Gold Precipitation Research Articles

Differentiated Archean Dolerites: Igneous and Emplacement Processes that Enhance Prospectivity for Orogenic Gold

Abstract Magnetite-bearing granophyre and quartz dolerite are the evolved fractions of differentiated dolerite (diabase) sills and are an important host to Archean gold deposits because they are chemical traps for orogenic fluids. Despite their economic importance, there is a poor understanding of how melt composition, crystal fractionation, sill geometry, and depth of emplacement increase the volume of host rock that is most favorable for gold precipitation during orogenesis. We use drill core logging, whole-rock geochemistry, magnetic susceptibility, gold assay, and thermodynamic modeling data from 11 mineralized and unmineralized ca. 2.7 Ga differentiated dolerites in the Eastern Goldfields superterrane (Yilgarn craton, Western Australia) to better understand the influence of igneous and emplacement processes on gold prospectivity. Orogenic gold favors differentiated dolerites, derived from iron-rich parental magmas, that crystallize large volumes of magnetite-bearing quartz dolerite (>25% total thickness). Mineralized sills are commonly >150 m thick and hosted by thick and broadly coeval sedimentary sequences. Sill thickness is an important predictor for gold prospectivity, as it largely controls cooling rate and hence fractionation. The parental melts of gold-mineralized sills fractionated large amounts of clinopyroxene and plagioclase (possibly up to 50%) at depth before emplacement in the shallow crust. A second fractionation event at shallow levels (<3 km) operated both vertically and laterally, resulting in an antithetic relationship between quartz (magnetite) dolerite and cumulates (pyroxenites and peridotites). By comparison with younger mafic sills emplaced in synsedimentary basins, we argue that the geometry of these high-level sills was more irregular than the often-assumed tabular form. Any irregularities in the lower sill margin act as traps for early formed (dense) ferromagnesian minerals, now represented by pyroxene and peridotite cumulates. In contrast, irregularities in the upper sill margin trap the buoyant fractionated liquids when the sill is more crystalline, through magma flow on the scale of <1 km. Sills derived from iron-poor melts are rarely mineralized and, all else being equal, probably have to be thicker than Fe-rich sills to be similarly prospective for orogenic gold. Finally, we provide a list of quantifiable parameters that can be incorporated into an exploration program targeting differentiated dolerites that host orogenic gold.

The extensive hydrocarbon-mediated fixation of hydrothermal gold in the Witwatersrand Basin, South Africa

There is a close spatial relation between high-grade gold mineralization in the Witwatersrand basin and carbonaceous nodules, veins and seams. Hydrocarbons thus may well have been essential in ore genesis. We have sampled four major gold-, uranium- and hydrocarbon-bearing ore horizons, namely the Carbon Leader, Vaal, B and Black reefs, to determine the role of hydrocarbons in the accumulation and hydrothermal fixation of gold. Our multipronged approach included high-resolution scanning and transmission electron microscopy (SEM, TEM), nanotomography with video clips, and geochemical modeling. Post-depositional hydrothermal activity at the peak of regional metamorphism produced an assemblage of quartz, phyllosilicates, brannerite, crandallite, florencite, monazite and gold in all four reefs. The gold, hydrocarbons and associated mineral assemblages are closely related on the micro to nano scale. Gold deposition occurred in interstices, as fracture fillings in detrital minerals, and on the surface of migrated solid hydrocarbon residues. The spherical to elliptical inclusions in the gold consist of an outer pyrobitumen phase and a central void space, partially associated with nanometric gold, uraninite, coffinite and silica. The hydrocarbon-bearing inclusion likely formed by the entrapment of a fossil liquid oil precursor during gold precipitation. The oil was subsequently thermally altered and converted into the final pyrobitumen and gaseous residues. Geochemical calculations to simulate the interaction of an invading hot hydrothermal fluid with the hydrocarbons in the reefs reveal that a very small amount of hydrocarbons will drastically decrease the aqueous solubility of gold and hence cause its instant precipitation.We extend our genetic model for the epigenetic formation of gold in the Witwatersrand. Regional metamorphism promoted the extensive and likely basin-wide circulation of hydrothermal fluids; these were capable of mobilizing substantial amounts of gold. The liquid, gaseous and solid hydrocarbons in the reefs acted as efficient chemical traps for the concentration of gold. Being strong chemical reductants, they caused the rapid precipitation and accumulation of gold on the surface of the fossil oil droplets and already solidified hydrocarbons. The release of the gases from accessible hydrocarbons into the sediments away from their source buffered the redox state of the hydrothermal solutions even at a considerable distance from the pyrobitumen seams and veins, likely resulting in the deposition of gold in the absence of visible hydrocarbons. Although our findings do not explain the ultimate origin and exceptional endowment of gold in the Witwatersrand, we do provide intriguing evidence for the large-scale hydrothermal mobilization, accumulation and fixation of gold mediated by hydrocarbons during post-depositional metamorphism.

