Gold-bearing Pyrite Research Articles

Quartz Rb-Sr Isochron Ages of Two Type Orebodies from the Nibao Carlin-Type Gold Deposit, Guizhou, China

The Nibao gold deposit, which includes both fault-controlled and strata-bound gold orebodies, constitutes an important part of the Yunnan–Guizhou–Guangxi “Golden Triangle” region. Defining the mineralization age of these gold orebodies may provide additional evidence for constraining the formation ages of low-temperature orebodies and their metallogenic distribution in South China. Petrographic studies of gold-bearing pyrites and ore-related quartz veins indicate that these pyrites coexist with quartz or filled in vein-like quartz, which suggests a possible genetic relationship between the two from Nibao gold deposit. Minerals chemistry shows that Rb and Sr are usually hosted in fluid inclusions in quartz ranging from 0.0786 to 2.0760 ppm and 0.1703 to 2.1820 ppm, respectively. The Rb–Sr isotopic composition of gold-bearing quartz-hosted fluid inclusions from the Nibao gold deposit were found to have Rb–Sr isochron ages of 142 ± 3 and 141 ± 2 Ma for both fault-controlled and strata-bound orebodies, respectively, adding more evidence to previous studies and thus revealing a regional gold mineralization age of 148–134 Ma. These results also confirm the Middle-Late Yanshanian mineralizing events of Carlin-type gold deposits in Yunnan, Guizhou, and Guangxi Provinces of Southwest China. In addition, previous studies indicated that antimony deposits in the region which were formed at ca. 148–126 Ma have a close affinity with gold deposits. This illustrates that the regional low-temperature hydrothermal gold mineralization is related in space and time to the Yanshanian (ca. 146–115 Ma) magmatic activity. Specifically, the large-scale gold and antimony mineralization are considered to be inherently related to mantle-derived mafic and ultramafic magmatic rocks associated with an extensional tectonic environment. Based on the initial 87Sr/86Sr ratios of 0.70844 ± 0.00022 (2σ) and 0.70862 ± 0.00020 (2σ) for gold-bearing quartz veins from fault-controlled and strata-bound gold orebodies, respectively, at the Nibao gold deposit, as well as the C, H, O, and S isotopic characteristics of gold deposits located in the Golden Triangle region, we suggest that the mantle-derived material can be involved in the formation of the Nibao gold deposit and that the ore-forming fluid can be derived from a mixed crust–mantle source.

Influence of grinding media, pyrite mineralogy and water chemistry on the galvanic interaction between grinding media and pyrite

ABSTRACTThe purpose of this study was to investigate the role of grinding media type, water chemistry and pyrite mineralogy on the galvanic interaction between grinding media and pyrite during grinding of a gold ore. The galvanic current between pyrite and grinding media increased with the electrochemical activity of the grinding media, while it decreased as a function of increasing chromium content in the media. Furthermore, changing the pyrite-based electrode from pure pyrite to a gold-bearing pyrite electrode increased the galvanic current between the pyrite and the grinding media electrodes; therefore, the gold-bearing pyrite electrode is more active than the pure pyrite one. In addition, using process water instead of tap water increased the galvanic current density between pyrite and the high-chromium grinding media electrode (30% chromium). However, when the forged steel or the low-chromium grinding media electrodes were used, the galvanic current decreased compared to the tap water condition. In addition, the results obtained from SEM-EDX analysis showed that in both tap and process water conditions, oxide species were identified on the surface of the forged steel and low chromium electrodes. However, no oxygen was detected on the surface of the 30% chromium grinding media.

Ore fluid, geochronology and tectonic setting of mesothermal gold metallogeny in southeastern Jilin Province, Northeast China: A case study of the Shajingou gold deposit

