Gold Precipitation Research Articles

THE MINERALOGY OF THE BELLEROPHON-NELSON TELLURIDE-BEARING GOLD DEPOSIT, ST. IVES CAMP, YILGARN CRATON, WESTERN AUSTRALIA

The newly discovered Bellerophon-Nelson telluride-bearing gold deposit at the St. Ives camp, Western Australia, is hosted by meta-sedimentary rocks of the lower Black Flag Group and alkaline intrusions. Four stages of mineralization are recognized; from oldest to youngest these are: quartz-carbonate veins (Stage I), quartz-albite-carbonate-pyrite veins and sericite-pyrite seams (Stage II), quartz-pyrite veins (Stage III), and carbonate ± chlorite veins (Stage IV). Stages II and III contain economic gold mineralization, and the gold grains are strongly associated with pyrite. Intense albite and hematite alteration surround the mineralized veins, and trace amounts of gold precipitated in these altered rocks. The albite and hematite alterations are synchronous and derived from the same oxidized fluid as the auriferous veins. The occurrence and absence of hematite within the alteration zone reflects variable amounts of magnetite in the precursor rocks. Thirteen species of telluride and sulfosalt minerals have been identified in Stages II and III. The most common telluride minerals include calaverite, petzite, tellurobismuthite, and altaite, and these minerals have similar occurrences to native gold. In addition to native gold, telluride and sulfosalt minerals are also major Au carriers and account for at least 15% of the gold in this deposit. The mineral associations of PbCl(OH)–Pb2Cl3(OH)–Te–TeO2 and BiOCl–BiO(OH,Cl)–Te–TeO2 were formed as replacement of earlier telluride minerals as the result of reactions with Cl-bearing fluids. The intergrowth between native gold and Cl-bearing minerals + native Te/Te-oxide indicates that Au in telluride minerals was remobilized and re-deposited. The phase diagram for the telluride and sulfosalt mineral association suggests that during Stage II logƒS2 decreased from −8 to −11, and that logƒTe2 increased from −8 to the level required for the formation of Te-oxide. The values of logƒTe2 and logƒS2 in the Stage III veins were −8 to −11 and −9 to −11.5, respectively. The highly oxidized, tellurium-enriched hydrothermal fluid, which formed the Bellerophon gold telluride deposit, is consistent with the involvement of magmatic fluid, and sulfidation is the likely cause of gold precipitation.

Constraints of mineralogical characterization of gold ore: Implication for genesis, controls and evolution of gold from Kundarkocha gold deposit, eastern India

Gold mineralization in Kundarkocha gold deposit occurs in the eastern Indian Craton that is hosted by sheared quartz–carbonate–sulfide veins emplaced within the graphitic schist, carbonaceous phyllite and talc–chlorite–serpentine schist belongs to Gorumahisani–Badampahar schist belt of Iron Ore Group. Gold mineralization exhibits both lithological and structural controls in the study area, albeit the stratigraphic control is more ubiquitously observed. Detailed mineralogical characterization coupled with electron probe microanalysis of the sulfide phases reveal the occurrences of gold in three distinct forms (i) as lattice-bound form within sulfides especially enriched in arsenopyrite, loellingite, pyrite, pyrrhotite and chalcopyrite in decreasing order of abundance; (ii) as micro inclusions or nano-scale gold inclusions within pyrite and arsenopyrite especially along the growth zones and micro-fractures as substrates and (iii) as free milling nugget gold grains either along the grain boundaries of sulfides or within the host rocks. Three generations of pyrite (Py-I, Py-II and Py-III) and arsenopyrite (Asp-I, Asp-II, Asp-III) have been identified based on textural, morphological characteristics and mineral chemistry. The lattice-bound gold content in pyrite and arsenopyrite varies from 600 to 2700ppm and 900 to 3600ppm respectively and increase in concentration of such refractory gold is seen in the order of chalcopyrite>pyrrhotite>pyrite>loellingite/arsenopyrite.The evolutionary stages of different forms of gold include remobilization of the lattice-bound grains in pyrite and arsenopyrite (Py-I and Asp-I) and re-concentration along the zoned-pyrite and arsenopyrite (Py-II and Asp-II) and ultimately as native gold/nuggets surrounding the sulfides as well as within the main mineralized zone. Lattice-bound gold distribution could have resulted due to metamorphic devolatilization reactions which are further aided by the influx of hydrothermal fluids. These reactions along with additional input of hydrothermal fluid paved the way for expulsion of lattice-bound gold from sulfides to concentrate as nuggets/main lode within shear fractures channelizing the fluid flow. Thermometry results of the arsenopyrite–pyrite–pyrrhotite assemblage yielded a temperature range from 375 to 390°C which is the ideal condition for gold precipitation. Native silver, gersdorffite and arsenian-ullmannite are being reported for the first time from this deposit indicating the complexity and wide variety of mineral phases associated with gold implying magmatic-hydrothermal input of the source fluid.

