Research subject. Zircon with high contents of P, Y, REE, and As from altered granitic pegmatites forming veins cutting through the amphibolites of the Kharbey metamorphic complex (Polar Urals). Aim. To study the morphological features, internal structure, and chemical composition of zircon, as well as to establish the mechanism of its formation.Methods. The study of zircon was carried out under binoculars, electron microscopes, and a Raman spectrometer. The internal structure of the mineral was analyzed using images obtained in the BSE and CL modes.Results. In granite pegmatites, i.e., biotite-quartz-oligoclase and biotite-microcline-quartz-oligoclase rocks with a high content of Na2O (about 6 wt %), two morphological types of zircon were identified – prismatic pink and long prismatic brown. Prismatic pink varieties have an internal structure and composition characteristic of “classical zircon”, having crystallized from a magmatic melt at temperatures of 700–750°C. In individual cases, such crystals are overgrown with a thin rim of zircon, which has a dark color in CL images with an increased content of Ca, Al, Fe, Na, P, Y, REE, and As. Brown zircons are characterized by growth areas and those with uneven blocky, mosaic, and porous structures that appear dark in CL images. The darkest areas of the mineral (in images in CL and BSE modes) show increased concentrations of P2O5 (up to 6 wt %), Y2O3 (up to 9 wt %), UO2 (up to 4 wt %), ThO2 (up to 3 wt %), REE, FeO (up to 3 wt %), Al2O3 (up to 3 wt %), CaO (up to 3 wt %), and Na2O (up to 1 wt %), with the degree of disorder of the mineral structure (metamictity) increasing. The above elements, as well as, apparently, the hydroxyl group, are included in the structure of zircon according to complex substitution patterns. Crystallization of this type of zircon and the mineral that forms rims around zircon of the first type occurred at the post-magmatic stage of transformation of granites from hydrothermal fluid of high alkalinity at temperatures of 550–600°C. Zircon was subjected to repeated changes under the influence of solutions according to the principle of dissolution–redeposition, which occurred under a decrease in temperatures down to 240–330°C. As a result, zircon acquired a spongy structure, in the pores of which hydrothermal minerals were formed – arsenic pyrite, quartz, monazite, xenotime, chernovite, ankerite, albite, etc.Conclusions. In the granitic pegmatites that form synmetamorphic veins in the amphibolites of the central area of the Kharbei metamorphic complex, three types of zircon are observed: magmatic (zircon of the first type), hydrothermal, and hydrothermally altered (zircon of the second type), differing in morphological features, internal structure, and composition. Judging by the chemical composition of hydrothermal minerals in the rocks, post-magmatic solutions were enriched in Na, P, As, and REE.

Active and abandoned mining sites are significant sources of heavy metals and metalloid pollution, leading to serious environmental issues. This study assessed the environmental risks posed by potentially toxic elements (PTEs), specifically arsenic (As) and antimony (Sb), in the Technosols (mining residues) of the former Pejão coal mine complex in Northern Portugal, a site impacted by forest wildfires in October 2017 that triggered underground combustion within the waste heaps. Our methodology involved determining the “pseudo-total” concentrations of As and Sb in the collected heap samples using microwave digestion with aqua regia (ISO 12914), followed by analysis using hydride generation-atomic absorption spectroscopy (HG-AAS). The concentrations of As an Sb ranging from 31.0 to 68.6 mg kg−1 and 4.8 to 8.3 mg kg−1, respectively, were found to be above the European background values reported in project FOREGS (11.6 mg kg−1 for As and 1.04 mg kg−1 for Sb) and Portuguese Environment Agency (APA) reference values for agricultural soils (11 mg kg−1 for As and 7.5 mg kg−1 for Sb), indicating significant enrichment of these PTEs. Based on average Igeo values, As contamination overall was classified as “unpolluted to moderately polluted” while Sb contamination was classified as “moderately polluted” in the waste pile samples and “unpolluted to moderately polluted” in the downhill soil samples. However, total PTE content alone is insufficient for a comprehensive environmental risk assessment. Therefore, further studies on As and Sb fractionation and speciation were conducted using the Shiowatana sequential extraction procedure (SEP). The results showed that As and Sb levels in the more mobile fractions were not significant. This suggests that the enrichment in the burned (BCW) and unburned (UCW) coal waste areas of the mine is likely due to the stockpiling of lithic fragments, primarily coals hosting arsenian pyrites and stibnite which largely traps these elements within its crystalline structure. The observed enrichment in downhill soils (DS) is attributed to mechanical weathering, rock fragment erosion, and transport processes. Given the strong association of these elements with solid phases, the risk of leaching into surface waters and aquifers is considered low. This work underscores the importance of a holistic approach to environmental risk assessment at former mining sites, contributing to the development of sustainable remediation strategies for long-term environmental protection.