The Shajingou gold deposit is a recently discovered medium-scale gold deposit in southeastern Jilin Province, Northeast China. The deposit is located in the easternmost segment of the northern margin of the North China Craton (NCC). This study investigated the mineralization age and tectonic setting of the Shajingou gold deposit based on geology, microthermometry and H–O isotopic analyses of fluid inclusions (FIs), pyrite Re–Os and zircon U–Pb dating, petro-geochemistry and zircon Hf isotopic analyses. The findings are placed in the wider context of mesothermal gold metallogeny in Northeast China.Metallogenesis of the gold deposit can be divided into three stages based on the ore mineral assemblage: quartz–K-feldspar–pyrite (stage I), quartz–polymetallic sulfide–native gold (stage II) and quartz–carbonate (stage III). Four types of primary FI were identified in quartz and calcite minerals: liquid-rich aqueous (L-type), CO2-bearing (C1-type), CO2-rich (C2-type) and CO2-pure (C3-type). From stage I to stage III, the fluid inclusion suite changes from L-type and C2-type in stage I, all four types in stage II and only L-type in stage III. Homogenization temperatures of stages I to III are 318–394 °C, 264–356 °C and 202–276 °C, respectively. Salinities of stages I–III are 7.31–12.42, 2.02–8.94 and 0.53–3.53 wt% NaCl equiv., respectively. The ore-forming fluid system evolved from a H2O–NaCl–CO2 system in stages I and II to a H2O–NaCl system in stage III. During the stage II, the addition of meteoric water led to decreases in temperature and pressure, resulting in an immiscible H2O–NaCl–CO2 fluid system. This immiscibility in turn led to the escape of CO2 and other volatiles, the increase of pH and the decrease in solubility and stability of Au–S clathrates, thus facilitating the deposition of gold and associated polymetallic sulfides.Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating of the Shajingou diorite yielded a weighted mean 206Pb/238U age of 171.7 ± 0.9 Ma (MSWD = 3.0). Re–Os dating of the gold-bearing pyrite yielded an isochron age of 166 ± 29 Ma (MSWD = 5.2). The Shajingou diorite has high concentrations of SiO2, Al2O3, Na2O, Mg#, Cr and Ni, weakly positive Eu anomalies, and is LREE- and LILE-enriched, HREE- and HFSE-depleted, as well as has positive εHf (t) values (+3.0 to +7.1). This indicates the diorite was sourced from the partial melting of a depleted-mantle wedge that had been modified by the subducted- slab-derived fluids. The deposit geology, ore genesis, geochronology and tectonic setting of the Shajingou gold deposit differ from those of other gold deposits in the eastern segment of the XMOB, but are similar to those of gold deposits in the Jiapigou–Haigou gold belt (JHGB) of southeastern Jilin Province at the northern margin of the NCC. These findings suggest the JHGB extends southeastward to the Helong area of Yanbian, which indicates the need for further research on the gold mineralization potential of the southeastern JHGB and further gold prospecting in the region. Comparative studies of Shajingou and other gold deposits in the JHGB have confirmed that a significant tectonic–magmatic–hydrothermal event occurred during the Middle Jurassic due to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.

Alteration of Eocene lamprophyres in the Zhenyuan orogenic gold deposit, Yunnan Province, China: Composition and evolution of ore fluids

The Zhenyuan orogenic gold deposit (>100 t Au), located in the northern part of the Ailaoshan shear zone (ASSZ) in South China, is characterized by extensive alteration and mineralization in a variety of host rocks. In the deposit, abundant Eocene lamprophyres have been infiltrated by ore fluids to produce carbonate veins with zones of disseminated fine-grained gold-bearing pyrite and arsenopyrite on both margins of the veins. Primary phlogopite, K-feldspar and magnetite have been altered to carbonate, sericite and fuchsite. Carbonate minerals and sericite in altered lamprophyre have higher Mg contents than in other host rocks, reflecting the primitive geochemistry of the lamprophyres. Since carbonatization is the primary alteration type in the lamprophyres, whose main primary components are low-Ca phlogopite and K-feldspar, the CaO content of the lamprophyres reflects their degree of alteration. As alteration increases, incompatible element ratios remain essentially constant whereas Sr is enriched with Ca. In addition, Au with Co, Ni, As, Ag, Sb, and Bi are enriched, reflecting the addition of these metal elements during alteration of the lamprophyres. Altered lamprophyres have slightly lower 143Nd/144Nd (t) (0.51240–0.51253) and substantially higher 87Sr/86Sr (t) (0.70716–0.71234) than unaltered or weakly-altered lamprophyres, indicative of crustal contamination. The auriferous ore-forming fluids are thus interpreted to have migrated upwards and interacted with the sedimentary host rocks before altering the lamprophyres. Lamprophyres commonly show a close spatial and commonly temporal association with orogenic gold deposits worldwide. At Zhenyuan, the lamprophyres shared a common deep source to the auriferous ore fluids but did not themselves contribute to this fluid or its gold content.

