As-rich Pyrite Research Articles

Gold mineralisation by pyrite recrystallisation and arsenopyrite sequestration in the Jiaochong Au deposit, Tongling ore district, eastern China: Implications for the formation of stratabound ore deposits

The Middle–Lower Yangtze River Valley metallogenetic belt (MLYRB) is a large and economically valuable porphyry–skarn Cu–Au ore belt in China with a total reserve of >600 tonnes of gold. Orebodies in the MLYRB are commonly stratabound and extend for several tens of metres in length and locally for more than 1000 m. However, the origin of the stratabound orebodies is uncertain, with proposals including a magmatic or sedimentary genesis, and the specific processes by which these gold deposits were formed are also unclear. Here we report a study of the representative stratabound Jiaochong Au deposit in the Tongling ore district, where stratified gold orebodies are hosted at the Triassic -Permian strata boundary. A detailed field survey, petrologic observations, in situ sulfur isotopic analysis, and major and trace element compositional analysis of gold and sulfide grains were carried out to determine how the gold was enriched. Field survey shows that precipitated sulphides exhibit obvious mineral and textural zonation from wall-rock to orebody centres as follows: Zone 1, colloform pyrite (Py1) ± sphalerite ± chalcopyrite → Zone 2, euhedral pyrite (Py2) ± pyrrhotite + arsenopyrite ± colloform pyrite (Py1) → Zone 3, pyrrhotite + arsenopyrite. From zone 1 to zone 3, the sulphides were progressively phase-transformed, with the earlier Py1 being gradually consumed and converted into Py2, which finally became pyrrhotite. The gold deposition occurs in Zones 2 and 3 and forms two generations of gold grains, one group being completely enclosed within euhedral Py2 with low As contents (<0.1 wt%) and another group infilling fractures between arsenopyrite and pyrrhotite or occurring as inclusions within arsenopyrite with high As contents (0.4 to 1.4 wt%). The comparable S-isotope compositions of the ore sulfides to those of Mesozoic intermediate-felsic intrusions and the veining characteristics of colloform pyrite in the Tongling ore district suggest that Mesozoic magmatic-hydrothermal fluids, rather than Carboniferous sedimentary rocks as usually thought, provided the principal ore-forming materials. The gold precipitation in this study followed two continuous processes: recrystallisation and arsenopyrite sequestration; in the first process, Au is remobilised from the pyrite structure at higher temperatures up to recrystallisation of fine-grained, colloform morphologies into euhedral crystals and formation of Au-bearing metallic particles (Au1); with the ongoing recrystallisation more As is released and accumulated into the equilibrated fluids, forming As-rich pyrite or arsenopyrite and a second period of gold deposition (Au2). The continuous sulphide zonation texture and phase transformation, similar to Jiaochong, have been observed in other gold deposits in the MLYRB, and our gold enrichment processes may also give an explanation for the formation of these stratabound gold deposits.

Gold and Arsenic in Pyrite and Marcasite: Hydrothermal Experiment and Implications to Natural Ore-Stage Sulfides

Hydrothermal synthesis experiments were performed in order to quantify the states of Au and As in pyrite and marcasite. The experiments were performed at 350 °C/500 bar and 490 °C/1000 bar (pyrite–pyrrhotite buffer, C(NaCl) = 15 and 35 wt.%). The synthesis products were studied by EPMA, LA-ICP-MS, and EBSD. The EPMA was applied for simultaneous determinations of Au, As, Fe, and S, with a Au detection limit of 45–48 ppm (3σ). The analyses were performed along profiles across zonal grains. The concentrations of As and Au up to 5 wt.% and 8000 ppm, respectively, were determined in pyrite and up to 6 wt.% and 1300 ppm in marcasite. In pyrite, the Au concentration decreases with fluid salinity and temperature increases. Strong positive Au–As correlation and strong negative Au–Fe and As–S correlation were identified in pyrite. Comparison of the correlations with theoretical lines implies Au–As clustering. The cluster stoichiometry is inferred to be [AuAs10]. Most probably, As in pyrite presents in the form of clusters and in the As→S solid solution. Incorporation of Au in As-rich pyrite can be controlled by the reductive deposition mechanism. In marcasite, the concentrations of Au are not correlated with the As content. The [AuAs10] clusters enrich the {210}, {113}, and {111} pyrite faces, where the former exhibits the highest affinity to Au and As. The affinity of {110} and {100} forms to Au and As is lower. Implication of the experimental results to data for natural auriferous pyrite shows that the increase of Au content at C(As) &gt; 0.5–1 wt.% is caused by the incorporation of the Au-As clusters, but not because of the formation of Au→Fe solid solution. Therefore, the concentration of “invisible” gold in pyrite is dictated solely by the hydrothermal fluid chemistry and subsequent ore transformations.

REE-bearing minerals in sediment-hosted stratiform pyrite mineralization zones of the Wiśniówka area (Holy Cross Mts., Poland)

There are two arsenical pyrite (As-FeS 2 ) mineralization zones cropping out in the Podwiśniówka and Wiśniówka Duża quarries where quartzites and quartzitic sandstones have been extracted for over a century. A large amount of pyrite in the Wiśniówka siliciclastics is unusual in the hard rock mining throughout the world. The pyritiferous beds contain a variety of REE-bearing minerals, including a crandallite series of aluminum-phosphate-sulfate (APS) minerals, e.g., predominant goyazite SrHAl 3 [(PO 4 ) 2 (OH) 6 ] with subordinate gorceixite BaHAl 3 [(PO 4 ) 2 (OH) 6 ] and very occasional crandallite CaHAl 3 [(PO 4 ) 2 (OH) 6 ]. By contrast, the other REE-phosphate minerals, e.g., xenotime YPO 4 , bur particularly monazite CePO 4 occur in a lesser amount. Goyazite prevails somewhat in the Podwiśniówka beds whereas xenotime in the Wiśniówka Duża beds. Of the other REE-bearing minerals, bastnäsite REECO 3 (F,OH), florencite (REE)Al 3 (PO 4 ) 2 (OH) 6 and synchysite CaCe[CO 3 ] 2 F occur in trace amounts. Interestingly, the quite common phosphate minerals, i.e., wavellite (Al 3 [(OH,F) 3 |(PO 4 ) 2 ]×5H 2 O and variscite Al[PO 4 ]×2H 2 O) are depleted in REEs with only Ce attaining 0.09 wt.% as documented by an electron-probe microanalysis. In contrast to quartzites/quartzitic sandstones, carbonaceous clayey-silty shales and bentonites/tuffites are distinctly enriched in REE-bearing minerals. This diversity is also mirrored in the mean total REE concentrations varying from 204 to 314 mg/kg, in clayey-silty shales and bentonites, attaining 457 mg/kg in some Podwiśniówka shale beds. Results of this and the previous petrographic, mineralogical and geochemical studies have indicated that REE-bearing minerals formed generally along with As-rich pyrite, nacrite/dickite and probably TiO 2 polymorphs as a result of multiphase hydrothermal vent activity that took place in the Wiśniówka Late Cambrian sedimentary basin. This evidence is also backed up by the values of LREE NASC /HREE NASC (1.44–1.75) and Eu/Eu NASC (1.24–1.30) coefficients in the clayey-silty shales. This positive Eu anomaly (³1.20) points to the formation of REE-bearing minerals in a reducing environment.

