Gold Precipitation Research Articles

Pyrite textures and compositions from the Zhuangzi Au deposit, southeastern North China Craton: implication for ore-forming processes

The Zhuangzi Au deposit in the world-class Jiaodong gold province hosts visible natural gold, and pyrite as the main ore mineral, making it an excellent subject for deciphering the complex hydrothermal processes and mechanisms of gold precipitation. Three types of zoned pyrite crystals were distinguished based on textural and geochemical results from EPMA, SIMS sulfur isotopic analyses and NanoSIMS mapping. Py0 has irregular shapes and abundant silicate inclusions and was contemporaneous with the earliest pyrite–sericite–quartz alteration. It has low concentrations of As (0–0.3 wt.%), Au and Cu. Py1 precipitated with stage I mineralization shows oscillatory zoning with the bright bands having high As (0.4–3.9 wt.%), Au and Cu contents, whereas the dark bands have low contents of As (0–0.4 wt.%), Au and Cu. The oscillatory zoning represents pressure fluctuations and repeated local fluid phase separation around the pyrite crystal. The concentration of invisible gold in Py1 is directly proportional to the arsenic concentration. Py1 is partially replaced by Py2 which occurs with arsenopyrite, chalcopyrite and native gold in stage II. The replacement was likely the result of pseudomorphic dissolution–reprecipitation triggered by a new pulse of Au-rich hydrothermal fluids. The δ34S values for the three types of pyrite are broadly similar ranging from + 7.1 to + 8.8‰, suggesting a common sulfur source. Fluid inclusion microthermometry suggests that extensive phase separation was responsible for the gold deposition during stage II mineralization. Uranium–Pb dating of monazite constrains the age of mineralization to ca. 119 Ma coincident with a short compressional event around 120 Ma linked to an abrupt change in the drift direction of the subducting Pacific plate.

Structures and Implications for Fluid Migration in the Jiadi Carlin‐Type Gold Deposit, Guizhou Province, Southwest China

AbstractThe Jiadi gold deposit is a newly discovered Carlin‐type gold deposit in the Guizhou Province, Southwest China. This deposit is structurally controlled by a shallow fold–fault superimposed system along the Lianhuashan trend. Field geological investigations, structural analysis, and mathematical research are conducted to study its structures and hydrothermal fluid flow process. Geological investigations (i.e. sections, stope, and drill hole) indicate that the structures are dominated by NE‐trending folds, NWW‐trending folds, and faults. A trend‐surface analysis of the low interformational fracture zone suggests that the overall distribution of the Lianhuashan trend is controlled by the NE‐trending Lianhuashan anticline. Nearly all primary Carlin‐type gold deposits are distributed along the southeastern flank where the fold axis changes from NE to EW. Gold orebodies are hosted by the interformational fracture zones and primarily situated at the transitions from the high‐value areas to the low‐value areas of the interformational fracture zones. A stress analysis on the hydrothermal veins indicates that they are of tenso‐shear properties. The ore‐forming elements (Au, As, Sb, Hg, and Tl) of the hydrothermal veins from the interformational fracture zones and intrusive breccia body present strong positive anomalies compared with those from the adjacent wall rocks. According to the development patterns of the ore‐forming elements and hydrothermal veins, a migration process of the ore‐forming fluids is proposed: the deeply sourced ore‐forming fluids migrated vertically to shallow crust along an intrusive breccia body, subsequently flowed horizontally along the interformational fracture zones accompanied by gold precipitation in the early stage, and finally migrated outward along steep micro‐fractures during following stages.

Shear-zone hosted gold mineralization of the Arabian–Nubian Shield: devolatilization processes across the greenschist–amphibolite-facies transition

Abstract Orogenic gold ores of the Arabian–Nubian Shield are structurally controlled by the Najd Fault System. The Najd Fault System controlled the exhumation of the metamorphic complexes and, as such, there is a genetic relationship between the metamorphism and the formation of the orogenic gold ores. In order to constrain this genetic relationship, field observations, petrography, geochemistry and fluid inclusions of four mines from the Egyptian side of the shield are presented. The studied gold-bearing dykes and veins are structurally controlled where the gold-bearing fluid precipitated in pre-existing second-order and third-order extensional faults of the major NW–SE Najd Fault System. Fluid inclusions indicate that the gold was precipitated at shallow- to medium-crustal levels, equivalent to a temperature range of 250–350°C, and from low salinity metamorphic fluids, possibly mixed with magmatic/meteoric water. Thermodynamic modelling suggests that gold-bearing fluids were generated due to metamorphic devolatilization processes across the greenschist–amphibolite-facies transition of ophiolitic and metasedimentary source rocks. The Najd Fault System enables the vertical transport of gold-bearing fluids from the source region to the depositional sites. Decreasing the temperature of the fluid is required to precipitate the gold. However, the gold precipitation process needs to be buffered by Fe-bearing wall rocks.