‘Indicator’ carbonaceous phyllite/graphitic schist in the Archean Kundarkocha gold deposit, Singhbhum orogenic belt, eastern India: Implications for gold mineralization vis-a-vis organic matter

Carbonaceous rocks in the form of graphitic schist and carbonaceous phyllite are the major host rocks of the gold mineralization in Kundarkocha gold deposit of the Precambrian Singhbhum orogenic belt in eastern India. The detection of organic carbon, essentially in the carbonaceous phyllite and graphitized schist within the Precambrian terrain, is noted from this deposit. A very close relationship exists between gold mineralization and ubiquitous carbonaceous rocks containing organic carbon that seems to play a vital role in the deposition of gold in a Precambrian terrain in India and important metallogenetic implications for such type of deposits elsewhere. However, the role played by organic matter in a Precambrian gold deposit is debatable and the mechanism of precipitation of gold and other metals by organic carbon has been reported elsewhere. Fourier transform infrared spectroscopy (FTIR) results and total organic carbon (TOC) values suggest that at least part of the organic material acted as a possible source for the reduction that played a significant role in the precipitation of gold. Lithological, electron probe analysis (EPMA), fluid inclusions associated with gold mineralization, Total Carbon (TC), TOC and FTIR results suggest that the gold mineralization is spatially and genetically associated with graphitic schist, carbonaceous phyllite/shale that are constituted of immature organic carbon or kerogen. Nano-scale gold inclusions along with free milling gold are associated with sulfide mineral phases present within the carbonaceous host rocks as well as in mineralized quartz-carbonate veins. Deposition of gold could have been facilitated due to the organic redox reactions and the graphitic schist and carbonaceous phyllite zone may be considered as the indicator zone.

A fluid inclusion study of the Hetai goldfield in the Qinzhou Bay–Hangzhou Bay metallogenic belt, South China

The Hetai goldfield is located in the southern section of the Qinzhou Bay–Hangzhou Bay metallogenic belt (QHMB), South China. Gold mineralization is controlled by NE-trending ductile shear zones. Gold grade is higher at the shear zone centers and decreases sharply away from the shear zones, regardless of the host rock type. Fluid inclusions (FIs) preserved in the auriferous quartz veins have been analyzed to constrain their genesis. Three types of gold mineralization-related FIs, including moderate-salinity aqueous (A-type), low-salinity H2O–CO2 (B-type) and CO2-dominated (C-type), have been newly identified. The measured homogenization temperatures (Th) range from 130°C to 310°C, with two peaks of about 245°C and 170°C. The calculated pressures of FIs range from 50MPa to 170MPa. Immiscibility and CO2 effervescence of fluids may have played an important role in gold precipitation during the ascent of the ore-forming fluids. The Hetai goldfield is a typical example of orogenic gold deposits originating from auriferous metamorphic fluids.

Novel process for recycling gold from secondary sources: Leaching of gold by dimethyl sulfoxide solutions containing copper bromide and precipitation with water

A novel method for recovering gold from secondary sources, the leaching of gold using dimethyl sulfoxide (DMSO) solutions containing copper bromide followed by the precipitation of gold with water, is presented. Gold dissolution was conducted in a DMSO solution with 0.1–0.2M of CuBr2 and 0–0.2M of KBr at 333–348K. The mechanism of gold dissolution was investigated by electrochemical measurements. The precipitation of dissolved gold was performed by the addition of water, during which the effects of the amount of water and the pH on the recovery rate were investigated. It was found that the initial gold dissolution rate in the DMSO solutions with CuBr2 was up to 37mgcm−2h−1, which is larger than those obtained with other leaching methods. The results of the electrochemical measurements indicate that the anodic dissolution of gold in DMSO containing CuBr2 occurs at relatively negative potentials and is paired with the cathodic reduction of Cu2+ to Cu+. A recovery rate of gold up to 87% was obtained by precipitation with water. A small amount of copper was precipitated with gold, which can be avoided by using water of lower pH, as is expected by the Eh-pH diagram. Our results demonstrate that a circulating system for gold leaching and recovery, which would offer a number of advantages, including eco-friendliness, easy operation, low costs, and minimization of chemical sludge production, can be developed.

Geology and fluid characteristics of the Ulu Sokor gold deposit, Kelantan, Malaysia: Implications for ore genesis and classification of the deposit

