyries and in spatially associated hydrothermal breccias (Au-mineralized and Au-barren). Three types of tourmaline (I, II and III) are recognized at Longtoushan, most of which being alkali-deficient schorl-dravites. The pre-ore Tourmaline I in the rhyolite porphyry is Fe-rich schorl, contrasting with the dravitic ore-stage Tourmaline II in the mineralized breccia and Tourmaline III in the granite porphyry. Locally, Tourmaline II shows concentric zoning which correlates with core-rim chemical variations (higher Fe, Al in the rim than in the core). Tourmalines from Longtoushan are characterized by a dominant MgFe -1 exchange vector and have relatively low Na contents (avg. 0.66 apfu; formula normalized on 15 total cations), indicating they possibly crystallized in reduced fluids of low-salinity. The δ 11 B values of tourmalines from this study fall in a narrow range of −14.5 to −10.2‰, indicating dominant magmatic-hydrothermal fluids of continent crust origin. The results suggest that tourmaline from Longtoushan may originate from boron-rich vapor fluids unmixed from felsic magmas. Tourmaline breccia was likely formed by hydraulic fracturing, with open spaces infilled by boron-rich boiling fluids released from the magma. The fluid boiling process during brecciation would be critical for Au-bearing pyrite precipitation, which could compete with coexisting tourmaline for Fe, leaving tourmaline Fe-depleted. We propose that the Fe-poor dravitic tourmaline in the hydrothermal breccia could be used as prospecting guide at the Longtoushan Au deposit. Supplementary material: major element and boron isotope composition analyses of tourmaline are available at https://doi.org/10.6084/m9.figshare.c.4468808

Incursion of meteoric water triggers molybdenite precipitation in porphyry Mo deposits: A case study of the Chalukou giant Mo deposit

There has been a long-term debate about the timing of the incursion of meteoric water into porphyry systems and whether it plays an important role in ore precipitation. To evaluate the role of meteoric water in the mineralization in porphyry Mo deposits, typical hydrothermal veins of different stages from the Chalukou giant porphyry Mo deposit are selected for combined scanning electron microscope cathodoluminescence (SEM-CL), fluid inclusion microthermometry and high-resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses. Multiple generations of quartz are identified in almost all of the veins. The barren quartz veins and quartz-magnetite veins of the early Mo mineralization stage are dominated by granular CL-bright quartz (EMQ1), which usually shows dissolution and re-deposition textures and are overprinted by CL-dark quartz (EMQ2)-filled healed fractures. The molybdenite precipitation in the quartz-molybdenite veins of the main Mo mineralization stage is synchronous with small volumes (<15%) of CL-dark quartz (MMQ2) and postdates the large volume (>85%) of CL-bright quartz (MMQ1). Similar to the quartz-molybdenite veins, the precipitation of pyrite in the quartz-pyrite veins during the transitional stage and sphalerite in the quartz-sphalerite-pyrite veins during the Zn-Pb mineralization stage are also directly associated with CL-dark quartz (TSQ2, ZPQ2). The SIMS oxygen isotopic values of EMQ1 and MMQ1 fall into the range of 5.42–9.79‰ with calculated equilibrium fluid compositions of 2.07–6.45‰, indicating a magma-dominated fluid composition, whereas MMQ2, TSQ2 and ZPQ2 have very low oxygen isotopic values from −4.68 to 4.29‰ with corresponding fluid compositions of −13.64 to −5.94‰, implying that the fluid formed by the mixing of magmatic water and a large proportion of meteoric water. The contrasting SEM-CL features and oxygen isotopic compositions of the two generations of quartz in the quartz-molybdenite veins demonstrate that the incursion of meteoric water into the hydrothermal system occurred earlier than the molybdenite precipitation. Based on these results and the crosscutting relationships and spatial distribution of the veins, we reconstructed the quartz evolutionary sequence during the vein development process. After the early precipitation of barren quartz, the reduction in fluid pressure from lithostatic to hydrostatic conditions allowed the incursion of convecting cool meteoric water into the relatively hot hydrothermal system, and the temperature of the evolved Mo-rich fluid decreased rapidly from >500 °C to <400 °C, leading to concentrated molybdenite precipitation with small volumes of quartz. Fluid cooling resulting from the incursion of meteoric water may have played a greater role in the metal precipitation in the Chalukou Mo deposit than wallrock interactions and fluid boiling. Moreover, this process may have also occurred in other porphyry Mo deposits.