The Shewushan gold deposit, situated in the western extension of Edongnan, marks the westernmost point of the Middle-Lower Yangtze Metallogenic Belt, is an important iron and copper mineralization area in the Middle-Lower Yangtze Metallogenic Belt (MLYB). Previous studies identified the Shewushan gold deposit as a Carlin-type gold ore, but its age remains controversial. This paper examines a set of widely distributed calcite veins in the Shewushan gold deposit. In hydrothermally altered carbonate rocks, calcite is texturally associated with ore-stage jasperoid and disseminated Au-bearing arsenian pyrite, suggesting synmineralization. Calcite veins concurrent with As- and Sb-sulfide mineralization are relatively enriched in middle rare earth elements and heavy rare earth elements. They yield Sm-Nd isochron age of 99.1 ± 2.4 Ma, with similar initial εNd values, ranging from −12.2 to −12.6. These ages are interpreted to record the age of decarbonation and gold deposition of the Carlin-type gold deposits in the MLYB, formed during the late stages of the Yanshanian Orogeny. This interpretation matches the Cretaceous Sporo-pollen fossil assemblages found in the Shewushan gold deposit in red laterite, siliceous rock, and limestone. This provides the first micropaleontological evidence showing that convecting meteoric water played an important role in mineralization. Late Cretaceous Carlin-type gold mineralization further supports the conclusion that late-stage copper-gold mineralization in the MLYB occurred more likely between ∼110 and 100 Ma, rather than ∼123–105 Ma. It also supports that ore-forming events in the MLYB were possibly contemporaneous with other parts of Eastern China and were also controlled by the drifting direction of the subducting Pacific plate. The tectonic and mineralization evolution during the Jurassic-cretaceous in the MLYB happened during the latest Cretaceous to early Tertiary. This indicates that the western extended part of Edongnan is not only a promising Carlin-type gold area, but also a potential target area for Cu–Mo–Au porphyry deposits, W–Cu–Au skarn deposits, Ag–Pb–Cu bearing veins, and epithermal Ag–Au deposits.

Epithermal Au–Ag mineralization of the Pechalnoe deposit is of considerable interest, since it was formed in carbonaceous terrigenous strata of the basement of a volcanic dome structure, at a distance of about 200 km from the border of the Okhotsk-Chukchi marginal continental volcanic belt.The geological structure of the Pechalnoye deposit is two–tiered: quartz-adularic and quartz Au–Agveins are localized in keratinized terrigenous rocks of the lower tier, and quartz rhyolites and komendites of the Pechalninsky strata of the upper tier contain potentially industrial REE mineralization. Productive veins form three zones of sublatitudinal strike, the length of the veins in which is 200‒300 m,sometimes 640, 840 m; average thickness 0.1–3 m, rarely up to 6.2 m, average contents: Ag — 266 g/t,Au — 4.4 g/t. The following mineralogical features of ores have been established: low sulfidity (1–2%);native Ag, low-grade Au, polybasite, and highly selenic acanthite act as productive minerals, in addition,arsenic pyrite, arsenopyrite, pyrrhotite, ferruginous sphalerite, chalcopyrite and marcasite are quite widely developed in ores. The geochemical features of the ores are in good agreement with the mineral composition. The ores are enriched with a fairly wide range of trace elements (according to the rating): Ag, Au, As, Sb, Se, W, Tl, Li, Be, Bi, Cs, Mo, the predominance of light lanthanides over heavy ones has been established, very low Eu/Sm ratios (

The Akcal epithermal Au–Ag deposit, located on the Biga Peninsula, southwestern Turkey, is estimated to contain 32.5 million tonnes of mineralization at an average grade of 1.03 g t −1 Au, equivalent to 1.2 million ounces of gold. Epigenetic mineralization is hosted by the lower Miocene Sapci volcanic suite, a calc-alkaline subduction-related volcanic arc consisting mainly of andesitic and basaltic andesitic rocks, while the coeval Soma Formation is composed of sandstone, claystone, conglomerate, marl-limestone, siltstone and tuffite. Near the base of the volcanic pile, the Karakaya Complex is represented by limestone blocks of various sizes (from a few millimetres to several kilometres) within the Sapci volcanic suite. Short-wave infrared and X-ray diffraction analyses have shown that kaolinite, illite–smectite, montmorillonite, illite, beidellite, gypsum, vermiculite, quartz and chlorite are the most common alteration minerals at Akcal. The alteration mineralogy indicates that the temperature of hydrothermal fluids broadly varied from 150° to 230°C, with a mildly acidic pH range of 5.0–6.5; these conditions accompanied by a lack of alunite suggest low-sulfidation epithermal conditions for the formation of the Akcal epithermal Au–Ag deposit. Petrographic examination and microanalysis by scanning electron microscopy indicate that mineralization at Akcal consists of pyrite and arsenopyrite with lesser hematite, ilmenite, rutile and apatite. The occurrence of pyrite and hematite on cleavage planes of hydrothermal biotite is evidence of replacement of an earlier potassic alteration phase. Further petrographic examination of the basal limestone reveals replacement by sulfides displaying complex colloform intergrowths of pyrite and marcasite. The lack of skarn alteration and marble supports a carbonate-replacement origin for sulfides occurring in the basal limestone and is likely related to epithermal mineralization at Akcal. Geochemical results show that an Au–Ag–As–Sb ± Pb ± Zn signature is dominant across the volcanic-hosted epithermal mineralization and carbonate-replacement occurrences in the basal limestone; the lack of Cu mineralization (<92 ppm) is typical of low-sulfidation epithermal systems. Strong positive correlations between Au and Ag ( r ′ = 0.77), As (0.80) and Sb (0.86) indicate that gold is likely hosted as a sub-microscopic phase within pyrite, arsenian pyrite and arsenopyrite; visible gold (or free gold) and (or) electrum were not identified at Akcal. A model of fluid mixing involving shallow circulating meteoric waters with deeper magmatic-hydrothermal fluids is proposed for the genesis of gold mineralization at Akcal; a positive Au correlation with Mo ( r ′ = 0.74) and Mo concentrations of up to 582 ppm support the involvement of magmatic-hydrothermal fluids in the epithermal system. Boiling as a mechanism for gold precipitation is unlikely given palaeodepths of greater than 250 m (3.0–5.0 MPa) and model temperatures of 150° to 230°C. Mineralogical and textural evidence of boiling, such as adularia, bladed calcite and lattice-texture quartz, have not been observed at Akcal. However, the presence of a silica-rich blanket with chalcedony (at the Akcal surface) and having formed near the palaeo water table may indicate some involvement of CO 2 -rich steam-heated waters, which could suggest some influence from boiling at depth, although evidence of boiling has not been confirmed in samples from Akcal. Precipitation of gold during the fluid mixing process would have occurred through a shift to higher f O 2 conditions and destabilization Au–thiosulfide complexes. This shift towards higher f O 2 is supported by the occurrence of gypsum and hematite with gold mineralization at depth.

