Bisulfide Complexes Research Articles

Hydrothermal Graphite as a Trigger for High-Temperature Orogenic Gold Mineralization at Haoyaoerhudong, Northern China

Abstract Carbonaceous materials are a key factor controlling mineralization processes in many world-class gold deposits. Haoyaoerhudong is the largest carbonaceous metasediment-hosted gold deposit on the north margin of the North China craton. Gold-bearing orebodies are hosted in carbonaceous slates and schists belonging to Mesoproterozoic rift-related successions. Typical hydrothermal minerals are pyrrhotite, quartz, biotite, graphite, apatite, titanite, and native gold. The ore mineralogy, combined with microthermometry and Raman spectra on fluid inclusions, has demonstrated three stages of hydrothermal activity: (I) quartz-biotite ± sulfide stage associated with gold mineralization (315°–510°C; ~4.8 wt % NaCl equiv; H2O-NaCl-CO2 ± CH4 ± N2 system); (II) quartz-sulfide stage, including quartz-sulfide stringers (IIa, 250°–334°C; ~5.4 wt % NaCl equiv; H2O-NaCl-CH4 ± CO2 ± N2 system) and fractured quartz-sulfide ores (IIb, 234°–308°C; ~4.1 wt % NaCl equiv; H2O-NaCl-N2 ± CH4 system); and (III) post-ore quartz-calcite stage (70°–219°C; ~4.8 wt % NaCl equiv; H2O-NaCl system). The molar ratios of CO2 and CH4 progressively decreased from stage I to II, consistent with the occurrence of graphite in alteration zones. Microscopic observation and Raman spectra suggest that the fine-grained graphite from altered schist (Gr-1/2) and coarse-sized graphite from gold-bearing veins (Gr-3/4) are of high crystallinity and exhibit characteristics indicating a hydrothermal origin. The δ13C values of graphite, varying from −27.1 to −26.0‰ Vienna-Pee Dee Belemnite (V-PDB), suggest that the carbon was of biogenetic origin. Apatite Sr isotopes (87Sr/86Sr: 0.708293–0.708842) and titanite Nd isotopes (εNdt: –11.76 to –14.84) also indicate contributions from carbonaceous rocks during mineralization. Thermodynamic modeling demonstrates that graphite may have precipitated at Haoyaoerhudong due to cooling and reduction of the H2O-CO2-CH4 fluids at high temperatures. Graphite precipitation would significantly consume CO2 and effectively destabilize Au bisulfide complexes, facilitating the codeposition of pyrrhotite, graphite, and native gold at high temperatures (≥379°C). We infer that deposition of hydrothermal graphite is a crucial process for mesothermal-hypothermal mineralization in sediment-hosted orogenic gold deposits.

Nature and conditions of gold mineralization at the Tau deposit, Mupane mine, Tati Greenstone Belt, Botswana: Evidence from fluid inclusion and arsenopyrite geothermometer

AbstractThe gold mineralization at the Tau deposit in northeastern Botswana can be distinguished into two stages. The first stage of mineralization consists of invisible gold associated with early, sulfur‐rich arsenopyrite (Apy1). The second stage is characterized by native gold associated with late, sulfur‐poor arsenopyrite (Apy2). This study aims to constrain the nature of the ore‐forming fluid at the Tau deposit on the basis of fluid inclusion microthermometry and arsenopyrite geothermometry. Quartz crystals closely associated with gold‐bearing sulfides (Apy1) from the host rock host three types of fluid inclusions occurring in the same assemblages: Type I, two‐phase aqueous fluid inclusions, Type II three‐phase aqueous‐carbonic fluid inclusions and Type III one‐ or two‐phase CO2‐rich fluid inclusions. Microthermometric and Raman spectroscopic studies revealed the dominance of coexisting H2O+NaCl (Type I) and CO2N2CH4 (Type III) fluid inclusions. The coexisting Type I and Type III fluid inclusions could represent products of the immiscibility of a homogenous H2OCO2NaClN2CH4 primary ore fluid. The salinity of Type I (calculated from melting temperature of ice) and Type II (calculated from dissolution temperature of clathrate) fluid inclusions vary from 3.4 to 9.2 wt% NaCl equiv. and 5.8 to 9.8 wt% NaCl equiv., respectively. The overall fluid inclusion salinity range (<10 wt% NaCl equiv.) and gas compositions of fluids suggest that the primary ore‐forming fluid at the Tau deposit was of low salinity and H2OCO2N2CH4NaCl composition. Integration of fluid inclusion microthermometry and arsenopyrite geothermometer results suggest that the first stage and second stages of gold mineralization and the associated alteration at the Tau deposit occurred at pressure and temperature conditions of 75–145 MPa and 290–370°C and 85–160 MPa and 330–370°C, respectively. These results suggest that there was an increase in temperature from the first stage to the second stage of mineralization, resulting in the recrystallization of invisible gold‐bearing arsenopyrite in the first stage to form native gold‐bearing arsenopyrite in the second stage. Fluid composition and the estimated physico‐chemical conditions at the Tau deposit confirm that gold was transported as reduced bisulfide complexes and initial gold deposition was likely caused by a reduction in sulfur contents as a result of sulfidation reactions and pressure‐induced phase separation (fluid immiscibility).