A novel framework for interpreting pyrite-based Fe isotope records of the past

Abstract The variation in the iron isotopic composition (δ56Fe) of sedimentary pyrite is often interpreted to reflect the degree of Fe redox cycling in modern and ancient environments. However, the degree to which precipitation pathways, isotopic exchange, and precipitation rates can affect the isotopic fractionation associated with pyrite precipitation from aqueous Fe(II) (Fe(II)aq) is poorly understood. In this study, pyrite is precipitated at 80 °C in batch reactors through the H2S and polysulfide pathways, in which the precipitation rates and the concurrent formation of a greigite (Fe3S4) phase is modulated by the amount of initially added elemental sulfur and aqueous molybdenum. Our results indicate an average apparent isotopic fractionation (δ56Fepyrite - δ56FeFeSx, where FeSx includes FeS, Fe(II)aq, and greigite) of −0.51 ± 0.22‰ throughout the experiments irrespective of precipitation pathways and greigite formation. Early-stage precipitation is associated with ∼0.3‰ larger isotopic fractionation than late-stage precipitation, possibly indicating either a rate-dependent kinetic isotope effect (KIE) or a different isotopic fractionation factor for early-stage pyrite nucleation compared to later-stage growth. Overall, the magnitude of the apparent isotopic fractionation is significantly smaller than the

The origin of the carbonate-hosted Huize Zn–Pb–Ag deposit, Yunnan province, SW China: constraints from the trace element and sulfur isotopic compositions of pyrite

The Huize Zn–Pb–Ag deposit in SW China has an ore reserve in excess of 5 Mt. with an ore grade of ≥25 wt% Zn + Pb and 46–100 g/t Ag. This deposit is hosted in Carboniferous dolostone and limestone. Sulfide mineralization is dominated by sphalerite, galena, and pyrite. Four types of pyrite (Py1 to Py4) are temporally and spatially related to mineralization distinguished on the basis of textural features and mineral associations. Pyrite 1 to 3 corresponds to the pyrite-sphalerite sub-stage, whereas Py4 corresponds to sphalerite-galena-pyrite sub-stage. Pyrite 1 shows zoned texture composed of an inclusion-rich core and an inclusion-free rim, whereas Py2, Py3, and Py4 show replacement relic or overgrowth textures. The zoned texture in Py1 was formed by multiple stages of ore fluids, whereas replacement relic texture in Py2 to Py4 was formed by replacement of pyrite by late Pb-Zn-rich fluids. Trace element variation in pyrite results from a combination of mineral inclusions, co-precipitating minerals, and various fluid compositions. Sphalerite, pyrite, and galena have δ34S values of 10.4–23.5‰, suggesting that sulfur was probably derived from the thermochemical reduction of marine sulfates. The Huize pyrite has Co and Ni concentrations (0.02–9.5 ppm and 0.08–143 ppm, respectively) and Co/Ni ratios (~0.01–2.63) similar to pyrite from sedimentary exhalative deposits, submarine hydrothermal vents, and sedimentary pyrite, which may be due to pyrite precipitation from low-temperature (<~250 °C), basin brines or seawater. The Co/Ni ratios of the Huize pyrite are lower than those (~0.2–7.2) of pyrite from Mississippi Valley-type Zn-Pb deposits. The Huize pyrite has Co and Ni contents lower than those associated with volcanogenic massive sulfide, iron oxide copper gold, and porphyry Cu deposits, arguing against the involvement of a magmatic/volcanic component and a direct genetic link between Permian Emeishan basalts and Pb-Zn mineralization. Combining with fluid inclusion temperatures and silver grade, we define the Huize deposit as a transitional type between Mississippi Valley-type and high-temperature carbonate-replacement Zn-Pb deposits.