The Ulu Sokor gold deposit is one of the most famous and largest gold deposits in Malaysia and is located in the Central Gold Belt. This deposit consists of three major orebodies that are related to NS- and NE-striking fractures within fault zones in Permian-Triassic meta-sedimentary and volcanic rocks of the East Malaya Block. The faulting events represent different episodes that are related to each orebody and are correlated well with the mineralogy and paragenesis. The gold mineralization consists of quartz-dominant vein systems with sulfides and carbonates. The hydrothermal alteration and mineralization occurred during three stages that were characterized by (I) silicification and brecciation; (II) carbonatization, sericitization, and chloritization; and (III) quartz–carbonate veins.Fluid inclusions in the hydrothermal quartz and calcite of the three stages were studied. The primary CO2–CH4–H2O–NaCl fluid inclusions in stage I are mostly related to gold mineralization and display homogenization temperatures of 269–389°C, salinities of 2.77–11.89wt.% NaCl equivalent, variable CO2 contents (typically 5–29mol%), and up to 15mol% CH4. In stage II, gold was deposited at 235–398°C from a CO2±CH4–H2O–NaCl fluid with a salinity of 0.83–9.28wt.% NaCl equivalent, variable CO2 contents (typically 5–63mol%), and up to 4mol% CH4. The δ18OH2O and δD values of the ore-forming fluids from the stage II quartz veins are 4.5 to 4.8‰ and −44 to −42‰, respectively, and indicate a metamorphic–hydrothermal origin. Oxygen fugacities calculated for the entire range of T-P-XCO2 conditions yielded log fO2 values between −28.95 and −36.73 for stage I and between −28.32 and −39.18 for stage II. These values indicate reduced conditions for these fluids, which are consistent with the mineral paragenesis, fluid inclusion compositions, and isotope values.The presence of daughter mineral-bearing aqueous inclusions is interpreted to be a magmatic signature of stage IIIa. Combined with the oxygen and hydrogen isotopic compositions (δ18OH2O=6.8 to 11.9‰, δD=−77 to −62‰), these inclusions indicate that the initial fluid was likely derived from a magmatic source. In stage IIIb, the gold was deposited at 263° to 347°C from a CO2–CH4–H2O–NaCl fluid with a salinity of 5.33 to 11.05wt.% NaCl equivalent, variable CO2 contents (typically 9–15mol%), and little CH4. The oxygen and hydrogen isotopic compositions of this fluid (δ18OH2O=8.1 to 8.8‰, δD=−44 to −32‰) indicate that it was mainly derived from a metamorphic–hydrothermal source. The CO2–H2O±CH4–NaCl fluids that were responsible for gold deposition in the stage IIIc veins had a wide range of temperatures (214–483°C), salinities of 1.02 to 21.34wt.% NaCl equivalent, variable CO2 contents (typically 4–53mol%), and up to 7mol% CH4. The oxygen and hydrogen isotopic compositions (δ18OH2O=8.5 to 9.8‰, δD=−70 to −58‰) were probably acquired at the site of deposition by mixing of the metamorphic–hydrothermal fluid with deep-seated magmatic water and then evolved by degassing at the site of deposition during mineralization. The log fO2 values from −28.26 to −35.51 also indicate reduced conditions for this fluid in stage IIIc. Moreover, this fluid had a near-neutral pH and δ34S values of H2S of −2.32 to 0.83‰, which may reflect the derivation of sulfur from the subducted oceanic lithospheric materials.The three orebodies represent different gold transportation and precipitation models, and the conditions of ore formation are related to distinct events of hydrothermal alteration and gold mineralization. The gold mineralization of the Ulu Sokor deposit occurred in response to complex and concurrent processes involving fluid immiscibility, fluid–rock reactions, and fluid mixing. However, fluid immiscibility was the most important mechanism for gold deposition and occurred in these orebodies, which have corresponding fluid properties, structural controls, geologic characteristics, tectonic settings, and origins of the ore-forming matter. These characteristics of the Ulu Sokor deposit are consistent with its classification as an orogenic gold deposit, while some of the veins are genetically related to intrusions.

Ore-forming process of the Huijiabao gold district, southwestern Guizhou Province, China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO2 inclusions, CO2–H2O inclusions, hydrocarbon inclusions, and hydrocarbon–H2O inclusions. The ore-forming fluids are characterized by a H2O+CO2+CH4±N2 system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H2O+NaCl→H2O+NaCl+CO2+CH4±N2→H2O+NaCl±CH4±CO2 with a slight decrease in homogenization temperature and salinity. The δ18O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δDH2O and calculated δ18OH2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ13CPDB and δ18OSMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H2O–NaCl–CO2 fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.

High-Sulfidation Epithermal Pyrite-Hosted Au (Ag-Cu) Ore Formation by Condensed Magmatic Vapors on Sangihe Island, Indonesia

