Invisible Gold Research Articles

Invisible gold in arsenian pyrite from the high-grade Hishikari gold deposit, Japan: Significance of variation and distribution of Au/As ratios in pyrite

Au and As concentrations in pyrite grains from the Hishikari deposit were extensively determined by secondary ion mass spectrometry (SIMS) and electron probe microanalysis (EPMA) to investigate the microscale relationship between Au and As. The low-sulfidation epithermal Hishikari deposit, which has produced more than 224 t of gold since 1985 (as of March 2016), is world-famous for its high-grade ore. Accurate quantitative SIMS analyses of Au in pyrite were possible by preparing an Au-implanted pyrite standard sample and by using a properly tuned SIMS analytical method. An analysis size of 3 μm was chosen since the pyrite grains from the Hishikari deposit have heterogeneous As concentrations. This paper presents the first comprehensive data set of Au and As concentrations in a 3 μm area of pyrite obtained from the widely distributed high-grade Au-bearing Hishikari veins.Microanalyses of arsenian pyrite from the Hishikari gold deposit by SIMS and EPMA revealed that Au concentrations of 0.1–2800 ppm are positively correlated with As concentrations of 0 to 7 wt% in a 3 μm area. The formation of Hishikari veins is divided into two stages that are further subdivided into the early and late sub-stages. The rim of a pyrite grain precipitates after formation of the core; however, the time scale of core and rim formation is far shorter than that of the entire mineralization sequence. The specific value of the Au/As ratio in pyrite proves to be characteristic of the specific Au-bearing vein and its specific depth. Therefore, it could be possible to infer the nature of unknown veins by the Au/As ratio obtained. Although the Au/As ratio varies from vein to vein, a general tendency is observed for each ore zone of the Hishikari deposit. The Au/As ratio is high in the Sanjin ore zone, which consists of numerous high-grade veins in or above the Cretaceous accretionary sedimentary rocks of the Shimanto Group. The Au/As ratio is moderate in the Honko ore zone, which consists of several high-grade veins and also occurs in or above the basement sedimentary rocks. On the other hand, the Yamada ore zone, which consists of relatively low-grade veins, occurs in Pleistocene andesitic pyroclastic rocks that overlie the basement sedimentary rocks unconformably. The Au/As ratio in pyrite of the Yamada ore zone is relatively low compared with that of other ore zones.Several previous papers have proposed that the As− in pyrite may contribute to the reduction of Au+ to Au0 as Au nanoparticles (NPs) by As oxidation; however, Au and As concentrations of arsenian pyrites fall on or below the Au solubility limit in this study. Moreover, the As and S concentrations in pyrite are inversely correlated, which is consistent with the substitution of As for S as anionic As− in the Fe(S1−xAsx)2 solid solution of natural arsenian pyrite under reducing environments. The present study suggests that the Hishikari Au exists as a solid solution in the Hishikari arsenian pyrite, although there remains a possibility that it exists partially in the form of NPs even though the Au and As concentrations of arsenian pyrites fall on or below the Au solubility limit.

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

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

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.

Alteration paragenesis and the timing of mineralised quartz veins at the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

The world-class Geita Hill deposit is one of the largest gold deposits located within the Geita Greenstone Belt in NW Tanzania. The deposit is hosted within a complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately NW dipping deformation zone consisting of a network of discontinuous shear fractures that record early thrusting overprinted by later strike-slip and normal events. The regional metamorphic assemblage in the meta-sediments is characterised by biotite+chlorite+actinolite+K-feldspar+magnetite±pyrrhotite±pyrite indicating upper greenschist facies conditions. The gold-related alteration overprints the regional metamorphic assemblage, and is characterised by silicification and sulfidation fronts that generally extend out from the mineralised zone by no more than one meter. The alteration assemblage includes sub-vertical, mineralised quartz veins that trend approximately E-W. The mineralised quartz veins are accompanied by alteration halos of quartz+biotite+K-feldspar+pyrite which overprints the peak metamorphic assemblage. Gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides (sylvanite, calaverite and nagyagite). It occurs mainly as inclusions in pyrite and as invisible gold in pyrite but also as gold inclusions in biotite and along quartz grain boundaries. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite, hosting invisible gold, and inclusion free euhedral pyrite, hosting visible gold grains. It is common for the inclusions rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multiphase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite+siderite+chlorite±quartz±pyrite±barite. Palaeostress analysis of mineralised shear fractures along the Geita Hill Shear Zone are indicative of sigma 1 being vertical and sigma 3 trending N-S, indicating N-S extension, which is consistent with the orientation of the mineralised quartz veins.