Physiological response of adult Salix aurita in wetland vegetation affected by flooding with As-rich fine pyrite particles

An uncontrolled, natural episode of flooding with waters contaminated with As-rich pyrite (FeAsS) particles caused serious ecological damage leading to necrosis of plants growing in a fresh wet meadow located in an area characterized by unique geological structures rich in arsenopyrites. One of the few plant species capable of surviving this event was Salix aurita L., which grew in numbers in the analyzed area, but individual plants were affected differently by toxic flooding. No significant phenotypic changes (Group I), through partial leaf and/or stem necrosis (Group II) up to necrosis of the whole parental plant and root suckers (Group III), were observed for various willow clumps. These varied phenotypic responses of S. aurita to As-rich sediments were compared with the biochemical status of the foliage of willow trees, and with their rhizosphere physiological parameters.Our in situ study revealed that the biochemical status of leaves reflects the phenotypic damage incurred by adult willows growing in their natural environment and affected by the flooding. In leaves of willows with increasingly negative phenotypic changes (Groups I → II → III) as well as increasing levels of reactive oxygen species, malondialdehyde and decreased levels of glutathione and thiol groups were detected. Phytochelatins, commonly considered major As chelators, were not detected in S. aurita leaves. Despite a decrease in the size of leaves with the intensity of tree damage, all leaves expressed a normal level of leaf pigments.Phenotypic changes observed for particular willow clumps were only partly related to soil As levels. Moreover, As and S (but not Fe) foliar levels were related but did not correspond strictly with foliar biochemical features, or with soil As levels, soil pH or soil microbial activity, with the latter two drastically decreased in the rhizospheres of willows from Groups II and III.

Tin mineralization in the giant Shuangjianzishan Ag-Pb-Zn deposit, Inner Mongolia

The newly discovered Shuangjianzishan giant Ag-Pb-Zn deposit which located in the Southern segment of the Great Xing’an Range (SGXR), has controlled more than 18 000 t reserves of Ag, 1.9 Mt reserves of Zn, and 3.3 Mt reserves of Pb. Previous research has shown that Shuangjianzishan is a typical epithermal Ag-Pb-Zn deposit, however, mid- to high-temperature Cu-Sn (-W) mineralization are few studies. In this paper, a detailed mineralogical study reveals that a remarkable Cu-Sn (-W) mineralization is observed both in the upper tunnel and the deeper drill core samples. In the shallow ore-bodies, tin dominantly occurs as canfieldite in Pb-Zn-rich ores, minor as cassiterite-quartz veinlets. In the deep drill cores, tin occurs as cassiterite in the Cu-rich ores with the assemblage of quartz-arsenopyrite-chalcopyrite-stannite-(wolframite). Four primary paragenetic stages have been recognized. Stage I: cassiterite-quartz-arsenopyrite-chalcopyrite, including two sub-stage: cassiterite-arsenopyrite-pyrrhotite-quartz (stage I1) and sphalerite-chalcopyrite-galena-pyrite (stage I2); Stage II: galena-sphalerite-silver minerals; Stage III: Quartz-silver minerals, and stage IV: pyrite-carbonate.Electron probe micro-analysis (EPMA) show that the early mineralization (stage I) is characterized by Fe-rich sphalerite, Co-rich pyrite, Se-rich canfieldite, and Se-Bi-rich galena, while the later mineralization (stage II-IV) contains Fe-depleted sphalerite, As-rich pyrite, and Sb-bearing silver minerals. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis show that cassiterite has high contents of W, Fe, Sb, and Ti and depleted in Nb and Ta. Mineral associations and compositions implies a cassiterite-sulfide type tin mineralization in the Shuangjianzishan Ag-Pb-Zn deposit.A concealed granite porphyry intrusion was exposed in deep drill cores in the northwest part of the ore-field, with zircon U-Pb ages of (137.8 ± 1.1) Ma and (136.8 ± 1.0) Ma for fine- and coarse-grains granite, respectively. Bulk analysis shows that concealed granites are characterized by a highly fractionated, relatively reduced, and high K-calc-alkaline to alkaline, which belongs to A-type granites, and the remnant of Sn-bearing titanite suggests that it has the potential to provide the source of tin. Zircon U-Pb age (138–130 Ma) is well coupled with the previous data (ore-forming age, 136–129 Ma) of Shuangjianzishan, suggesting a close temporal and genetic relationship between tin mineralization and emplacement of the granite. This contribution implies that the Shuangjianzishan deposit may exist a middle- to high-temperature Cu-Sn (-W) mineralization center rather than a simple low-temperature hydrothermal system, and shows there is significant Sn-Cu mineralization potential in the peripheral and root zone of ore district.

Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments

Although it has been suggested that the trace element patterns of sedimentary pyrite can be affected by early diagenesis, detailed studies on modern marine environments addressing this hypothesis are scarce. Organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) are the dominant sulfate reduction processes in organic-rich and methane-rich marine sediments, and commonly result in the formation of authigenic pyrite. To better understand how OSR and SD-AOM affect the trace element geochemistry of pyrite, we applied laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) to analyze trace element contents of pyrite from two methane-seep sites (DH-CL11 and HS148) of the South China Sea. The trace element data are complemented by the measurement of iron isotope compositions of authigenic pyrite. This new data set, supplemented by published sulfur isotope data, improves our understanding of the diagenetic processes during early diagenetic pyrite formation. High δ56Fe and δ34S values of pyrite reflect the dominance of SD-AOM within sulfate-methane transition zones (SMTZs) at both sites (i.e., from 705 to 767 cm below seafloor at site DH-CL11 and from 490 to 690 cm below seafloor at site HS148), whereas pyrite from shallow depth characterized by low δ56Fe values points to early OSR processes. Within the SMTZs, pyrite exhibits highly variable but overall high manganese (Mn) contents (12 to 732 mg/kg at site DH-CL11; 12 to 1145 mg/kg at site HS148), compared to OSR-derived pyrite from shallow depth. Similar to this pattern, pyrite formed at the SMTZ shows higher lead (Pb), zinc (Zn), copper (Cu), and vanadium (V) contents than pyrite formed at shallower depths above the SMTZ. Enrichments of these metals in pyrite forming at the SMTZ are best explained by enhanced reductive dissolution of iron and manganese (oxyhydr)oxides under sulfidic conditions induced by SD-AOM, and subsequent trace element incorporation into pyrite. Arsenic (As), molybdenum (Mo), and antimony (Sb) distributions in pyrite vary between the two study sites. Pyrite from shallow depth of site HS148 has high contents of all three elements, indicating effective immobilization of dissolved As, Mo, and Sb during pronounced OSR. In contrast, apparently less intense OSR at shallow depth of site DH-CL11 allowed for more dissolved As, Mo, and Sb to diffuse downward to greater depth, resulting in the enrichments of these elements in pyrite formed at or close to the SMTZ. Positive correlations between As and cobalt (Co), and between As and nickel (Ni) typify pyrite derived from both OSR and SD-AOM for both study sites, and As-rich pyrite from site DH-CL11 is characterized by a distinctly high Co/Ni ratio. Both patterns suggest that the presence of As affects the uptake of Co and Ni into pyrite. Our study documents that pyrite element contents in methane-rich environments are significantly influenced by different sulfate reduction processes during early diagenesis, allowing the differentiation of OSR-derived pyrite from SD-AOM-derived pyrite by means of trace element geochemistry.

Polymetallic nanoparticles in pyrite from massive and stockwork ores of VMS deposits of the Iberian Pyrite Belt

This paper reports the first-ever study on nanoscale mineralogy in pyrite from the volcanogenic massive sulfide (VMS) deposits of the Iberian Pyrite Belt, southwestern Iberian Peninsula. It targeted colloform-textured grains formed at low temperature in the distal part of a polymetallic (Pb-Zn) massive sulfide lens hosted in felsic volcanoclastic rocks from the Masa Valverde deposit, and euhedral-textured grains (re)-deposited by higher temperature fluids in the Co-Au rich stockwork hosted in black shales of the Filón Norte orebody of the Tharsis deposit. The results acquired by a combination of techniques for mineral microanalysis and characterization (i.e., reflected light, FE-SEM, EPMA, LA-ICP-MS, HRTEM-STEM and TEM-EDS) show that trace amounts of metals (Au, Ag, As, Pb, Sb, Cu, Co) are incorporated as both lattice-bound and into nanoparticles (NPs). The mode of occurrence is strongly related with the evolutionary history of the mineralization. In the colloform pyrite collected from the massive sulfide lens, a rhythmic banding/oscillatory zonation with up to 3 wt% As, 5,000 ppm Pb, 1,070 ppm Sb and 750 ppm Cu is defined by the coexistence of several nano-sized layers (5 to 100 nm) and NPs (<100 nm) containing all these metals. The NPs include galena [PbS], tetrahedrite [(Cu,Fe)12Sb4S13)] and arsenopyrite [FeAsS] that exhibit euhedral and less frequently anhedral (i.e., droplet-like) morphologies being both randomly and preferentially oriented with respect to As-rich pyrite bands they are usually associated with. These features suggest formation of the NPs via direct deposition from the hydrothermal fluid(s) or low-temperature melts entrained in them as well as exsolution of trace elements originally dissolved in the As-rich pyrite structure. Additionally, some of these NPs are connected to late fractures disrupting the chemical zoning in colloform pyrite documenting a third genetic type of NPs related to late infiltration of fluids post-dating pyrite formation. In contrast, euhedral pyrite from the stockwork form well-developed homogeneous grains with discrete porous areas relatively depleted in Fe (45.20 wt%), and As (8,800 ppm) but enriched in Co (5,900 ppm). At the nanoscale, Co-enriched domains show patchy zoning defined by irregular distribution of Co– and As-rich bands of 200–500 nm in thickness. These nanometer Co– and As-rich bands are often disrupted by micron-to-nano-sized polycrystalline Au-Ag-Hg particles that fill voids in porous areas. Contact morphology anatomy between Co-rich pyrite and inclusions suggests that the Au-Ag-Hg particles are negative crystals occupying spaces originated in pyrite by coupled dissolution-reprecipitation reaction. Likewise, HRTEM observations along such pyrite-inclusion contacts show the existence of polycrystalline matrices in both pyrite and Au-Ag-Hg inclusions, the former consisting of nano-sized domains of arsenian pyrite and/or arsenopyrite in As-free pyrite and the Au-Ag-Hg inclusions made up of multiple crystal domains including nano-crystallites of Au0/Ag0 or electrum. Recognition of crystalline nanodomains and NPs in these polycrystalline matrices raises the possibility that Au NPs or nanomelts already present in the hydrothermal fluid catalyzed the formation of these heterogeneous crystals.