Although gold in high-sulfidation epithermal deposits generally occurs as the native metal or electrum, in some deposits, a significant proportion of the gold is hosted in pyrite. Here we use a combination of petrography, whole-rock geochemistry, pyrite chemistry, crystallography, and phase stability relationships to determine how gold was transported and incorporated into pyrite in two relatively young high-sulfidation epithermal deposits, where the gold occurs almost exclusively in solid solution or as nanoparticles in pyrite. The genetically related Bawone and Binebase Au (Cu-Ag) deposits, located 1 km apart on the volcanic island of Sangihe, northeastern Indonesia, are hosted by andesitic volcaniclastic rocks that were altered to a proximal advanced argillic association of quartz + pyrite (py I) + pyrophyllite + natroalunite + alunite + dickite + kaolinite and a more distal intermediate argillic association of quartz + pyrite (py I) + kaolinite + dickite + illite. The economic mineralization takes the form of multiple generations of auriferous pyrite, the first of which, pyrite I (py I), developed during advanced argillic alteration. Mass balance calculations show that all elements were mobile with the exception of Nb, Ti, some rare earth elements, and possibly Al. The highest gold concentration is in pyrite II (py II), which occurs in veins that cut pyrite I. This drusy variety of pyrite is characterized by complex growth and sector zoning, and contains as much as 6.0 wt % Cu. The elevated Cu concentrations correlate positively with Au and As concentrations, whereas the Ag concentration correlates strongly with Au but not Cu. Later barite-enargite mineralization exploited py II veins and vugs, and significant concentrations of Ag and Au are hosted by enargite, although the Au concentration in enargite is lower than in py II or py I. A model is presented in which the fluid responsible for advanced argillic and intermediate argillic alteration and associated stage 1 gold mineralization was a condensed magmatic vapor derived from an oxidized magma. The gold and other metals were transported as hydrated species that ascended through the volcanic pile via fractures and zones of enhanced permeability to a depth between 900 and 1300 m, where the vapor condensed at a temperature between 250 and 340°C to form an acidic liquid with a pH of ~2.5; f o2 ranged up to four log units above the hematite-magnetite buffer. Interaction of this liquid with the host andesites caused advanced argillic and intermediate argillic alteration, including sulfidation of mafic minerals to form py I. During crystallization of py I, Au, Cu and Ag were adsorbed onto the surface of the pyrite and deposited as nanoparticles, or were incorporated in the pyrite structure. Adsorption of Au, Cu, and Ag from the condensed vapor reached a peak during the crystallization of vein-hosted py II, and the uptake of Ag and minor Au continued during later crystallization of enargite. From the distribution of metals among growth and sector zones in py II, incorporation of gold and other metals appears to have been maximized when physicochemical conditions were relatively stable. This is in contrast to the requirement for native gold precipitation, namely that physicochemical gradients be steep to ensure supersaturation of gold in the ore fluid.

Geology, timing, and genesis of the high sulfidation Au (–Cu) deposit of Touzlar, NW Iran

The Touzlar Au (–Cu) deposit in northwestern Iran is hosted by pyroclastics and lavas of andesitic to trachytic composition and by the coeval microdiorite stocks. The EW to NW trending host volcano-plutonic complex is part of the Cenozoic Urumieh–Dokhtar Magmatic Arc (UDMA) of the Zagros orogenic belt, a stratovolcano formed within the shallow marine environment of the Oligo-Miocene Red Beds of northwestern Iran that overlay a metamorphic core complex in an extensional regime. The zircon U–Pb ages of the host volcanic and subvolcanic rocks are 18.1±1.0 to 19.3±0.3Ma.The deposit with an estimated reserve of 1.5Mt of ore @ 2.2g/t gold and 0.3% Cu consists of a number of sub-parallel, massive silicified zones and breccia quartz veins within a hydrothermal alteration zone that follows the E-W trending extensional structures. The hydrothermal alteration assemblages consist of extensive propylitic and argillic alteration, advanced argillic alteration, silicification and sulfidation in the core zone, and of phyllic alteration. The multi-stage mineralization consists of disseminations, veins–veinlets and open space fillings. Advanced argillic alteration and silicification associated with different degrees of Cu-rich sulfidation occurred in phases (A) and (B). Phase (C) is characterized by hydrothermal brecciation and precipitation of sulfides in open spaces that was followed by the precipitation of barite in phase (D). Gold is found as free grains associated with quartz (phase B) and as overgrowth on pyrite grains in phase (C). Supergene Fe-oxyhydroxides formed with minor amounts of azurite and malachite, as well as gypsum, and is associated with enrichment in mineable gold grades of up to 5ppm.Homogenization temperatures of the (LV) fluid inclusion from the mineralized quartz veins range from 211 to 310°C and salinities from 0.3 to 8wt.% NaCl equiv. These fluid inclusions coexist with low-density, CO2-bearing fluid inclusions suggesting low-pressures, dilation associated with the precipitation of gold at these lower temperatures. Isotope compositions of quartz and barite are similar and vary within a short range of δ18OH2O 5.58 to 8.95‰ and 6.8 to 8.77‰ respectively suggesting that the ore fluids formed from mixing of magmatic fluids with meteoric waters. Calculated δ34SH2S composition (−2.68 to 15.59‰) of the sulfides indicate an igneous source for the sulfur that highly exchanged with a sedimentary source.The Touzlar deposit shows characteristic alteration assemblages and ore minerals of high sulfidation deposits. It has been formed in relation with the lower Miocene, high-level magmatic–hydrothermal activity within an extensional regime at the last stages of the UDMA activity of northwestern Iran.