Origin of the Kyaukmyet Low-Sulfidation Epithermal Gold Prospect, Monywa District, Central Myanmar

The Kyaukmyet prospect is one of the principal epithermal gold prospects in the Monywa District, Central Myanmar; its gold- and base metal-bearing quartz veins contain around 3 g/t gold. Ore minerals are mainly hosted by volcanic and volcaniclastic rocks of the Late Oligocene to Middle Miocene Magyigon Formation. The distribution of magmatic intrusions in the area is controlled by ENE-WSW trending faults; these faults are likely related to ore mineralization. Common ore minerals at the Kyaukmyet prospect include pyrite, sphalerite, galena, chalcopyrite, and electrum. They occur in mineralized crustiform-textured brecciated quartz veins and banded (colloform) and massive quartz veins. Mineralized rock is accompanied by silicification and propylitic and argillic alterations. The alteration mineral assemblages include quartz, adularia, calcite, chlorite, illite/smectite, sericite, and illite. Fluid inclusions in the quartz veins have homogenization temperatures ranging from 148 °C to 304 °C and salinities from 0.35 wt % to 2.75 wt % NaCl equiv. The quartz in the mineralized quartz veins was most likely precipitated at a depth ranges165-256 m below the paleosurface. The precipitation of gold at the Kyaukmyet prospect may have been formed by mixing large amounts of meteoric fluid with small amounts of magmatic fluid. The coexistence of liquid-rich and vapor-rich inclusions and presence of adularia and bladed calcite indicate that fluid boiling is caused the main mechanism of ore formation. The vein textures, ore mineral assemblages, alteration minerals and fluid inclusion data suggest that the Kyaukmyet prospect is a polymetallic low-sulfidation epithermal gold deposit.

Obtaining of Nano Gold from Concentrate (Banská Hodruša, Slovakia) Using Algae with Utilization of Mechanochemical Activation

The difficult and predominantly toxic processing of sulphide concentrates with a low gold content has prompted the finding of a more efficient and environmentally friendly method of obtaining this noble metal. This work was therefore focused on the recovery of gold from the concentrate of refractory sulphide complex (Banská Hodruša, Slovakia) using a non-cyanide mechanochemico-biological process. Gold in this complex concentrate is physically located in the intercrystalline space of sulfide minerals and fills in structural defects in sulfides. The precipitation of gold from the complex mineral matrices of the concentrate can be achieved using a mechanochemico-biological process. This innovative process makes it possible to recover gold by applying a thiourea solution, which is a preferred alternative to toxic a cyanidation. The use of a biological process with the application of algae has shown that algae with siliceous structures make it possible to obtain gold from a complex concentrate (Banská Hodruša, Slovakia) with dimensions of the nano. Limnetic algae (diatoms, golden algae) are part of aquatic ecosystems and form the largest of biomass of all plants on Earth. Mechanochemicobiological process is a method that allows to obtain 100 nm gold nanoparticles from refractory sulphide concentrate (Banská Hodruša, Slovakia).

Textural, fluid inclusion, and in-situ oxygen isotope studies of quartz: Constraints on vein formation, disequilibrium fractionation, and gold precipitation at the Bilihe gold deposit, Inner Mongolia, China

Abstract Bilihe is a porphyry gold deposit located in the northern margin of the North China Craton (NCC), Inner Mongolia, China. Different stages of quartz are well developed at this deposit. To document the history of quartz deposition, the fluid evolution and gold precipitation events of the deposit and the detailed oxygen isotope signatures of quartz from Bilihe were studied using high-resolution secondary ion mass spectroscopy (SIMS), then integrated with scanning electron microscope-cathodoluminescence (SEM-CL) and fluid inclusion microthermometry. The SEM-CL features show that the hydrothermal veins at Bilihe have a complex growth history, with multiple generations of quartz developed in each set of veins. Fluid inclusions in different quartz stages yield variable homogenization temperatures, ranging from 178 °C to above 600 °C. These quartz stages exhibit variable δ18O values of 3.5–15.4‰, corresponding to δ18Ofluid ranging from –8.7 to 12.0‰. There are two abnormal peaks of δ18Oquartz and δ18Ofluid values occurring in a sub-generation of A type veins and auriferous-banded quartz veins, suggesting that the vein quartz may have experienced sporadic disequilibrium oxygen fractionation with water when crystallizing, thus resulting in local 18O-enrichment. The overall δ18Ofluid values, which show a gradual decrease from early to late stages, suggest a progressive decrease in the proportion of magmatic hydrothermal fluids. The relationship between quartz textures and gold occurrence shows that gold precipitated twice at Bilihe. The first precipitation in the UST quartz may have resulted from rapid cooling and indicates that the addition of meteoric water was not necessary for gold precipitation, whereas the progressive incursion of meteoric water probably had a significant effect on the second gold precipitation event.