Ore fluid characteristics and depositional processes of the hydrothermal gold mineralisation at Damang, Southwestern Ghana

Gold mineralisation at Damang in the southwestern part of Ghana occurs in two types of geological settings: (1) as paleoplacer in the Banket Series conglomerates of the Tarkwaian system and (2) in hydrothermal sheeted quartz vein system that crosscuts all the Tarkwaian lithologies including the palaeoplacer units. This paper focused on the second type because not much research has be conducted on the fluid inclusion and stable isotope characteristics of the Damang deposit to reveal the nature of the hydrothermal ore-forming associated the quartz veins and gold deposition. Microthermometry measurements of fluid inclusions in mineralised quartz veins indicated two sets of temperature-salinity variations: a relatively low temperature (142–292 °C) and moderate salinity aqueous fluids that range from 2.4 to 8.9 wt% NaCl equivalent, and a high temperature (210–393.6 °C) with low salinity aqueous-carbonic fluids with the salinity ranging from 1.0 to 2.6 wt% NaCl equivalent. These fluids are inferred to have unmixed from a homogeneous fluid with an average salinity of 3.85 ± 2.42 wt % NaCl equivalent. Laser Raman spectroscopic data on the fluid inclusions confirmed an aqueous-carbonic (H2O–CO2–NaCl) composition of the fluids with minor amounts of H2S, CH4 and SO2 as volatile species. The temperature of gold precipitation based on overlapped temperatures of chlorite, fluid inclusions and stable isotope geothermometers, indicated a mean temperature of 324.7 ± 20 °C and corresponding pressures from 1.1 to 3.5 kbar. The calculated oxygen isotopic composition of the ore fluids, δ18Ofluid using the estimated mineralisation temperature indicated average values of 9.3 ± 1.2‰. This isotopic range and the low salinity of the fluids (<10 wt % NaCl equiv.) are suggestive of metamorphic origin, which may be attributed to devolatisation reactions in the underlying Birimian greenstones. This is consistent with orogenic lode gold mineralisation in many Precambrian greenstone belts worldwide. The close association of gold with sulphides is consistent with gold transport as bisulphide complexes. A combination of factors including sulphidation and phase separation, concomitant with temperature decrease are inferred to be the principal factors that controlled the hydrothermal gold precipitation at Damang.

The roles of hydrocarbons on the mineralization of Carlin-type gold deposits, Nanpanjiang Basin, South China

The spatial association of hydrocarbons with metalliferous ore deposits, like MVT lead–zinc deposits, sandstone uranium deposits and various type gold deposits (epithermal Au-Ag (-Hg) deposits, orogenic gold deposits and Carlin-type gold deposits) is found worldwide. However, the genetic relationship between hydrocarbon evolution and mineralization is still under controversy. Integrating isotope dates related to timing of petroleum evolution and gold mineralization from published data, as well as high-resolution transmission electron microscopy (TEM) analysis of pyrobitumen, electron probe microanalysis (EPMA) on both pyrobitumen and pyrite, the In-situ S isotope analyses of the gold-bearing pyrite in this study, we consider different type hydrocarbons (liquid oil and gas) played different roles on the mineralization of Carlin type gold deposits. As the residue of liquid oil cracking, the widespread high maturity pyrobitumen within the reservoir indicate both oil and gas has ever accumulated in the Nanpanjiang Basin. The high gold abundance (EPMA: ∼120 ppm; TEM: ∼1500 ppm) within the pyrobitumen in agreement with the metal solubility experiment suggest liquid oils could have acted as gold carrier before metal precipitation. The late Triassic timing overlapping between gas generation and gold mineralization and the δ34S features with gold bearing pyrite ∼ 10 ‰ lighter than the coeval seawater sulfate indicate gas dominated TSR process may play an important role on the supply of gold bisulfide complex (Au(HS)2−) in the ore-forming fluid and directly involved the precipitation of the Carlin-type gold deposit.

Metallogenesis and depositional environment of the Archean-Proterozoic carbonaceous phyllites from the Dharwar Craton, India