Abstract Gold deposits of the Nadaleen trend in central Yukon host over 1.7 million ounces (Moz) of Au and share many characteristics in common with Nevada’s Carlin-type deposits, including similar host rock types, structural setting, alteration, and geochemistry, as well as the occurrence of gold in hydrothermal arsenian pyrite. We examined the textures, minor and trace element geochemistry, and δ34S signatures of precursor pyrite and hydrothermal pyrite overgrowths in samples grading over 35 g/t Au from the Sunrise and Conrad deposits. In the Osiris limestone at Sunrise, hydrothermal pyrite occurs as rims ranging from <1 to 5 µm overgrowing subhedral to euhedral sedimentary pyrite grains that are 20 to 100 µm in diameter; as rims (<1 to 3 µm thick) of hydrothermal pyrite that cement together the individual aggregates (measuring <1 to 5 µm) in framboidal pyrite; and as disseminated hydrothermal pyrite grains (<1 µm) that may be single stage. The hydrothermal pyrite in our Sunrise samples contains up to 45 ppm Au, 29 ppm Cu, 1,053 ppm As, and 15 ppm Ag, with δ34S compositions that are 1 to 8‰ higher than the sedimentary pyrite. The hydrothermal pyrite is zoned at the nanoscale, with the highest Au concentrations typically in the outermost portion of the rims. In the Conrad gabbroic dike, hydrothermal pyrite occurs as rims ranging from <1 to 5 µm overgrowing earlier pyrite grains that are 5 to 100 µm in diameter. The inner rims of the hydrothermal pyrite contain up to about 20 ppm Au, 900 ppm As, 60 ppm Ag, and 50 ppm Cu, whereas the outer margins of the hydrothermal pyrite contain up to about 670 ppm Au, 23,400 ppm As, 385 ppm Ag, and 115 ppm Cu. Relatively coarse hydrothermal rims (up to 5-µm) occur on the coarsest grains of precursor pyrite, suggesting that the substrate partially controls the texture of the hydrothermal pyrite, potentially due to the availability of Fe during sulfidation. The δ34S plateau values of the hydrothermal rims range from 1.2 to 11.0‰. Bayesian stable isotope modeling shows that the δ34S compositions of the hydrothermal pyrite can be generated by mixing the locally present sedimentary rocks with locally present magmatic sulfur. The modeling indicates that additional sources are not required, although they cannot be ruled out. At high Au concentrations, the modeling shows that most of the sulfur in the hydrothermal pyrite comes from a magmatic source, potentially from buried plutons visible as aeromagmetic anomalies. The modeling does not differentiate between whether (1) these magmatic rocks contributed sulfur and metals during passive leaching by an amagmatic hydrothermal fluid or (2) cooling magmas exsolved a sulfur- and metal-bearing fluid that led to magmatic-hydrothermal mineralization. We favor the latter interpretation, since the available geochronological evidence suggests that mineralization on the Nadaleen trend occurred during or shortly after Late Cretaceous emplacement of volumetrically limited, mantle-derived gabbroic dikes. Collectively, the evidence supports a Carlin-type origin for the gold deposits on the Nadaleen trend. Continued study is needed to link site-specific characteristics and processes to the regional metallogenic setting.