Textural and compositional evolution of Au-hosting Fe-S-As minerals at the Axi epithermal gold deposit, Western Tianshan, NW China

The Axi deposit is the largest low-sulfidation epithermal gold deposit in Chinese Western Tianshan, Central Asian Orogenic Belt. In this contribution, combined petrological observations, electron microprobe analysis (EMPA) and X-ray backscattered electron images (BSE) have been conducted on Fe-S-As minerals to reveal their textural and compositional evolution. Pyrite and arsenopyrite are ubiquitous. Their texture and composition show gradual changes with time. Four generations of pyrite can be identified, named Py1, Py2, Py3 and Py4 from early to late. The coarse-grained, euhedral to subhedral Py1 (mostly 100–300μm) from the quartz-chalcedony stage is the earliest, and predates gold mineralization. Sometimes they contain silicate and arsenopyrite (Apy1) inclusions. Compositionally, they are enriched in Pb (up to 0.60wt%), but depleted in As (0.06–7.61wt%, mostly <0.30wt%) and Au (0.01wt%–0.25wt%, mostly <0.10wt%). Subhedral to anhedral Py2 grains (50–1000μm) coexist with other sulfides such as chalcopyrite, sphalerite, galena and tetrahedrite in the quartz-polymetallic sulfides stage. They are unusually rich in Au (with a mean value of 0.06wt%), Ag (with a mean value of 0.04wt%), Cu (with a mean value of 0.21wt%) and As (with a mean value of 3.06wt%), implying a syn-mineralization feature. Subsequent Py3 occurs as very fine-grained disseminations of euhedral crystals (10–50μm) in quartz-polymetallic sulfides stage and carbonate-quartz stage. They display clear oscillatory zoning and contain arsenopyrite (Apy3), sphalerite and Sb-Te mineral inclusions. Their Au content vary greatly from 0.02wt% to 1.02wt%, with corresponding As content varying from 0.07wt% to 8.28wt%. The post-mineralization Py4 crystals (30–5000μm) mainly occur as foliated, sheaf, or radiaxial aggregates in carbonate-quartz stage. They have the lowest Au (with a mean value of 0.04wt%), As (with a mean value of 0.32wt%), Co (with a mean value of 0.04wt%) and Ni (with a mean value of 0.04wt%) concentrations, although dramatically high S contents (with a mean value of 52.72wt%). Correspondingly, there are three generations of arsenopyrite: Apy1 (20–50μm) and Apy3 (3–15μm) mainly occur as inclusions in Py1 and Py3, respectively, whereas the latest Apy4 (<5–60μm) can occur either as serrated grains surrounding Py4 or overgrowth Py1 and Py3, or as individual euhedral to sub-euhedral grains. They also contain considerable, but much higher Au contents than pyrite, with the values of 0.6wt%–1.1wt% for Apy1, 0.18wt%–1.32wt% for Apy3 and 0.50wt%–1.75wt% for Apy4, respectively. Besides, they also contain considerable amounts of trace elements such as Co, Zn, Pb, Sb and Se. Arsenopyrite geothermometer constrains the ore-forming temperature responsible for Apy3 formation to 325–385°C. Our result reveals that significant invisible gold accommodated by sulfides is the main cause of refractory ores at the Axi deposit.

Gold- and Silver-Rich Massive Sulfides from the Semenov-2 Hydrothermal Field, 13°31.13′N, Mid-Atlantic Ridge: A Case of Magmatic Contribution?

The basalt-hosted Semenov-2 hydrothermal field on the Mid-Atlantic Ridge is host to a rather unique Cu-Zn– rich massive sulfide deposit, which is characterized by high Au (up to 188 ppm, average 61 ppm, median 45 ppm) and Ag (up to 1,878 ppm, average 490 ppm, median 250 ppm) contents. The largest proportion of visible gold is associated with abundant opal-A, which precipitated after a first generation of Cu, Fe, and Zn sulfides and before a second generation of Fe and Cu sulfides. Only rare native gold grains were found in earlier sulfides. Fluid inclusions in opal-A associated with native gold indicate precipitation at 300° ± 40°C from fluids of salinity higher than that of seawater (3.5–6.8 wt % NaCl equiv). According to laser ablation-inductively coupled plasma-mass spectrometry analyses, invisible gold is concentrated in secondary covellite (23–227 ppm) rather than in the primary sulfides ( 1,000 ppm) than all other sulfides (<250 ppm). Covellite replacing Zn sulfides (covellite-A) is enriched in all analyzed trace elements relative to covellite replacing Cu-Fe sulfides (covellite-B). The enrichment of covellite-A in trace elements may be related to the dissolution of inclusions of various minerals hosted in former sphalerite, which were the source for Au and Ag (native gold), Pb and Tl (galena), Se (chalcopyrite, Se-bearing galena, naumannite), Te and Bi (Bi tellurides), As (tennantite, chalcopyrite), and Sb (tennantite). The formation of covellite-A was favored by hydrothermal fluid/seawater mixing or direct oxidation of sulfides by seawater, as suggested by the relatively high contents of typical “seawater” elements (U and V). The degree of seawater involvement was apparently lower for covellite-B. Although the Semenov-2 field is basalt hosted, several geochemical features of the massive sulfides studied are similar to those of the Mid-Atlantic Ridge ultramafic-hosted Cu-Zn–rich massive sulfides, such as Fe:Cu:Zn ratios close to 1:1:1, high Sn, Se, Au,and Ag contents, and high Au/Ag ratios. However, the strong enrichment in SiO2, the moderate Mn and Co contents, very low Ni contents, and the Co/Ni ratio >1 are more consistent with a mafic signature. Thermodynamic modeling of hydrothermal fluids produced by reactions between various proportions of seawater and basalt or peridotite at 350°C shows that mineral assemblages broadly similar to those of the Semenov-2 deposit can precipitate from fluids produced in a mafic environment, but that Au and Ag minerals are not predicted to precipitate from such fluids over a wide temperature range. These results suggest that an additional contribution to the hydrothermal system is required in order to achieve saturation in precious metals. A magmatic input is suggested by the occurrence of plagiogranites and tonalites dredged on sea floor in the Semenov area, which could be a potential source of Au-rich magmatic fluids, and by mineralogical and geochemical similarities with magma-related, low- to intermediate-sulfidation epithermal systems, namely high Au and Ag grades, high Au/(Cu + Zn + Pb) and Au/Ag ratios, and presence of Ag, Bi, and Te minerals. The likely crucial role of silicic melts in producing high Au and Ag grades suggests that exploration for precious metal-rich, volcanic-hosted massive sulfide deposits should be primarily directed to sites in which evolved igneous rocks occur on sea floor. Both in modern and ancient mafic-hosted deposits, zones characterized by abundant deposition of silica could be good clues to the presence of significant gold.