Gold-Bearing Pyrrhotite Ores in East Sayan: Composition and Formation Conditions (by the Example of the Ol’ginskoe Ore Occurrence)

Abstract —We present results of study of the chemical composition, formation conditions, and genesis of gold-bearing pyrite–pyrrhotite ores widespread within the ophiolite belts in the southeast of East Sayan. The study was performed at the Ol’ginskoe ore occurrence localized in the Ol’gino gold ore zone. Sulfide mineralization zones are composed of lenticular interbeds and bodies subconfoimable with the bedding of the enclosing schist strata. The ores are carbonaceous siliceous garnet–biotite schists variably enriched in sulfide minerals, mainly pyrrhotite, with impurities of other silicate minerals: tremolite, chlorite, albite, plagioclase, diopside, and epidote. In addition, sphalerite, chalcopyrite, pyrite, and arsenopyrite are present in small amounts in the ores. The established Р–Т conditions of ore metamorphism (T = 430–540 °C, P ~ 5 kbar) correspond to the lower boundary of the epidote–amphibolite facies. Thermobarogeochemical studies of fluid inclusions in vein quartz showed close temperatures (536–340 °C) but lower pressures, 200–800 bars, which indicates that the quartz vein formation was related to a pressure drop. The predominant salts of the fluid inclusions are Fe and Mg chlorides with Na and K impurities. The geologic location, structures, textures, and mineral, geochemical, and isotope compositions of the studied sulfide ores indicate their formation in submarine deep-water environments as a result of the activity of hydrothermal systems, analogues of “black smokers”, later subjected to metamorphism. During metamorphism, primary ores underwent mineral and chemical transformations: pyrrhotitization of pyrite, appearance and coarsening of native gold particles, and redistribution of components (Mn, As, etc). The low contents of gold and some ore-forming elements (Zn, Cu, and Pb) in pyrrhotite ores testify to the removal of these elements with a metamorphogenic fluid. The removed ore-forming components might have been a source of material for later gold–sulfide–quartz deposits of the Urik–Kitoi zone in East Sayan.

ON THE INFLUENCE ORGANIC COMPONENTS OF THE EXTRACT OF HERACLEUMON FLOTATION OF GOLD-BEARING SULPHIDES

The complexing, sorption and flotation properties of the organic extract of the green mass of Heracleum Sosnovsky (BO) containing sulphide gold-bearing minerals were investigated. The sorption and elemental composition of the surface of the minerals before and after contact with the reagent BO were studied on polished sections using an LEO 1420VP analytical scanning electron microscope (ASEM) equipped with an X-ray energy dispersive microarray analyzer INCA 350 and a KEYNCE laser microscope with VK-9700. The sorption of furocoumarines on the surface of chalcopyrite and gold particles after their contact with the organic extract of BO was determined by a spectrophotometric method and thin-layer chromatography (TLC). By flotation studies using monomineralic powders FeS2 and CuFeS2, as well as minerals with artificially deposited gold (FeS2Au and CuFeS2Au), the flotation activity of the BO extract with respect to the gold-bearing pyrite and chalcopyrite was revealed. When flotation gold-bearing sulphide ore with the use of BO as an additional collector, an increase in the quality of the concentrate with respect to gold content up to 10.8 g / t and gold recovery into concentrate to 82.69% was found, which is 3% higher than in the base experiment. The obtained results give grounds to talk about the possibility of using the flotation of a poisonous species of Heracleum after isolation from it of natural chemical compounds possessing surface-active properties with respect to gold-bearing sulfide minerals. The extraction of an extract containing furocoumarines from the Heracleum is not a problem.

Timing and genesis of ore formation in the Qarachilar Cu-Mo-Au deposit, Ahar-Arasbaran metallogenic zone, NW Iran: Evidence from geology, fluid inclusions, O–S isotopes and Re–Os geochronology