Textural and geochemical analysis of celestine and sulfides constrain Sr-(Pb-Zn) mineralization in the Shizilishan deposit, eastern China

Breccia-hosted Sr-(Pb-Zn) mineralization from the Shizilishan deposit (Edong district, eastern China) is located at the contact between a Lower Cretaceous (∼139 Ma) quartz diorite porphyry stock and Triassic evaporite-bearing limestone/dolomite sequences. Backscatter electron imaging, electron probe microanalysis, and laser ablation (multicollector-) inductively coupled-plasma mass spectrometry constrain ore genesis based on the textural evolution and trace element and isotope geochemistry in multiple stages of celestine and sulfides (pyrite and marcasite). Complex growth and/or dissolution and/or recrystallization textures of celestine and superposition of pyrite and marcasite are suggestive of evolving fluid compositions and physicochemical conditions during which pH first decreased (favorable for marcasite precipitation) and then increased, facilitating precipitation of Ba-rich celestine and As-rich pyrite. Sphalerite is characterized by enrichment in structurally-bound Sb, Cu, Ag, As, and Tl. Celestine displays Sr and S isotope signatures comparable to Middle Triassic seawater, indicating derivation of both elements from evaporite sequences. Decreasing δ34S in sulfides through the paragenetic sequence may reflect multiple sulfur sources produced by thermochemical reduction of sulfate and/or bacterial sulfate reduction processes. This research highlights that low to moderate salinity fluids, expected to carry significant concentrations of Sr, are able to form large celestine deposits through extensive water–rock interaction when fluid flow is either protracted, or there are superimposed episodes of fluid introduction with high Sr/Ba ratios and moderate dissolved sulfate. The Shizilishan deposit shares some alteration features and geochemical signatures with low-temperature Carlin-type Au deposits (e.g., relative enrichment in As and Tl in ore-stage pyrite) and Fe-poor MVT-type Pb-Zn ores (e.g., high content of Ga and Ge but low Fe, Mn, and In in sphalerite).

Genesis of the Gaolong gold deposit in Northwest Guangxi Province, South China: Insights from in situ trace elements and sulfur isotopes of pyrite

The Guangxi-Yunnan-Guizhou district, also named Dian-Qian-Gui “Golden Triangle”, in Southwest China has a large amount of Carlin-type Au deposits. Gaolong is one source of Carlin-type Au deposit in the northwestern Guangxi Province. Pyrites are the major Au-bearing sulfides. Majority of Au are structurally bound Au+, with a few nanoparticles (Au0) occurring in the sulfides. In this study, in situ chemical and S isotopes of pyrites in the Gaolong Au deposit were analyzed by laser ablation inductively-coupled plasma mass spectrometry and laser ablation multi-collector inductively-coupled plasma mass spectrometry, respectively. Backscattered electron (BSE) images and chemical composition analysis showed that the pyrites were divided into four main types: (1) As-poor Py1 (pre-ore sedimentary or diagenetic stage, with prominent spotted and aggregate structures. These are fine-grained pyrite aggregates with rough surfaces and low brightness under the BSE, with edges often eroded by hydrothermal fluid); (2) As-weak Py2 (early ore stage, the core of the rim pyrite, with good crystallization and characteristic corrosion spots in the particle, and with the edge dissolved to show irregular structure); (3) As-rich Py3 (main ore stage, the narrow pyrite rims overgrowing Py1 and Py2, irregularly-shaped and less than 50 μm in width); and (4) As-rich Py4 (fine-grained pyrite, with good crystallization, high brightness, and symbiotic with Py3 and Py1, or an isolated distribution). Py1 contains higher concentrations of S, Fe, and W, and lower Co/Ni ratios (average of 0.91), suggesting a pre-ore origin. The ore-stage pyrites Py2, Py3, and Py4, have elevated mineralization elements—Au of 0.25–72 ppm, As of 2214–40613 ppm, Tl of 0.95–109 ppm, Sb of 31.16–1851 ppm, Cu of 49.09–910 ppm, with average Co/Ni ratios of 0.43. Au was positively correlated with As, Tl, Cu, and Ni in these pyrites. Electron micro probe analyzer (EMPA) elemental mappings show that: 1) Py1 has a low Au content, while Py2, Py3, and Py4 have high Au, As, Cu, Ti, and Mg contents; 2) Au occurs in As-rich pyrite in the form of invisible gold; 3) Rim pyrite contains high concentrations of Cu, which has a good positive correlation with As, while As and Au are decoupled on a smaller scale. The Au contents and Au/As ratios show that the Au-carrying fluids are unsaturated with Au. In situ S isotopes show that the δ34S values range from 10.19‰ to 13.79‰ with a mean value of 12.07‰ for Py1, 8.60‰ to 10.78‰ with a mean value of 9.77‰ for Py2, 1.42‰ to 8.29‰ with a mean value of 6.81‰ for Py3, and − 0.49‰ to 4.42‰ with a mean value of 1.67‰ for Py4. The δ34S data decreased from Py1 to Py4, whereas the As content increased significantly. This indicates that Au and As are enriched in ore-forming fluids, and the ore-forming fluids have low S isotope values. Based on a comprehensive study of geological and geochemical data, we believe that the magmatic hydrothermal genetic model can explain the genesis of the Gaolong gold deposit. We conclude that the S isotope of pyrites resulted from the mixing of magmatic S and sedimentary S, and the Au-bearing fluid came from deep-concealed granite, establishing a magma genesis of the Gaolong Au deposit.

Effects of arsenic on the distribution and mode of occurrence of gold during fluid-pyrite interaction: A case study of pyrite from the Qiucun gold deposit, China

Abstract This paper presents the results of an investigation of the incorporation of Au within pyritic ore from the Qiucun epithermal Au deposit, China. The new data provide insights into the mode of occurrence of Au during fluid-rock interactions within epithermal systems. The distribution and mode of occurrence of Au within arsenian pyrite were investigated using a chemical and structural characterization-based approach combining laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analysis, megapixel synchrotron X-ray fluorescence analysis (MSXRF), and atom probe tomography (APT). The resulting data indicate that invisible Au is present at elevated concentrations in the form of a homogeneous solid solution within As-rich pyrite domains, which yields Au concentrations that positively correlate with As. Arsenic-induced lattice defects, such as stacking faults and the expansion of the pyrite unit cell, provide evidence of the effect of As on the incorporation of Au into pyrite. The nucleation and crystallization of electrum preferentially occurred at the fluid-pyrite reaction interface or along fractures and grain boundaries within the pre-existing pyrite. This study indicates that changes in physicochemical conditions (e.g., temperature, pH, and sulfur fugacity) during fluid-pyrite interactions are key controls on the development of nanometer- or micrometer-scale clusters of gold. The systematic compositional and textural observations documented in this study provide new insights into the mechanisms responsible for the different modes of occurrence of Au (ionic vs. particulate) and enable us to further understand the processes involved in the formation of Au mineralization.