Selective crystallization and precipitation of authigenic pyrite during diagenesis in uranium reservoir sandbodies in Ordos Basin

The occurrence characteristics of authigenic pyrite during diagenesis in uranium reservoir sandbodies of Zhiluo Formation in Ordos Basin are investigated. Based on outcrop area investigations and drilled cores observations, the pyrite is closely related to carbonaceous debris. Microscopically, the host components for pyrite consist of carbonaceous debris, clay minerals, biotite, the earlier pyrite, and ilmenite by emphasizing on δ34S values via LA-ICP-MS, as well as observing through optical microscope and scanning electron microscope equipped with energy dispersive spectroscope. Three kinds of distribution pattern of pyrite are classified as: (i) periphery; (ii) infilling; (iii) the combined form of the above two. In-situ micrometer-scale sulfur isotope analyses of pyrite demonstrated that broad-scale isotopes are heterogeneous (from −52.5‰ to +35.4‰ of δ34S) and sulfur is derived from more than a single source. Significant differences of host components effects on authigenic pyrite formation are discussed. The carbonaceous debris can supply the energy for the bacterial sulfate reduction process, and the biotite can provide the source of iron for pyrite precipitation. The ferrous ion can be adsorbed on the surface of the clay minerals. Meanwhile, the authigenic pyrite is distributed in or around the earlier pyrite and ilmenite by the indirect adsorption of ferrous ion, and the ilmenite may also provide the source of iron. Futhermore, the uranium minerals are mainly associated with the authigenic pyrite relevant to carbonaceous debris and clay minerals, which indicates there exists constraints of authigenic pyrite during diagenesis on the uranium mineralization.

Gold behavior in intermediate sulfidation epithermal systems: A case study from the Zhengguang gold deposit, Heilongjiang Province, NE-China

The Zhengguang gold deposit, a typical intermediate-sulfidation epithermal deposit, is located in the southeastern part of the Duobaoshan orefield, west of the Hegenshan-Heihe suture zone, in the eastern part of the Central Asian Orogenic Belt. The deposit comprises five ore zones with total Au reserves exceeding 35 tonnes, with potential additional resources at depth. All vein-type orebodies are hosted by Paleozoic volcanic rocks and comprise multiple vein sets 1–100 cm in thickness. Although gold generally occurs in native form, or as electrum in epithermal deposits like Zhengguang, both pyrite and sphalerite are known to accommodate modest concentrations of invisible gold. This study employs a combination of petrography and sulfide chemistry to determine the role of invisible gold in the Zhengguang ores and the mechanisms of gold incorporation into epithermal sulfides. Three sulfide stages are identified: an early quartz + pyrite (Py1a, Py1b) ± chalcopyrite (Ccp1) stage; a subsequent quartz + sphalerite (Sph2a, Sph2b) + pyrite (Py2a, Py2b, Py2c, Py2d) + chalcopyrite (Ccp2a, Ccp2b) ± galena ± calcite stage; and a late stage containing deformed quartz + pyrite (Py3a, Py3b) ± sphalerite. Petrography and sulfide chemistry allow three groups of pyrite (Au-poor, Au-rich, and a distinct Sb-rich group) to be distinguished, alongside three groups of chalcopyrite (Bi-rich, intermediate-Bi, and Bi-poor), and two groups of sphalerite (Au-poor, Au-rich). A potential porphyry system is indicated beneath the epithermal system by the appearance of Au-poor pyrite and Bi-poor chalcopyrite. After precipitation of early Au-poor sulfides, inflow of relatively low temperature epithermal fluids led to alteration and replacement of early porphyry-related sulfides, and to precipitation of Au-rich pyrite, Bi-rich and intermediate-Bi chalcopyrite, and sphalerite. Gold-rich pyrite contains up to 140 ppm Au, interpreted as both as lattice-scale substitution (Au1+) and as included particles of native gold (Au0). Epithermal chalcopyrite is an important silver carrier but, although Au is measurable, it is a not a good carrier for gold. A strong positive correlation between Au and Cu in pyrite from the first two stages indicate that gold and other metals were likely sourced from magma-derived hydrothermal fluids. The deposit was formed in the Early Paleozoic but some gold ores appear deformed and partially destroyed by a later metamorphic event during which a distinct Sb-rich pyrite crystallized. This study should catalyze exploration in the orefield as it provides further support for an as-yet undiscovered porphyry system close to the Zhengguang deposit.