Processing of gold-antimony concentrates

This paper describes the results of a study that looked at processing of goldantimony concentrates with selective extraction of antimony and gold as commodities. The common global practice of processing antimony sulphide concentrates (20–30% Sb) is based on alkaline sulphide leaching followed by precipitation of metallic antimony by electrowinning. However, application of this technique to process sulphide concentrates that, apart from antimony, also contain gold, can be difficult as, together with antimony, up to 10–15% of gold can leach to the solution. It takes a special process during final refining of cathode antimony to recover that gold. This paper describes a process that involves two stages of atmospheric leaching of antimony. The gold that leached to the solution is precipitated with zinc after the first stage of antimony leaching. Together with atmospheric leach tailings, it then goes to the pressure oxidation unit. This process helps oxidize the rest of the sulphides and release refractory gold. The resultant cake is processed following a standard sorption cyanidation technique. The paper looks at the antimony leaching rate and the rate at which gold leaches to the solution during this process. The paper describes the results of selective precipitation of gold from gold-antimony solutions and highlights certain features of this process. A series of tests was conducted to test the techniques of pressure oxidation of atmospheric leach tailings and cyanidation of the residue. The paper also describes a process that was developed for processing of goldantimony concentrates and precipitation of antimony and gold. An antimony recovery exceeding 90–95% can be achieved when using this process. At the same time, the percent of dissolved gold can be reduced from 10–15 tо 1–3%.

Sources of auriferous fluids associated with a Neoarchean BIF-hosted orogenic gold deposit revealed by the multiple sulfur isotopic compositions of zoned pyrites

Internal textures, multiple sulfur isotopic compositions, and contents of gold, selenium, and molybdenum of ore-related pyrites from a Neoarchean carbonate-facies BIF-hosted gold deposit in Quadrilatero Ferrifero were investigated to elucidate the source(s) of sulfur. Sodium hypochlorite etching and BSE imaging revealed pervasive zoning in pyrites. Five different growth zones (Py1a, Py1b, Py2, Py3, and Py4) and six types of zoning (Type A–Type F) were identified. Two pyrite generations were distinguished in ores: G1 (Py1a and Py1b) and G2 (Py2, Py3, Py4). Both G1 and G2 have positive Δ33S, but the magnitudes of G1 are higher. The G1-relevant fluids can be shallow-sourced, whereas the G2-related auriferous fluids are most likely deep-sourced metamorphic fluids derived from devolatilization of the lower succession of the Nova Lima Group (metavolcanics and metasedimentary rocks) during metamorphism, with fluid–rock interactions during fluid ascent and at the depositional site. The negative δ34S, higher selenium contents, and carbonates inclusions of Py1a contrast with the positive δ34S, lower selenium contents, and carbonaceous material inclusions of Py2, Py3, and Py4. The Lamego system possibly started with shallow-sourced oxidized and low-gold fluids (Py1a), closely followed by mixing in of deep-seated reduced auriferous fluids (Py2, Py3, Py4) initiated by tectonic activities. The oscillatory zoning of Py2 with the highest gold contents consists of alternating gold-rich and gold-poor laminae, indicating that fault-valve activity is a trigger of gold deposition. The sulfidation of siderite in BIF (desulfidation of auriferous fluids) related to the formation of ore-related pyrites also contributed to gold precipitation.

Gold mineralization of the Thone Myae Song area, Sagaing Region, northern Myanmar

• Central Magmatic-Volcanic Belt (CMVB) hosts several Au-Cu ore types and Thone Myae Song deposit. • Formation of CMVB is related to the eastward subduction of Indian Ocean floor during the Cretaceous. • Sericite-carbonate alteration is important as confines to the margins of mineralized veins. • Fluid immiscibility was a major trigger to the precipitation of gold and base metals at EE3 deposit. Several gold deposits are found in the Central Magmatic-Volcanic Belt (CMVB) of Myanmar, which forms a nearly N–S-trending, 1500-km-long arc extending from the Andaman Sea to northern Myanmar. The CMVB consists of Cretaceous granitoids intruded into folded andesites and pillow basalts (Mawgyi Andesite) which rest on cherts, talc-schists, mudstones and phyllites. The EE3 gold deposit is part of gold mineralization in the Thone Myae Song area, located in the Kawlin–Wuntho block in the northern part of the CMVB. The orebodies occur as sulfide-bearing massive quartz veins hosted by Mawgyi Andesite. Based on the mineral assemblages and cross-cutting relationships, three mineralization stages are identified in the EE3 gold deposit: Stage-I quartz-carbonate-sulfide veins; Stage-II quartz-carbonate veins intersecting the Stage-I veins; and Stage-III quartz-carbonate-sulfide veins parallel to the Stage-I veins. The main hydrothermal alteration minerals are sericite, epidote, chlorite and calcite. Three main types of fluid inclusions were distinguished from the mineralized quartz veins hosted by Mawgyi Andesite: liquid-rich aqueous fluid inclusions (Type A), vapor-rich aqueous inclusions (Type B), and H₂O-CO₂-NaCl inclusions (Type C). The latter occurs only in the Stage-I veins. The ore-forming conditions are estimated from fluid inclusions in the Stage-I, Stage-II and Stage-III veins to be at 180 °C and 28 bars, 176 °C and 26 bars, and 158 °C and 21 bars, respectively. The formation pressures of the Stage-I, Stage-II and Stage-III mineralization correspond to a shallow depth of 300 m, 280 m, and 225 m, respectively. Based on the ore and alteration mineral assemblages, and estimated temperature and pressure conditions of the mineralization, the EE3 gold deposit was formed in an epithermal condition.