Supported Gold Nanoparticles for Furfural Valorization in the Future Bio-based Industry

Gold nanoparticles prepared by sol immobilization using PVA as protecting agent were supported over different oxides (CeO2, ZrO2 and TiO2). The activity of the gold catalysts was tested in the oxidative esterification of furfural by an efficient and sustainable process. The samples supported on ceria and zirconia (AuCe and AuZr) have shown complete conversion and selectivity under mild reaction conditions. The catalytic performances are better than those obtained with the reference commercial AuTiWGC. The DR UV–Vis spectroscopic results indicate that gold nanoparticles that give rise to similar plasmonic band, as in the case of AuCe and AuZr, are very active and highly selective. The extent of crystallinity of the gold particles seems to have no influence on the activity of the sol immobilized catalysts. The null selectivity of the catalyst supported on titania is probably due to the presence of residual sulphate groups. The effect of the oxygen pressure on the furfural conversion is almost negligible for all the samples and, despite of the oxygen pressure lowering, it is possible to obtain always high conversion and selectivity. The selectivity of the process is unaffected by using air instead of oxygen, even at very low pressures. On the contrary, the presence of PVA seems to influence the conversion when working in air at low pressures, due to a dilution effect of the oxidant atmosphere. This is confirmed by the results obtained for a sample synthesized by deposition precipitation of gold on zirconia, where the high metal dispersion and the absence of the protecting agent allowed to reach high conversion even in low air pressure.

Atomic scale origins of sub-band gap optical absorption in gold-hyperdoped silicon

Gold hyperdoped silicon exhibits room temperature sub band gap optical absorption, with potential applications as infrared absorbers/detectors and impurity band photovoltaics. We use first-principles density functional theory to establish the origins of the sub band gap response. Substitutional gold AuSi and substitutional dimers AuSi − AuSi are found to be the energetically preferred defect configurations, and AuSi gives rise to partially filled mid-gap defect bands well offset from the band edges. AuSi is predicted to offer substantial sub-band gap absorption, exceeding that measured in prior experiments by two orders of magnitude for similar Au concentration. This suggests that in experimentally realized systems, in addition to AuSi, the implanted gold is accommodated by the lattice in other ways, including other defect complexes and gold precipitates. We further identify that it is energetically favorable for isolated AuSi to form AuSi − AuSi, which by contrast do not exhibit mid-gap states. The formation of dimers and other complexes could serve as nuclei in the earliest stages of Au precipitation, which may be responsible for the observed rapid deactivation of sub-band gap response upon annealing.

Investigation of Reduction and Precipitation Rate of Colloidal Gold Particles Obtained in the Process of Electrical and Electronic Waste Recycling

This paper studies the influence of process parameters: temperature, the weight of the added precipitant agent, pH and reducing agent on the kinetics of the process of reduction and precipitation of gold of 98.44% purity obtained in the process of electrical and electronic waste recycling. The optimal conditions of reduction and precipitation of colloidal gold particles in the process of recycling of different waste types in order to obtain high-purity gold are the following: an optimal process time (τ = 11minutes), temperature (t=5°C), the weight of the added precipitant (m = 6g of pure gold) and K2S2O5 as the preferred reducing agent. The results have contributed to a better understanding of the reduction and precipitation rate of colloidal gold particles in order to manage and control this process. The granulometric analysis confirms the existence of colloidal gold particles. SEM analysis indicates spherical particles and good homogeneity of the sample. XRF analysis shows that gold obtained in dis process of recycling is highly pure (99.99%).

