Pyrite Crystals Research Articles

Formation of Pyrite Spherules From Mixtures of Biogenic FeS and Organic Compounds During Experimental Diagenesis

AbstractPyrite () is the most common iron sulfide on the Earth's surface and has widely been used as a paleo‐environmental proxy. Yet the information recorded by pyrite depends on whether it was formed through abiotic or biogenic routes. It is thus of importance to properly identify its origin. Here, we investigate the final morphology of pyrite produced upon a simulated diagenetic history from biogenic and abiotic iron sulfide/phosphate systems. Abiotic starting material obtained by chemical synthesis and biogenic starting material produced from pure culture of Desulfovibrio desulfuricans were submitted to increasing diagenetic conditions (75°C or 150°C from 1 to 10 days). Mineralogical products were characterized by X‐ray diffraction and electron microscopy. For both biogenic and abiotic starting materials, the final state was characterized by the association of pyrite and lipscombite (), the most stable phases in these conditions. Intermediate phases such as greigite for iron sulfides and beraunite/wolfeite for iron phosphates were present in the abiotic residue but were not detected in the biogenic residue. Distinct pyrite morphologies were observed depending on the presence of organic matter. Indeed, while abiotic starting material led to the formation of submicrometric single crystals of pyrite with euhedral shapes similar to the subunits of well crystallized framboids, biogenic starting material produced micrometric spherulitic clusters of pyrite resembling the so‐called pseudo‐framboids. Although further experiments are required to ensure that it can be used as biosignatures, such specific morphologies, likely related to the presence of organic matter, may help recognizing biogenic pyrite in the geological record.

Geneze fosfátu v jeseneckých vápencích konicko-mladečského pruhu (drahanský faciální vývoj moravskoslezského paleozoika na Moravě)

Jesenec Limestone of the Famennian to Tournaisian age belongs to the Drahany Facies Domain (Development) of the Moravian-Silesian Paleozoic sedimentary sequence. Massive micritic and biosparitic limestone layers occasionally alternate with sandy limestone and limestone with an admixture of pyroclastic material.Several mm to 25 mm angular and rounded fragments of black phosphorites were found in the investigated limestones crop out in the Konice-Mladeč Belt (sediments and volcanites) north of the villages of Dzbel and Jesenec. We use an optical microscope and electron probe microanalysis to classify and interpret these phosphorites. Based on shape, phosphorite particles we divided into two principal groups: angular intraclasts and rounded concretions. Fine-grained brecciated intraclasts are fragments of phosphorite layers formed on the surface of carbonate sediment (probably near the volcanoes) in a shallow-water environment. The destruction of these phosphate crusts may have been due to waves associated with storm activity or volcanic eruptions.Fine-grained phosphorite concretions comprise predominantly dark-colored subhedral to anhedral fluorapatite (0.07–0.14 wt. % FeO; up to 0.04 wt. % MnO) with interspersed calcite (0.06–0.14 hm. % FeO; up to 0.43 wt. % MgO) and quartz crystals. Apatite spherulites up to 0.2 mm in diameter are locally visible. Rare present pyrite crystals and their relics indicate polyphase evolution, including partial replacement of pyrite by apatite. We assume the crystallization of pyrite in an anoxic environment as the first phase of the process. The low pH of pore waters due to pyrite oxidation led to the dissolution of the biogenic phosphate and its subsequent precipitation in the form of concretions.

Trace elements mineral chemistry of sulfides from the Woxi Au‐Sb‐W deposit, southern China

AbstractThe Woxi Au‐Sb‐W deposit is one of the largest polymetallic ore deposits in the Xuefengshan Range, southern China, hosted in low‐grade metamorphosed Neoproterozoic volcaniclastic rocks. The orebodies of the deposit are predominantly composed of banded quartz veins, which are strictly controlled by bedding and faults. Petrographic observations and geochemical results are reported on the occurrence of Au and properties of the ore‐forming processes for different stages in the deposit. The veins extend vertically up to 2 km without obvious vertical metal zoning. The ore‐forming process can be subdivided into four mineralization stages: Pre‐ore stage; Early stage (scheelite‐quartz stage); Middle stage (pyrite‐stibnite‐quartz stage); and Late stage (stibnite‐quartz sage). Four types of pyrite (Py0, Py1, Py2, and Py3) were identified in the ores and host‐rock: Py0 occurs as euhedral grains with voids in the core, ranging in size from 50 to 100 μm and formed mainly in the Pre‐ore stage and Early stage; Py1 occurs as subhedral grains. Small grains (around 10 μm) of Py1 form irregularly shaped clusters of variable size ranging from tens to hundreds of μm and mainly formed in the Middle stage; Euhedral‐subhedral fine‐grained Py2 formed in the Late stage; Minor subhedral fine‐grained Py3 was deposited in the Late‐stage. Stibnite is widely distributed in the Middle and Late stage ore veins. No systemic difference was recognized in mineralogical features among stibnite formed in different stages. In addition to native gold, the lattice bound Au+1 widely exists in Py1 and Py2 in the deposit, and widespread Py1 is considered as the main Au‐bearing mineral with the highest Au contents. Most elements (such as Co, Ni, Cu, As, Sb, Ba, and Pb) are considered to occur as solid solution within the crystal lattice and/or invisible nanoparticles in sulfides minerals. The Co/Ni ratio of most pyrite is lower than 1, suggesting that the metals in the ore‐forming fluid are sourced from sedimentary rocks. The coupled behavior between Au and As; Au and Sb suggests that the substitution of As and Sb in pyrite can enhance the incorporation of Au. Variation of trace elements in pyrites of different stages suggests some information on the mineralization processes: Large ion lithophile elements (such as Ba and Pb) are enriched in Py0 indicating that water‐rock reaction occurred in the Early stage; Fine‐grained Py1 with a heterogeneous distribution of elements suggests fast crystallization of pyrite in the Middle stage.