Geology, geochemistry and genesis of the Huachanggou gold deposit, western Qinling Orogen, central China

The large Huachanggou gold deposit, on the southern margin of the Mian‐Lue Suture in western Qinling Orogen (central China), is hosted in the Devonian Sanhekou Group composed of spilite, limestone and phyllite. The mineralization is considered to have occurred in three stages: (1) Early quartz–pyrite veins, (2) Middle quartz–sulphides veins, and (3) Late carbonate–quartz veinlets, with gold being introduced mainly in the second stage. Quartz formed in the two earlier stages contains four types of fluid inclusions, namely (1) Pure CO2, (2) CO2–H2O, (3) Solid‐bearing and (4) NaCl–H2O, whereas the late‐stage minerals contain only the NaCl–H2O inclusions. The inclusions in quartz formed in the early‐, middle‐ and late stages have yielded total homogenization temperatures of 272−385 °C, 232−395 °C and 118−228 °C, respectively, with salinities no higher than 12 wt.% NaCl equiv. For the middle‐stage, trapping pressures estimated from the CO2–H2O inclusions for the altered spilite‐type (123−326 MPa) and quartz vein‐type (132−342 MPa) orebodies correspond to mineralization depths of 12 km and 13 km, respectively. We suggest that fluid boiling and mixing may have caused rapid precipitation of metallic sulphides and native gold. Through boiling and influx of meteoric water, the ore‐forming fluid system has evolved from CO2‐rich to CO2‐poor and from metamorphic to meteoric, as indicated by the decreasing δ18Owatervalues from the early‐ to late stages. Integrating the evidence obtained from regional geology, ore deposit geology, fluid inclusion and C–H–O isotope geochemistry, we conclude that the Huachanggou gold deposit is best ascribed to be an orogenic‐type system formed in the tectonic setting of the north‐vergent oceanic plate subduction along the Mian‐Lue Suture during the Late Triassic. Copyright © 2014 John Wiley & Sons, Ltd.

Genesis of two different types of gold mineralization in the Linglong gold field, China: Constrains from geology, fluid inclusions and stable isotope

The Dongfeng and Linglong gold deposits are located in the northwest Jiaodong Peninsula, North China Craton. The deposits are mainly hosted in the Mesozoic granitoids and structurally controlled by the Zhaoyuan–Pingdu fault zone. Gold mineralization at Dongfeng occurs as disseminated ores and sulfide stockworks, typically enveloped by broad alteration selvages. In contrast, mineralization at Linglong is characterized by massive auriferous quartz veins with narrow alteration halos. Three stages of mineralization were identified in both deposits, with the early stage represented by quartz±pyrite, the middle stage by gold+quartz+pyrite or gold+quartz+base metal sulfides, and the late stage by quartz+carbonate±pyrite, respectively. Four types of fluid inclusions were distinguished based on petrography, microthermometry, and laser Raman spectroscopy, including (1) pure CO2 fluid inclusions (type I), (2) H2O–CO2–NaCl fluid inclusions (type II), (3) H2O–NaCl fluid inclusions (type III), and (4) daughter mineral-bearing or multiphase fluid inclusions (type IV).In the Dongfeng gold deposit, the early- and middle-stage quartz mainly contains primary type II fluid inclusions that completely homogenized at temperatures of 276–341°C with salinities of 2.8–11.7wt.% NaCl equivalent, and temperatures of 248–310°C with salinities of 3.3–10.8wt.% NaCl equivalent, respectively. A few primary type I fluid inclusions could be observed in the early-stage quartz. In contrast, the late-stage quartz contains only the type III fluid inclusions with homogenization temperatures of 117–219°C, and salinities of 0.5–8.5wt.% NaCl equivalent. The estimated pressures for the middle-stage fluids are 226–338MPa, suggesting that gold mineralization mainly occurred at paleodepths of deeper than 8.4–12.5km. The mineralization resulted from extensive water–rock interaction between the H2O–CO2–NaCl fluids and wallrocks in the first-order fault.In the Linglong gold deposit, the early-stage quartz mainly contains primary type II fluid inclusions and a few type I fluid inclusions, of which type II fluid inclusions have salinities of 3.3–7.5wt.% NaCl equivalent and homogenization temperatures of 271–374°C. The middle-stage quartz mainly contains all four types of fluid inclusions, among which the type II fluid inclusions yield homogenization temperatures of 251–287°C and salinities of 5.5–10.3wt.% NaCl equivalent, while the type III fluid inclusions have homogenization temperatures of 244–291°C and salinities of 4.1–13.3wt.% NaCl equivalent. Fluid inclusions in the late-stage quartz are type III fluid inclusions with low salinities of 0.3–8.2wt.% NaCl equivalent and low homogenization temperatures of 103–215°C. The trapping pressure estimated for the middle-stage fluids is 228–326MPa, suggesting that the gold mineralization mainly occurred at paleodepths of about 8.4–12.1km. Precipitation of gold is possibly a consequence of phase separation or boiling of the H2O–CO2–NaCl fluids in response to pressure and temperature fluctuations in the open space of the secondary faults.The δ34S values of pyrite are similar for the Dongfeng and Linglong deposits and show a range of 5.8 to 7.0‰ and 5.9 to 7.4‰, respectively. Oxygen and hydrogen stable isotopic analyses for quartz yielded the following results: δ18O=−3.8 to +6.4‰ and δD=−90.5 to −82.7‰ for the Dongfeng deposit, and δ18O=0.0 to +8.9‰ and δD=−77.4 to −63.7‰ for the Linglong deposit. Stable isotope data show that the ore-forming fluids of the two gold deposits are of magmatic origin, with gradual incorporation of shallower meteoric water during/after mineralization.