Delivery of Interference RNA Molecules to Ovarian Cancer Cells Using Gold‐Liposome Nanoparticle Conjugates

RNA interference is a therapeutic modality in which RNA molecules, such as small interference RNAs (siRNAs) and microRNAs (miRNAs) (inhibitors and mimics), are used to regulate the expression of target mRNAs. MiRNAs are multitargeting RNA molecules that are involved in the regulation of several targets and can therefore influence multiple pathways when deregulated. On the other hand, siRNAs repress a specific gene by sequence complementarity to the target mRNA. Although both miRNAs and siRNAs have an increasing therapeutic potential, systemic delivery of RNA‐based technologies is limited by the poor stability of RNA molecules in circulation. Hence, our main goal is to develop a carrier for the efficient and targeted delivery of RNA molecules to ovarian cancer cells. To achieve this, we propose a nanocarrier formulation of gold nanoparticle (AuNP)‐RNA conjugates encapsulated on folate receptor targeted liposomes that could achieve a double protection layer for the RNA while increasing delivery to ovarian cancer cells that overexpress the folate receptor on their surface. We prepared carriers for siRNAs and miRNA oligonucleotide mimics by conjugation of the RNA molecules to the surface of 15 nm diameter AuNPs via thiol linkages. We performed this conjugation in presence of polyethylene glycol (PEG‐2000) to avoid gold precipitation. Physical characterization of the carriers was conducted by dynamic light scattering (DLS) and quantification of conjugated RNA was assessed by fluorescence. The conjugation resulted in approximately 47 to 75 RNA molecules linked per nanoparticle and conjugates of 20 to 25 nm diameter with a slightly negative charge. In addition, encapsulation of the AuNP‐RNA conjugates into liposomes was attained with more than 70% encapsulation efficiency. These results indicate the feasibility of the development and production of our nanocarrier. As a proof of principle we are using this nanoparticle conjugate to target Integrin‐linked kinase (ILK). ILK is a pseudokinase that has been found to be aberrantly expressed in ovarian tumors. Further experiments of internalization, stability and toxicity demonstrate the efficacy of this nanocarrier as a delivery system for miRNA mimics and siRNAs to ovarian cancer cells. We conclude that siRNA‐based technologies are an alternative strategy to target ILK and other target proteins lacking specific chemical inhibitors.Support or Funding InformationRCMI: U54 MD007600This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Fault geometry and fluid-rock reaction: Combined controls on mineralization in the Xinli gold deposit, Jiaodong Peninsula, China

The structures and fluid-rock reaction in the Xinli gold deposit, Jiaodong Peninsula, were investigated to further understand their combined controls on the development of permeability associated with ore-forming fluid migration. Orebodies in this deposit are hosted by the moderately SE-to S-dipping Sanshandao–Cangshang fault (SCF). Variations in both dip direction and dip angle along the SCF plane produced fault bends, which controlled the fluid accumulation and ore-shoot formation. Gold mineralizations occurred in early gold–quartz–pyrite and late gold–quartz–polymetallic sulphide stages following pervasive sericitization and silicification alterations. Theoretical calculation indicates that sericitization caused 8–57% volume decrease resulting in the development/enlargement of voids, further increase of grain-scale permeability, and resultant precipitation of the early gold–quartz–pyrite pods, which destroyed permeability. The rock softening produced by alterations promoted activities of SCF secondary faults and formation of new fractures, which rebuilt the permeability and controlled the late gold–quartz–polymetallic sulfide veins. Quantitative studies on permeability distributions show that the southwestern and northeastern bend areas with similar alteration and mineralization have persistent and anti-persistent permeability networks, respectively. These were likely caused by different processes of rebuilding permeability due to different stress states resulting from changes in fault geometry.

Mineralogy of the epithermal precious and base metal deposit Banská Hodruša at the Rozália Mine (Slovakia)

The Au-Ag-Pb-Zn-Cu epithermal deposit Banska Hodrusa of intermediate-sulphidation type is located in the Middle Miocene Stiavnica stratovolcano on the inner side of the Carpathian arc in Slovakia. This deposit represents an unusual subhorizontal multi-stage vein system, related to processes of underground cauldron subsidence and exhumation of a subvolcanic granodiorite pluton. Veins are developed on a low-angle normal shear zone, possibly representing a detachment zone in andesitic wall rocks that formed during emplacement and exhumation of the granodiorite pluton. The deposit consists of two parts, separated by a thick sill of quartz-diorite porphyry. The eastern part is currently mined, and the western part has already been depleted. The Banska Hodrusa mineralization was formed during four stages: (1) low-grade silicified breccia at subhorizontal structures at the base of the deposit; (2) stockwork of steep veins with rhodonite-rhodochrosite, quartz-sulphide-carbonate and quartz-gold assemblages; (3) thin quartz-gold veins with medium dip in tension cracks inside the shear zone and complementary detachment hosted quartz-base metals-gold veins; (4) Post-ore veins. Gold and electrum (920–730) occur as intergrowths with base metal sulphides or hosted in quartz and carbonates, accompanied by Au-Ag tellurides (hessite, petzite). Rare Te-polybasite and Cu-cervelleite result from re-equilibration of early Te-bearing minerals during cooling. Sulphide minerals include low Fe sphalerite (~ 1.25 wt%), galena, chalcopyrite, and pyrite. The wall rock alteration is represented mostly by adularia, illite, chlorite, quartz, calcite and pyrite. Precipitation of gold, Au-Ag tellurides, Mn-bearing minerals and adularia resulted from boiling of fluids due to hydraulic fracturing, as well as opening of dilatational structures within the shear zone.