Organic carbon rich meta-sedimentary rocks are important repositories of gold, PGE and vestiges of the biogeochemical cycles that prevailed during the Precambrian time. In this study, we report gold, PGE and δ34S concentrations of carbonaceous phyllites from Neoarchean Chitradurga, Gadag greenstone belts of western Dharwar Craton and Mangampeta of Proterozoic Cuddapah basin, eastern Dharwar Craton to understand their metallogenesis and depositional environments. Chitradurga phyllites (avg. Au = 180 ppb) show elevated concentrations of refractory gold compared to Gadag (avg. Au = 38 ppb) and Mangampeta shales (avg. Au = 23 ppb). The Archean carbonaceous phyllites display three generations of pyrites and their mineral chemistry suggests hydrothermal imprints while Proterozoic shales have two generations of diagenetic pyrites. Gadag has high PGE concentration (ΣPGE = 55 ppb) compared to Chitradurga (ΣPGE = 37 ppb) and Mangampeta (ΣPGE = 27 ppb). The PGE geochemistry is characterized by enrichment of PPGE over IPGE with Au > Pt > Pd and the Au/Pd, Pd/Pt, Ru/Ir, Ir/Pd ratios coupled with negative Pt (Pt/Pt*) anomalies indicate hydrothermal source. The Chitradurga and Gadag samples yielded low sulphur isotopic signatures (δ34S = 1.8–2 and 1.3–2‰ respectively) whereas the Proterozoic Mangampeta samples have comparatively higher values (δ34S = 8.4–34.3‰) suggesting hydrothermal and sea water sulphate sources respectively. Sulphate present in Archean and Proterozoic sea water was converted to sulphur by bacterial sulphate reduction which led to the formation of pyrites. In the Archean phyllites, the metal bearing bisulphide complexes were adsorbed into the pyrites while the Proterozoic shales are characterized by the partitioning of organo-metallic complexes into the pyrite during the diagenesis. Strong positive correlations of transition metals, redox sensitive elements with PGE indicate hydrothermal source and V/(V + Ni), U/Th, Th/U, authigenic U attest euxinic paleo-redox conditions. Thus, biogenically mediated redox depositional environments which prevailed during Archean and Proterozoic times have enhanced the metal sequestration processes that subsequently resulted in the ore formation in these carbonaceous phyllites.

Characteristics and nature of gold-bearing fluids in Fatira area, North Eastern Desert of Egypt: possible transition from intrusion-related to orogenic deposits

The gold lode in Fatira area is represented in three modes of occurrence: (1) altered felsite dykes, (2) massive quartz veins that are cutting through the hosting granitic rocks, and (3) siliceous-hybrid shear zone which is trending in NNW-SSE direction and separating between metavolcanic and granitic rocks. Metavolcanic rocks are described by meta-andesite and meta-dolerite varieties; they exhibit calc-alkaline magma type related to island arc system. However, granitic rocks are of I-type granodiorite composition with calc-alkaline affinity developed in convergent plate environment (island arc/active continental margins) during the late stage of orogeny. The wallrock alteration around the ore body includes propylitization, sericitization, and silicification facies. The ore minerals were observed as disseminated sulfides (pyrite, sphalerite, chalcopyrite, and covellite). Goethite, jarosite, and ilmenite are the main iron-rich mineral phase as well as malachite and rutile. Gold was detected by microprobe analysis within the internal composition of pyrite and/or jarosite. Two groups of fluid inclusions could be recognized in samples from different mineralized zones: (a) aqueous fluid inclusions (H2O–NaCl/KCl) and (b) aqueous-carbonic fluid inclusions (H2O–CO2–NaCl/KCl). Isochore construction from fluid inclusion data, combined with chlorite thermometry from alteration zone, revealed that Fatira gold deposits were formed at temperature range of 275–297 °C and pressure between 0.2 and 1.1 kb). The Fatira gold deposits exhibit similar features of both intrusion-related and orogenic deposits. Hence, the initial ore-bearing fluid was proposed to have originated from magmatic source at deeper levels related to the granitic emplacement (5–10 kb); moreover, the granitic batholith acted as an important vehicle for driving of the metamorphic hydrothermal currents and probably as a cause for the metamorphic process itself. Then, the migration of gold was taken place in form of bi-sulfide complexes and the final deposition occurred through fluid uplifting consequent with a possibility of mixing with meteoric water at shallow depths, pressure/temperature drop, and fluid/wallrock interactions.

Biogeochemical cycling of heavy metals in lake sediments: impact of multispecies diffusion and electrostatic effects

Fate and transport of heavy metals is controlled by the biogeochemical processes in the environment. Reactive transport modeling is particularly important for capturing the complex interplay between the microbial community dynamics and redox-stratified environments. The focus of this study is to investigate the impacts of (i) multicomponent diffusion (MCD) and (ii) electrical double layer (EDL) on reactive diffusive transport of heavy metals at Lake Coeur d’Alene (LCdA) sediments. The solute benthic fluxes at LCdA sediments are controlled by diffusion, and therefore, the biogeochemical model is focused purely on diffusive transport. In diffusive transport-dominated multicomponent systems, species-specific multicomponent diffusion (i.e., Nernst-Planck representation of diffusion) and the explicit treatment of electrostatic effects can play an important role on the overall dynamics of biogeochemical cycling of metals in the system. The results of this study demonstrate that the use of single uniform diffusion coefficient for all species in purely diffusion-dominated sediments may underestimate the mobility of heavy metals undergoing complex reaction network. This outcome is further signified when explicit treatment of EDL effects is considered in addition to MCD. The simulation results also illustrate the importance of aqueous metal (bi)sulfide complexes, especially when MCD and EDL effects are implemented in reactive transport simulations, impacting the solubility and dynamics of heavy metals in diffusion-dominated systems. The competitive effects of Fe-reducing bacteria FRB and sulfate reducing bacteria SRB activities on pH and overall biogeochemical processes are also demonstrated with multispecies diffusion and explicit treatment of electrostatic effects in the system.