H–O, S and Pb isotope geochemistry of the Awanda gold deposit in southern Tianshan, Central Asian orogenic belt: Implications for fluid regime and metallogeny

The Awanda gold deposit is located within the southern Tianshan Accretionary Complex, a late Paleozoic orogenic belt in the northern margin of Tarim block in NW China. The gold mineralization here occurs in primarily within shear zones and secondary s faults with quartz mica schist as the principal host rock. The sulfide mineral assemblage is dominated by arsenopyrite, and pyrrhotite, with minor pyrite and gudmundite. Alteration includes silicification, sericitization and rare carbonatization. The measured δ18O values for the gold-bearing quartz range from 15.9 to 20.3‰, and the estimated δ18O fluid values range from 7.8 to 12.2‰. The δD values for fluid inclusions from quartz range from −116.3 to −87.6‰. The δ34S values of gold-associated sulfide minerals range from −3.6 to +6.2‰, and the calculated δ34SH2S values of H2S for the hydrothermal fluids range from −4.8 to +5.8‰, with an average of −0.4‰. Lead isotopic compositions of sulfides from gold-bearing ores show 206Pb/204Pb ratios ranging from 17.4625 to 18.5356, 207Pb/204Pb ratios from 15.4943 to 15.6468, and 208Pb/204Pb ratios from 37.8825 to 38.8238, respectively. Collectively, the isotope (H, O, S, Pb) characteristics suggest that the ore-forming fluids were of metamorphic origin derived through dehydration of carbonaceous black shale with the sulfur sourced from crustal components, and the lead from a mixture of host rocks and ore-forming metamorphic fluids. This deposit can be classified as orogenic gold deposit and share many major characteristics with those of Muruntau-type gold deposits. During late Paleozoic, the southern Tianshan Accretionary Complex was intensely deformed into an orogenic belt by folding and thrusting, generating regional thrust faults at different scales. The ore-controlling subsidiary faults also formed subsequently, with the ore-forming fluids derived from dehydration of deeply buried carbonaceous black shale and the ore fluids channeled into shear zones leading to gold precipitation.

Gold nanoparticle formation via microbial metallophore chemistries

Microbes are evolved to live in almost every environment and often produce small molecules to protect themselves, communicate, and sequester nutrients and minerals from their surroundings. Our recent discovery that the gold-associated microbe, Delftia acidovorans, overcomes Au3+ toxicity through complexation and precipitation of soluble gold by the nonribosomal peptide metallophore, delftibactin, revealed a new mechanism for gold biomineralization and a possible industrial application for gold sequestration. In this work, we have sought to define more the scope of nanoparticle formation by other microbial metallophores and the physical properties of gold nanoparticles created by microbial natural products (hydroxamates, catechols, citrates, mixed ligands) and synthetics.

Constraints on the Genesis of the Archean Oxidized, Intrusion-Related Canadian Malartic Gold Deposit, Quebec, Canada

With a current resource of 13.4 Moz Au, plus past production of 5.1 Moz Au, the Canadian Malartic deposit represents the first bulk tonnage (measured and indicated resources of 372.9 Mt at 1.02 g/t Au) mine in the Superior province. Canadian Malartic is thus an important example of a large-tonnage, low-grade Archean gold deposit in which the mineralization is disseminated (or in fine veinlets) and hosted partly by felsic to intermediate intrusions. The deposit is located in the Abitibi greenstone belt, Quebec, within and immediately south of the Cadillac-Larder Lake tectonic zone, and occurs in porphyritic monzodiorite intrusions as well as clastic metasedimentary rocks of the Pontiac Group and mafic-ultramafic volcanics of the Piche Group. These rocks have undergone pervasive potassic alteration, carbonatization, pyritization, and local silicification. The main ore minerals are native gold and subordinate gold tellurides, accompanied by pyrite and minor chalcopyrite, galena, sphalerite, hematite, molybdenite, and Ag-Pb-Bi-bearing tellurides. Gold is concentrated in two generations of thin, discontinuous veins, and as finely disseminated grains in alteration envelopes around the main ore-stage veinlets. The main-stage veinlets consist of quartz-carbonate ± biotite ± albite surrounded by alteration haloes of K-feldspar-biotite ± pyrite ± calcite, whereas later veins are dominated by quartz-pyrite-calcite ± muscovite ± biotite ± chlorite. Alteration and gold mineralization were accompanied by large mass gains in K and S, extremely large mass gains in Ag, Te, and Au, and significant mass gains in Sb, W, Bi, and Pb; Cu underwent significant mass loss. The oxygen and hydrogen isotope composition of the mineralizing fluid determined from the corresponding compositions of quartz, biotite, and hematite ( δ 18Ofluid of 5.2 to 9.8‰, δ Dfluid of −52.0 to −45.0‰) is consistent with a predominantly magmatic source. A magmatic fluid source is supported by the composition of fluid inclusion leachates from quartz. Based on the isotopic composition of pyrite, sulfur was dominantly of magmatic origin but included a small contribution from a sedimentary source ( ![Formula][1] from −4.5 to +3.3‰, with small, positive Δ33S values). The deposit is interpreted to have formed at a temperature of ~475°C, based on oxygen and sulfur isotope geothermometry. Using this temperature in conjunction with the titanium-in-quartz geothermobarometer, the deposit is interpreted to have been emplaced at a pressure of ~3 kbar or a depth of ~10 km. Consistent with the constraints noted above, we propose a genetic model for the Canadian Malartic deposit in which felsic to intermediate, borderline alkaline to subalkaline magmas, emplaced at midcrustal levels, exsolved relatively oxidized ![Formula][2] , CO2- and sulfur-rich (∑ a S ≥ 0.1) auriferous fluids. These fluids rose to higher levels (~10 km; ~3 kbars), where they interacted with associated porphyritic monzodiorite intrusions, clastic Pontiac metasedimentary and Piche Group mafic to ultramafic rocks. The porphyries and metasedimentary rocks buffered the fluids to near-neutral pH, whereas the mafic-ultramafic rocks buffered the fluids to higher pH and lower ![Formula][3] . Ore deposition resulted from pyritization of the host rocks and oxidation of the mineralizing fluid, which reduced ![Formula][4] and caused destabilization of gold bisulfide species, leading to precipitation of native gold. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif

Direct observation of liquid pre-crystallization intermediates during the reduction of aqueous tetrachloroaurate by sulfide ions

Phenomena preceding precipitation of gold(i) sulfide in the reaction of aqueous tetrachloroauric acid with sodium sulfide have been studied applying several time-resolved in situ techniques. As can be deduced from UV-vis absorption spectroscopy, soluble species ("pre-nucleation clusters") emerge within several seconds and slightly change with time; the spectra, along with the previous data for the immobilized products, are indicative of disordered matter with Au(I)-S bonding and the gap of about 1.5 eV. In situ tapping-mode atomic force microscopy (AFM) directly imaged "soft" droplet-like species from 20-50 nm to about 200 nm in the lateral size and the continuous film condensed on the highly oriented pyrolytic graphite (HOPG) support. Small-angle X-ray scattering (SAXS) revealed quasi-particles ("liquid clusters") growing to about 12 nm for 30 min, and slowly afterwards. Dynamic light scattering (DLS) was preferentially due to the larger species, although the liquid clusters within 20 nm comprise more than 80% of the total volume of the fluid products. The entities, which exist in solution for many hours before sudden gold sulfide sedimentation, are considered as "dense liquid" intermediates involved in a non-classical nucleation pathway.

Physical-chemical properties and metal budget of Au-transporting hydrothermal fluids in orogenic deposits

Abstract We use a set of analytical techniques for fluid inclusion analysis to determine the bulk properties of the ore fluids from five orogenic gold deposits. The most common ore fluid is a low salinity (0.4–6.5 wt% NaCl) two- to three-phase aqueous-carbonic fluid. Its T h(total) range is 200–400 °C, and total homogenization occurs by bubble or liquid disappearance. A less common aqueous fluid of similar bulk salinity and T h(total) is also documented. Microchemical data show that the ore fluid in the five deposits is chemically uniform and made of Na, subordinate K and B and minor to trace amounts of Cu, As, Li, Sr, Rb, Ba, Cs, Sb and Au (range 0.5–5 µg/g). Thermodynamic modelling carried out for one of the studied deposits indicates that the ore fluid was in equilibrium with vein minerals at the time of gold deposition, and that its vapour phase was distinctly enriched in Au, B, As and Sb. The proposed mechanism of gold precipitation is a combination of fluid decompression and boiling. Combining these results with 3D geometrical reconstructions of one of the studied deposits (Sigma), we estimate that a minimum ore fluid volume of c. 0.1–1.0 km 3 is sufficient to generate a world-class orogenic deposit.

SULFIDE COMPOSITION AND ISOTOPIC SIGNATURE OF THE ALTAR Cu-Au DEPOSIT, ARGENTINA: CONSTRAINTS ON THE EVOLUTION OF THE PORPHYRY-EPITHERMAL SYSTEM

Sulfides and sulfosalts from the porphyry, transitional, and epithermal high sulfidation stages of the Altar Cu-(Au) deposit (San Juan, Argentina) were investigated by electron microprobe, laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS), and S isotopic analysis. Compositional differences found in the trace-element content in pyrite, chalcopyrite, and enargite from different alteration zones and depths of the system have implications in the physicochemical evolution of the hydrothermal fluids and might have potential for exploration in similar porphyry-high sulfidation epithermal environments. Low contents of most analyzed trace elements were found in chalcopyrite ± pyrite from the potassic alteration, except for Zn, Sn, and Ag in chalcopyrite, and Co and Ni in pyrite. During the phyllic alteration, cooling of the fluids and a decrease of its pH ( In the transitional veins and their selvages, early precipitation of bornite + pyrite and the weak kaolinite + quartz alteration envelope indicate a decrease in the temperature ( f S 2 . This stage was followed by a sudden increase in the f O 2 of the fluid and a slight decrease in the f S 2 , and bornite was replaced by tennantite (vein type E1). A trend toward higher f S 2 favored enargite precipitation (vein type E3). Traces of Au found in tennantite and enargite therefore indicate that fluctuations in the oxidation state of the fluids promoted gold precipitation in the Altar porphyry system. Negative S isotope values (δ 34 S: −1.7 to −3.0‰) obtained for the sulfides and sulfosalts from the potassic, phyllic, and transitional stages at Altar are consistent with oxidizing (SO 4 2− dominant) mineralizing fluids. The precipitation of sphalerite (δ 34 S = −4.4‰ and low concentration of FeS) and galena associated with calcite + anhydrite in late fractures (vein type 8) reflect cooling of the fluids and a slight increase of its pH at higher f O 2 , as the system moved to greater depths. The typical advanced argillic assemblages are not developed at the Altar porphyry deposit, indicating that fluids did not attain very low pH conditions. In contrast, enargite from the epithermal siliceous ledges located above the porphyry deposit precipitated with low pH mineral assemblages (quartz + alunite) and has a distinctive lighter δ 34 S value of −5.4‰, consistent with a decrease of temperature, pH, and/or an increase in the oxidation state of the fluids during its deposition. Enargite from the transitional stage veins of the porphyry deposit is enriched in Zn, Sb, Ag, and Bi, whereas enargite from the epithermal environment has much lower concentrations of trace elements, except for isolated inclusions of Au-Bi.