Multistage gold mineralization in the Hadamengou gold deposit in the northern margin of the North China Craton: Insights from in-situ trace element contents and sulfur isotope analyses of pyrite

Abstract The Hadamengou gold deposit is one of the largest gold deposits in the Wulashan-Daqingshan gold mineralization cluster, in the northern margin of the North China Craton. The gold deposit, which is hosted by Archean gneiss, comprises potassic alteration and abundant quartz-pyrite veins, with five types of hydrothermal pyrite associated with gold, tellurides, and sulfides. The earliest stage of pyrite (Py1) occurs as random disseminations within the limited volume of modified quartz-pyrite veins and was formed during or prior to the last episode of metamorphism. Py2, from the potassic and chlorite-sericite alteration zones, is related to the activity of potassic fluids, and contains inclusions of native gold, galena, melonite, electrum, and petzite. Py3, without inclusions, occurs as randomly- disseminated grains within the limited volume of grey quartz-pyrite veins. A few native gold grains are found along fractures of quartz associated with Py3. Py4 occurs as veinlets or aggregates in the milky quartz-pyrite veins and contains the highest Au concentrations. Abundant native gold, calaverite, and electrum occur along the grain boundaries and fractures, or as inclusions within Py4 and quartz. Py5 occurs as rounded and cataclastic shapes within the limited volume of quartz-carbonate veins and is depleted in Au. LA-ICP-MS trace element analysis of the pyrite shows that Py1 has a low content of gold (median of 0.07 ppm) and other trace elements (Ag, Te, Pb, Cu, Bi, and As). Potassic alteration, grey quartz-pyrite veins, and milky quartz-pyrite veins make a very large contribution to the gold endowment, with a relatively high gold content in pyrite, including Py2 (median of 0.8 ppm), Py3 (4.3 ppm), and Py4 (2.5 ppm). Other trace elements (e.g., Ag, Te, Pb, Mo, and Bi) are also significantly enriched within Py2 to Py4. Little or no gold was detected in Py5. Thus the potassic alteration and milky quartz-pyrite veins were major contributors to gold mineralization, based on the volume and Au content of pyrite within the Hadamengou deposit. In-situ LA-MC-ICP-MS sulfur isotope analyses of pyrite show negative values and a narrow range of variation of δ34S (averages of −11.4‰, −11.8‰, −10.5‰, and −9.4‰ for Py2-Py5, respectively), indicating an oxidized magmatic sulfur source for the pyrites in Hadamengou. Additionally, the occurrence of tellurium minerals associated with various pyrites, and the high Te concentration of various pyrites, also suggest that ore-forming fluids were derived from an oxidized alkaline magma. These results, combined with published geochronological data, suggest that during two episodes, Au-rich potassic and SiO2-rich fluids separated from the deep-lying Devonian alkaline magma during ascent along the EW-trending faults, and reacted together with the wall rocks to form potassic and chlorite-sericite alteration zones, resulting in gold precipitation.

Origin and evolution of ore fluids of the jinwozi gold deposit, beishan orogen, NW China

The Jinwozi gold deposit is one of the most economic gold deposits in the Beishan Orogen, NW China. The gold orebodies mainly occur as quartz veins hosted in Jinwozi granite. In this contribution, based on a comprehensive study including field investigation of ore geology, mineralogy, geochemistry, geochronology, and fluid inclusion microthermometry, we discuss the origin and evolution of the ore fluids and the gold precipitation mechanism at the Jinwozi deposit. The mineralization process includes three stages, i.e., the early quartz-pyrite veins, main-stage quartz-sulfide veins, and late carbonate-quartz veinlets, with gold being introduced in the main stage. Four types of fluid inclusions, i.e., PC-, C-, W-, and S-types, were observed and investigated in microthermometry. The early-stage quartz contains C-type FIs with a salinity of 3.9 to 9.0 (av. 5.8 ± 1.2) wt.% NaCl eqv., CO2 content of 10 to 85 vol%, and total homogenization temperature (Th) of 254 to 358 °C (av. 302 ± 17 °C). It shows that the ore fluids are low salinity, CO2-rich and mesothermal, and identical to those of orogenic-type gold deposits. The C-type FIs in the main stage quartz show salinity of 3.0 to 9.0 (av. 4.9 ± 1.2) wt.% NaCl eqv., CO2 contents of 8 to 90 vol% and Th of 210 to 326 °C (av. 256 ± 29 °C). The W-type FIs yield salinity of 5.5 to 10.2 (av. 8.5 ± 0.9) wt.% NaCl eqv.. and Th of 197 to 305 °C (av. 247 ± 25 °C). Gold precipitation was caused by phase separation that supported by plentiful immiscibility fluid inclusion assemblages observed in main stage quartz. Trapping pressures of C-type FIs are estimated at 90.5–286 MPa and 57.7–279 MPa for the early and main stages, corresponding to mineralization depths of ~10.4 km and ~5.8 km, respectively. Last but not least, the homogenization temperatures of the FIs in the main stage quartz gradually increase with depth (from 1590 m, through 1510 m, to 1477 m ASL) at the #3p4 orebody, but suddenly decrease at depth of 1442 m ASL. This temperature decreases possibly indicates the appearance of a new orebody beneath the currently exploited #3p4 orebody occurring in echelon formation.