Geology, mineralogy, and cassiterite geochronology of the Ayawilca Zn-Pb-Ag-In-Sn-Cu deposit, Pasco, Peru

The Ayawilca deposit in Pasco, Peru, represents the most significant recent base-metal discovery in the central Andes and one of the largest undeveloped In resources globally. As of 2018, it hosts an 11.7 Mt indicated resource grading 6.9% Zn, 0.16% Pb, 15 g/t Ag, and 84 g/t In, an additional 45.0 Mt inferred resource grading 5.6% Zn, 0.23% Pb, 17 g/t Ag, and 67 g/t In, and a separate Sn-Cu-Ag inferred resource of 14.5 Mt grading 0.63% Sn, 0.21% Cu, and 18 g/t Ag. Newly obtained U–Pb dates for cassiterite by LA-ICP-MS (22.77 ± 0.41 and 23.05 ± 2.06 Ma) assign the Ayawilca deposit to the Miocene polymetallic belt of central Peru. The polymetallic mineralization occurs as up to 70-m-thick mantos hosted by carbonate rocks of the Late Triassic to Early Jurassic Pucará Group, and subordinately, as steeply dipping veins hosted by rocks of the Pucará Group and overlying Cretaceous sandstones-siltstones of the Goyllarisquizga Group. Relicts of a distal retrograde magnesian skarn and cassiterite (stage pre-A) were identified in the deepest mantos. The volumetrically most important mineralization at Ayawilca comprises a low-sulfidation assemblage (stage A) with quartz, pyrrhotite, arsenopyrite, chalcopyrite, Fe-rich sphalerite, and traces of stannite and herzenbergite. Stage A sphalerite records progressive Fe depletion, from 33 to 10 mol% FeS, which is compatible with the observed transition from low- to a subsequent intermediate-sulfidation stage (B) marked by the crystallization of abundant pyrite and marcasite. Finally, during a later intermediate-sulfidation stage (C) sphalerite (up to 11 mol% FeS), galena, native bismuth, Cu-Pb-Ag sulfosalts, siderite, Mn-Fe carbonates, kaolinite, dickite, and sericite were deposited. This paragenetic evolution shows striking similarities with that at the Cerro de Pasco Cordilleran-type polymetallic deposit, even if at Ayawilca stage C did not reach high-sulfidation conditions. The occurrence of an early retrograde skarn assemblage suggests that the manto bodies at Ayawilca formed at the transition between distal skarn and skarn-free (Cordilleran-type) carbonate-replacement mineralization. Mineral assemblages define a T-fS2 evolutionary path close to the pyrrhotite-pyrite boundary. Buffering of hydrothermal fluids by underlying Devonian carbonaceous phyllites of the Excelsior Group imposed highly reduced conditions during stage A mineralization (logfO2 < − 30 atm). The low fO2 favored efficient Sn mobility during stages pre-A and A, in contrast to other known ore deposits in the polymetallic belt of central Peru, in which the occurrence of Sn minerals is minor. Subsequent cooling, progressive sealing of vein walls, and decreasing buffering potential of the host rocks promoted the shift from low- (stage A) to intermediate-sulfidation (stages B and C) states. LA-ICP-MS analyses reveal significant In contents in Fe-rich sphalerite (up to 1.7 wt%), stannite (up to 1908 ppm), and chalcopyrite (up to 1185 ppm). The highest In content was found in stage A sphalerite that precipitated along with chalcopyrite and stannite, thus pointing to the early, low-sulfidation assemblage as prospective for this high-tech metal in similar mineral systems. Indium was likely incorporated into the sphalerite crystal lattice via Cu+ + In3+ ↔ 2 Zn2+ and (Sn, Ge)4+ + (Ga, In)3+ + (Cu + Ag)+ ↔ 4 Zn2+ coupled substitutions. Indium incorporation mechanisms into the stannite and chalcopyrite crystal lattices remain unclear.