Extraction of Au(III) by ionic liquid from hydrochloric acid medium

The ionic liquid, 1-hexadecyl-3-methylimidazolium chloride/chloroform system was investigated for Au(III) extraction. In the extraction system, [C16mim]Cl bears a function of extractant. The effects of extraction time and [C16mim]Cl concentration were examined for Au(III) extraction. The anion-exchange mechanism of Au(III) extraction by [C16mim]Cl was confirmed by the method of continuous variation, UV–Vis and infrared spectrum analysis. According to the thermodynamics analysis, thermodynamic parameters of the Au(III) extraction reaction were obtained, which illustrates that higher temperature show negative effect for the Au(III) extraction. Under the experimental conditions, the [C16mim]Cl/chloroform system shows a high extractability for Au(III), also has high selectivity over some metals (Cu(II), Co(II), Ni(II), Fe(III), Al(III) and Sn(IV)). Moreover, oxalic acid is explored to reduce the Au–IL complex [C16mim]+AuCl4-, by which Au(III) is striped in the form of metallic gold precipitate and [C16mim]Cl also can be recycled.

Does gold in orogenic deposits come from pyrite in deeply buried carbon-rich sediments?: Insight from volatiles in fluid inclusions

The origin of volatiles in fluid inclusions was reviewed for testing the involvement at depth of carbonaceous-pyritic sedimentary rocks as the source for orogenic gold mineralization. Fluid inclusions from selected deposits were analyzed by solid-probe mass spectrometry. Fluids are mostly aqueous-carbonic, with variable amounts of N 2 , CH 4 , C 2 H 6 , Ar, H 2 S, H 2 and He. For fluids with CH 4 and C 2 H 6 , their ratios (C 1 /C 2 ) range from 2.6 to 25.5, indicating that C 2 H 6 is sourced from thermally degraded organic matter. Proportions of CO 2 , CH 4 , C 2 H 6 and H 2 are highly variable and can be explained by hydrothermal reactions where C 2 H 6 is degraded to CO 2 by water consumption. Such reactions may account for the problematic CO 2 -rich, H 2 O-poor fluids associated with some of the richest gold districts. Conditions needed for C 2 H 6 degradation are also fundamental for forming gold deposits, such as HS – -enriched fluids for carrying gold and local weakly oxidizing conditions for promoting gold precipitation. The C 2 H 6 content is recorded in fluids from Mesoarchean to Cretaceous gold deposits, providing support for a general model where fluids and gold were sourced from deeply buried, carbon-rich, and pyrite-gold–bearing sedimentary rocks.

The immobilization of gold from gold (III) chloride by a halophilic sulphate-reducing bacterial consortium

Abstract A consortium containing halophilic, dissimilatory sulphate-reducing bacteria was enriched from Basque Lake #1, located near Ashcroft, British Columbia, Canada to evaluate the role these bacteria have on the immobilization of soluble gold. The consortium immobilized increasing amounts of gold from gold (III) chloride solutions, under saline to hypersaline conditions, over time. Gold (III) chloride was reduced to elemental gold in all experimental systems. Salinity did not affect gold immobilization. Scanning electron microscopy and transmission electron microscopy demonstrated that reduced gold (III) chloride was immobilized as c. 3–10 nm gold colloids and c. 100 nm colloidal aggregates at the fluid–biofilm interface. The precipitation of gold at this organic interface protected cells within the biofilm from the ‘toxic effect’ of ionic gold. Analysis of these experimental systems using X-ray absorption near-edge spectroscopy confirmed that elemental gold with varying colloidal sizes formed within minutes. The immobilization of gold by halophilic sulphate-reducing bacteria highlights a possible role for the biosphere in ‘intercepting’ mobile gold complexes within natural, hydraulic flow paths. Based on the limited toxicity demonstrated in this experimental model, significant concentrations of elemental gold could accumulate over geological time in natural systems where soluble gold concentrations are more dilute and presumably ‘non-toxic’ to the biosphere.