Two types of sediment-hosted Pb-Zn deposits in the northern margin of Lanping basin, SW China: Evidence from sphalerite trace elements, carbonate C-O isotopes and molybdenite Re-Os age

Lanping basin is a large Pb-Zn-Cu-Ag polymetallic mineral province in SW China, with major deposit types including vein-type Cu deposits, basinal brine-related Pb-Zn deposits (e.g., Liziping, Fulongchang) and minor quartz-vein-type Pb-Zn deposits (e.g., Shangnuluo, Baiji). In this study, we have conducted EPMA and LA-ICP-MS sphalerite trace element analyses, together with gangue carbonate C-O stable isotope analysis and molybdenite Re-Os dating. Using these data, we discuss the mineral assemblage, sphalerite minor/trace element composition, ore-fluid source, and mineralization age of these two types of Pb-Zn deposits. Except sphalerite and galena, major ore minerals from Liziping and Fulongchang are As-rich (e.g., gratonite, jordanite, bournonite, As-rich pyrite), whereas those from Shangnuluo and Baiji are Sb-rich (e.g., boulangerite, jamesonite, stibnite, zinkenite). The trace element compositional characteristics in sphalerite of these deposits (low Mn, Fe, Co, In and Sn, and high Cd) mimic those of medium-low-temperature hydrothermal deposits (MVT and Jinding deposits). For the Liziping zoned sphalerite, the dark-brown zones have relatively high Co, Cu, Ga, As, and Sb; whilst the light-colored zones have relatively high Fe, Mn and Cd. The calculated sphalerite formation temperature based on sphalerite geothermometer – GGIMFis (Frenzel et al., 2016) are basically in agreement with those homogenization temperature of inclusions, indicating that the contents of these elements (Ga, Ge, Fe, Mn and In) in sphalerite are mainly controlled by temperature. Moreover, the estimated sulfur fugacity (log fS2) for Liziping (−14.7) and Fulongchang (−13.6) are distinctly lower than those for Shangnuluo (−7.1) and Baiji (−8.5). Carbonate C–O isotope compositions indicate that the dissolved carbon and oxygen of ore forming-fluids of these Pb-Zn deposits were mainly derived from sedimentary carbonate dissolution. Early-ore-stage molybdenite from Baiji deposit yielded two Re-Os weighted average model ages of 21.4 Ma and 12.93 Ma, and a Re-Os concordia age of 11.88 ± 0.77 Ma was obtained from four younger molybdenite samples. These ages are distinctly younger than those reported mineralization age (ca. 30 Ma) for basinal brine-related Pb-Zn deposits. All these results indicate that there are two different types of Pb-Zn metallogenic events in the northern margin of Lanping Basin.

Arsenian Pyrite and Cinnabar from Active Submarine Nearshore Vents, Paleochori Bay, Milos Island, Greece

Active, shallow-water (2–10 m below sea level) and low temperature (up to 115 °C) hydrothermal venting at Paleochori Bay, nearshore Milos Island, Greece, discharges CO2 and H2S rich vapors (e.g., low-Cl fluid) and high-salinity liquids, which leads to a diverse assemblage of sulfide and alteration phases in an area of approximately 1 km2. Volcaniclastic detritus recovered from the seafloor is cemented by hydrothermal pyrite and marcasite, while semi-massive to massive pyrite-marcasite constitute mounds and chimney-like edifices. Paragenetic relationships indicate deposition of two distinct mineralogical assemblages related to the venting of high-Cl and low-Cl fluids, respectively: (1) colloform As- and Hg-bearing pyrite (Py I), associated with marcasite, calcite, and apatite, as well as (2) porous and/or massive As-rich pyrite (Py II), associated with barite, alunite/jarosite, and late-stage hydrous ferric oxides. Mercury, in the form of cinnabar, occurs within the As-rich pyrite (Py I) layers, usually forming distinct cinnabar-enriched micro-layers. Arsenic in colloform pyrite I shows a negative correlation with S indicating that As1− dominates in the pyrite structure suggesting formation from a relatively reducing As-rich fluid at conditions similar to low-sulfidation epithermal systems. On the contrary, As3+ dominates in the structure of porous to massive pyrite II suggesting deposition from a sulfate-dominated fluid with lower pH and higher fO2. Bulk sulfide data of pyrite-bearing hydrothermal precipitates also show elevated As (up to 2587 ppm) together with various epithermal-type elements, such as Sb (up to 274 ppm), Tl (up to 513 ppm), and Hg (up to 34 ppm) suggesting an epithermal nature for the hydrothermal activity at Paleochori Bay. Textural relationships indicate a contemporaneous deposition of As and Hg, which is suggested to be the result of venting from both high-salinity, liquid-dominated, as well as CO2- and H2S-rich vapor-dominated fluids that formed during fluid boiling. The CO2- and H2S-rich vapor that physically separated during fluid boiling from the high-salinity liquid led to calcite formation upon condensation in seawater together with the precipitation of As- and Hg-bearing pyrite I. This also led to the formation of sulfuric acid, thereby causing leaching and dissolution of primary iron-rich minerals in the volcaniclastic sediments, finally resulting in pyrite II precipitation in association with alunite/jarosite. The Paleochori vents contain the first documented occurrence of cinnabar on the seafloor in the Mediterranean area and provide an important link between offshore hydrothermal activity and the onshore mercury and arsenic mineralizing system on Milos Island. The results of this study therefore demonstrate that metal and metalloid precipitation in shallow-water continental arc environments is controlled by epithermal processes known from their subaerial analogues.