Gold occurrences of the Woumbou–Colomine–Kette district, eastern Cameroon: ore-forming constraints from petrography, SEM–CL imagery, fluid inclusions, and C–O–H–S isotopes

The Woumbou–Colomine–Kette district is located in the East Metallogenic Province of the Republic of Cameroon. Northeast- to E-striking splays of the regional Pan-African crustal-scale Sanaga Shear Zone control widespread gold mineralization. Gold occurs within local shear zones developed along granite-gneiss contacts, as well as within the margins of the Pan-African intrusions. Field investigations coupled with mineralogical and textural studies reveal two stages of gold mineralization. Stage 1 led to pyrite, chalcopyrite, sphalerite, pyrrhotite, bismuthinite, tellurobismuthite, galenobismuthite, and native gold deposition with quartz, sericite/muscovite, chlorite, calcite, and ankerite as gangue minerals. Stage 2 included the precipitation of pyrite, hematite, tellurobismuthite, and native gold. A SEM–CL study distinguished the two stages of quartz, as well as an early remnant barren quartz depositional stage. Two fluid inclusion assemblages (FIAs) with four types of fluid inclusions can be observed. The FIA 1 is aqueous-carbonic (±N2±CH4), with carbonate daughter minerals, which is related to stage 1 of the gold mineralization and composed of low salinity (~7.5 wt% NaCl equiv) H2O-CO2-NaCl (type 1), CO2-rich (type 2), and H2O-rich (type 3a) fluid inclusions. The inclusions provide evidence of episodic phase separation, with unmixing at P~2 kbar, T~300 °C, and at a paleodepth of ~7 km. The FIA 2 consists of aqueous (type 3b) fluid inclusions with a salinity of 0.2 to 11.7 wt% NaCl equiv and trapping temperatures between 205 and 245 °C. The δ18Oquartz (+11.0 to +11.7‰), δDfluid inclusions (−46.2 to −40.9‰) and δ13Cfluid inclusions (−4.9 to −4.1‰) for gold-bearing quartz veins suggest a metamorphic source for the ore-forming fluids, although some contribution from the mantle and/or magmatic source(s) cannot be ruled out. The δ34S (+6.5 to +7.0‰) for gold-bearing pyrites suggests a metamorphic source for the sulfur that transported gold and was involved in the precipitation of sulfide minerals. These data are consistent with a mesozonal orogenic gold deposit model for the Woumbou–Colomine–Kette district, which is an important part of the emerging greenfields exploration region for Pan-African orogenic gold and with the more significant targets defined by granite-gneiss contacts along NE- and E-striking shear zones.

Genesis of the Hebaoshan gold deposit in Fujian Province of Southeast China: constraints from a combined fluid inclusion, H-O-C-S-Pb-He-Ar isotope and geochronological study

The large Hebaoshan gold deposit (41.5 t Au, average grade: 3.5 g/t) is located in a hitherto poorly documented gold province in the northeastern part of the South China Block. It is hosted by Precambrian metasedimentary rocks that experienced Triassic greenschist- to amphibolite-facies metamorphism. Three hydrothermal stages can be distinguished: quartz + sericite + pyrite + pyrrhotite (stage I), quartz + magnetite + sulfide (stage II), and quartz + carbonate + pyrite + hematite (stage III). Auriferous pyrite samples yielded a Rb-Sr isochron age of 234 ± 3 Ma (2σ, MSWD = 0.24), and ore-related hydrothermal sericite yielded a 40Ar-39Ar plateau age of 226.4 ± 2.0 Ma (2σ, MSWD = 1.01), which are distinctly younger than ca. 430 Ma granitic rocks in the mining district and older than 170–130 Ma granites in the region, ruling out any genetic link between granite emplacement and gold mineralization. The mineralizing fluid system can be described as carbonic-aqueous with low to moderate salinity (2.2–10.1 wt% NaCl equiv.) and medium temperature of 290 to 350 °C (stage I) and 230 to 280 °C (stage II). The δ18O quartz ranges from 13.3 to 15.4 ‰, and the δD values for fluid inclusions in quartz range from − 97 to − 60 ‰. The calcite has C-isotopes ranging from − 6.1 to − 3.8 ‰ and O-isotopes from 2.8 to 13.6 ‰. H-O-C isotope data are consistent with a metamorphic fluid derived from devolatilization of Neoproterozoic basement rocks during regional metamorphism in the Late Triassic Indosinian period. The δ34S values of sulfides for stages I, II, and III are 0.1 to 7.8 ‰, − 10.6 to 5.1 ‰, and − 18.9 to − 14.1 ‰, respectively, indicating an increase in oxygen fugacity during fluid ascent which resulted in more negative sulfur isotope values of sulfides and precipitation of gold. Pyrite separates have 206Pb/204Pb ratios of 17.340 to 17.687, 207Pb/204Pb ratios of 15.539 to 15.604, and 208Pb/204Pb ratios of 37.749 to 38.094. The S-Pb isotope data suggest derivation of S and Pb from the Precambrian metasedimentary country rocks. Auriferous pyrite yielded elevated 3He/4He ratios (0.78–1.46 Ra), which suggest a mantle component as can be expected from fluids derived from a subduction setting. It is concluded that Hebaoshan is an orogenic gold deposit that formed during Triassic flat-slab subduction of the paleo-Pacific plate beneath the South China Block and thus constitutes the first Triassic deposit of this type recognized in the coastal area of Southeast China.