Ore geology, fluid inclusion, and stable isotope constraints on the origin of the Damoqujia gold deposit, Jiaodong Peninsula, China

The Damoqujia gold deposit within the Zhaoping Fault Zone on Jiaodong Peninsula in eastern China is hosted primarily by Mesozoic granitoids and contains &gt;60 t of gold, making it an important gold producer. Three mineralization stages are distinguished (early, middle, and late): (K-feldspar)–sericite–quartz–pyrite, quartz – gold – polymetallic sulfides, and quartz–carbonate. Gold deposition occurred mainly in the middle stage. The primary fluid inclusions of three stages are mainly homogenized at temperatures of 236–389, 191–346, and 104–251 °C, with salinities of 2.96–11.33, 1.39–17.28, and 0.53–11.48 wt.% NaCl equivalent, respectively. Fluid inclusion studies indicate that the metallogenic system evolved from CO2-rich mesothermal homogeneous fluids to CO2-poor aqueous fluids due to inputs of meteoric waters. The gold was carried as a bisulfide complex in the ore-forming fluids. Precipitation of gold was caused by a combination of fluid immiscibility and water–rock interaction. Studies of the fluid inclusion characteristics (medium temperature, CO2-rich, and low salinity H2O–CO2–NaCl homogeneous system), hydrogen and oxygen isotopes ([Formula: see text] = –1.0‰ to 7.6‰, δD = –109‰ to –77‰), sulfur values ([Formula: see text] = 4.5‰ to 8.5‰), and regional geological events show that the ore-forming fluids reservoir was likely metamorphic in origin. Based on the immiscibility of fluid inclusion assemblages, the estimated depth and pressure of trapping are 8.3–10.2 km and 83–276 MPa, respectively, corresponding to the depth and pressure of mineralization.

The role of Pb(II) complexes in hydrothermal mass transfer: An X-ray absorption spectroscopic study

We studied the nature and geometry of Pb(II) complexes in chloride-, bromide-, and bisulfide-bearing solutions in situ at pressures of 600 and 800 bar and temperatures up to 489 °C using in situ XAS spectroscopy combined with ab initio molecular dynamics (MD) simulations. In halide-free acidic solutions, Pb(II) complexes to approximately seven water ligands (predominantly [Pb(H2O)7]2+) at 30 °C; this hydration number reduces to approximately six ligands at 191 °C. Due to the influence of a stereochemically active electron lone pair, Pb(II) halide complexes are characterised by a broad bond distance distribution, highly flexible structures, and a distinct gap for excess electron density. Residual maps (based on R-factor) of parameter space confirmed that many possible parameter combinations resulted in equally satisfactory fits of the EXAFS data. Thus, EXAFS interpretation was constrained using MD simulations and results of experimental speciation and solubility studies available in the literature. The results indicate that in chloride-bearing solutions, at T > 200 °C, Pb(II) has a maximum coordination of four to five ligands, and the Pb-complex is always coordinated to a combination of water and chloride; even at 400 ≤ T ≤ 500 °C in an acidified solution containing ~10 m Cl−, the predominant Pb(II) complex is [Pb(H2O)1-2Cl3]−. The highest order bromide complex similarly contains three halide ligands ([Pb(H2O)0.5Br3]− in a ~4 m Br− acidified solution above 400 °C), but the hydration number is lower than in the case of chloride. With regard to bisulfide complexing, the solubility of galena in the presence of bisulfide was lower than expected from extrapolations using available low temperature solubility data, but the formation of Pb(II) bisulfide complexes was confirmed by the increase in solubility with increase in bisulfide concentration.The new data show that in terms of coordination geometry, Pb(II) behaves like semi-metals such as Sb(III), As(III), Bi(III), or Te(IV), which are affected by a stereochemically active lone pair, rather than like divalent first row transition metals such as Zn. Pb(II)-chloride complexes with a maximum of three chloride ligands are the dominant complexes responsible for Pb transport. In contrast, Zn(II) forms tetrahedral chloride complexes, and ZnCl42− is stable in brines up to temperatures ≥300 °C. These differences in coordination chemistry, combined with the differences in solubilities of Zn and Pb minerals, account for Zn/Pb fractionation in hydrothermal fluids.