The Shuiyindong deposit is one of the ultralarge Carlin-type gold deposits in Southwest Guizhou Province, China. Gold mineralization mainly occurs in the Permian Longtan Formation and the early Triassic Yelang Formation. It is controlled by both strata and faults. Detailed studies of the mineralogy and geochemistry characteristics of the Shuiyindong deposit are conducted to investigate the ore-forming process. Arsenian pyrite and arsenopyrite are the main Au-hosting minerals. Three types of pyrite can be recognized, including euhedral and subhedral pyrite, framboidal pyrite, and bioclastic pyrite. The euhedral and subhedral pyrite is the main Au-hosting type. The Au appears as a solid solution (Au+) and natural nanoscale gold (Au0) in the sulfide minerals. The Co/Ni ratios of sulfides (0.07-3.13) reveal that the ore-forming fluids were mainly affected by hydrothermal activity, but magmatic activity cannot be excluded. Organic matter in the ores is abundant (0.11-3.04%), which might provide sulfur for pyrite and favor an increase in the porosity and permeability of the host rocks by releasing organic acids. The REE and trace element results suggest that halogens (F and Cl) were contained in the reducing magmatic hydrothermal fluids. The sulfur isotopic data (from -8.64‰ to 27.17‰) suggest that the source of sulfur is complicated and is probably a combination of a magmatic source, the reduction of marine sulfate, and bacterial sulfate reduction. The Pb isotopic data of the sulfides indicate that Pb is from a mixture of crust and mantle sources. The obvious enrichment zones exist along the boundary faults in the geochemical map of As, implying that As may originate from the deep crust and then move to the strata with basinal fluids. By combining these results, it can be inferred that the ore-forming fluids were a mixture of basinal and deep source fluids. A probable ore-forming model of the Shuiyindong gold deposit is established.

Abstract Understanding the complex interplay between the processes of mineral crystallization and the incorporation of trace elements, particularly in economically significant deposits like Carlin-type gold systems, is essential for unraveling geological processes. This study investigates the microscale to nanoscale texture and composition of weakly deformed arsenian pyrite from the Shangmanggang Carlin-type Au deposit in Southwest China, employing advanced techniques such as scanning transmission electron microscopy and atom probe tomography. Trace element–rich oscillatory zones in the pyrite are characterized by ~30-nm-thick bands enriched in As, Au, and Cu. Cu, As, Sb, Pb, Hg, and Tl are distributed heterogeneously and form clusters and discontinuous planar features on the outer edge of As-rich oscillatory bands. Discontinuous planar features, nucleating from trace element–enriched clusters, are oriented approximately in line with the direction of epitaxial growth. The nanoscale epitaxial growth zones are likely the result of the incorporation of impurity defects coupled with diffusion-limited self-organization and fluctuations in fluid composition. Arsenic-induced lattice distortion facilitates surface adsorption of dopant trace metals, which leads to “unstructured” impurities (Sb, Pb, Hg, and Tl) clustering locally in misfit crystal defects. The transition from homogeneous element distribution in As-rich bands to clustered trace elements suggests a Stranski-Krastanov growth process. Discontinuous planar features may represent the propagation of crystal defects locally and the further incorporation of trace elements. Our study provides insights into the factors governing the heterogeneous incorporation of trace elements, particularly Au, into pyrite during epitaxial growth.