Pyrite Morphology and δ34S as Indicators of Deposition Environment in Organic-Rich Shales

This study is focused on the mineralogical, chemical, and isotopic characterization of pyrites from the rocks of the Bazhenov Formation (Upper Jurassic–Lower Cretaceous organic-rich shales, Western Siberia, Russia). Scanning electron microscopy (SEM) revealed pyrites of different morphologies: small and large framboids, small crystals, and large euhedral crystals; all morphotypes were usually combined into aggregates. Isotope ratio mass spectrometry (IRMS) and secondary ion mass spectrometry (SIMS) showed that small framboids and microcrystalline pyrite are isotopically light, with δ34SCDT varying from −55 to −20‰. Large framboids and euhedral crystals of pyrite are isotopically heavy with δ34SCDT up to +26‰. Both morphology and δ34S were suggested to be controlled by the redox conditions and sedimentation regime. The abundance of small framboids suggests that pyrite sedimentation occurred under anoxic conditions; the presence of the large framboids and euhedral crystals of pyrite suggest the accumulation of sediments occurred at suboxic conditions, possibly in the presence of oxygen.

Constraints of Fe-S-C stable isotopes on hydrothermal and microbial activities during formation of sediment-hosted stratiform sulfide deposits

Microbial and hydrothermal venting activities on the seafloor are important for the formation of sediment-hosted stratiform sulfide (SHSS) deposits. Fe isotopic compositions are sensitive to both microbial and hydrothermal activities and may be used to investigate the formation of these deposits. However, to the best of our knowledge, no Fe isotopic studies have been conducted on SHSS deposits. In the Devonian Dajiangping SHSS-type pyrite deposit (389 Ma), South China, laminated pyrite ores were precipitated from exhalative hydrothermal fluids, whereas black shales were deposited during intervals with no exhalation. Pyrite grains from black shales mostly display positive δ56Fe-py (0.01–0.73‰), higher than marine sediments (ca. 0‰), due to pyrite deriving Fe from basinal shuttled Fe(III) (hydr-)oxides and slowly crystallizing in pores of sediments with equilibrium fractionation, except for negative δ56Fe-py (−0.17‰ to −0.24‰) of two samples caused by mixing of Fe from underlain laminated ores. The positive δ34S-py (3.50–24.5‰) of black shales reflect that sulfur of pyrite originated from quantitative reduction of sulfate in closed pores of sediments. In contrast, pyrite grains of laminated ores have negative δ56Fe-py (−0.60‰ to −0.21‰), which were not only inherited from the negative δ56Fe of hydrothermal fluids but also caused by kinetic fractionation during rapid precipitation of a pyrite precursor (FeS) in hydrothermal plumes. These ores have negative δ34S-py (−28.7‰ to −1.82‰), because H2S for pyrite mineralization was produced by bacterial sulfate reduction (BSR) in a sulfate-rich seawater column or shallow sediments.The δ56Fe-py values of laminated ores co-vary positively with δ34S-py and δ13C-carbonate along the ore stratigraphy, with δ13C-carbonate values ranging from −12.0‰ to −2.50‰. However, they correlate negatively with aluminum-normalized total organic carbon (TOC/Al2O3). Organic carbon is thus considered to enhance the production of H2S by BSR activities, increase pyrite precipitation rates and promote the expression of kinetic fractionation of Fe isotopes. Intriguingly, in the ore units with vigorous hydrothermal venting activities, δ56Fe-py, δ34S-py and δ13C-carbonate values display a consistently increasing trend. Such results suggest that venting hydrothermal fluids significantly inhibited the H2S production of BSR, which then reduced the pyrite crystallization rate and decreased the kinetic fractionation of Fe isotopes. Our study reveals that the formation of SHSS deposits relies on H2S from microbial activities and metals from hydrothermal exhalation on the seafloor, but that vigorous exhalation can inhibit microbial activities and thus sulfide precipitation rates. The integrated use of Fe, S, and C isotopes can effectively elucidate these dynamic interactions between hydrothermal venting and microbial activities during the formation of SHSS deposits.