The groundwater monitoring system in Tuscany according to the EU Directive 2000/60 and the groundwater As contamination in the Versilia coastal plain

The EU Water Framework Directive 2000/60/EC commits European Union Member States to achieve good qualitative and quantitative status for all water bodies. Its application requires periodic monitoring of selected water bodies. Concerning groundwater bodies, the Tuscany Region selected monitoring wells for periodic sampling and performing chemical analyses. Results of these analyses are used to assess the qualitative status according to the EU Directive. In some cases, this groundwater monitoring can highlight some water pollution, hence activating more detailed investigations. In this work, we present the case of high arsenic concentrations in groundwater, reaching about 1000 μg/L, from domestic wells located in southern Versilia (Tuscany, Italy), close by the Baccatoio stream which receives acidic waters from a former mining site. Initially, the arsenic has been related to acid mine drainage from the nearby sites in the Baccatoio Valley. As-rich pyrite weathering was supposed to be the primary source for As in groundwater despite the Baccatoio stream is As-free. Although the evaluation of the amount of pyrite in sediments needs further investigations, alternative hypothesis were formulated. The highest As concentration is often accompained by high amounts of iron and ammonium ions reflecting reducing environments. This suggests the occurrence of secondary As-sources such as ferric oxyhydroxide and organic matter, likely in peat layers, undergoing reductive dissolution. The aim of this work is to carry out a preliminary assessment of the origin of the As contamination in groundwater of the Versilia coastal plain.

Gold and antimony metallogenic relations and ore-forming process of Qinglong Sb(Au) deposit in Youjiang basin, SW China: Sulfide trace elements and sulfur isotopes

In the northwestern margin of the Youjiang basin (NWYB) in SW China, many Carlin-like gold deposits are highly antimony (Sb)-rich, and many vein-type Sb deposits contain much Au. These deposits have similar ages, host rocks, ore-forming temperatures, ore-related alterations and ore mineral assemblages, but the Au and Sb metallogenic relations and their ore-forming process remain enigmatic. Here we investigate the large Qinglong Sb deposit in the NWYB, which has extensive sub-economic Au mineralization, and present a new metallogenic model based on in-situ trace elements (EPMA and LA-ICP-MS) and sulfur isotopes (NanoSIMS and fs-LA-MC-ICP-MS) of the ore sulfides. At Qinglong, economic Sb ores contain coarse-grained stibnite, jasperoid quartz and fluorite, whilst the sub-economic Au–Sb ores comprise dominantly veined quartz, arsenian pyrite and fine-grained stibnite. Three generations of ore-related pyrite (Py1, Py2 and Py3) and two generations of stibnite (Stb1 and Stb2) are identified based on their texture, chemistry, and sulfur isotopes. The pre-ore Py1 is characterized by the lower ore element (Au, As, Sb, Cu and Ag) contents (mostly below the LA-ICP-MS detection limit) and Co/Ni ratios (average 0.31) than the ore-stage pyrites (Py2 and Py3), implying a sedimentary/diagenetic origin. The Py2 and Py3 have elevated ore element abundance (maximum As ​= ​6500 ​ppm, Au ​= ​22 ​ppm, Sb ​= ​6300 ​ppm, Cu ​= ​951 ​ppm, Ag ​= ​77 ​ppm) and Co/Ni ratios (average 1.84), and have positive As vs. Au–Sb–Cu–Ag correlations. Early-ore Stb1 has lower As (0.12–0.30 ​wt.%) than late-ore Stb2 (0.91–1.20 ​wt.%). These features show that the progressive As enrichment in ore sulfides is accompanied by increasing Au, Sb, Cu and Ag with the hydrothermal evolution, thereby making As a good proxy for Au. As-rich, As-poor and As-free zones are identified via NanoSIMS mapping of the Au-bearing pyrite. The As-rich zones in the Qinglong Au-bearing pyrites (Py2 and Py3) and ore stibnites (Stb1 and Stb2) have narrow δ34SH2S ranges (−8.9‰ to +4.1‰, average −3.1‰) and −2.9‰ to +6.9‰, average ​+ ​1.3‰), respectively, indicating that the Au-rich and Sb-rich fluids may have had the same sulfur source. Published in-situ sulfur isotopic data of pyrite As-rich zones from other Carlin-like Au deposits (Shuiyindong, Taipingdong, Nayang, Getang and Lianhuashan) in the NWYB have similar ore-fluid δSH2S34 values (−4.5‰ to +6.7‰, average −0.6‰) to those of Qinglong. Therefore, we infer that the sulfur of both Au and Sb mineralization was derived from the same magmatic-related source (0 ​± ​5‰) in the NWYB.Moreover, the core of pyrites (Py1) has variable S isotope fractionation (−18.9‰ to +18.1‰, mostly +3‰ to +12‰), suggesting that the higher-34S H2S was produced by bacterial sulfate reduction (BSR). The hydrothermal pyrite (Py2 and Py3) δ34S values gradually decrease with increasing As concentrations, and ultimately, within the restricted range (−5‰ to +5‰) in As-rich zones. This variation implies that the As-rich pyrite was formed through ongoing interactions of the magmatic-hydrothermal fluid with pre-existing sedimentary pyrites, causing the progressive decreasing δ34S values with As content increase, Hence, the fluid/mineral interaction may have generated the observed variation in δ34S and As contents. Overall, comparing the Au and Sb deposits in the NWYB, we favor a magmatic-related source for the Au–Sb–As-rich fluids, but the Au- and Sb-ore fluids were likely evolved at separate stages in the ore-forming system.

High-sulfidation veins in the Jiama porphyry system, South Tibet

The giant Jiama polymetallic ore deposit is a porphyry–skarn system in the eastern Gangdese porphyry belt of the Tibetan Plateau. Steeply dipping high-grade massive sulfide veins are classified as lode-type high-sulfidation (HS) mineralization comprising mainly Cu-sulfosalts in Au-poor and Au-rich veins. Au-poor veins contain an As-rich pyrite–enargite–tennantite assemblage, whereas Au-rich veins contain an Sb-rich enargite–luzonite–tetrahedrite–chalcostibite–watanabeite assemblage. The coexistence of pyrite, dendritic muscovite, and euhedral quartz in the Cu-sulfosalts indicates slightly acidic or near-neutral formation conditions. Most Au- and Ag-bearing minerals are inclusions of tellurides (hessite, sylvanite, and petzite) in tennantite–tetrahedrite, and watanabeite. Au and Ag concentrations in tennantite–tetrahedrite, watanabeite, and chalcostibite are much higher than in enargite and luzonite, confirming that they were controlled by intermediate-sulfidation state Cu–As–Sb–S solid solutions, with Au tending to be enriched in Sb-rich minerals compared with As-rich solid solutions. The HS veins overlying the known porphyry intrusion and cutting skarn orebodies in the Jiama deposit constitute a porphyry–skarn–high sulfidation system. The occurrence of HS veins suggests that channels in paleo-fumaroles and the mineralization that resulted may be ascribed to expansion of magmatic vapor. HS veins close to the porphyry intrusion exposed at the surface indicate that shallow epithermal disseminated mineralization was likely removed by rapid regional uplift and erosion of the Gangdese porphyry belt.