Abstract Carlin-type gold deposits are renowned for hosting gold in finely zoned hydrothermal pyrite, but the characteristics of this zonation are incompletely understood. We use new depth profile techniques in nanoscale secondary ionizing mass spectrometry (NanoSIMS) to characterize the Au, Cu, As, Ag, and δ34S zoning in auriferous pyrite from eight gold deposits in Nevada: Carlin-type pyrite from Carlin, Deep Star, Beast, Turquoise Ridge and Getchell; Eocene dike pyrite from Beast, Betze Post, and Deep Star; and auriferous hydrothermal pyrite from the Lone Tree distal disseminated gold deposit and the Red Dot sedimentary rock-hosted deposit at Marigold. All of the hydrothermal pyrite types are characterized by hundreds of nanoscale zones with varied Cu, As, Ag, and Au. Most samples show concentric zoning, although patchy alteration or sectoral zoning can also be present. The number, sequence, and thickness of the zones is inconsistent throughout the data set. Correlations among the trace and minor elements vary among pyrite types, deposits, and between grains in the same sample. In different grains from the same sample, the Pearson correlation between Au and As varied from strongly negative (–0.7) to no correlation (0.0) to strongly positive (1.0). The sedimentary and magmatic precursor pyrite grain cores contain minor Au, Ag, As, and Cu, as well as Sb where analyzed. These trace elements are universally more enriched in hydrothermal pyrite overgrowths, except for Ag, which can be more enriched in some of the grain cores of magmatic origin. The maximum trace element concentrations in our Carlin-type hydrothermal pyrite are 2,600 ppm Cu and 17,290 ppm As (Turquoise Ridge); 2,050 ppm Ag (Beast); and 1,960 ppm Au (Deep Star). The maximum values from the entire sample suite are in Lone Tree arsenian pyrite with 70,080 ppm As; 9,790 ppm Ag; and 2,022 ppm Au; and Red Dot hydrothermal pyrite with 26,700 ppm Cu. Transmission electron microscopy data indicates that the Au occurs as nanoparticles at Red Dot. We combine new and previously published NanoSIMS δ34S data to show that Carlin-type pyrite grains with high δ34S sedimentary pyrite grain cores have rims with lower δ34S, whereas those with isotopically negative δ34S sedimentary pyrite grain cores have positive δ34S in the rims, due to mixing between sulfur in the sedimentary pyrite and sulfur from a magmatic-hydrothermal fluid. At high Au content, the Carlin-type hydrothermal rim δ34S values are close to the mean (7.1‰) of Tertiary magmas in the Great Basin, and within the range of Eocene mineralizing magmatic-hydrothermal fluids in the region (pyrite in equilibrium with this fluid has a δ34S of 0 to 8.8‰). At Lone Tree the δ34S values of the hydrothermal rims are slightly greater than the pyrite grain cores, and at Red Dot the rims have δ34S that is lower than the cores. The presence of As assisted with incorporation of Au in the Carlin-type pyrite, although Au was inconsistently available during pyrite growth. Our data show a wide range of As/Au molar ratios, indicating that the gold occurs as both Au+1 and Au(0) in different zones of the same grain. Variation in the form of Au may have resulted from fluctuations in the saturation state of Au, temperature changes during pyrite growth, or the presence of electrical potential differences caused by heterogeneous As and Cu concentrations in the pyrite. Local-scale mixing with meteoric fluids resulted in successive hydrothermal pyrite growth zones, iteratively upgrading the Au content of the pyrite to achieve the large Au endowment of the deposits. Despite many commonalities between Carlin-type hydrothermal pyrite and distal disseminated hydrothermal arsenian pyrite at Lone Tree, the metal sources or processes of fluid evolution are not identical. Hydrothermal arsenian pyrite at Red Dot has characteristics intermediate between distal disseminated and Carlin-type pyrite.

Pyrite is the most common sulfide mineral in hydrothermal ore-forming systems. The ubiquity and abundance of pyrite, combined with its ability to record and preserve a history of fluid evolution in crustal environments, make it an ideal mineral for studying the genesis of hydrothermal ore deposits, including those that host critical metals. However, with the exception of boiling, few studies have been able to directly link changes in pyrite chemistry to the processes responsible for bonanza-style gold mineralization. Here, we report the results of high-resolution secondary-ion mass spectrometry and electron microprobe analyses conducted on pyrite from the Brucejack epithermal gold deposit, British Columbia. Our δ34S and trace element results reveal that the Brucejack hydrothermal system experienced abrupt fluctuations in fluid chemistry, which preceded and ultimately coincided with the onset of ultra-high-grade mineralization. We argue that these fluctuations, which include the occurrence of extraordinarily negative δ34S values (e.g., -36.1‰) in zones of auriferous, arsenian pyrite, followed by sharp increases of δ34S values in syn-electrum zones of nonarsenian pyrite, were caused by vigorous, fault valve-induced episodic boiling (flashing) and subsequent inundation of the hydrothermal system by seawater. We conclude that the influx of seawater was the essential step to forming bonanza-grade electrum mineralization by triggering, through the addition of cationic flocculants and cooling, the aggregation of colloidal gold suspensions. Moreover, our study demonstrates the efficacy of employing high-resolution, in situ analytical techniques to map out individual ore-forming events in a hydrothermal system.

The mechanisms responsible for invisible gold enrichment driven by coupled dissolution-reprecipitation reaction (CDR) are debated. Here we report the micro- to nano-scale textures of arsenian pyrite in a high-grade (>10 g/t) gold ore from the Chang’an deposit to trace the gold enrichment process. Our study records a CDR-driven evolution of mineral growth from an As-rich, Au-poor pyrite core, with numerous fine arsenopyrite inclusions, to an inclusion-free, As-Au-rich oscillatory pyrite rim. The reaction occurred at ~260 °C under 4.7 to 5.8 pH and –36.6 to –32.9 logfO2 conditions. The elevated As but depleted S contents in the pyrite core indicate a combined elevation of S fugacity and solubility of Au. The coprecipitation of arsenopyrite inclusions in the core caused a depletion of S fugacity to –13.8 ~ –11.7, triggering Au enrichment in the rim. This non-unique process has the potential to explain the upgrade of invisible Au in arsenian sulfides, worldwide.