Hydrothermal processes at the Axi epithermal Au deposit, western Tianshan: Insights from geochemical effects of alteration, mineralization and trace elements in pyrite

As the largest low-sulfidation (LS) epithermal deposit in the Tulasu basin, western Tianshan, NW China, the Axi gold deposit has a poorly understood geologic history. It is not clear whether hydrothermal mineralization occurred during a single event or multiple events. In this paper, new data consist of whole rock geochemistry and mass-change calculations for hydrothermal alteration. LA-ICP-MS trace element analysis of pyrite was conducted in order to develop a comprehensive metallogenic model. The pyrite-sericite-quartz alteration at the Axi deposit is characterized by replacement of plagioclase and mafic phenocrysts by sericite, quartz, pyrite (py1), smectite, and illite, indicating slightly acidic and reducing conditions during the early ore stage. Medium subhedral pyrite crystals (py1) with porous texture are typically sited adjacent to iron-rich minerals (e.g., hornblende) and are characterized by Co-Ni-Mn enrichment with high ratios of Co/Ni (avg. 1.114) and Cu/As (avg. 0.655), implying preferential sulfidation reactions in magmatic hydrothermal systems. The quartz ± calcite ± pyrite ± adularia assemblages in silicified rocks and the strong mobility of LREE both reflect a slightly acidic chloride-rich solution phase during the later ore stage. Coarse anhedral pyrites with cataclastic texture (py2) and medium euhedral pyrites (py3) have a mixed hydrothermal origin and were deposited with quartz during Au-pyrite-quartz and Au-polysulfide-quartz stages, respectively. Py2 and py3 both have significantly lower Cu/As ratios (avg. 0.103 and avg. 0.088, respectively) than py1, indicating the input of meteoric fluid during the later ore stage. The three generations of pyrite, especially py3, have significant lattice-bound gold contents (py1: 0.01–38.86 ppm, median 1.51 ppm; py2: 0.03–234.37 ppm, median 1.21 ppm; py3: 3.31–231.56 ppm, median 42.72 ppm). Gold precipitation is associated with the destabilization of Au-bearing chlorine (Au(Cl)2−) and bisulfide complexes (Au(HS)2−) via sulfidation and fluid boiling. All of these characteristics indicate the Axi epithermal deposit is the result of two stages of mineralization.

The solubility of silver in magmatic fluids: Implications for silver transfer to the magmatic-hydrothermal ore-forming environment

Silver is a common and economically important constituent of many magmatic-hydrothermal ore deposits, yet little is known about its mobility during magma degassing. We performed experiments to determine the effect of various chemical components (NaCl, KCl, LiCl, HCl, CaCl2, H2S) on the solubility and speciation of Ag in high-temperature, low-density fluids (≈0.38–0.54 g/cm3). The experiments were conducted at T = 900 °C, P = 2000 bar and oxygen fugacity (fO2) of 0.5 log units below Ni-NiO buffer (NNO-0.5) in rapid-quench Molybdenum-Hafnium Carbide externally-heated pressure vessel assemblies. The fluid phase was sampled at run conditions by the entrapment of synthetic fluid inclusions (SFI) in in situ fractured quartz chips. As capsule material, Au97Ag2Cu1 alloy was used, which imposed an Ag activity of 0.005. The apparent Ag solubility, defined as the equilibrium concentration of Ag in the fluid phase at an Ag activity of 0.005, exponentially increases as a function of total chloride concentration in the NaCl-H2O and NaCl-HCl-H2O systems. In the mixed NaCl-HCl system, at a fixed total Cl concentration of 1 mol/kg (1 m) H2O and varying NaCl/HCl ratios, the apparent solubility of Ag reaches a maximum (432 ± 123 μg/g) at equal NaCl and HCl concentration (0.5 m each) and then decreases towards both end-members (1 m for NaCl or HCl) following a parabolic function. Apparent silver solubilities predicted using the HKF model and thermodynamic properties for traditional Ag-chloride and bisulfide complexes greatly underestimate the measured values. Model calculations suggest that instead of charged AgCl2−, a neutral NaAgCl2 species is the dominant dissolved Ag species in S-free, chloride-bearing fluids with the additional presence of NaAgHSCl in S-bearing fluids. Comparison of the solubility data in high-temperature fluids to measured Ag solubilities in silicate melts suggests that efficient Ag partitioning into magmatic fluids will only take place when the composition of the silicate melt is felsic enough (rhyodacitic to rhyolitic), similarly to the behavior of Cu but in contrast to that of Au.

Fluid evolution of Au-Cu zones in Um Balad area, North Eastern Desert of Egypt: Implications from mineral chemistry and fluid inclusions

Scanning electron microscope (SEM), Electron microprobe (EMPA) and fluid inclusion studies of the ore body, as well as geochemical analyses of country rocks were performed to determine the nature and characteristics of the mineralizing fluid responsible for Au-Cu deposits in Um Balad area, Northern Eastern Desert of Egypt. The Um Balad Au-Cu deposits are confined to well developed-quartz veins and veinlets cutting through the hosting country rocks. Petrographic and geochemical investigations of the hosting rocks distinguished between two main rock units; 1) metagabbro-diorite rocks with tholeiitic nature derived in island arc/continental margin tectonic regime, and 2) granodiorite rocks formed from calc-alkaline magma in continental margin regime. Wallrock alterations are represented by propylitic and argillic types. The mineralized quartz veins are striking in NE-SW direction and dipping between (35°–45°) in SE direction, other mineralized mafic dykes enriched with auriferous quartz veinlets are trending NE-SW and dipping 70°/SE. The main ore minerals are represented by gold, chalcopyrite, pyrite, sphalerite, malachite, covellite and goethite. While, geffroyite, cuprite, chrysocolla, pseudomalachite, britholite, wolframite, scheelite, hematite and rutile are detected as minor constituents. Fluid inclusions microthermometry and isochore calculations combined with chlorite geothermometry revealed that the Um Balad deposits were formed at temperature ranging from 305 °C to 325 °C and pressure between (100–500 bar). The mineralization had been developed in the shallow levels, beneath the water table at depth of 350–1760 m, rather than common mesothermal vein-type deposits in Egypt. Magmatic water have been suggested as the main source for the mineralized fluid. The transportation of the gold metal seems to be happen as bisulfide complexes in moderately acidic environment. The deposition was resulted from combination of changes in physico-chemical parameters, temperature and pressure plus the instability of the reduced sulfur complexes. A contamination with metamorphic and/or meteoric water was also proposed that has strong influence during the depositional process.