In situ chemical and isotopic analyses and element mapping of multiple-generation pyrite: Evidence of episodic gold mobilization and deposition for the Qiucun epithermal gold deposit in Southeast China

Abstract Gold deposits are often the result of complex mineralization and remobilization processes. Interpretation of bulk geochemical and sulfur isotope data of the gold deposits is frequently hampered by complex zoning in pyrite, which calls for in situ determination of geochemical and sulfur isotope composition of sulfide minerals. The Qiucun deposit is a good representative of epithermal gold deposits in the Mesozoic Coastal Volcanic Belt of southeastern China. It represents a complex mineralization history, comprising three hydrothermal stages: (I) early stage of pyrite-quartz-chalcedony; (II) main ore stage of quartz-polymetallic sulfide; and (III) post-ore stage of quartz-carbonate. Detailed backscattered electron imaging (BSE) and in situ trace element and sulfur isotope analyses using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and LA-multicollector (MC)-ICP-MS were applied to reveal the gold mineralization and remobilization history of this deposit. Four texturally distinct generations of pyrite were identified, all of which host invisible gold: Py1a and Py1b in Stage I and Py2a and Py2b in Stage II. A detailed study of the texture, chemistry, and sulfur isotopic composition as well as hydrothermal evolution of auriferous pyrite from the Qiucun deposit revealed the behavior of gold in the course of pyrite evolution. Pyrite of Stages I and II contains invisible gold, whereas later-stage visible native gold and re-enrichment in invisible gold is associated with alteration rims around the primary pyrite grains. Py1a is rich in silicate inclusions, enriched in Co and Ni, and depleted in As and Au relative to later pyrite generations. This redistribution is attributed to the alteration of biotite in the sub-volcanic host rocks that effectively destabilized gold in the ore fluid during Py1a deposition. Py1b and Py2a show oscillatory zoning with bright bands having elevated As and Au contents. The oscillatory zoning is interpreted to reflect pressure fluctuations and repeated local fluid boiling around the pyrite crystals. These three pyrite generations (Py1a, Py1b, Py2a) record a narrow range of δ34SV-CDT values between –3.6 and 4.6‰, consistent with a magmatic sulfur source. Gold and some trace elements (As, Ag, Sb, Pb, Tl, and Cu) that were initially incorporated into Py2a became partially exsolved and remobilized during the replacement of porous and invisible gold-rich Py2b. This replacement was likely due to coupled dissolution and re-precipitation reactions triggered by oxidation of the mineralizing fluids. Fluid oxidation is further supported by a general decrease trend of δ34SV-CDT from Py2a (–3.2 to 4.6‰) to Py2b (–15.2 to –2.3‰). Last, previously formed auriferous pyrite underwent post-mineralization fracturing, causing local pulverization of pyrite. Thus, newly created porosity facilitated fluid circulation, hydrothermal alteration of the pyrite, and remobilization of invisible gold, which re-precipitated with pyrite in the form of electrum as small inclusions or as larger grains within fractures. Our study emphasizes that pressure-driven hydrothermal processes play a vital role in the initial enrichment and re-concentration of gold and some other trace metals during episodic deposition, replacement, and hydrothermal alteration of gold-bearing pyrite in epithermal gold deposits, ultimately forming visible gold and high-grade ore shoots as exemplified by the Qiucun deposit.

Rapid preparation and characterization of pyrite materials under HPHT: A new method

This paper reports the synthesis of bulk pyrite (FeS2) polycrystals at high pressure and high temperature (HPHT) for the first time. The HPHT method markedly improves the speed of synthesis, reducing the time for synthesizing bulk pyrite from several hundred hours to several hours. A unique honeycomb-type structure was introduced into the system by the primary reaction to refine the size of the reactants. On this basis, the sulfur-deficient contamination phase Fe7S8 was effectively eliminated by secondary synthesis. Using Raman spectroscopy and X-ray diffraction (XRD), the crystalline phase of the product was confirmed to be the cubic pyrite phase. The crystal structure and initial crystal parameters of pyrite crystals synthesized under HPHT conditions were obtained for the first time by Rietveld refinement with GASA software. In combination with a scanning electron microscope (SEM) assay, the reaction process was traced, and the significance of secondary synthesis for HPHT was analyzed. Seebeck data showed that as the temperature increased, the sample changed from p-type to an n-type semiconductor and the conductivity increased monotonically, revealing the typical semiconductor characteristics of pyrite. UV reflectance spectra and VSM tests revealed the bandgap width and magnetic properties of pyrite.

Depositional environment of Middle Triassic organic‐rich shales in the Ordos Basin, Northwest China

The Middle Triassic Tongchuan Formation in the Ordos Basin of northwestern China is a typical lacustrine deposit. A major component of this formation is thick layers of organic‐rich shales that are probably a potential hydrocarbon source and preserve the earliest known Mesozoic‐type lacustrine ecosystem. The exact depositional environment of the shales in the Tongchuan Formation, however, remains unknown. To address this question, we carried out high‐resolution carbon (δ13Corg) and sulphur (δ34Spy) isotope analysis as well as undertook total organic carbon (TOC)/pyrite contents and pyrite morphology investigation, and framboidal pyrite size measurements in shales of the Bawangzhuang section of the southern Ordos Basin. Remarkably high TOC (23 ± 9%) and pyrite (7 ± 3%) contents were obtained from the shales, which indicate a large amount of organic carbon and pyrite burial during shale deposition. Framboids are the dominant pyrite morphology in the pyrite crystals and show large and variable mean diameters (7.0 ± 1.7 μm to 14.3 ± 6.8 μm) across the section, indicating oxic–dysoxic bottom water during shale deposition. δ13Corg and δ34Spy revealed narrow and less variable values, ranging from −31.8‰ to −28.1‰, and −4.1‰ to 4.9‰, respectively. The δ13Corg values suggest balanced and consistent carbon cycles. Integrated with pyrite content and morphological patterns, consistent δ34Spy values probably demonstrate a relatively open environment for the formation of sedimentary pyrite, and thus a shallow chemocline that was quite close to the water‐to‐sediment interface during shale deposition. Overall, the organic‐rich shales of the Tongchuan Formation were probably deposited under oxic–dysoxic bottom‐water conditions. Shallow chemocline depth combined with moderately high sedimentation rate and high primary productivity may have played crucial roles in the deposition and formation of the organic‐rich shales in the Tongchuan Formation. The shallow chemocline also facilitates the fossil preservation in a lacustrine environment.