Minerals and/or their compositions (substituted minor elements) can become metastable in changing conditions or if formed outside of equilibrium. Unstable minerals undergo chemical and/or structural modifications at rates determined by re-equilibration processes, such as diffusion, coupled dissolution-reprecipitation and recrystallization. However, re-equilibrated domains with sharp contacts that lack porosity or deformation microstructures are difficult to reconcile with previously documented processes. In this study, we investigate the mechanism by which Au-rich pyrite re-equilibrates to Au-poor pyrite. Gold and As-rich {100} oscillatory bands are truncated by Au-As-poor pyrite along {100} re-equilibration interfaces. At the nanoscale, dislocations oriented consistently along <100>, are enriched in Ni, As, Cu, Sb, Pb, and Au. Dislocations are located at the re-equilibration interfaces between the Au-As-rich and Au-As-poor pyrite. Quantitative crystallographic orientation maps do not show the presence of deformation-related boundaries along the re-equilibration interfaces, indicating that the dislocations are not deformation-related but are misfit dislocations to accommodate for lattice stain between As-rich and As-poor pyrite. The co-location of steps along the re-equilibration interfaces and dislocations suggests that pyrite can re-equilibrate by the migration of dislocations. The process is likely driven by lattice strain minimisation induced by As impurities. Element transport is achieved by a two step process with (1) capture of impurities by dislocation-impurity pair diffusion during the migration of dislocations and (2) pipe diffusion along the dislocation network towards the exterior of the crystal. We propose that re-equilibration of Au-rich arsenian pyrite, and the resulting remobilisation of Au, can operate through a dislocation-mediated interfacial re-equilibration (DMIR) process. This new mechanism may be active in a range of mineral reactions, particularly in metamorphic settings where limited fluid availability precludes interface-coupled dissolution-reprecipitation processes.

Abstract The Stibnite-Yellow Pine district of central Idaho was mined from the early 1900s until the 1990s, extracting gold, antimony, tungsten, and mercury from veins and disseminated and replacement ores in mountainous terrain along the headwaters of the Salmon River. Mining during the two World Wars supplied critical antimony and tungsten to the war efforts. Recent exploration has delineated mineral resources of over 187 metric tons Au, 274 metric tons Ag, and 93,000 metric tons Sb. Mineralization is hosted in Cretaceous Idaho batholith granitic rocks and a sequence of Neoproterozoic to Paleozoic metasedimentary strata of carbonate and siliciclastic compositions. Historic studies outlined some of the complex paragenesis but debated the absolute age of mineralization. New petrographic and geochronologic work documents a sequence of five hydrothermal events in the Stibnite-Yellow Pine district. Event 1 is related to Cretaceous magmatic and hydrothermal activity and includes events ranging in age from 86 to 75 Ma, including sparse quartz-molybdenite veins dated at 86 Ma. Disseminated gold mineralization of Event 2 is associated with sericitic alteration and sulfidation of igneous biotite and replacement of plagioclase by potassium feldspar, largely in granodiorite. Gold is present in zoned arsenian pyrite in both disseminated ores and in crosscutting carbonate-quartz veins containing pyrite and arsenopyrite. The large Yellow Pine deposit, localized at a dilatant bend in the Meadow Creek fault, hosts such disseminated and vein gold. Event 2 is interpreted as the major gold-forming event; 40Ar/39Ar ages of sericite and potassium feldspar alteration range, respectively, from 70 to 59 and 66 to 56 Ma. The long span is interpreted to reflect the age of gold mineralization and local overprinting by Event 3. A narrower range from 66 to 61 Ma is interpreted to date the peak of gold mineralization and alteration. Event 3, tungsten mineralization with scheelite, is texturally later than Event 2 gold and localized along the Meadow Creek structure. Event 3 scheelite has been dated by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb methods at 57 Ma. Event 4, best developed in the West End area, includes gold-silver–bearing quartz-carbonate-pyrite veins and breccias with epithermal textures and potassium feldspar alteration envelopes. Adularia from Event 4 yields 40Ar/39Ar plateau ages of 52 to 51 Ma. Event 5 antimony and mercury mineralization consists of stibnite veins and breccia cements at the Yellow Pine and Hangar Flat deposits as well as cinnabar veins and replacements at the peripheral Fern and Hermes deposits; it is constrained by an LA-ICP-MS U-Pb date on scheelite (ca. 47 Ma) intergrown with stibnite. Minor propylitic and argillic alteration is evident in 47 Ma igneous dikes, which do not contain economic mineralization. The Au-Sb-W ores in the Stibnite-Yellow Pine mining district formed over an extended time period from about 70 to 45 Ma in multiple pulses that were localized along the Meadow Creek fault zone. Each event corresponds to episodes of magmatism and/or hydrothermal activity in the region. Insignificant Event 1 skarn and molybdenum mineralization is similar in age to the Thompson Creek porphyry molybdenum deposit in central Idaho. Event 2 gold mineralization occurred during a magmatic gap in central Idaho but was synchronous with magmatism in the Bitterroot lobe further north; Event 2 is similar in age to orogenic gold-arsenic mineralization at the Beartrack mine in eastern Idaho. Event 3 scheelite mineralization coincides with tungsten mineralization at the Quartz Creek deposit, late magmatism in the Bitterroot lobe, and rapid exhumation of the Atlanta lobe of the Idaho batholith. Event 4 gold mineralization is coincident with the onset of regional Challis magmatism and extension. Event 5 antimony and mercury mineralization is time-equivalent to epithermal gold mineralization in the nearby Thunder Mountain volcanic field and the peak of Challis magmatism.