Geochemistry and mineralogy of contrasting supergene gold alteration zones, southern New Zealand

Supergene alteration of gold deposits has had important geochemical effects that changed the mineralogy and textures of near-surface exposures and can affect the economics of gold mines by enhancing gold liberation and increasing gold grades. This study links observed mineralogy with 655 water analyses from environments relevant to supergene processes in the Otago Schist goldfield of southern New Zealand, and these analyses are used as proxies for past supergene processes. There are two contrasting supergene alteration styles in otherwise identical host rocks and mineralisation zones in this area. Long-term groundwater alteration of sediments and underlying basement near to a regional unconformity developed progressively since the Cretaceous. This alteration has affected orogenic gold deposits exposed at the unconformity, and has resulted in supergene enrichment of gold with localised formation of centimetre scale gold nuggets. Alteration processes occurred under circumneutral pH conditions (pH 6–8.5), and was accompanied by extensive oxidation and clay alteration of basement rocks and sediments. Gold was mobilised as thiosulphate and bisulphide complexes and was reprecipitated by redox changes near the unconformity. The chemical processes of supergene enrichment were primarily abiotic, with minor bacterial facilitation of late stage gold mobility at the micron scale. In contrast, supergene alteration of eroding freshly-exposed mountain outcrops has occurred in the late Cenozoic without Au enrichment apart from liberation and minor particle size enhancement. The ambient pH of the associated waters was also circumneutral, but localised and temporary acidification (to pH ∼3) has occured on the metre scale or less in sulphidic outcrops. This supergene alteration was dominated by oxidation but some low redox minerals, senarmontite and secondary stibnite, have formed during the early stages of oxidation or subsequent reduction. Sulphide decomposition, with associated localised acidification and gold remobilisation was probably enhanced by bacterial processes in these exposed outcrops.

Nature of ore-forming fluid and formation conditions of BIF-hosted gold mineralization in the Archean Amalia Greenstone Belt, South Africa: Constraints from fluid inclusion and stable isotope studies

Orogenic gold mineralization in the Amalia greenstone belt is hosted by oxide facies banded iron-formation (BIF). Hydrothermal alteration of the BIF layers is characterized by chloritization, carbonatization, hematization and pyritization, and quartz-carbonate veins that cut across the layers. The alteration mineral assemblages consist of ankerite-ferroan dolomite minerals, siderite, chlorite, hematite, pyrite and subordinate amounts of arsenopyrite and chalcopyrite. Information on the physico-chemical properties of the ore-forming fluids and ambient conditions that promoted gold mineralization at Amalia were deduced from sulfur, oxygen and carbon isotopic ratios, and fluid inclusions from quartz-carbonate samples associated with the gold mineralization.Microthermometric and laser Raman analyses indicated that the ore-forming fluid was composed of low salinity H2O-CO2 composition (~3wt%NaCl equiv.). The combination of microthermometric data and arsenopyrite-pyrite geothermometry suggest that quartz-carbonate vein formation, gold mineralization and associated alteration of the proximal BIF wall rock occurred at temperature-pressure conditions of 300±30°C and ∼2kbar. Thermodynamic calculations at 300°C suggest an increase in fO2 (10−32–10−30 bars) and corresponding decrease in total sulfur concentration (0.002–0.001m) that overlapped the pyrite-hematite-magnetite boundary during gold mineralization. Although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site, the calculated low fO2 values are consistent with previously determined high Fe/Fe+Mg ratios (>0.7) in associated chlorite, absence of sulfates and restricted positive δ34S values in associated pyrite. Based on the fluid composition, metal association and physico-chemical conditions reported in the current study, it is confirmed that gold in the Amalia fluid was transported as reduced bisulfide complexes (e.g., Au(HS)2−). At Amalia, gold deposition was most likely a combined effect of increase in fO2 corresponding to the magnetite-hematite buffer, and reduction in total sulfur contents due to sulfide precipitation during progressive fluid-rock interaction.The epigenetic features coupled with the isotopic compositions of the ore-forming fluid (δ34SΣS=+1.8 to +2.3‰, δ18OH2O=+6.6 to +7.9‰, and δ13CΣC=−6.0 to −7.7‰ at 300–330°C) are consistent with an externally deep-sourced fluid of igneous signature or/and prograde metamorphism of mantle-derived rocks.