In the Qarachilar Cu-Mo-Au deposit of the Ahar–Arasbaran metallogenic zone (AAMZ), northwest Iran, mineralization occurs as three quartz-sulfide veins that cut granodiorite-quartz monzodiorite rocks of the Qaradagh batholith (QDB). Ore formation can be divided into three stages, with chalcopyrite, molybdenite, and gold-bearing pyrite appearing mainly in the first two stages. The main wall-rock alteration is silicification, and intermediate argillic, carbonate, and propylitic alteration. Fluid inclusion microthermometry indicates trapping of medium- to high-salinity (9.2–55 wt% NaCl equiv.) fluids at Qarachilar. Fluid inclusion trapping conditions are estimated to be 190 °C–530 °C and 0.1–3 kbar. The variable phase ratios as well as spatial coexisting of liquid- and vapor-rich two-phase and halite-bearing multiphase fluid inclusions homogenizing over the same temperatures are consistent with fluid boiling during ore formation. Obtained δ18OH2O values of quartz from ore-stage veins are +5.7‰ to +9.7‰, signifying that the ore–fluid system was predominantly magmatic water. The average calculated δ34SH2S values are 1 ± 1‰ for pyrite, chalcopyrite and molybdenite, consistent with a magmatic source for sulfur. Combined, the fluid inclusion and stable isotope data indicate that the ore-forming fluids at Qarachilar were magmatic in origin and were subsequently cooled and diluted by meteoric water. Fluid boiling and mixing facilitated hydrothermal alteration and mineralization. Molybdenite Re–Os dating shows that mineralization occurred at 42.35 ± 0.16 Ma, coincident with formation of porphyry Cu-Mo mineralization at Agarak deposit, and Hanqasar, Aygedzor and Dastakert prospects in the Lesser Caucasus. However, Qarachilar is older than all porphyry Cu-Mo mineralization in the AAMZ and Urumieh-Dokhtar magmatic arc (UDMA), which suggests that collision between Arabia and Eurasia were oblique and thus diachronous. Our data suggest that mineralization at Qarachilar is related to collisional Eocene magmatic–hydrothermal activity related to Neo-Tethys subduction, and shares a number of similarities with the vein-type Cu-Mo-Au mineralization related to Cu-Mo porphyries.

Control of Shear-Zone-Induced Pressure Fluctuations on Gold Endowment: The Giant El Callao District, Guiana Shield, Venezuela

The El Callao district, with a total endowment of more than 2000 t Au, is considered to be the most prolific gold resource in Venezuela. Mineralization is hosted by a vein system that is genetically associated with the El Callao transpressional shear zone. This vein system consists of a network of interconnected quartz–albite–ankerite veins enveloping a large number of metabasaltic fragments that host gold-bearing pyrites. Based on detailed mineralogical, microstructural, and fluid inclusion studies, a pressure-temperature pathway was established for the evolution of the mineralizing fluid during shear-zone development and exhumation. This path is characterized by repeated episodes of fluid pressure fluctuation from lithostatic (higher than 1.6 kbar) to near-hydrostatic values (<0.4 kbar), recorded throughout the transition from the quasi-plastic to frictional deformation cortical domains. Each successive pressure drop induced boiling of the hydrothermal fluid, with the resulting fluid phase separation controlling: (i) pyrite and invisible gold crystallization, which occurred during ductile and ductile-brittle transition strain conditions, and (ii) primary gold remobilization with consequent native-refined gold precipitation, occurring mainly under brittle conditions. The metallogenic framework that was proposed for the El Callao shear zone can be used as a vector to explore and characterize other mineralized shear zones in the Guiana Shield and analogous orogenic systems worldwide.

Early Cretaceous gold mineralization in the Lesser Xing’an Range of NE China: the Yongxin example

ABSTRACTThe northern Lesser Xing’an Range in NE China hosts many gold deposits. However, genesis and tectonic background for the mineralization remain unclear. The newly discovered Yongxin gold deposit in this region provides a good example for understanding the related issues. Two economic orebodies have been recognized at Yongxin and they are mainly hosted in the hydrothermal breccias. Zircon U–Pb ages of granite porphyry and diorite porphyry are 119.3 ± 0.7 Ma and 119.9 ± 0.6 Ma, respectively. These data provide constraints to the upper limit of ore-forming age. The δ34S values of pyrite from orebodies range from 2.3‰ to 5.1‰. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the pyrite are of 18.126–18.255, 15.492–15.537 and 37.880–38.019, respectively. The δ18OH2O and δD values of ore-forming fluids range from −12.5‰ to 1.8‰ and from −124.8‰ to −102.1‰, respectively. The REE compositions of gold-bearing pyrite are similar to those of the volcanic rocks of the Longjiang formation, diorite porphyry and granite porphyry. The combined geological, geochronological and geochemical characteristics of the Yongxin gold deposit indicate that the ore-forming materials were likely sourced from the volcanic rocks of the Longjiang formation, diorite porphyry and granite porphyry, whereas the ore-forming fluids are dominated by meteoric water. The Yongxin gold deposit could be a product of the Early Cretaceous large-scale gold mineralization in northeast China which occurred in an extensional tectonic setting and were related to the rollback of the subducted Paleo-Pacific Oceanic Plate beneath the continental margin of northeast China.