A new kind of invisible gold in pyrite hosted in deformation-related dislocations

Abstract Mining of “invisible gold” associated with sulfides in gold ores represents a significant proportion of gold production worldwide. Gold hosted in sulfide minerals has been proposed to be structurally bound in the crystal lattice as a sulfide-gold alloy and/or to occur as discrete metallic nanoparticles. Using a combination of microstructural quantification and nanoscale geochemical analyses on a pyrite crystal from an orogenic gold deposit, we show that dislocations hosted in a deformation low-angle boundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulative trace-element enrichment in the dislocations is 3.2 at% higher compared to the bulk crystal. We propose that trace elements were segregated during the migration of the dislocation following the dislocation-impurity pair model. The gold hosted in nanoscale dislocations represents a new style of invisible gold.

Geological, mineralogical and geochemical characteristics of Mississippian K-bentonites from southern Turkey: A correlation with coeval tephras from Gondwana-derived terranes

Five thin (5–15 cm) and yellowish green tephra (K-bentonite) layers are exposed along two sections in the south of Akşehir (Konya, southern Turkey) within the Early Carboniferous (Mississippian) red dolomite-dolomitic limestone succession of the Sultandağ Unit in the central Tauride Belt. K-bentonites exhibit typical porphyritic texture with euhedral plagioclase phenocrysts replaced by fine-grained white K-micas and sparry dolomite crystals. The intensively sericitized volcanic matrix has vitrophyric texture and contains hydroxidized euhedral pyrite crystals. Illite is the only clay mineral in all K-bentonite samples. Non-clay minerals are dolomite, calcite, quartz, pyrite, rutile and zircon. Illites have sheet-like flakes (1–4 μm sizes) showing well-developed orientations and partly slaty cleavage. Illite Kübler index values (KI, Δ°2θ) of K-bentonites reflect high-grade diagenetic to anchizonal conditions indicating temperatures that reached up to 200 °C. Illite d060 values (1.4990–1.5007 Å) corresponding to dioctahedral composition display differences in coexisting slates and shales of the same age. Illite polytypes are characterized by the association of 2M1, 1M and 1Md. The presence of 1M illites is indicative of a volcanic (tephra) origin. Immobile element compositions suggest that tephras have been derived from trachyandesitic volcanism. K-bentonites exhibit some differences from the coeval pelitic occurrences in the Hadim-Bozkır areas, S Turkey, and the Late Devonian-Early Carboniferous K-bentonites in Bartın and Zonguldak areas of the Istanbul-Zonguldak Terrane, NW Turkey. A geochemical evolution of the studied K-bentonites suggests that they were possibly originated from the Early Carboniferous (Mississippian) intra-continental back-arc magmatism at the northern margin of Gondwana-derived Tauride-Anatolide microplate.

Stranger than a scorpion: a reassessment of Parioscorpio venator, a problematic arthropod from the Llandoverian Waukesha Lagerstätte

AbstractA relatively uncommon arthropod of the Waukesha lagerstätte, Parioscorpio venator, is redescribed as an arthropod bearing a combination of characters that defy ready classification. Diagnostic features include sub‐chelate ‘great appendages’, a lack of antennae, multiramous anterior trunk appendages, filamentous fan‐like rear trunk appendages, and apparently thin and poorly preserved pleural fields. Phylogenetic analysis resolves this organism as basal to crown‐group Mandibulata and Chelicerata, but its exact placement is inconclusive. Thus, we compare its morphology to several stem groups of arthropods in a discussion of its plausible taxonomic affinities. The examined specimens are probably carcasses and preserve a variety of soft‐tissue details, including muscle blocks in the head, eyes and eye facets, likely ventral nerve cords, a central gut tract and trunk legs with multiple filamentous elements organized into stiff bundles. The preservation habits of P. venator are characterized and compared to previous assessments of Waukesha lagerstätte taxa. Four preservation habits are observed: a phosphatized habit showing flattened to partly three‐dimensional mineralization in francolite; a mouldic habit largely left behind by removed francolite that shows no carbon enrichment despite a darkened colour; sheet‐like or speckled carbonaceous compressions; and scattered pyrite crystals. This redescription highlights both the palaeobiological value of ‘small’ lagerstätten typical of the middle Palaeozoic and the caution that must be taken when interpreting their more enigmatic constituents.