Two episodes of sulfide mineralization at the Yuhuang-1 hydrothermal field on the Southwest Indian Ridge: Insight from Zn isotopes

The newly discovered Yuhuang-1 hydrothermal field (YHF) on the Southwest Indian Ridge primarily contains two different sulfide distribution areas: the southwest sulfide (SWS) and the northeast sulfide (NES) distribution areas. In this study, Zn isotope compositions and element ratios of Zn, Fe, Cu, and Cd in the surface sulfides were analyzed to characterize Zn isotopic fractionation, ore-forming conditions, and mineralization processes of the YHF. The results indicate that sulfide-rich samples in the YHF exhibit intensive Zn isotopic fractionation, with δ66Zn values ranging from −0.43‰ to 1.24‰. These values are not only larger than those currently reported for seafloor sulfides but are also larger than the values reported for the deposits on land. Remarkably, the obtained δ66Zn value (−0.43‰) is the lowest one observed in seafloor sulfides (−0.26‰–1.17‰), currently. A study of Zn isotope systematics revealed that Zn isotope variations of sulfide-rich samples in the YHF were controlled by the sphalerite and pyrite content; the study also revealed that early minerals are rich in light Zn isotopes, but later minerals are rich in heavy Zn isotopes. A comparative study of previous data on Zn isotopes shows that Zn isotope fractionations in the YHF are likely attributed to Rayleigh fractionation, caused by mineral precipitation and multiple generations of pyrite. Further, Zn/Cd ratios of sulfide-rich samples indicate that the ore-forming temperature of the silicified sulfide-rich samples in the SWS probably formed under lower temperatures as compared with the silicified sulfide-rich chimney in the NES. Furthermore, sulfur and Zn isotope compositions in the SWS are significantly different from those in sulfide-rich samples of the NES, which suggests pervasively different metallogenic conditions, further indicating that they are probably products of separate mineralization episodes.

Multiple mineralization events of the Paleozoic Tuwu porphyry copper deposit, Eastern Tianshan: evidence from geology, fluid inclusions, sulfur isotopes, and geochronology

The Tuwu porphyry Cu deposit, located in Eastern Tianshan, NW China, is hosted by a plagiogranite porphyry and Carboniferous Qi’eshan Group volcanic rocks. Based on crosscutting relationships and mineral assemblages, hydrothermal alteration and mineralization processes at Tuwu can be divided into four stages: early propylitic alteration (stage I), porphyry mineralization (stage II), overprinting mineralization (stage III), and post-mineralization (stage IV). The porphyry mineralization stage (stage II) contributed to the majority of the Cu–Mo resource, with Cu mineralization occurring mainly as quartz-chalcopyrite veins. Stage III also produced minor Cu mineralization, characterized by chalcopyrite–anhydrite–chlorite–calcite assemblages. Fluid inclusion (FI) study reveals that stage II is characterized by a high-temperature, high-salinity, highly oxidized, and K-rich H2O–NaCl–CaCl2 fluid. Fluid boiling and mixing likely occurred during the porphyry mineralization stage, leading to the precipitation of chalcopyrite and pyrite. Alteration and mineralization in stage III were derived from a S-rich H2O–NaCl–CaCl fluid, with fluid boiling leading to the precipitation of chalcopyrite. The δ34S values of chalcopyrite from stages II and III are − 0.8–0.6 ‰ and 1.1–1.3 ‰, respectively, suggesting magmatic sources for the ore-forming components of both stages. 40Ar/39Ar dating indicates that stage II likely occurred at 328.1 ± 1.4 Ma, around the age emplacement of the causative plagiogranite porphyry (ca. 337.7–330.3 Ma). We suggest the overprinting mineralization event occurred at ca. 323 Ma, spatially and genetically related to the emplacement of the quartz albite porphyry at 323.6 ± 2.5 Ma.