Geologic structure of the Mukodek gold field (northern Transbaikalia) and sources of matter (Pb-Pb and Sm-Nd data)

The Mukodek gold field is located in the Yana zone of the Baikal-Muya fold belt (northern Transbaikalia). Its orebodies are gold-bearing metasomatites confined to the fault of NE strike. It is shown that pre-ore faults played a crucial role in the spatial localization of the orebodies, as they favored the concentration of ore-bearing fluids. Results of geological and structural studies, along with available geochronological data (40Ar-39Ar dates for micas in gold-bearing pyrite), testify to ore-forming processes in the ore field during two Late Paleozoic stages. Early gold-bearing metasomatites of sericite-chlorite-albite-ankerite-quartz composition formed at 320 Ma, and late gold-bearing vein metasomatites of ankerite(calcite)-quartz composition originated at 285 Ma. A Pb-Pb isotope study of gold mineralization has shown that ore lead was supplied into the mineral-forming system mainly from a mantle source, most likely, regional Neoproterozoic igneous rocks. According to Sm-Nd isotope data, these rocks formed with the participation of a material of mantle genesis.

Constraints on depositional conditions and ore-fluid source for orogenic gold districts in the West Qinling Orogen, China: Implications from sulfide assemblages and their trace-element geochemistry

Yangshan, Mian-Lue-Ning and Manaoke represent three large orogenic gold districts in the Triassic West Qinling Orogen, the third largest gold province in China (∼1100 t gold resource). Although their component gold deposits have similar geological characteristics, representative deposits from these three gold districts have contrasting ore-mineral assemblages and gold speciation suggestive of diversity in gold source and/or ore-forming depositional conditions or processes. Decrease in P-T conditions of ore formation and metamorphic grades of host rocks in deposits from the Mian-Lue-Ning through Yangshan to Manaoke districts is matched by variation in their mineral paragenesis from pyrite–chalcopyrite–sphalerite–galena–native gold through pyrite–arsenopyrite–stibnite to scheelite–pyrite–arsenopyrite–stibnite–realgar. Invisible gold in the deposits of the Yangshan and Manaoke gold districts, contrasts with native gold in the Mian-Lue-Ning gold district. Concentrations of Ag–As–Au-Bi-Cu–Pb–Sb-Tl in hydrothermal pyrite show a gradual systematic increase from Mian-Lue-Ning through Yangshan to Manaoke. Pyrite in the Huachanggou gold deposit from the Mian-Lue-Ning district has high Co and low As and Au concentrations, in contrast with pyrite which is zoned with respect to As and Au in gold deposits from the other two districts. These variations between gold deposits in the different districts could be due to variable local ore–fluid sources derived from the different host-rock sequences during metamorphic devolatilization, as also allowed by mass-balance calculations. However, previously unmetamorphosed voluminous source rocks that have experienced amphibolite facies metamorphism, with liberation of gold and other metals during conversion of gold-bearing pyrite into pyrrhotite, are limited. It appears more likely that an external deeply-sourced fluid, most logically from devolatilization of the pyrite-bearing sediment wedge above the Triassic subduction zone associated with the formation of the West Qinling Orogen, was responsible for gold mineralization. In this model, variation between districts was due to fluid interactions with contrasting host-rock sequences under contrasting P-T conditions.

The effect of breakage energies on the mineral liberation properties of ores

A number of previous researchers have noticed that the degree of mineral liberation in a given size fraction is the same regardless of where in a comminution circuit a sample is collected. This behaviour has been observed for a range of ore types, and was found to be independent of the mode of breakage. This provides a useful heuristic for modelling the particle characteristics in mineral comminution and separation circuits. However, the published research does not explicitly consider the effect on the liberation behaviour of minerals of the amount of energy applied to break the ore. In this study, a gold-bearing pyrite ore and a copper sulphide ore were comminuted over a wide range of energy levels, using impact breakage. The liberation properties of the product particles were characterised using a Mineral Liberation Analyser (MLA). It was found that that the amount of impact energy applied did not significantly affect the degree of liberation of the minerals in a given size fraction, measured using particle sections. This provides sound experimental evidence to support the heuristic model.