Uranium incorporation in fluorite and exploration of U–Pb dating

Abstract. The age of ore deposits constitutes a decisive element in understanding their formation. Deciphering their precise chronology may be a challenge in the absence of mineral phases that can be dated by conventional geochronometers. Fluorite is very common either as the major or accessory mineral in a wide variety of ores and may provide information regarding the origin and timing of mineralizing fluid flows. In this contribution, we explore U–Pb dating on fluorite crystals from the world-class carbonate strata-bound fluorite ore of Pierre-Perthuis in Burgundy (Morvan massif, France). The uranium distribution within fluorite is mapped using induced fission-track and synchrotron radiation X-ray fluorescence nano-imaging, showing that higher U content is measured in an overgrowth of fluorite (Flog) as a discrete band. Preservation of a micrometer-thick zonation in U, associated with other substituted elements such as Sr, Y, Fe and Zr, implies that neither solid-state diffusion nor dissolution–recrystallization occurred. These U-bearing external fluorite overgrowths contain solid inclusions of about 30 µm globular pyrite crystals with a mean δ34S of −23.6 ± 0.4 ‰V-CDT. We propose that the U incorporation in the fluorite lattice results from the development of a redox front during bacterial sulfate reduction. Flog generation sampled and analyzed by laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) on four different crystals provides identical U–Pb ages within the limits of analytical uncertainty. Considered altogether, these four crystals yield an age estimate of 40.0 ± 1.7 Ma, not corrected for matrix-related elemental fractionation. Our results show that fluorite LA-ICP-MS U–Pb geochronology has potential for dating distinct crystal growth stages, although further research should be conducted to evaluate its accuracy.

Decoupling of Au and As during rapid pyrite crystallization

Abstract Gold (Au) is largely hosted by pyrite in a variety of hydrothermal systems, but the incorporation of Au into pyrite under disequilibrium conditions remains poorly understood. We integrate synchrotron X-ray fluorescence microscopy, electron backscatter diffraction, nanoscale secondary ion mass spectrometry, and laser ablation–multicollector–inductively coupled plasma–mass spectrometry to constrain the processes that sequester Au into zoned pyrite in the hydrothermal cement of breccia ores from the world-class Daqiao orogenic Au deposit, central China. Euhedral pyrite cores with oscillatory and sector zoning, variable δ34S values, and lower Au-As contents than the mantles are attributed to crystallization during oxidation of metal-depleted ore fluids with local variation in fluid conditions. The isotopically uniform colloform mantles are formed by pyrite crystallites separated by low-angle boundaries and are characterized by unusual decoupling of Au and As. Mantle formation is attributed to rapid disequilibrium precipitation from a metal-rich FeS2-supersaturated fluid. Incorporation of Au into the pyrite mantles was facilitated by abundant lattice defects produced by rapid nucleation. Gold-As–poor pyrite rims were deposited from an evolved ore fluid or other metal-depleted fluids. These results show that chemical variations recorded by fine layering within minerals can provide valuable insights into disequilibrium mass transfer and ore formation. The decoupling between Au and As in pyrite mantles indicates that As is not always a reliable proxy for Au enrichment in rapidly crystallized porous pyrite.

First-principles characterisation of structural and electronic properties of some RuO2 crystals

Density functional theory at the level of LDA, GGA, LDA + U, GGA + U and hybrid functionals is applied to investigate structural and electronic properties of three RuO2 crystals. The rutile structure, and the pyrite and flourite modifications of RuO2 are undertaken. The structural properties, enthalpy-pressure curves, electronic states, and Fermi surfaces are presented. The enthalpy-pressure curves show that pressure causes the rutile-RuO2 to transform into pyrite and flourite phases. The pyrtie phase transforms in the fluorite phase. All calculations point out pressure induced rutile → pyrite phase transition in confirmation with the experimental studies. The pyrite → fluorite transition is pointed out by current calculations. The rutile and pyrite crystals are metals while hypothetical fluorite is a semiconductor. All calculations show s that the fluorite has an indirect bandgap in the 0.57–2.96 eV range. The Fermi surface of metallic rutile structure using GGA + U shows improvement over GGA on comparison with the measurement. The GGA + U calculations suggest that rutile → fluorite and pyrite → fluorite metal-insulator transitions are accompanied by orbital ordering.