Distribution and composition of gold in porphyry gold systems: example from the Biely Vrch deposit, Slovakia

The Biely Vrch deposit in the Western Carpathians is assigned to the shallow, sulfide-poor porphyry gold deposit type and has an exceptionally low Cu/Au ratio. According to 3-D geochemical models, there is a limited spatial correlation between Au and Cu due to the primary introduction of gold by a salt melt and Cu by low-density vapor. Despite a rough spatial correlation of gold grades with quartz stockwork intensity, gold is hosted mostly by altered rock, exclusively in native form. Three main gold mineral assemblages were recognized here. In the deepest parts of the system, the K- and Ca-Na silicate gold assemblage is associated with minerals of high-temperature alteration (plagioclase, K-feldspar, actinolite), with gold grades and fineness depending on depth and potassium content of the host rock: K-silicate alteration hosts the lowest fineness gold (~ 914), whereas Ca–Na silicate alteration has the highest (~ 983). The intermediate argillic gold assemblage is the most widespread, with gold hosted mainly by chlorite, illite, smectite, and interstratified illite–chlorite–smectite minerals. The gold fineness is mostly variable (875–990) and inherited from the former gold mineral assemblages. The latest advanced argillic gold assemblage has its gold mostly in kaolinite. The extremely high fineness (~ 994) results from gold remobilization by late-stage aqueous magmatic-hydrothermal fluids. Uncommon bonanza-grade appears where the earlier gold mineral assemblages were further enriched by this remobilized gold. Primary precipitation of gold occurred during ascent and cooling of salt melts at 450 to 309 °C, mostly during retrograde quartz solubility.

Geology and Mineral Chemistry of Gold Mineralization in Mirge‐Naqshineh Occurrence (Saqez, NW Iran): Implications for Transportation and Precipitation of Gold

AbstractThe Mirge‐Naqshineh gold district is situated at northwest of Iran with a NW‐trending brittle‐ductile shear zone. It is hosted by Precambrian meta‐sedimentary and meta‐volcanic units traversed by mineralized quartz veins. In terms of cross‐cutting relationships and sulfide content three types of quartz veins are identified in the region. Among those, parallel to bedding quartz vein (type I) is the main host for gold mineralization. Gold is found in three different forms: 1) submicrometer‐size inclusions of gold in arsenian pyrite, 2) as electrum and 3) in the crystal lattice of sulfides (pyrite, galena and chalcopyrite). Six types of pyrite (Py1‐Py6) were identified in this ore reserve. Py3 coexists with arsenopyrite and contains the greatest As‐Au concentrations. There is a negative correlation between the As and S contents in Py2 and Py3, implying the substitution of sulfur by arsenic. Pyrites and mineralized quartz veins were formed via metamorphic‐hydrothermal fluid and reflect the gold‐transportation as Au(HS)−2 under reducing and acidic conditions. The gold precipitation mainly controlled by crystallization of arsenian pyrite during fluid/rock interactions and variation of fO2. The volcanic host rock has played an important role in gold concentration, as Py3 in this rock contains inclusion of gold particles, but gold is within the lattice of pyrite in phyllite or other units.

An experimental study on gold precipitation from leach solutions of technogenic gold-bearing raw materials

This paper presents the results of the study dedicated to the determination of the optimum parameters for the electrolytic gold precipitation from thiourea leach solutions. The leaching was carried out using technogenic gold-bearing raw materials (gold-bearing sands) of the Far East of the Russian Federation. The study focused on the influence of the below parameters of electrolytic precipitation: electrolyte composition, overall cathodic current density, electrode voltage, and structural design of an electrolytic cell. As the results of the experiments, the optimum electrolyte composition and parameters of the current (overall cathodic current density 1000 А/m 2 , electrode voltage 6 V) were determined. Under these parameters, it is possible to recover up to 93 % of gold from thiourea leach solutions.