Pyrite from Zarshuran Carlin-type gold deposit: Characterization, alkaline oxidation pretreatment, and cyanidation

Different types of pyrite exist within a refractory ore body and certain types possess a relative abundance of gold compared to other forms. Accordingly, identification and characterization of the different types of pyrite may represent a beneficial effect on the gold recovery by downstream processes. The main purpose of this study was to establish a connection between the mineralogy, leaching behavior and gold recovery of such gold bearing pyrites during alkaline pretreatment and cyanidation processes. The pyrite concentrate used in this study was obtained from the refractory Carlin-type gold deposit of Zarshuran, Iran. Characterization studies were done using SEM/EDX and EPMA analyses which revealed that pyrite was generally in the two form of euhedral pyrite including pyritodrone and cubic pyrite and subhedral to anhedral pyrite including large particles or fine-grained aggregates. The later type of pyrite was identified as the best host with gold occurring more likely in rims rather than the center of pyrite particles. A positive correlation of Au and As, Ni, and Pb was also found in gold-bearing rims. Alkaline oxidation experiments were conducted to liberate gold from pyrite context. The results showed that leaching rate increases with pH, temperature, oxygen overpressure and surface area where orders of reaction equal to 0.24 and 0.22 were calculated with respect to [OH−] and PO2 respectively. Kinetic analysis showed that pyrite leaching fitted well to the shrinking core model with surface chemical reaction control with the activation energy of 38.41 kJ/mol. Gold cyanidation experiments after alkaline oxidation pretreatments correlated well with the results of characterization and leaching studies in which heterogeneous distribution of Au was identified in pyrite context. It was found that oxidative pretreatment is required for the gold-bearing pyrite to improve gold recovery. However, because much of the gold is in the rims, high gold recovery is possible by the application of only partial oxidation.

Sulfur isotope and metal variations in sulfides in the BIF-hosted orogenic Cuiabá gold deposit, Brazil: Implications for the hydrothermal fluid evolution

Textural, chemical and multiple sulfur isotope analyses of sulfides from the Archean world-class Cuiabá orogenic Au deposit, Quadrilátero Ferrífero district, Brazil, were conducted to track distinct trace element signatures from syngenetic to epigenetic-hydrothermal pyrite types in different host units, and to deduce the nature and source of the mineralizing fluids. Gold mineralization in Cuiabá is hosted in greenschist facies volcanic and sedimentary host rocks belonging to the Archean Rio das Velhas Supergroup, especially in association with quartz veins and sulfide minerals. The BIF-hosted Fonte Grande Sul orebody is controlled by quartz veins and disseminated gold-bearing pyrite, pyrrhotite and arsenopyrite in the wall rocks. Five pyrite types are classified based on textural relations as spongy, syngenetic (Py1), porous early-(Py2), smooth main-(Py3) and smooth isolated and overgrown late-stage (Py4 and Py5) types variably present in metamorphosed carbonaceous pelites, BIF and andesite. Besides pyrite, pyrrhotite and arsenopyrite are also defined according to textural and paragenetic differences, and they display significant trace element contents. Trace element abundance maps and LA-ICP-MS analyses display trace element incorporations in Py1 (present only in carbonaceous pelite), Py2 and Py3 (in all host rocks), and Py4 and Py5 (only in BIF). Py1 yields high As, Co, Ni, Pb and Ag concentrations, whereas Py2 (formed by agglomeration of Py1) maintains high trace element concentrations with slightly less of those elements. Trace element incorporation in Py3 is similar to Py1 in carbonaceous pelite. Early-stage Py2 and main-stage Py3 in BIF and andesite have increased Co and Ni, but have less Au and As. Late-stage Py4 is characterized by lower trace element concentrations, whereas Py5 is further enriched in As, Bi, Co, Ni and Pb. The hydrothermal alteration is divided into early-, main- and late-stage, which correlate to the sulfide hydrothermal evolution. The study shows that carbonaceous pelite is pre-enriched in Co, Ni and Pb, whereas certain elements like Ag, Au, Bi and As are only hydrothermally concentrated either during the early, main or late stage in BIF and andesite. This geochemical pattern supports a syngenetic versus hydrothermal origin of distinct elements, including some trace elements now in the ore zones. Multiple sulfur isotope data suggest that mineralizing fluids at the Cuiabá gold deposit reflect a complex mixing of sulfur evolved from three possible sources: seawater, mantle, and reduced elemental sulfur. Spongy, syngenetic Py1 yields Δ33S values ranging from −2.28 to −0.25‰, separated into two ranges, the first from −2.28 to −1.97‰, and the second from −0.96 to −0.25‰. The first range suggests Py1 in the carbonaceous pelite was deposited in a seawater environment. These pyrites probably mixed with later mantle sulfur carrying a near-zero Δ33S signature, which is indicated by the second Δ33S range, or could have mixed with fluids sourced from sedimentary rocks at depth carrying a positive Δ33S signature. The second scenario is more common in sedimentary and diagenetic pyrites. An unequivocal contrast is shown by early-, main- and late-stage pyrite types (Py2, Py3, Py4 and Py5) in carbonaceous pelite, BIF and andesite, which present a continuous process of crustal assimilation values towards increasingly positive δ34S and Δ33S values. This confirms evidence for a sedimentary-derived sulfur source as also indicated by fluid inclusion data of the andesite-hosted orebody at the Cuiabá deposit.