Arsenic release from pyrite ash waste over an active hydrogeological system and its effects on water quality.

Arsenic is a potentially toxic element of concern for environmental compartments, and it is a frequent pollutant in many abandoned industrial sites. In this study, geochemical and hydrogeological tools were used to determine the long-term effects of As-rich pyrite ash disposal (83,000 m3 as estimated by geostatistical tools) in a brownfield located over a quaternary alluvial aquifer. Throughout the site, soil pollution and water table oscillation led to leachates in the form of both run-off and infiltration waters, thereby reducing (ground)water quality (e.g. pH, electrical conductivity) and, in particular, increasing the concentration of arsenic (average approx. 4000 μg/l for one hydrological year). By means of laboratory and in situ measurements, the main mechanisms through which the sulphide remaining in the pyrite ash leaches were identified. In addition, to evaluate the effects of the polluted groundwater on the nearby main river, a mathematical approach using the Domenico analytical groundwater transport model revealed potential concentrations of 49 μg/l of arsenic in the junction between the study aquifer and the river, equivalent to an annual quantity of 49 kg of this element.

Sulfur isotope and trace element compositions of pyrite determined by NanoSIMS and LA-ICP-MS: new constraints on the genesis of the Shuiyindong Carlin-like gold deposit in SW China

The Shuiyindong Carlin-like gold deposit, the largest one of this type in China, is located in the Late Paleozoic–Early Mesozoic Youjiang basin. The pyrites in the studied ore bodies (No. I to V) can be divided into four main types plus several sub-types: (1) As-poor pyrites include Py1 (Py1a and Py1b), Py2, and Py3; and (2) As-rich pyrite Py4 (Py4a and Py4b). In situ LA-ICP-MS analyses show that the As-rich pyrite also contains higher Au, Cu, Sb, and Tl than the other pyrite types. In situ NanoSIMS analyses show the δ34S variation of Py1 (48.1~67.5‰), Py2 (2.4~7.6‰), Py3 (9.2~14.2‰), and Py4 (− 3.0~6.6‰), with the average values of 58.1‰, 5.1‰, 11.4‰, and 3.1‰, respectively. The large δ34S variations of As-poor pyrites may reflect a sedimentary source. However, the narrow ranges of δ34S values for the As-rich pyrite (Py4) may not reflect a sedimentary but a magmatic or metamorphic origin. A metamorphic sulfur source of Py4 is favored due to paucity of coeval magmatism in the region and decreasing variations of δ34S values from Py1 to Py4. Integrated transmission electron microscope (TEM) and NanoSIMS element mapping show that the distribution and concentration of Au are discordant with As and Cu at the submicron scale, also implied by the occurrence of some of Au as nano-submicron particles in Py4. Overall, our new data indicate that the ore fluids were not only enriched in Au but also in As, Cu, and S. Taking an alternative of fluid oxidation probably resulting in the decreasing δ34S values of Py4, our result of I to V orebodies indicates the mixing between the underlying metamorphic fluid and meteoric water. Integrated variations in δ34S and As-Au-Cu correlations across individual pyrite grains infer the physicochemical changes of aqueous and/or aqueous-carbonic fluids along migration paths.

The origin of pyrite mineralization: Implications for Late Cambrian geology of the Holy Cross Mountains (south-central Poland)

This paper presents results of a petrographic, mineralogical and geochemical study of ore, gangue and rock-forming minerals that occur within the Upper Cambrian (Furongian)-age siliciclastic rocks of the Wiśniówka massif (western part of the Main Range, Holy Cross Mountains, south-central Poland). This area is characterized by the presence of ubiquitous pyrite that makes up two stratiform mineralization zones assigned to the early and middle Late Cambrian, giving rise to acid mine drainage. The oldest zone comprises additionally sedimentary breccias with intraclast-supported pyrite cement. The predominant microscopic (colloform) and partly framboidal pyrite typically shows high concentrations of As (up to 8.23 wt%) and a distinct enrichment in the light sulfur isotope (δ34S from −41.4 to −23.6‰). The occurrences of large amounts of As-rich pyrite showing strongly negative δ34S values are rather uncommon in Paleozoic sedimentary formations worldwide that exhibit distinctly smaller amounts of As and mostly enrichments in heavier sulfur isotope (δ34S up to 63‰). Another specific feature of the Wiśniówka rock series is the presence of REE-bearing aluminophosphate and phosphate minerals that occur primarily in clayey-silty shales and bentonites. Based on the results obtained, a sequence of hydrothermal index minerals was discriminated, including pyrite (whose grains are usually made of alternating arsenic-rich and arsenic-poor bands), as well as paragenetic assemblages of dickite, nacrite, xenotime and the crandallite series of aluminum-phosphate-sulfate minerals. Both mineral composition and specific microtextures are indicative of multistage hydrothermal activity that took place in the Wiśniówka depositional basin during the Late Cambrian time. This is in distinct contrast to the remaining part of the Main Range, which is devoid of the hydrothermal pyrite imprint. Moreover, the results of this study give insight into the complex sedimentary and early diagenetic history of the southwestern passive margin of the Baltica craton. This currently makes up a part of the Łysogóry Block (Terrane) that borders on the Trans-European Suture Zone separating the Precambrian East-European Craton from the Paleozoic Platform of Central Europe. This study is important because we still lack the full knowledge about the hydrothermal processes that took place in Early Paleozoic depositional basins.