Application of La-Icp-Ms Sulfide Analysis and Methodology For Deciphering Elemental Paragenesis and Associations In Addition To Multi-Stage Processes In Metamorphic Gold Settings

Orogenic gold deposits span a spectrum in regards to setting and style of mineralization, nature of gold (e.g., invisible, coarse) and favourable host rock. In addition, deposit formation is often attributed to protracted multi-stage hydrothermal processes. That gold mineralization also depends on a variety of features, such as metal-source reservoirs, metal-transport processes and wall-rock stratigraphy, which add to the challenge of investigating origin of paleo-mineralized settings. Recent work using quantitative laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) element distribution maps/profiles and their corresponding time slice datasets (TSD) provides new insight into identifying and assessing elemental paragenesis, multi-dimensional element coupling/decoupling processes, and corresponding mineralizing events. To further assess complexities of mineralization, application of geostatistical tools (e.g., multidimensional scaling, principal component analysis, linear discriminant analysis) and various innovative multi-element binary plots (e.g., Ag versus Au, Ni versus Co) is advised. To illustrate the application of this methodology, the results LA ICP-MS mapping and data processing for Fe sulphides from several metasedimentary-rock-hosted Canadian gold systems are presented: three Archean Algoma-type BIF-hosted gold deposits (~ 4 Moz Au Meadowbank, ≥ 2.8 Moz Au Meliadine district, ~ 6 Moz Au Musselwhite) and eight slate-belt style vein gold deposits from the Paleozoic Meguma terrane (Nova Scotia). The maps and derived elemental plots generated from the various settings demonstrate that: 1) the gold mineralization present is the product of multi-stage processes; 2) elemental associations vary as mineralization progresses, such as the early growth history of sulphides versus later coupled dissolution-precipitation reactions; and 3) different metal-source reservoirs and stratigraphy influence the fluid signature. These results contribute to better deciphering the complex processes involved in the protracted evolution of these and other orogenic-type gold systems (e.g., remobilization of invisible gold from early sulphide, precipitation of visible gold in later sites, increase of Au fineness).

Hydrothermal fluid evolution of the Jintingling gold deposit in the Jiaodong peninsula, China: Constraints from U-Pb age, CL imaging, fluid inclusion and stable isotope

The Jintingling gold deposit, located in the Jiaodong peninsula, north China, contains vein-, dissemination- and stockwork-style ores hosted in the Jurassic Linglong granitic pluton. Hydrothermal activities in this deposit are divided into seven stages, including pre-ore massive K-feldspar alteration, specularite-quartz vein (V1), milky quartz ± sulfide vein (V2), quartz-pyrite vein (V3), massive quartz-sericite-pyrite alteration and gold mineralization, quartz-polymetallic sulfide vein (V4), and quartz-calcite vein (V5). Rhenium-Osmium dating was conducted on molybdenite from V2, yielding an isochron age of 123.3 ± 3.6 Ma. LA-ICP-MS zircon U-Pb dating yielded 121.5 ± 1.5 Ma (1σ, MSWD = 0.010) for a pre-ore diorite porphyrite dyke and 117.6 ± 1.2 Ma (1σ, MSWD = 0.018) for a post-ore diorite porphyry dyke, respectively. Thus, gold mineralization occurred at 122–118 Ma.Four types of fluid inclusions were identified in quartz veins, including CO2-H2O ± CH4 inclusions (type I), pure CO2 inclusions (type II), mineral-bearing inclusions (type III) and aqueous inclusions (type IV). Fluid inclusion studies demonstrate that the primary ore-forming fluids contain CO2-rich vapor (357–420 °C, 7.2–11.3 wt% NaCl equivalent) and critical fluids (368–400 °C). The two phases were likely formed by immiscibility of magmatic-hydrothermal fluids. Later on, the fluids evolved to a H2O-CO2-NaCl ± CH4 system which had temperatures of 300–364 °C and salinities of 2.0–9.7 wt% NaCl equivalent. The H2O-CO2-NaCl ± CH4 fluid system may experience phase separation, forming coexisted CO2-H2O ± CH4 inclusions (homogenized to vapor or liquid at temperature of 265–309 °C and 265–287 °C, salinities of 3.1–7.1 wt% and 4.3–7.1 wt% NaCl equivalent, respectively) and aqueous inclusions (homogenized to liquid at temperature of 255–294 °C and 3.7–7.6 wt% NaCl equivalent). Along with further evolution, the fluids shifted to a H2O-NaCl system, with temperatures of 163–258 °C and salinities of 0.5–9.0 wt% NaCl equivalent.Pyrite from the Jintingling deposit has δ34S values (5.5‰–6.1‰) similar to those of pyrite from other gold deposits in the Jiaodong district, indicating that they were related to the same hydrothermal ore-forming system. The δ13CPDB values of calcite (−4.5‰ to −5.4‰) in V5 are generally located in the range of magmatic-derived carbon. Fluids of V2, V3, V4 have δD from −64.4‰ to −87.1‰, and δ18O from 8.4‰ to 0.2‰, located in the compositional fields between magmatic water and meteoric water. Collectively, microthermonetric results and isotopic data indicate that the primary ore-forming fluids were most probably magmatic in origin, with incorporation of meteoric water in the later ore-forming stage. Precipitation of gold was triggered by cooling of fluids, fluid immiscibility and decrease of sulfur in the fluids.