The Hydrothermal Fluid Evolution of Vein Sets at the Pipeline Gold Mine, Nevada

The origin of sediment-hosted Nevada gold deposits has been highly debated, especially regarding the relative contribution of multiple mineralizing events, particularly relating to the Cretaceous. We examined the Pipeline gold mine in north-central Nevada, focusing on data from the four vein sets in this atypical deposit where there is evidence for Cretaceous gold mineralization. Only the third, a quartz-sericite-pyrite-calcite vein set, has any link with the alteration styles and gold mineralization within the Pipeline deposit. Our geochemical results from fluid inclusion microthermometry and gas analysis show that the fluids from which quartz deposited were sourced from condensing magmatic volatiles and were trapped at ~300 °C and 2 kbar lithostatic pressure (~8 km). 40Ar/39Ar dating of sericite demonstrates that the quartz-sericite-pyrite veins formed at ~92 Ma, matching the dates of gold-associated epigenetic illite. Ore fluids enriched in CO2 and H2S caused decarbonation thereby releasing Fe2+ that reacted with H2S to form pyrite. Decreasing H2S destabilized gold bisulfide complexes and deposited gold. We conclude that this process can occur in a single Cretaceous event in advance of potential Tertiary mineralization.

Silver–bearing minerals in the Xinhua hydrothermal vein-type Pb-Zn deposit, South China

Electron probe microanalysis (EPMA) results are reported for newly identified silver–bearing minerals from the Xinhua deposit, Yunkaidashan area, South China. The Xinhua deposit is a hydrothermal vein–type Pb–Zn deposit and is hosted in the Pubei Complex, which consists of a cordierite–biotite granite with a U–Pb zircon age of 244.3 ± 1.8–251.9 ± 2.2 Ma. The mineralization process is subdivided into four mineralization stages, characterized by the following mineral associations: mineralization stage I with quartz, pyrite, and sphalerite; mineralization stage II with siderite, galena, and tetrahedrite; mineralization stage III with quartz and galena; and mineralization stage IV with quartz, calcite, and baryte. Tetrahedrite series minerals, such as freibergite, argentotetrahedrite, and tennantite are the main Ag–bearing minerals in the Xinhua deposit. The greatest concentration of silver occurs in phases from mineralization stage II. Microscopic observations reveal close relationship between galena and tetrahedrite series minerals that mostly occur as irregular inclusions within galena. The negative correlation between Cu and Ag in the lattices of tetrahedrite series minerals suggests that Cu sites are occupied by Ag atoms. Zn substitution for Fe in argentotetrahedrite and Cd substitution for Pb in tetrahedrite are also observed. Micro–thermometric data reveal that both homogenization temperatures and calculated salinities of hydrothermal fluids decrease progressively from the early to the later mineralization stages. The metal ions, such as Ag+, Cu+, Pb2+, and Zn2+, are transported as chlorine complex ions in the early mineralization stage and as bisulfide complex ions in the late mineralization stage, caused by changes in oxygen fugacity, temperature, and pH of the hydrothermal fluids. Because of the varying solubility of different metal ions, Pb2+, Zn2+, and Cu2+ ions are initially precipitated as galena, sphalerite, and chalcopyrite, respectively. With decreasing temperature of the fluids, Pb2+ ions are incorporated along with Cu+, Sb3+, and As3+ ions into sulfosalt minerals, and Ag+ ions are coprecipitated with Cu+, Sb3+, and As3+ ions forming tetrahedrite series minerals or replacing earlier sulfides and sulfosalts.

Resolving the role of carbonaceous material in gold precipitation in metasediment-hosted orogenic gold deposits

Abstract Carbonaceous material (CM) is commonly associated with gold and sulfides in metasediment-hosted orogenic gold deposits. The role of CM in Au deposition is controversial; CM has been proposed to contribute to gold deposition by reducing Au bisulfide complexes, or by facilitating sulfidation, which destabilizes Au in bisulfide complexes with resultant Au deposition. Integration of petrographic observations, thermodynamic models, and geochemical data from metasediment-hosted orogenic gold deposits in New Zealand, Australia, Canada, and West Africa reveals genetic links between sulfides, CM, and mineralization. The results are consistent with the coexistence of CM and pyrite as a consequence of their codeposition from ore fluids, with a minor proportion of CM originally in situ in the host rocks. Au is deposited when pyrite and CM deposition decreases H2S concentration in ore fluids, destabilizing Au(HS)2– complexes. Most CM in gold deposits is deposited from CO2 and CH4 in ore fluids. These findings are applicable to similar deposits worldwide.