Diagenesis and its impact on a microbially derived carbonate reservoir from the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The uppermost Middle Triassic Leikoupo Formation in the western Sichuan Basin of China has recently been shown to host as much as 5.3 tcf (1.5 × 1012 m3) of natural gas resources. The reservoir rocks, composed mainly of microbially derived dolomudstone (e.g., thrombolites and stromatolites), are characterized by low porosity (<8%) and permeability (<0.001 to 10 md). The limestone is commonly tight and not of reservoir quality because of abundant meteoric calcite cementation, whereas the dolostone has various types of pores dominated by solution-enlarged pores and vugs, microbial framework pores, and micropores. Breccias are well developed in places, probably because of dissolution of underlying evaporites (e.g., anhydrite) by an influx of low-salinity fluids (e.g., freshwater and seawater) during an early burial stage. Early dolomitization created micropores in the dolomudstone, and subsequent diagenetic events were dominated by calcite, dolomite, quartz cementation, pyrite replacement, compaction, fracturing, and development of stylolites. Localized hydrothermal activity has been evidenced by high homogenization temperatures (∼160°C–200°C) obtained from fluid inclusions in fracture-filling cements. Bacterial sulfate reduction probably resulted in H2S generation, pyrite precipitation, and solution-enlarged pore and vug formation, whereas part of the current H2S in these reservoirs may have been sourced from thermochemical sulfate reduction or an underlying formation (e.g., the Feixiangguan Formation). Development of microfractures and associated micropores was probably the final diagenetic event, which improved pore interconnectivity. This study confirms the effect of diagenesis on the development of a microbial dolomudstone reservoir, which may be applicable to other similar microbial carbonate reservoirs elsewhere, for example, Middle Triassic sections of the Tethys region and offshore Brazil.

U-Pb Geochronology of Apatite and Calcite at The Ernest Henry Deposit, NW Queensland; Implications for Hydrothermal Evolution and Ore Genesis

The Ernest Henry deposit represents the largest known Iron Oxide Copper Gold (IOCG) deposit in the Eastern Succession of the Mount Isa inlier. The orebody consists of a structurally controlled pipe-like breccia hosted in complexly altered Proterozoic volcanics with mineralization occurring post-peak metamorphism during a regional transpressional deformational event (D3). Ore formation was controlled by the mixing of magmatic, metamorphic and basal fluids, resulting in the precipitation of chalcopyrite, pyrite, calcite, quartz, magnetite and accessory gold. Coarse-grained apatite is present as an accessory mineral in areas of high sulphide mineralization and in shear zones adjacent to the orebody. The paragenesis and relative timing of the alteration and mineralisation stages have been well constrained by previous workers. However, advances in U-Pb geochronology via the in-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) method means that for the first time apatite and calcite from the ore body and adjacent structures may be suitable for age dating. This project will use an established apatite dating technique and aims to develop a calcite dating technique to provide dates for individual paragenetic stages. These time constraints will improve the current understanding hydrothermal evolution and ore genesis at Ernest Henry. As calcite is a common accessory mineral in ore deposits, this technique could be widely implemented to date mineralization events and may allow hydrothermal events at different deposits to be linked.