Mineralogy, ore-forming fluids and geochronology of the Shangmachang and Beidagou gold deposits, Heilongjiang province, NE China

The newly discovered Shangmachang epithermal gold (>11t Au; Heilongjiang province, NE China) and Beidagou epithermal tellurium-gold (>5t Au) deposits are located at the northeastern margin of the Central Asian Orogenic Belt and are hosted in Early Cretaceous volcanic rocks. At the Shangmachang deposit, gold occurs largely as electrum microparticles (5–120μm) with a fineness of approximately 401–697 in quartz, calcite, and pyrite. Silver-bearing minerals include electrum, polybasite, and minor argentite. The main mineral paragenetic assemblages consist of pyrite-electrum and galena-polybasite, while the hydrothermal alteration minerals include quartz, calcite, sericite, chlorite, and pyrite. Trace elements in the gangue minerals show that the Al contents of gold-bearing quartz veins are lower than those of pure quartz veins (early stage). Fluid inclusions in quartz and calcite mainly comprise liquid-vapor, and the components contain a liquid phase consisting of Na+, Cl−, and SO42− and a gas phase consisting of N2, CO2, and CH4. The Shangmachang fluid inclusions homogenize at 142.8°–303.0°C and are of low salinity (0.2–8.8wt% NaCl equiv.), while the Au mineralization formed at 150°–225°C. The calcite δ18Ofluid values range from −17.8‰ to −13.1‰, and the carbon isotope compositions range from −8.2‰ to −4.5‰. Zircon U-Pb dating of the Longjiang Formation andesite yields 119.4±1.9Ma, and the granite yields 118.6±1.2Ma, suggesting that local magmatism occurred during the Early Cretaceous. Calcite Sm-Nd dating suggests that the Au mineralization has an age of approximately 113.6±4.0Ma.At the Beidagou deposit, the main mineral paragenetic assemblages consist of sphalerite-galena-hessite and chalcopyrite-altaite-petzite-hessite, while the hydrothermal alteration minerals include quartz, calcite, anhydrite, chlorite, and pyrite. The components of the fluid inclusions in quartz contain a fluid phase consisting of Na+, K+, Cl−, and SO42− and a gas phase consisting of N2, CO2, CH4, and C2H6. The Beidagou fluid inclusions homogenize at 133.8°–370.1°C and are of low salinity (0.2–8.7wt% NaCl equiv.), while the Au mineralization formed at 175°–250°C. The δ18Ofluid and δDSMOW values of the quartz samples from Beidagou range from −8.2‰ to −5.0‰ and −101.6‰ to −92.8‰, respectively. The Al contents of gold-bearing quartz veins are higher than those of pure quartz veins. Rb-Sr dating of ore-bearing quartz veins suggests that the Te-Au-Ag mineralization has an age of about 115.5±4.4Ma.In summary, the interpretation of new data from the geological setting, fluid inclusions, O-H-C isotopes, and geochronology, suggests that the ore-forming fluids predominantly comprised meteoric fluids, while the metal content was derived from a mantle source at both the Shangmachang and Beidagou deposits. A geochemical model suggests that the fluids responsible for quartz deposition had a neutral pH and that adiabatic boiling and mixing triggered the gold, silver and tellurium precipitation. Both Shangmachang and Beidagou are interpreted as low-sulfidation epithermal gold deposits.