Mineralogical characteristics of silver minerals from the Dongyang Gold deposit, China: Implications for the evolution of epithermal metallogenesis

The Dongyang gold deposit is located in Fujian Province in southeastern China, mainly hosted by a NW–SE trending Late Jurassic porphyritic rhyolite, and classified as a low-sulfidation epithermal gold deposit. The units associated with the gold mineralization in this area are highly altered and have undergone sericitic and argillic alteration as well as silicification, with the latter two types of alteration especially associated with gold enrichment. The deposit contains pyrite, marcasite, and arsenopyrite, with lesser amounts of chalcopyrite, galena, sphalerite, and silver minerals. The sulfides and silver minerals within the deposit provide insight into the evolution of the ore-forming fluid and the processes that caused the gold enrichments to form in the study area. In addition, the geochemical similarity of gold and silver means these elements tend to migrate and precipitate simultaneously, in turn indicating that sulfides and silver minerals can provide insights into the processes that concentrate gold. Here, we present chemical compositional, and crystal structural data for sulfides and silver minerals within the Dongyang gold deposit by scanning electron microscopy (SEM), electron microprobe analyser (EMPA), X-ray diffraction (XRD) and laser Raman microprobe, and use these data to gain insights into the characteristics and evolution of the fluids that formed the deposit. We also outline key concepts for future gold deposit exploration in this region. Four types of pyrite in the deposit are recognized based on arsenic content and crystal morphology, evolving from early As-poor pyrite (Py1) to As-rich pyrite (Py2) and/or arsenopyrite, to non-cubic As-poor pyrite (Py3), and finally to cubic As-poor pyrite (Py4), with an inverse correlation between sulfur and arsenic concentrations. Pyrite crystal morphologies vary from earlier pentagonal dodecahedron forms to later cube forms, indicating a decrease in degree of supersaturation (as controlled by the activity of dissolved Fe and sulfide) and/or temperature conditions for pyrite growth. In addition, the presence of As-poor marcasite associated with early pyrite (Py1) suggests that the early ore-forming fluid was relatively acidic. Arsenopyrite is generally concentrated in the shallower parts of the deposit, indicating that later hydrothermal fluids were enriched in arsenic. This study also identified a number of key silver mineral phases that formed in a sequence from chalcopyrite, to a galena–fahlore–allargentum–dyscrasite–polybasite–electrum assemblage, to stephanite, and finally to chlorargyrite. Chalcopyrite replaced by fahlore suggests increased oxygen fugacity and decreased temperature. Fahlore Sb / (As + Sb) (>90%) and Ag / (Ag + Cu) ratios (6%–27%) are indicative of relatively high temperatures (>200 °C), and allargentum is thought to form in elevated pH conditions (pH > 7). Stephanite and chlorargyrite precipitated under relatively low temperature (<200 °C). Finally, the presence of chlorargyrite suggested that silver was transported as chloride complexes, indicating relatively high oxygen fugacity. The sequence of silver mineral precipitation suggests that the ore-forming fluids showed a temporal increase in pH and oxygen activity, and a decrease in temperature and sulfur fugacity. This indicates that the hydrothermal fluids that formed the deposit can be divided into early and late stage fluids, with the former being a relatively higher-temperature and higher‑sulfur fugacity fluid compared with the lower-temperature and higher‑oxygen-fugacity late-stage fluid. Finally, the presence of silver minerals with high gold contents (up to 2 wt%), as well as arsenian pyrite and arsenopyrite, may be useful indicators of prospective areas for gold exploration in this region.

Compositional evolution of the tetrahedrite solid solution in porphyry-epithermal system: A case study of the Baimka Cu-Mo-Au trend, Chukchi Peninsula, Russia

The Peschanka deposit and Nakhodka ore field occurring in the Baimka Cu-Mo-Au porphyry-epithermal trend in the western Chukchi Peninsula, Russia are spatially related to monzonitic rocks of the Early Cretaceous Egdykgych Complex. Tetrahedrite solid solution (Tts) was recognized at the deposit and within the ore field in the following assemblages: (1) porphyry stage bornite, chalcopyrite, and molybdenite, (2) transitional (subepithermal) stage sphalerite, galena, chalcopyrite, and As-free pyrite, (3) HS epithermal stage enargite, chalcopyrite and high-fineness native gold, and (4) IS epithermal stage As-rich pyrite, galena, sphalerite, and low-fineness native gold and electrum. The porphyry and subepithermal Tts crystals are oscillatory zoned because of variable contents of Sb and As. The literature data show that similar zoning has been recognized in the Tts crystals in the other porphyry Cu-Mo-Au and transitional assemblages and differs from the other type deposits. Therefore, such a zoning is considered to be a guide to these two mineralization types. No zoning was found in Tts referred to the HS and IS epithermal assemblages within the Baimka trend. The porphyry stage Tts evolves from Fe-rich tennantite ((sb=Sb/(Sb+As) below 0.01, fe=Fe/(Fe+Zn) 0.60–0.80) through oscillatory zoned tennantite enriched in Sb and Zn (sb 0.19–0.37, fe 0.56–0.66) to tetrahedrite enriched in Zn (sb 0.51–0.70, fe 0.39–0.66). This trend is caused by the Sb accumulation and increased fS2. The review of published data shows that such trend is typical of the other porphyry deposit worldwide. Therefore, it is considered to be a guide to distinguish porphyry deposits from the other type deposits containing fahlores and to distinguish porphyry-stage fahlores. The composition of the transitional stage Tts evolves from Zn-rich tetrahedrite (sb 0.56–0.82, fe 0.03–0.05) through Zn-rich tennantite (sb 0.03–0.19, fe 0.11–0.13) followed by Zn-rich oscillatory zoned solid solution (sb 0.03 to 0.69, fe 0.09 to 0.11) to goldfieldite. This evolution testifies to increased fTe2 to the end of transitional stage. The HS stage Tts corresponds to Fe-rich tennantite (sb below 0.05, fe 0.65–1.00) containing high Cuexcess (1.43 apfu). The IS stage Tts evolves from Zn-rich tennantite (sb=0, fe=0.44) to Zn-rich tetrahedrite (sb=0.97, fe=0.03). The latest Zn-rich tetrahedrite of this assemblage is enriched in Ag (up to 4.1wt.%) testifying to increasing Ag activity to the end of mineralizing process. The review of literature data shows the similar trend for the transitional and IS Tts in the other porphyry-epithermal systems. Therefore the Tts evolution trend is a criterion to separate porphyry, transitional, IS, and HS Tts.