Textures, trace elements and Pb isotopes of pyrite from the Donggushan tungsten polymetallic deposit, eastern China: Deciphering the source of a skarn tungsten polymetallic deposit

The Donggushan tungsten polymetallic deposit is located near the northern margin of one of the most important ore deposit belts in China; the Middle - Lower Yangtze Metallogenic Belt (MLYB). The source and genesis of mineralization in the Donggushan deposit is poorly understood and is investigated using pyrite textures and geochemistry. Two types of pyrite and two subtypes were recognized, based on the location within the ore body, mineral assemblages, textures and geochemistry. Pyrite 1a occurs in the cores of pyrite crystals in the tungsten orebody, at depth within the deposit, near the intrusive rocks. It is characterized by high Ni and low contents of other trace elements and is overgrown by Py1b which has low Ni and higher trace elements. Pyrite 2a is characterized by moderate As and higher trace elements than those in Pyrite 1. It occurs in the cores of pyrite crystals in the shallow zinc-lead orebody. This pyrite is overgrown by Pyrite 2b characterized by higher trace elements. The trace elements, textures and inclusions in these pyrites suggest a progressive decrease in temperature, with Pyrite 1a forming at 350 °C through to Pyrite 2b forming at < 250 °C. The Pb isotopic compositions in the pyrites indicate that Pb in the deep tungsten ore body was derived from Yangtze upper crust (Dongling basement) but that Pb in the shallower ore body was sourced from the Dabie Orogen lower crust. The data supports a model whereby intra-continental subduction was the main geodynamic process controlling the mineralization of the Donggushan deposit.

Mitigation of the internal p-n junction in CoS2 -contacted FeS2 single crystals: Accessing bulk semiconducting transport

Pyrite ${\mathrm{FeS}}_{2}$ is an outstanding candidate for a low-cost, nontoxic, sustainable photovoltaic material, but efficient pyrite-based solar cells are yet to materialize. Recent studies of single crystals have shed much light on this by uncovering a $p$-type surface inversion layer on $n$-type (S-vacancy doped) crystals, and the resulting internal p-n junction. This leaky internal junction likely plays a key role in limiting efficiency in pyrite-based photovoltaic devices, also obscuring the true bulk semiconducting transport properties of pyrite crystals. Here, we demonstrate complete mitigation of the internal p-n junction in ${\mathrm{FeS}}_{2}$ crystals by fabricating metallic ${\mathrm{CoS}}_{2}$ contacts via a process that simultaneously diffuses Co (a shallow donor) into the crystal, the resulting heavy $n$ doping yielding direct Ohmic contact to the interior. Low-temperature bulk transport studies of controllably Co- and S-vacancy doped semiconducting crystals then enable a host of previously inaccessible observations and measurements, including determination of donor activation energies (which are as low as 5 meV for Co), observation of an unexpected second activated transport regime, realization of electron mobility up to $2100\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$, elucidation of very different mobilities in Co- and S-vacancy-doped cases, and observation of an abrupt temperature-dependent crossover to bulk Efros-Shklovskii variable-range hopping, accompanied by an unusual form of nonlinear Hall effect. Aspects of the results are interpreted with the aid of first-principles electronic structure calculations on both Co- and S-vacancy-doped ${\mathrm{FeS}}_{2}$. This work thus demonstrates unequivocal mitigation of the internal p-n junction in pyrite single crystals, with important implications for both future fundamental studies and photovoltaic devices.

Fluid-rock reactions of the Triassic Taiyangshan porphyry Cu-Mo deposit (West Qinling, China) constrained by QEMSCAN and iron isotope

The multiphase hydrothermal superposition phenomena on porphyry deposits, the mineral complexity and characteristics of the alteration-associated assemblages limit a further determination for the magmatic-hydrothermal evolution in porphyry deposits. The Taiyangshan porphyry Cu-Mo deposit in the northern margin of the Triassic West Qinling Orogenic Belt exhibits a multi-stage, alteration-related quartz sulfide veins that recorded element migration processes during fluid-rock reactions. In this study, we investigated hydrothermal veins in the Taiyangshan deposit focusing on vein compositions and structures as well as the evolution of ore formation, using quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and iron isotope analyses. The formation of the early K-feldspar-biotite-quartz-magnetite-apatite veins is closely associated with the early potassic alteration, and during the fluid-rock reactions, Si, S, Fe, and K migrated in, while Mg, Ca and Na migrated out. K-feldspar-chlorite-pyrite-chalcopyrite-quartz veins correlated to the propylitic alteration, dominantly caused the migration of Si, Fe, S, Ca Mg, Al and K. Quartz-sericite-pyrite veins corresponding to the phyllic alteration processes that occurred in a relatively late stage, during the fluid-rock reactions of which, Si, Fe, S, and K migrated into the system. The δ56Fe whole-rock values of altered porphyries range from 0.08‰ to 0.31‰, and δ56Fe values of pyrite from quartz veins range from 0.30‰ to 0.57‰. Iron isotope compositions of both pyrites of the ore-bearing wall rocks and the quartz-sulfide veins of the Taiyangshan Cu-Mo deposit are generally uniform, which indicate the same source origin between the ore-bearing porphyry and the iron isotope compositions of the metal sulfide in the quartz veins, and additionally highlight that the ore-forming source is closely related to the ore-bearing porphyry. Magnetite precipitated in the early stage of potassic alteration, resulting in the enrichment of the light Fe isotope in the early fluids. Crystallization and precipitation of pyrite mainly occurred during processes of propylitic and phyllic alteration, leading to an absorption of the light Fe isotope, and elsewhere, an enrichment of the heavy Fe isotope in the fluids. During the fluid-rock reactions of propylitic and phyllic alteration, a large amount of S migrated into the veins, thereby promoting the fractionation of Fe isotope. Moreover, the development of the ore-forming fluids enriched with heavy Fe isotope from early potassic- to late phyllic alteration, are indicative of isotopic progressions from the early lithostatic pressure to the late hydrostatic pressure. Our research on the Taiyangshan Cu-Mo porphyry deposit helps to explain the evolutionary processes of the magmatic–hydrothermal system, and improves our understanding of the porphyry metallogenic evolution model.