Neoarchean supra-subduction gold in Mesoarchean tonalite-granodiorite: Two separate mineralization events at Hammond Reef defined by disseminated and channelized fluid flow

Declining gold discoveries underscore the importance of informed geological models for future exploration. The genesis of many gold deposits remains ambiguous, but integrated structural and chronological studies shed light on key questions of source, timing and geodynamic setting. The Hammond Reef gold deposit is unusual in that it is hosted in Mesoarchean tonalite of the Wabigoon superterrane, rather than in Neoarchean greenstone belt terranes, as is common elsewhere in the Superior Province. Gold deposition at Hammond Reef occurred in two stages: The main stage of disseminated low-grade mineralization was followed by discrete, channelized veins, both tied to regional tectonic events. Disseminated mineralization is associated with localized sulfidation and chlorite breakdown within a sericitized deformation zone adjacent to the locked greenstone-gneiss boundary. Chlorite-leached halos around small fractures at the deposit margins indicate that fluid penetration into the tonalite was guided by fractures concentrated in the broad deformation corridor. Pervasive alteration was enhanced by grain-scale flow. Hammond Reef may have been particularly susceptible to sulfidation and associated gold precipitation due to the greater local abundance of mafic dykes and chlorite-rich enclaves in the tonalite. The secondary gold mineralization phase is associated with younger and discrete auriferous quartz-carbonate veins that were episodically emplaced along pre-existing structures. Cross-cutting relationships show that both mineralization phases post-date the emplacement of the 2.889 Ga Diversion Stock. The kinematics of mineralized structures relate Hammond Reef to the 2.7 Ga northward subduction of the Wawa terrane beneath the Wabigoon superterrane. This places Hammond Reef in a supra-subduction setting, where an older dormant fault zone with enhanced permeability was infiltrated by auriferous fluids during the an orogeny up to 200 million years later. The two phases of gold enrichment present disseminated and channelized fluid flow through the available permeable pathways within the same regional tectonic setting.

Au crystal growth on natural occurring Au—Ag aggregate elucidated by means of precession electron diffraction (PED)

In the present work, a lamella from an Au—Ag aggregate found in Ni-laterites has been examined using Transmission Electron Microscope to produce a series of Precision Electron Diffraction (PED) patterns. The analysis of the structural data obtained, coupled with Energy Dispersive X-ray microanalysis, made it possible to determine the orientation of twinned native gold growing on the Au—Ag aggregate. The native Au crystal domains are found to have grown at the outermost part of the aggregate whereas the inner core of the aggregate is an Au—Ag alloy (∼4 wt% Ag). The submicron structural study of the natural occurring Au aggregate points to the mobilization and precipitation of gold in laterites and provides insights on Au aggregates development at supergene conditions. This manuscript demonstrates the great potential of electron crystallographic analysis, and in particular, PED to study submicron structural features of micron sized mineral aggregates by using the example of a gold grain found in a Ni-laterite deposits.

Geology, hydrothermal fluids, H-O-S-Pb isotopes, and Rb-Sr geochronology of the Daxintun orogenic gold deposit in Heilongjiang province, NE China

The newly discovered Daxintun Sb-Au deposit (Heilongjiang province, NE China) is located at the north-eastern margin of the Central Asian Orogenic Belt, and it is hosted by Neoproterozoic to Early-Cambrian two-mica schist and marble. The antimony-bearing minerals include stibnite, famatinite, tetrahedrite, boulangerite, freibergite, and minor andorite. Gold occurs largely as native gold microparticles (0.1–20 μm) with high fineness (974–983) accompanied by quartz, stibnite, calcite and pyrite. Fluid inclusions in ore-bearing quartz and calcite phases mainly comprise liquid–vapor and minor CO2-bearing three-phase types, and the gas phases contain CO2, CH4 and H2S. The Daxintun fluid inclusions homogenize at 111.8–353.4 °C and are of low salinity (0.53–12.51 wt% NaCl equiv.). The δ18OSMOW of the Daxintun quartz falls into a wide range (4.2‰–16.9‰), yet the δDSMOW falls into a narrow range (−108.0‰ to −87.3‰), suggesting that the ore-forming fluids formed from metamorphic fluids that mixed with meteoric water. The stibnite δ34S ranges between −0.9‰ and 5.1‰ (mean = 2.8‰). The stibnite has 206Pb/204Pb of 18.234–18.306, 207Pb/204Pb of 15.537–15.561 and 208Pb/204Pb of 38.103–38.227, implying a metamorphic fluid source. The conditions of gold and antimony precipitation are comprised of neutral pH and fluid mixing, with decreasing fS2 and fO2.Zircon U-Pb dating of the fine-grained Daxintun granite yields a Late-Jurassic age (154.0 ± 2.0 Ma), and the Daxintun diorite dike yields a weighted mean age of 122.1 ± 1.3 Ma. Stibnite Rb-Sr dating suggests that the Sb-Au mineralization has an age of approximately 152.8 ± 2.4 Ma. The observed ore deposit geology, as well as new data on the ore-forming fluids, H-O-S-Pb isotopes, and Rb-Sr geochronology, suggest that the Daxintun Sb-Au deposit is best described as an orogenic type gold deposit that formed during subduction of the Palaeo-Pacific and the Mongolia–Okhotsk Oceans during the Late-Jurassic.