Evidence supporting micro-galvanic coupling in sulphides leads to gold deposition

Electrical micro-junctions in metal sulfides drive electrochemical reactions with passing gold-bearing fluids, resulting in the deposition of gold, even from under-saturated ore fluids. Understanding the role micro-junctions play in the deposition of gold requires (a) imaging the electric field distribution of a galvanic couple near the surface to qualify the existence of an active micro-geo-battery and (b) correlating it with gold precipitation on the p-type cathode side of the junction by mapping the host at minor and trace levels. Here we report on correlating electron back scattered diffraction (EBSD), particle induced X-ray emission (PIXE) elemental maps including micron-scaled gold hot spots with laser beam induced current (LBIC) photocurrent maps of galvanic coupling in natural arsenian pyrite from the Otago Schist in New Zealand. The results provide convincing evidence that sulphide electrochemical interactions can lead to gold electro-deposition. We finish by discussing a simplistic model of the processes involved in reference to the original model of Moller and Kersten (Miner Deposita 29(5):404–413. 1994), and discuss the effects of temperature in light of recent-reported evidence of electrochemical gold deposition in the formation of hydrothermal gold deposits.

An Application for Thermodynamic Calculation of AuxAg1 – x-Fluid Equilibria: Super-Pure Native Gold in Woxi Au–Sb–W Deposit, Western Hunan, China

The native gold in the Woxi Au–Sb–W deposit is characterized by remarkably high purities (99.9%), called as super-pure native gold. The deposit can thus be used as the suitable application example for analyzing ore-forming conditions by using the thermodynamic calculation of AuxAg1 – x-fluid equilibria. According to experimental results of minerals and fluid inclusions, some physicochemical conditions are assumed in the thermodynamic calculation of the fluids equilibrated with Au–Ag solid solutions of XAu = 0.999. Calculated results in log fO2-pH diagrams show dissolved gold and silver concentrations, as well as stability fields of pyrite and sericite associated with the super-pure native gold. On the other hand, LA-ICP-MS analysis indicates that both Au and Ag concentration are determined in the fluid inclusions of scheelite, showing varied Au/Ag ratios with different homogenization temperatures. The calculated results assumed by different data of inclusions in scheelite, indicate two fluid inclusions showing higher homogenization temperatures (303°C and 356°C) and higher Au/Ag ratios are possible for precipitation of the super-pure native gold coexisting with pyrite and sericite, whereas one fluid inclusion showing a lower homogenization temperature (198°C) and a lower Au/Ag ratio is impossible for the super-pure native gold. Therefore, the super-pure native gold in the Woxi deposit was probably formed by the hydrothermal fluid with higher Au/Ag ratios and high temperatures in the mineralization stage of scheelite.

Gold remobilization in gossans of the Amani area, southwestern Tanzania

Oxide facies gold deposits are characterised by high Au-tenor free-milling gold nuggets and differ to their associated sulphide assemblages which have a lower Au-tenor. These changes point to an active gold geochemical cycle during the transitioning from sulphide- to oxide facies, which commonly also correlates with the formation of an iron oxide-rich gossan. Surficial gold occurrences in the Amani area of southwestern Tanzania are hosted by gossanous outcrops comprising weathered- to highly weathered quartz-carbonate veins and high Au-tenor gold, and thus represent an ideal natural laboratory in which to study supergene gold enrichment processes. The gossan mineralogy comprises goethite, hematite, malachite, azurite, siderite and quartz assemblages, with supergene gold particles up to 6 mm and with an average gold concentration of 95 wt%. The supergene gold particles have irregular and elongate morphologies, with elongate gold occurring exclusively along narrow ribs of neoformed goethite. The crystal habit of this gold shows a persistent orthogonal relationship between adjacent sub-micrometre-sized terraces, reflecting a possible crystallographic control on supergene gold precipitation. Novel results from 3-D X-ray computed tomography reveal that the micrometre- to millimetre scale distribution of gold follows a near-orthogonal pattern controlled by the textures and fluid flow pathways within the developing gossan. These elongate gold particles form interconnected 3-D skeletal gold networks which coalesce at discrete intersections to form larger nuggets (up to 6 mm in length) of high Au-tenor supergene gold. A detailed geochemical consideration of the mineralogy of the gossan, its precursor vein paragenesis and the surrounding host rocks suggest that the physicochemical conditions within the developing gossan favour supergene gold growth by dissolution-re-precipitation mechanisms involving an intermediate gold thiosulphate ligand complex.