Records of carbon and sulfur cycling during the Silurian Ireviken Event in Gotland, Sweden

Abstract Early Silurian (∼431 Ma) carbonate rocks record a ca. 4.5‰ positive excursion in the stable isotopic composition of carbonate carbon (δ13Ccarb). Associated with this isotopic shift is a macroevolutionary turnover pulse known as the ‘Ireviken Event’. The onset of this carbon isotope excursion is commonly associated with a shallowing-upward facies transition that may have been accompanied by climatic change, as indicated by a parallel positive shift (∼0.6‰) in the stable isotopic composition of carbonate oxygen (δ18Ocarb). However, the relationships among carbon cycle perturbations, faunal turnover, and environmental changes remain enigmatic. Here we present a suite of new isotopic data across the Ireviken Event from multiple sections in Gotland, Sweden. These samples preserve no systematic change in δ18Ocarb but show positive excursions of equal magnitude in both carbonate (δ13Ccarb) and organic (δ13Corg) carbon. In addition, the data reveal a synchronous perturbation in sulfur isotope ratios, manifest as a ca. 7‰ positive excursion in carbonate-associated sulfate (δ34SCAS) and a ca. 30‰ positive excursion in pyrite (δ34Spyr). The increase in δ34Spyr values is accompanied by a substantial, concomitant increase in stratigraphic variability of δ34Spyr. The relatively constant offset between the δ13Ccarb and δ13Corg excursions throughout the Ireviken Event could be attributed to increased organic carbon burial, or possibly a change in the isotopic composition of CO2 sources from weathering. However, a positive correlation between carbonate abundance and δ13Ccarb suggests that local to regional changes in dissolved inorganic carbon (DIC) during the shallowing-upward sequence may have been at least partly responsible for the observed excursion. The positive excursion recorded in δ34SCAS suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir. An inverse correlation between CAS abundance and δ34SCAS supports the notion of decreased sulfate concentrations, at least locally, consistent with a concomitant increase in pyrite burial. A decrease in the offset between δ34SCAS and δ34Spyr values during the Ireviken Event suggests a substantial reduction in the isotopic fractionations (epyr) expressed during microbial sulfur cycling and pyrite precipitation through this interval. Decreased epyr and the concomitant increase in stratigraphic variation in δ34Spyr are typical of isotope systematics observed in modern shallow-water environments, associated with increased closed-system behavior and/or oxidative sedimentary reworking during early sediment diagenesis. While the isotopic trends associated with the Ireviken Event have been observed in multiple locations around the globe, many sections display different magnitudes of isotopic change, and moreover, are typically associated with local facies changes. Due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the Ireviken Event, and their relationship to changes in local depositional environment, we surmise that these patterns more closely reflect biogeochemical processes related to deposition and lithification of sediment than global changes in carbon and sulfur burial fluxes.