Biohydrometallurgical treatment of old flotation tailings of sulfide ores containing non-nonferrous metals and gold

Proposed in the work is mining waste treatment using a three-step biohydrometallurgical process. The process includes: (i) a step of acid leaching (with sulfuric acid solution) to remove highly soluble copper and zinc forms, (ii) a step of leaching with biogenic ferric iron for the oxidation of sulfide minerals containing copper and zinc, as well as for the partial oxidation of gold-bearing pyrite, and (iii) a biooxidation step for a more complete pyrite oxidation, providing the release of gold associated with it. The subject of the research was a sample of pyrite flotation tailings containing copper, 0.26%, zinc, 0.22%, and gold, 0.67 g/t. Investigated was an acid leaching of pyrite tailings under different conditions. The leaching allowed to recover up to 35–36% of copper and zinc after 10 min. Leaching of pyrite tailings with biogenic ferric iron was studied. The highest efficiency shown was for the process carried out at the temperature of 80 °C, the pulp density of 30%, the ferric iron concentration of 30 g/L in the leaching solution, and the process duration of 8 h. As a result, approximately 14% of the pyrite oxidized, and the total zinc and copper recovery increased up to 86% and 64%, respectively. The highest efficiency of tailings treatment was shown for the biooxidation during the 12-day three-step process, which allowed to oxidize 97% of pyrite and to recover 96% of gold by carbon-in-pulp cyanidation. The total copper and zinc recovery during the three-step process was 79% and 96%, respectively. A flowsheet was proposed for the three-step process.

New insights into the extraction of invisible gold in a low-grade high-sulfur Carlin-type gold concentrate by bio-pretreatment

To extract gold from a low-grade (13.43 g/t) and high-sulfur (39.94wt% sulfide sulfur) Carlin-type gold concentrate from the Nibao deposit, Guizhou, a bio-pretreatment followed by carbon-in-pulp (CIP) cyanide leaching process was used. Various methods were used to detect the low-grade gold in the concentrate; however, only time-of-flight secondary-ion mass spectrometry (TOF-SIMS) was successful. With bio-pretreatment, the gold recovery rate increased by approximately 70.16% compared with that obtained by direct cyanide leaching of the concentrate. Various attempts were made to increase the final gold recovery rate. However, approximately 20wt% of the gold was non-extractable. To determine the nature of this non-extractable gold, mineralogy liberation analysis (MLA), formation of secondary product during the bio-pretreatment, and the preg-robbing capacity of the carbonaceous matter in the ore were investigated. The results indicated that at least four factors affected the gold recovery rate: gold occurrence, tight junctions of gold-bearing pyrite with gangue minerals, jarosite coating of the ore, and the carbonaceous matter content.

Electronic structure and flotability of gold-bearing pyrite: A density functional theory study

Various incorporation of Au in pyrite and its effects on the geometrical structure, electronic structure and flotability of pyrite were theoretically investigated and fully discussed by performing density functional theory (DFT). The calculated incorporation energy shows that gold would most likely exist in pyrite via incorporating into interstitial lattice sites in the absence of As impurity. As a result of incorporated Au, the covalence levels of the S—Fe and S—S bonds are changed, and the tonicity of Au—S bonds and antibonding of Au—Fe bonds are found to form in the pyrite, which would change the natural flotability of pyrite. The Au impurity energy levels are introduced into the energy band and result in the transformation of pyrite semiconductivity type. The calculated band-gap value suggests that the incorporated Au significantly decreases pyrite semiconductivity level, which enhances the formation and the adsorption stability of dixanthogen during pyrite flotation. The DOS results reveal that the stability and depression difficulty level of pyrites increases in the following order: Fe32S63As < Fe32S64 < Fe32S63AsAu < Fe32S64Au.