Concomitant behavior of arsenic and selenium from the karst infillings materials of the fractured carbonate Dogger Aquifer (Hydrogeological Experimental Site, Poitiers, France)

Petrographic and mineralogical analyses combined with sequential extractions and leaching experiments as a function of pH were performed on black clayey sediments fulfilling karsts in the Hydrogeological Experimental Site (HES) of Poitiers (France) to investigate the behavior of arsenic and selenium in a fractured limestone aquifer.Sequential extractions showed that arsenic is mainly associated with pyrite (about 35%) and secondary iron oxyhydroxides (around 13%), along with a substantial exchangeable fraction (about 13%). The soluble fraction and the fraction associated to organic matter are ∼2% and ∼5%, respectively. The distribution of selenium is mainly pyritic (around 39%) or associated with organic matter (about 18%). Its association to secondary iron oxyhydroxides minerals is low (around 2%), whereas its soluble fraction is around 5%.SEM analyses revealed the presence of arsenic “hot spots” into euhedral pyrite crystals surrounded by a halo of iron oxyhydroxides resulting from their alteration, and both are enriched with arsenic. Selenium has a similar pyritic origin but after alteration, it is predominantly associated with organic matter.Despite different distributions, the leaching experiment as a function of pH showed that the mobilization of arsenic and selenium overlapped below pH 2 and above pH 8. The main differences were observed between pH 2 and 8 with a plateau at 5% of released selenium, whereas the amount of mobilized arsenic continuously decreased. The pH-dependence of both elements is attributed to the partial dissolution of pyrite in acidic conditions combined with desorption processes at higher pH values.

ОСОБЕННОСТИ ХИМИЧЕСКОГО СОСТАВА ПИРИТА НАДРУДНОЙ ЗОНЫ ТАСЕЕВСКОГО ЗОЛОТОСЕРЕБРЯНОГО МЕСТОРОЖДЕНИЯ (РОССИЯ, ЗАБАЙКАЛЬЕ)

The relevance lies in the need to have objective data to assess the belonging of pyrite outcrops to the day surface to the supra-ore zones of gold deposits of a shallow gold-silver formation based on objective data on its chemical composition and forms of crystals and their aggregates. The purpose of the study is to determine the chemical composition of pyrite, the forms of its crystals and aggregates as typomorphic signs of their belonging to the supra-ore zone of the deposit of this formation. The object of the research is pyrite of the supra-ore zone of the Taseevskoye deposit, which is unique in terms of gold content and reserves. The subject of the research is the chemical composition and forms of pyrite separation. The method and methodology are presented by optical and electron microscopy with the determination of variations in the chemical composition of pyrite and morphological features of crystals and their aggregates. Results. The determination of the chemical composition of pyrite was carried out and it was found that almost all crystals are characterized by a sulfur deficiency, the formula coefficients of which are 1.801–1.939 with a change in content from 48.39 to 52.62 %. In 70 % of the measured pyrite crystals, arsenic is present from 0.72 to 4.78 % the formula coefficients of which are 0.007 - 0.076. In 2 % of pyrite crystals, along with As, antimony was found in an amount of 0.21 - 0.71 %, formula coefficients are 0.006 - 0.007. The forms of pyrite crystals are cubic, cubopentagondodecahedral, rarely cuboctahedral and subspherical aggregates, less often lenticular and veinlet, predominate. Conclusions. For the first time, original data on the chemical composition of pyrite in the supra-ore zone of the Taseevskoe gold-silver deposit were obtained. It is shown that its feature is high arsenic content (0.72 - 4.78 %), which can be an important typochemical sign for determining the belonging of chalcedony-like quartz containing it to the supra-ore zone of a shallow gold-silver formation