Geochemical and micro-textural fingerprints of boiling in pyrite

Abstract The chemical composition, textures and mineral associations of pyrite provide key information that help elucidate the evolution of hydrothermal systems. However, linking the compositional and micro-textural features of pyrite with a specific physico-chemical process, e.g., boiling versus non-boiling, remains elusive and challenging. In this study we examine pyrite geochemical and micro-textural features and relate these results to pyrite-forming processes at the active Cerro Pabellon Geothermal System (CPGS) in the Altiplano of the northern Chile. We integrate electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data with micro-textural observations of pyrite and associated gangue minerals recovered from a ∼500 m long drill core that crosscuts the argillic, sub-propylitic and propylitic alteration zones of the CPGS. Additionally, we carried out a Principal Component Analysis (PCA) in order to inspect and understand the main data structure of the pyrite geochemical dataset. The concentrations of precious metals (Au and Ag), metalloids (As, Sb, Se, Bi and Tl), and base and heavy metals (Cu, Co, Ni and Pb) in pyrite from the CPGS are significant. Among the elements analyzed, As, Cu and Pb are the most abundant with concentrations that vary from a few parts per million (ppm) to wt% levels (up to 4.4 wt% of As, 0.5 wt% of Cu and 0.2 wt% of Pb). Based on contemporaneous gangue mineral associations and textures, the mechanisms of pyrite precipitation in the CPGS were inferred. Pyrite formed during vigorous boiling is characterized by relatively high concentrations of As, Cu, Pb, Ag and Au and lower concentrations of Co and Ni compared to pyrite formed under different conditions. These anhedral to euhedral pyrite grains display zones with a porous texture and abundant mineral micro- to nano-inclusions (mainly galena and chalcopyrite) indicating a formation by rapid crystallization. In contrast, pyrite formed under gentle boiling (more gradual cooling and less abrupt physico-chemical variations than in vigorous boiling) to non-boiling conditions is characterized by a higher concentration of Co and Ni, and relatively low concentrations of As, Cu, Pb, Ag and Au. Texturally, these pyrites form aggregates of euhedral and pristine pyrite crystals with scarce pores and mineral inclusions suggesting formation under steadier physico-chemical conditions. Our results show that pyrite can not only record the chemical evolution of hydrothermal fluids, but can also provide critical information related to physico-chemical process such as boiling and phase separation. Since boiling of aqueous fluids is a common phenomenon occurring in a variety of pyrite-forming environments, e.g., active continental and seafloor hydrothermal systems, and porphyry Cu-epithermal Au-Ag deposits, pyrite compositional and textural features are a valuable complement for discriminating and tracking boiling events in modern and fossil hydrothermal systems.

Metal remobilization and ore-fluid perturbation during episodic replacement of auriferous pyrite from an epizonal orogenic gold deposit

Mineral-scale episodic replacement of auriferous pyrite by texturally-complex pyrite, marcasite and minor arsenopyrite occurred in breccia ores from the Daqiao epizonal orogenic gold deposit, West Qinling Orogen, China. This study uses a novel combination of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), Nanoscale secondary ion mass spectrometry (NanoSIMS), and secondary ion mass spectrometry (SIMS) to investigate the remobilization and re-concentration of gold and other trace elements during this complex replacement process and the probable mechanism. Several lines of evidence including some degree of preservation of external morphology, sharp contacts and compositional differences between the parent pyrite and product pyrite and marcasite, and reaction-induced porosity suggest that the replacement of parent pyrite proceeds via a two-step replacement via a dissolution and reprecipitation mechanism, plus an additional marcasite overgrowth. During the replacement of euhedral pyrite, depletion of gold and other trace elements (Te, Se, Zn, Co, Tl, Ni, W, and As) in porous product pyrite relative to its precursor indicate exsolution and remobilization of these metals from crystal lattice of the original pyrite. In the subsequent replacement of porous pyrite by two types of marcasite and minor arsenopyrite, euhedral product marcasite contains low contents of trace elements, possibly due to high metal solubility in the acidic fluids favorable for marcasite precipitation. The complex-zoned marcasite significantly enriched in gold and other metals relative to porous pyrite (W, Tl, As, Sb, Ag, Se, and Zn) is thought to have formed via precipitation triggered by further oxidation and/or immediate reduction in threshold supersaturation. Dissolution of the impurity-rich pyrite and precipitation of new pyrite and marcasite generations could have occurred at low pH plus high concentrations of dissolved Fe2+ condition caused by partial oxidation of aqueous H2S and/or S2− in ore fluids. The fluid oxidation is evidenced by a general decreasing trend of δ34S values from the parent euhedral pyrite, to product porous pyrite, euhedral marcasite, and complex-zoned marcasite. The isotopic results are consistent with ore fluid oxidation controlled by pressure fluctuations during multistage hydraulic fracturing in a fault-valve regime at Daqiao deposit. This quantitative study emphasizes that the pressure-driven hydrothermal process plays a key role in the micron- to nano-scale redistribution and re-enrichment of gold and other trace metals during episodic replacement of auriferous pyrite in brittle rheological zones from epizonal orogenic gold systems.