As a result of generalization of geophysical studies, petro-paleomagnetic and structural-geomorphological analyses, as well as thermodynamic modeling, some features of ore formation in the hydrothermal system of Cape Fiolent (southwestern Crimea) under island arc conditions were revealed. It has been established that the main transformations of rocks of the Middle Jurassic igneous complex of Cape Fiolent occurred under the influence of hydrothermal fluids during the introduction of felsic intrusions during 168–140 Ma. The zones contain sulfide mineralization, the main minerals of which are pyrite, sphalerite, pyrrhotite, galena, chalcopyrite and arsenic pyrite. In the central parts of the hydrothermal alteration zone, massive sulfides are strongly weathered; these zones contain many secondary sulfates. In the marginal parts of hypergenic limonite, yellow-brown goethite prevails in the oxidation zone, yellow jarosite in the center, which is probably due to the large amount of pyrite in the center of the system, which gave more sulfuric acid during oxidation. The presence of native sulfur in the section testifies to the mixing of the acidified hydrothermal solution with seawater. Complex petro-paleomagnetic and magnetometric studies have shown that contact changes and transformation of the contrasting basalt-rhyolite formation occurred along the NNW-trending faults.

The Badran orogenic gold deposit is located in the Yana-Kolyma belt, Eastern Siberia; it has proven reserves of ∼9.3 t of gold and an average grade of 7.8 ppm. The total gold production at the Badran deposit since 1984 amounts to ∼34 t. Despite many years of study, the origin of the gold deposits of the Yana-Kolyma metallogenic belt, one of the world’s largest belts, and the Badran deposit is controversial. Synthesis of regional geology and geology of the Badran deposit, fluid inclusion analysis, mineral and (S-O) isotope chemistry defines the genetic model, origin of fluids, and source of metals in the evolution of the ore-forming system, equivalent to other orogenic gold deposits on the margin of the Siberian craton. The deposit is localized in the Upper Triassic clastic rocks and is controlled by the NW-trending thrust. Polyphase mineralization occurs as disseminated arsenian pyrite and arsenopyrite ores with invisible gold, quartz veins with native gold and Fe, Pb, Zn, Cu sulfides and sulfosalts of orogenic type, and locally post-ore Ag, Sb-bearing minerals and Hg epithermal features. The quartz veins with native gold were formed from low-medium saline (1.5–10 wt% NaCl eq.) aqueous-carbonic fluids boiling at temperatures of 290°C to 210 °C and pressures of 300–250 to 125 bar. The δ34S values of pyrite and arsenopyrite vary from −1.1‰ to +2.4‰, with an average of +0.4‰; the δ18О of quartz from +15.1‰ to +17.5‰ at constant δ18ОH2O about +7.5‰ (±1.0‰). High contents of As (up to 2.4 wt%) and Co/Ni ∼ from 0.3 to 9.9 in pyrite of proximal alteration are typical for hydrothermal systems. The results obtained confirm that the ore-forming fluids did not have a single origin, but were formed from a mixture of subcontinental lithospheric mantle and metamorphic sources. The subcrustal lithospheric mantle was fertilized in the time preceding mineralization (Late Jurassic) and was derived directly from the down-going subduction slab and overlying sediment wedge at the closure of the Oymyakon Ocean.

Gold-arsenic sulfide type mineralization has first been revealed for the Gorny Altai region. The mineralized zones associate with lamprophyre dikes and are controlled by steeply dipping faults transecting the Vendian-Cambrian terrigenous-carbonate-volcanogenic sequence. The ore bodies are localized in horizons of basic metavolcanic rocks, enclosed between limestone layers. The near-ore metasomatites are composed of the paragenesis of albite, magnesiosiderite, and sericite. The gold content is related to fine dissemination of arsenic pyrite and arsenopyrite. Gold in the primary and oxidized ores is&#x0D; submicroscopic and finely dispersed. The sulfur isotopic composition of the sulfides (δ34SCDT = 0 ‰) indicates an association of the mineralization with a deep-seated magmatic source. The Kokpatas gold deposit in western Uzbekistan is considered as an analogue.