Atmospheric oxidation of pyrite with a novel catalyst and ultra-high elemental sulphur yield

The effectiveness of two different carbon based catalysts, Lewatit® AF 5 (AF 5) a new microporous carbonaceous bead, and a granular coconut shell based activated carbon, in aiding atmospheric oxidation of a gold-bearing pyrite concentrate was tested. Oxidation in ferric sulphate media unassisted by a catalyst was incomplete after 96h and resulted in only 60% pyrite oxidation, with an elemental sulphur yield ranging between 16% and 28%. Under the same test conditions, oxidation tests assisted by AF 5 saw approximately 96% pyrite oxidation with elemental sulphur yields ranging between 65% and 74%, while activated carbon assisted oxidation reached 100% pyrite oxidation with elemental sulphur yields between 63% and 64%. Testwork indicated that not only were pyrite oxidation kinetics greatly enhanced by the addition of carbon based catalysts leading to almost if not complete oxidation, but drastic increases in elemental sulphur yields and thus reductions in sulphate yields were also experienced.It is shown that the AF 5 catalyst adsorbs all of the elemental sulphur produced on its surface. The AF 5 beads are large enough (approximately 0.5mm) that after the leach process they can be screened out of the slurry, resulting in negligible elemental sulphur within the residue after solid/liquid separation. AF 5 also showed much stronger mechanical properties than the activated carbon, resulting in only approximately 0.40% AF 5 loss to fines compared to approximately 21.5% activated carbon loss to fines.

Geology and geochemistry of the Early Permian Axi low-sulfidation epithermal gold deposit in North Tianshan (NW China)

The Axi gold deposit containing 12Mt of ore with an average Au grade of 5g/t is the largest low-sulfidation epithermal gold deposit in Chinese Tianshan. It is hosted by Late Devonian to Early Carboniferous continental arc volcanic rocks in the Tulasu volcanic-sedimentary Basin in North Tianshan (NW China). The orebody is controlled by annular and radial faults associated with volcanic crater, and later NW-trending faults. Four stages of ore-formation could be identified based on petrographic studies: (I) chalcedony; (II) gold-bearing pyrite-quartz veins and breccias; (III) gold-bearing marcasite-quartz-carbonate veins; and (IV) quartz±barite-calcite veins. Electrum precipitated in stage II (285–195°C, logfS2=−6.7 to −13) and stage III (190–95°C, logfS2=−15.8 to −25.6). The chalcedony formed in stage I is strongly enriched in LREE (La/Yb=4.5–36.2) compared with later gold-bearing pyrite-quartz veins and quartz±barite-calcite veins (La/Yb=1.2–2.0). O–H isotopic data indicate that fluid mixing is a major mechanism causing electrum precipitation in stage II. Hydrothermal breccias consisting of chalcedony, altered wall-rocks and gold-bearing pyrite-quartz veins (stage II) are cemented by gold-bearing marcasite-quartz-carbonate assemblages (stage III), which implies that brecciation plays an important role in stage III ore-formation.Re-Os isotopic analyses of gold-bearing pyrite samples yield an 187Re/188Os–187Os/188Os isochron age of 299±35Ma with an initial 187Os/188Os ratio of 0.57±0.37. K-Ar ages of four intensively sericitized andesitic wall rocks range between 286 and 293Ma. These Early Permian ages are much younger than the Late Devonian-Early Carboniferous host volcanic rocks, but overlap the age range of postcollisional granitoid in the Tulasu Basin. The Re-Os isotopic compositions and the ages suggest that mantle-derived components were involved in the Axi ore-forming system in a postcollisional setting.

Influence of clays on the slurry rheology and flotation of a pyritic gold ore

The presence of clay minerals in ores has been known to adversely affect the flotation performance. This work investigated the influence of kaolin and bentonite clays on the flotation of gold-bearing pyrite in the gold ore. The changes in rheological properties of the flotation pulp under normal operating conditions were also studied with the additions of kaolin and bentonite clays at various solid concentrations. The zeta potential measurements also were carried out to understand the mechanism of adsorption of potential determining ions. It was found that bentonite clay reduced the pyrite recovery, and modified the pulp rheology more than kaolin. The addition of Ca2+ ions to the pulp containing bentonite improved the pyrite flotation recovery by its ability to modify the rheological characteristics. It is consistent with the published literature that Ca2+ ions suppressed the swelling properties of bentonite, and altered the surface charge properties of bentonite clay particles. Additionally, the rheological characteristics such as the flow index and the consistency index affected the flotation performance as the former indicates the extent of movement of bubbles and particles within flotation pulp. This suggests the possibility of using the rheological measures to monitor the changes in the flotation performance in routine plant operations.