Mineralogical Characteristics and in-situ Sulfur Isotopic Analysis of Gold-Bearing Sulfides from the Qilishan Gold Deposit in the Jiaodong Peninsula, China

Large-scale gold mineralization during the Early Cretaceous is identified in the Jiaodong Peninsula of China. Sources of ore-forming fluids remain debated. We study the Qilishan gold deposit in the northwestern Jiaodong Peninsula with detailed mineralogical observation and in-situ sulfur isotope analyses, in order to reveal the gold occurrence and the origin of ore-forming fluids. The Qilishan gold deposit is mainly clastic altered rock-type in mineralization, and ore minerals are visible native gold, electrum, pyrite, chalcopyrite and galena, gangue minerals as quartz, sericite and calcite. The gold occurrence includes inclusion and intergranular types, formed within pyrites and chalcopyrites and along their fissures. In-situ sulfur isotope analysis of gold-bearing sulfides suggests that the Qilishan deposit is enriched in heavy sulfur, with δ34S values mainly from +8.0‰ to +12.0‰. δ34S values increase gradually with the fluid evolution from the early to late stages, which is interpreted to be related to the loss of sulfur via sulfide precipitation. The crystallization of sulfides from hydrothermal fluids may have triggered the instability of Au(HS)2, and finally led to gold precipitation. Combined with sulfur isotope compositions of other gold deposits (n=43) and wall-rocks in the Jiaodong Peninsula, it is proposed that the ore-forming fluids were probably not directly originated from metamorphic wall-rocks (e.g., Jiaodong Group). Moreover, the relatively long time interval rules out the possibility that the gold mineralization (ca. 120 Ma) was associated with granitic magma activities (mostly 160–150 Ma). Possible ore genesis scenario is that, long-term subduction of slabs (e.g., the Paleo-Pacific) with gold-enriched pyritic materials and crustal sedimentary rocks resulted in both high Au contents and positive δ34S values of sulfur in the lithospheric mantle below the North China Craton. Subsequently, devolatilization of the metasomatized mantle produced auriferous fluids that migrated upward along translithospheric fault systems, and gold finally precipitated in favorable structural positions, generating the world-class Jiaodong deposits in the Early Cretaceous.

Support vector machine and artificial neural network modelling of orogenic gold prospectivity mapping in the Swayze greenstone belt, Ontario, Canada

Exploration for new mineral deposits has become increasingly difficult as new discoveries are being found under progressively deeper cover. To better understand and predict orogenic gold mineralization in the Archean Swayze greenstone belt, the essential ingredients of a mineral system are considered: 1) the source of gold and transport fluid ligands, 2) fluid pathways, 3) traps, and 4) the processes responsible for gold precipitation. The aim of this study is to use a mineral systems approach to help generate exploration targeting models using a spatial statistical method - weights of evidence (WofE) and data-driven machine learning tools, namely radial basis function neural networks (RBFNN) and support vector machine (SVM). The mineral prospectivity maps generated using the RBFNN and SVM machine learning methods were trained using the K-Fold cross-validation approach whereby 10 subsets of the data were used to train and test the model performance. The mean area under receiver operator curve after 10-fold cross-validations were 91% and 94% for the RBFNN models, and the SVM models obtained accuracies of 91% and 87%. Feature importance estimations obtained from both methods indicate that D2 and D3 high-strain zones, lithological contacts and D2 folds (i.e., synclines and anticlines) were found to be important predictor layers for targeting potential prospective zones of gold mineralization. The machine learning algorithms used in this study are novel and pragmatic methods that use the full potential of geoscience datasets in mapping orogenic gold prospectivity.

Genesis and ore-forming process of the Benqu mesothermal gold deposit in the Jiapigou ore cluster, NE China: Constraints from geology, geochronology, fluid inclusions, and whole-rock and isotope geochemistry

The Jiapigou ore cluster (JOC) located on the northeastern margin of the North China Craton (NCC) is a major producer of gold in China and is famous for its quartz vein-type gold deposits. It currently contains a dozen gold deposits with total proven reserves of >180 t Au, among which the Benqu deposit is the earliest discovered and most representative. The deposit is hosted within Neoarchean granitic gneisses and metamorphosed volcanic-sedimentary sequences. Gold orebodies mainly occur as gold-bearing quartz veins controlled by NE–NEE-trending ductile–brittle faults. Hydrothermal mineralization can be divided into early (quartz–pyrite), main (quartz–polymetallic sulfide–native gold), and late (quartz–carbonate) stages, with gold precipitation occurring in the main stage. Three types of primary fluid inclusions (FIs) are identified in quartz formed at different stages: CO2–H2O–NaCl (C-type), H2O–NaCl (W-type) and pure CO2 (PC-type). The early-stage quartz contains C- and W-type FIs, which have homogenization temperatures of 269–298 °C and salinities of 8.8–16.7 wt% NaCl equiv. The main-stage quartz contains all three types of FIs, with homogenization temperatures of 197–268 °C and salinities of 5.1–15.2 wt% NaCl equiv. The late-stage quartz contains only W-type FIs with homogenization temperatures of 145–198 °C and salinities of 3.2–12.4 wt% NaCl equiv. The ore-forming fluid system evolved from a CO2–H2O–NaCl system in the early stage to a H2O-dominated system in the late stage. The addition of meteoric water led to fluid immiscibility, facilitating the precipitation of gold and associated sulfides. The H–O–S–Pb–Sr isotope results indicate that the ore-forming fluids were derived from magmatic water and that the ore-forming materials were mainly extracted from a deep magmatic reservoir sourced mostly from the lower crust, with trace mantle components. Rb–Sr dating of the gold-bearing sulfides yields an isochron age of 175.8 ± 4.6 Ma (MSWD = 1.9). Zircon U–Pb dating of the Benqu diorite yields a weighted mean 206Pb/238U age of 176.5 ± 2.4 Ma (MSWD = 1.2). The Benqu diorite is high-K calc-alkaline, with enriched LREEs and LILEs (e.g., Rb, Pb, and K) and depleted HFSEs (e.g., Nb, Ta, and Ti), which is typical of the geochemical signature of arc magmas. The zircons have negative εHf(t) values of −11.3 to −8.9, indicating strong involvement of old crustal materials. These data suggest that the primary magma originated mainly from the partial melting of ancient lower crust that mixed with minor underplated mantle-derived magma. The above results systematically reveal that the Benqu is a mesothermal magmatic–hydrothermal lode gold deposit that formed in an active continental margin environment in the late Early Jurassic. Comparative studies of Benqu and other gold deposits in the JOC confirm that a significant magmatic–hydrothermal event occurred during the conversion from compression to extension that followed the first subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.