Speciation and thermodynamic properties of zinc in sulfur-rich hydrothermal fluids: Insights from ab initio molecular dynamics simulations and X-ray absorption spectroscopy

Chlorine and sulfur are the main elements involved in the complexing of metals in ore-forming fluids. The nature and thermodynamic properties of the Zn(II)–Cl complexes have been investigated by previous experimental and theoretical studies and are now well established up to high temperatures (600°C). In contrast, the role of bisulfide complexes for zinc speciation in sulfur-bearing fluids remains poorly known, and a better understanding of Zn(II)–HS complexation is required for modeling zinc transport in magmatic and metamorphic fluids and for optimizing the hydrometallurgical processing of sulfide ores.We have conducted ab initio molecular dynamics (MD) simulations to calculate the speciation of Zn(II)–HS complexes from ambient to hydrothermal-magmatic conditions (25–600°C, up to 2000bar). These theoretical calculations were complemented by X-ray absorption spectroscopy (XAS) measurements of Zn(II) in HS−-rich solutions at 200–500°C and 600–1000bar. The speciation and geometrical properties predicted by the ab initio MD simulations and the in situ XAS data are in excellent agreement. Upon heating from room temperature to 250°C, Zn(II) speciation in HS−-rich solutions shows a transition from the sixfold octahedral hexaaquo complex [Zn(H2O)6]2+ to fourfold tetrahedral [Zn(HS)n(H2O)4−n]2−n complexes (n=1–4). Ab initio MD simulations also show that at temperatures>250°C, the threefold trigonal-planar [Zn(HS)3]− complex becomes increasingly stable, and predominates in S-rich solutions; in contrast, chloro-complexes display a tetrahedral geometry at 25–500°C, while trigonal planar ZnCl3− predominates at temperatures>500°C. The stability constants of Zn(II)–HS complexes were calculated by thermodynamic integration of constrained ab initio MD simulations at 200, 350 and 600°C. The stability constants generated from this study predict that zinc can be transported by HS− at high temperature in reduced, neutral to alkaline solutions, while Zn chloride complexes dominate in acidic fluids.

Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt

The Las Cruces deposit is in the eastern end of the Iberian Pyrite Belt (SW Spain). It is currently being mined by Cobre Las Cruces S.A. The main operation is focused on the supergene Cu-enriched zone (initial reserves of 17.6 Mt @ 6.2% Cu). An Au-Ag-Pb–rich gossan resource (3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag) occurs in the upper part of the deposit. The Au grade ranges from 0.01 ppm to >100 ppm, and occurs as three different Au ore types: (1) Au mineralization in the upper part of the gossan linked to Fe-oxides lithofacies, (2) Au concentration in the lower part of the gossan associated with leached black shales, and (3) Au ore in the cementation zone related to subvertical fractures. A hydroseparation device has been used to obtain heavy mineral concentrates from selected samples of different ore types. Reflected-light microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA) were used to study the separated Au particles. Significant differences between the defined ore types include the Au-bearing lithologies, mineral associations, textural features, particle sizes, morphologies, and fineness. Au-rich minerals include native Au, Au-Ag electrum, and Au-Ag-Hg amalgams. Gold-bearing mineral associations include Pb-oxihalides, Fe-oxides, galena, pyrite, cinnabar, and Ag-sulfosalts. The Au enrichment mechanism in the supergene profile involves (1) dissolution of Au from the primary sulfides as chloride-rich ionic complexes during the weathering of the deposit under subaerial exposure; dissolved Au is transported downward through the supergene profile under acidic and oxidized conditions; (2) destabilization of the Au complexes by Fe-controlled redox reactions; as a consequence, coarse-grained, high-fineness Au particles precipitated in association with Fe-oxyhydroxides. This resulted in secondary concentration in the upper gossan; and (3) after deposition of cover sediments took place a progressive change in the system conditions resulting in a later Au remobilization as hydroxidehalide, hydroxide, thiosulfate, and bisulfide complexes in the lowermost gossan and cementation zone. The main pathways for migration of enriched fluids to the cementation zone are secondary permeability zones linked to Alpine reactivated faults. Deposition of Au seems to be related to fluid interaction with reductant lithologies, including black shales and the primary sulfides.

Rare greenockite (CdS) within the chromite–PGE association in the Bangur Gabbro, Baula-Nuasahi Complex, Eastern India

We report the presence of greenockite from the chromite–PGE–base metal sulfide association in the Bangur Gabbro, Baula-Nuasahi mafic–ultramafic complex. The CdS phase occurs, sans any precursor Cd-bearing phase, as minute grains within siderite micro-veins in close proximity to chlorite in the chalcopyrite+pyrrhotite+pentlandite+violarite assemblage. Using various calibrations, chlorite compositions yielded temperature ranges of 245 to 325°C. The associated siderite might have formed at or little below the above temperature. Electron probe micro-analyses (EPMA) of greenockite reveal small amounts of Fe (1.01–1.61wt.%), Zn (1.89–4.54wt.%), and substitution of Zn for Cd. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of chalcopyrite nearer to and away from siderite micro-veins that host greenockite confirms maximum Cd concentrations of 78 and 144ppm, respectively, entailing possible derivation of Cd from chalcopyrite. We propose the transport of Cd as bisulfide complex in a low temperature alkaline and reducing fluid and the simultaneous precipitation of greenockite and siderite by reaction with Fe-rich minerals, aided by decrease in pH and/or increase in fO2.