Abstract Gegalaw is one of the gold deposits situated in the Sagaing Fault zone, which marks the boundary between the Central Myanmar Basin to the west and the Mogok metamorphic belt to the east. The gold mineralization is mainly confined along northeast striking faults between carbonaceous silty limestone of the Kywethe Chaung Formation and carbonaceous and calcareous shale of the Eocene Male Formation. Hydrothermally induced dissolution breccia associated with progressive silica replacement increase near the northeast trending faults. The hydrothermal alteration associated with the mineralization is silicification in addition to illite‐kaolinite argillic alteration, of which illite is dated by the K‐Ar method as 40.26 ± 0.91 Ma. Sulfide minerals contained in the ores are mainly pyrite and arsenopyrite, with minor to trace amounts of marcasite, chalcopyrite, and sphalerite, among which pyrite and arsenopyrite are the hosts of gold. Pyrite and arsenopyrite are divided into diagenetic and hydrothermal stages, based on their mineral textures. Diagenetic stage pyrites are framboidal (Py1A) and coalescent framboidal (Py1B). Those of the hydrothermal stage are zoned pyrite with porous cores (Py2) and compact rims (Py4). In addition, euhedral homogeneous pyrites with or without compact rim are present (Py3). Some pyrites show pseudomorphs of ilmenite (Py3i) and carbonate minerals, possibly ankerite (Py3c). Arsenopyrites are also divided into porous, sooty arsenopyrite (Apy1), and compact arsenopyrite (Apy2) that co‐exists with Py3 and Py4 pyrites. Electron probe microanalysis for sulfides indicates that Py1A and Py1B are deficient in Fe while Py2, Py3 and Py4 are deficient in S compared to stoichiometry of pyrite. Such deficiencies are compensated by metal ions such as Co2+ and Ni2+ for Py1A, As2+ for Py1B, and As1− for Py2, Py3, and Py4. Arsenic equilibrium temperature of Apy2‐Py3 pairs shows a mode range of 250–270°C. Detected gold concentrations (~650 ppm) in arsenian pyrite (Py2, Py3, and Py4) indicate that gold in pyrite are below the solubility, suggesting that gold is present as solid‐solution in pyrite. Sulfur isotopic values of the pyrites in the unaltered host rocks are −18.9‰ and −8.9‰, and those of the pyrites from the hydrothermal altered rocks range from −17.3‰ to −0.5‰. These similar negative δ34S ranges of the pyrites suggest that the source of sulfur is dominated in biogenic reduction origin. Nevertheless, the two relatively heavier sulfur isotopic values (−4.9‰ and −0.5‰) suggest a contribution of an external hydrothermal fluid source. The research results indicate that gold mineralization occurred by infiltration of auriferous reduced hydrothermal fluids into the carbonaceous silty limestone and carbonaceous and calcareous shale through northeast trending secondary faults of the dextrally moving Sagaing Fault system and the fluids reacted with framboidal pyrite as well as iron‐bearing minerals in the host rocks to precipitate gold‐bearing arsenian pyrite. The age of hydrothermal illite suggests a link to the right‐lateral strike‐slip movement of the Sagaing Fault that is related to the collision of the Indian and Asian plates.

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.

Origin and evolution of gold-bearing fluids in a carbon-rich sedimentary basin: A case study of the Algamarca epithermal gold-silver-copper deposit, northern Peru

The Zhen’an-Xunyang Basin is a late Paleozoic rifted basin with a series of Au-Hg-Sb deposits that have been found, mostly along the Nanyangshan fault. Recently discovered large- and medium-sized gold deposits such as the Xiaohe and Wangzhuang deposits exhibit typical characteristics of Carlin-type gold deposits. Therefore, it is imperative to select a typical deposit for an in-depth study of its metallogenic mechanism to support future prospecting efforts targeting the Carlin-type gold deposits within the area. Based on detailed field investigation and microphotographic observation, four ore-forming stages are identified: I, low-sulfide quartz stage, characterized by euhedral, subhedral pyrite, and fine veins of quartz injected parallel to the strata; II, arsenopyrite–arsenian pyrite–quartz stage, the main mineralization stage characterized by strongly silicified zones of reticulated quartz, disseminated arsenopyrite, fine-grained pyrite; III, low-sulfide quartz stage, characterized by large quartz veins cutting through the ore body or fine veins of quartz; Ⅳ, carbonate–quartz stage, characterized by the appearance of a large number of calcite veins. In situ analysis of trace elements and S isotopes of typical metal sulfides was carried out. The results show significant variations in the trace element compositions of metal sulfides in different stages, among which the main mineralization stage differs notably from those of the Au- and As-low surrounding strata. In situ S isotope analysis reveals δ34S values ranging from 15.78‰ to 28.71‰ for stage I metal sulfides, 5.52‰ to 11.22‰ for stage II, and 0.3‰ to 5.25‰ for stage III, respectively, revealing a gradual decrease in S isotopic values from the pre-mineralization stage to post-mineralization stage, similar to those observed in the Xiaohe gold deposit. These features indicate a distinct injection of relatively low 34S hydrothermal fluids during the mineralization process. The element anomalies of the 1:50,000 stream sediment in the region revealed ore-forming element zonation changing in W→Au (W)→Hg, Sb (Au) anomalies from west to east, manifested by the discovery of tungsten, gold, and mercury–antimony deposits in the area. Moreover, conspicuous Cr-Ni-Ti-Co-Mo anomalies were observed on the western side of the Wangzhuang and Xiaohe gold deposits, indicating a potential concealed pluton related to these deposits. These lines of evidence point to a magmatic–hydrothermal origin for the Carlin-type gold deposits in this area. Furthermore, hydrothermal tungsten deposits, Carlin-type gold deposits, and low-temperature hydrothermal mercury–antimony deposits in this region are probably controlled by the same magma–hydrothermal system.