Hydrothermal alteration and mineralization of Baiyun gold deposit in Liaodong Peninsula, North China Craton

Baiyun deposit is a large gold deposit in the Liaodong Peninsula (North China Craton), containing over 31.7 t of gold. Silicic alteration, K‐feldspar alteration, and pyritization are well developed and closely related with mineralization in Baiyun deposit. Based on field and optical microscopic observations, the alteration styles include quartz + K‐feldspar zone, pyrite + sericite +quartz zone, and chlorite zone from orebody outward. In this study, the quartz, K‐feldspar, and pyrite of pre‐ore, syn‐ore stages 1 and 2 were selected for LA‐ICP‐MS trace element analysis and electron probe microanalysis (EPMA) to reveal the alteration‐mineralization link. Results show that the albite was altered to mainly K‐feldspar and minor secondary quartz at syn‐ore stage 1. Fluid‐rock reaction of the silicic‐potassic alteration may have decreased the fluid oxygen fugacity (fO2), which triggered the gold precipitation. Most of invisible gold occurred in the form of solid solution. At syn‐ore stage 2, albite altered to mainly quartz and minor K‐feldspar. The syn‐ore stage 1 K‐feldspar also altered to minor secondary quartz at syn‐ore stage 2. In addition, the release of sulphur from the syn‐ore stage 1 pyrite was likely another trigger for the gold precipitation. Most of syn‐ore stage 2 invisible gold occurred as nano‐particles. The ore‐forming fluids consist mainly of primary magmatic fluids mixed with minor metamorphic fluids. The ore metals may have sourced mainly from the magmatic fluid and the sillimanite‐mica schist of the Gaixian Formation.

Temperature-Controlled Ore Evolution in Orogenic Gold Systems Related to Synchronous Granitic Magmatism: An Example from the Iron Quadrangle Province, Brazil

Abstract Against the background of an ongoing debate on the genetic relationship between orogenic gold and granitic magmatism, we studied the evolution of a gold-mineralizing system in shear zone-hosted veins that are spatially associated with a syn- to late tectonic 2.69 Ga granite at the Satinoco deposit in the Archean Pitangui greenstone belt (Iron Quadrangle, Brazil). Detailed underground mine mapping, petrography, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and elemental mapping of sulfide grains revealed a complex polyphase history, with at least three gold-bearing stages: (1) A syntectonic arsenopyrite (Apy1)-löllingite-pyrrhotite assemblage I that formed during prograde metamorphism from ca. 475° to 650°C at 3 to 4 kbars; (2) further fluid circulation during syn- to late tectonic retrograde metamorphism from ca. 510° to 445°C, which led to a pyrrhotite-arsenopyrite (Apy2) ± löllingite ± galena assemblage II; and (3) a pyrite-pyrrhotite-galena ± chalcopyrite ± sphalerite ± ullmannite assemblage III formed during a later post-tectonic brittle deformation event from ca. 350° to 300°C at 1 to 2 kbars. Ascending granitic magma acted as a heat engine for peak metamorphic temperatures locally, that is at the immediate contact, reaching as much as 730°C and, together with the dehydration of mafic and ultramafic host rocks, generated the sulfide assemblage I. Crosscutting field relationships, together with existing S isotope data, indicate a metamorphic origin for all the sulfide stages with a contribution from granite-derived magmatogenic fluids to the mineralizing system at stage II. New X-ray element maps together with LA-ICP-MS spot analyses document oscillatory zoning of As, Fe, and Ni in Apy1 and enrichment in Ni, Mn, Zn, and V. The large amount of pyrrhotite in assemblage II is explained by granite-related Fe metasomatism. This stage led to an enrichment in Co, Sb, and Pb and the redistribution of Au. Assemblage III involved the formation of pyrite veins and precipitation of free gold and Pb-Sb-Cu-Zn sulfides in microfractures of this pyrite. This stage was likely responsible for the remobilization of Ni, Cu, Zn, and Bi-Te minerals. Based on existing geochronological data, at least most of the gold formed between ~2.72 and 2.68 Ga. This study revealed that gold mineralization took place over a wide range of temperatures—a finding that might be not only specific to the Satinoco deposit but possibly to other orogenic-type gold deposits elsewhere as well.