Multiple sulfur isotopes monitor fluid evolution of an Archean orogenic gold deposit

The evolution of a gold-bearing hydrothermal fluid from its source to the locus of gold deposition is complex as it experiences rapid changes in thermochemical conditions during ascent through the crust. Although it is well established that orogenic gold deposits are generated during time periods of abundant crustal growth and/or reworking, the source of fluid and the thermochemical processes that control gold precipitation remain poorly understood. In situ analyses of multiple sulfur isotopes offer a new window into the relationship between source reservoirs of Au-bearing fluids and the thermochemical processes that occur along their pathway to the final site of mineralisation. Whereas δ34S is able to track changes in the evolution of the thermodynamic conditions of ore-forming fluids, Δ33S is virtually indelible and can uniquely fingerprint an Archean sedimentary reservoir that has undergone mass independent fractionation of sulfur (MIF-S). We combine these two tracers (δ34S and Δ33S) to characterise a gold-bearing laminated quartz breccia ore zone and its sulfide-bearing alteration halo in the (+6 Moz Au) structurally-controlled Archean Waroonga deposit located in the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia. Over 250 analyses of gold-associated sulfides yield a δ34S shift from what is interpreted as an early pre-mineralisation phase, with chalcopyrite-pyrrhotite (δ34S = +0.7‰ to +2.9‰) and arsenopyrite cores (δ34S = ∼−0.5‰), to a syn-mineralisation stage, reflected in Au-bearing arsenopyrite rims (δ34S = −7.6‰ to +1.5‰). This shift coincides with an unchanging Δ33S value (Δ33S = +0.3‰), both temporally throughout the Au-hosting hydrothermal sulfide paragenesis and spatially across the Au ore zone. These results indicate that sulfur is at least partially recycled from MIF-S-bearing Archean sediments. Further, the invariant nature of the observed MIF-S signature demonstrates that sulfur is derived from a homogeneous MIF-S-bearing fluid reservoir at depth, rather than being locally sourced at the site of Au precipitation. Finally, by constraining the MIF-S-bearing sulfur source to a fixed reservoir, we are able to display the thermochemical evolution of the ore fluid in δ34S space and capture the abrupt change in oxidation state that causes Au precipitation. Our results highlight the importance of constraining multiple sulfur isotopes in space and time in order to elucidate the source and evolution of any given Au-bearing fluid.

Ore genesis of the Xiadian gold deposit, Jiaodong Peninsula, East China: Information from fluid inclusions and mineralization

Orebodies in the Xiadian gold deposit in the Jiaodong Peninsula, China are mainly hosted in the Mesozoic granitoids, controlled structurally by the Zhaoyuan–Pingdu Fault Zone, and occur as disseminated and cataclastic altered type. Four mineralization stages were identified as follows: quartz–pyrite stage (I), gold‐bearing fine‐grained pyrite–quartz stage (II), polymetallic sulfide–quartz stage (III), and quartz–carbonate stage (IV). Quartz was classified as including quartz granules with dentation boundaries (I), cataclastic quartz grain assemblages (II and III), and rod‐like quartz grains (IV). Petrography, laser Raman analysis, and microthermometry of fluid inclusions in these stages (in both tunnel and borehole samples) reveal (a) CO2–H2O fluid inclusions (C–H type), (b) CO2–H2O ± CH4fluid inclusions (C–H–CH4type), and (c) aqueous fluid inclusions (H type). Fluid immiscibility caused by fluid mixing caused rapid precipitation of gold. The ore‐forming fluid of the Xiadian gold deposit evolves from an H2O–CO2–NaCl ± CH4system with medium temperature and salinity to an H2O–NaCl system with low temperature and salinity, from CO2‐rich to CO2‐poor in composition and from a mixture of magmatic water with increasing meteoric water as δ18OH2Ovalues. Sulphur isotope compositions suggest a mixed source of ore metal, and the Jiaodong Group may be a major source for sulphur. Fluid parameters of borehole samples indicate that there is the same fluid system for Au precipitation at different depths and fault gouge with poor permeability may play a crucial role in forming a relatively closed semi‐open space for Au precipitation. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusions and stable isotope geochemistry, the Xiadian gold deposit is concluded as an orogenic‐type gold deposit formed in the tectonic transition from compression to extension.