Covellite of the Semenov-2 hydrothermal field (13°31.13′ N, Mid-Atlantic Ridge): Enrichment in trace elements according to LA ICP MS analysis

As a result of LA ICP MS analysis of sulfides of the Semenov-2 hydrothermal field, it is established that covellite, which replaces Zn sulfides, is enriched in most trace elements. The Ga, Ni, and In contents in it do not vary, whereas Mn, Co, and Cd are lower than in sphalerite. The distribution of trace elements in covellite, which replaces Cu–Fe sulfides, is distinct: it is enriched in Cd, Sb, Pb, and Bi, whereas the contents of other elements are either lower or invariant. Covellite, which replaces Zn sulfides, is enriched in all trace elements relative to that replacing Cu–Fe sulfides. Enrichment of covellite in trace elements relative to primary sulfides was favored by oxidation of the hydrothermal fluid by seawater, which is similar to the processes of submarine oxidation of ancient massive sulfide deposits. Covellite is also a host to invisible gold and silver in ores of the Semenov-2 field along with toxic elements such as As, Se, Te, Tl, and Cd.

Texture and chemistry of pyrite at Chah Zard epithermal gold–silver deposit, Iran

Gold mineralization at Chah Zard, Iran, is mostly concentrated in breccia and veins, and is closely associated with pyrite. Optical and scanning electron microscopy-backscattered electron observations indicate four different pyrite types, each characterized by different textures: porous and fractured py1, simple-zoned, oscillatory-rimmed, framboidal and fibrous py2, colloform py3, and inclusion-rich py4. Laser ablation ICP–MS analysis and elemental mapping reveal the presence of invisible gold in all pyrite types. The highest concentrations (161–166ppm Au) are found in py2 and py4, which correlate with the highest As concentrations (73,000–76,000ppm). In As-poor grains, Au concentrations decrease by about two orders of magnitude. Copper, Pb, Zn, Te, Sb, and Ag occur with invisible gold, suggesting that at least part of the gold occurs in nanoparticles of sulfosalts of these metals and metalloids. Gold distribution patterns suggest that only negligible Au was originally trapped in py1 from the initial ore fluids. However, most, if not all, Au was transported and deposited during subsequent overprinting hydrothermal fluid flow in overgrowth rims around the margins of the py2 and within microfractures of py4 grains. Oscillatory zonation patterns for Co, Ni, Sb, Cu, Pb, and Ag in pyrite reflect fluctuations in the hydrothermal fluid chemistry. The LA-ICP–MS data reveal that Cu, Pb and Ag show systematic variations between different pyrite types. Thus, Cu/Pb and Pb/Ag ratios in pyrite may provide a potentially powerful exploration vector to epithermal gold mineralization at Chah Zard district and elsewhere.

Laser Ablation ICPMS Analysis of Pyrite and U-Pb Zircon Dating of Host Rocks From the Tersang Gold Deposit, Malaysia

The Central gold belt of peninsular Malaysia comprises a number of gold deposits located in the east of the N-S striking Bentong-Raub Suture Zone. The Tersang gold deposit is a mesothermal, orogenic gold deposit, which is hosted in sandstone, shale, rhyolite and breccia. The deposit has an inferred and indicated resource of 120,000 ounces of gold in the JORC category. Detrital zircon grains from the host sandstones returned a maximum depositional age ranging from Early Carboniferous to Early Permian (333.5 ± 2.5 Ma-261.5 ± 4.9 Ma) and those of the rhyolite indicated a crystallization age of 218.8 ± 1.7 Ma. Pyrite trace element mapping has revealed four pyrite phases. Laser Ablation ICPMS analyses on pyrite has shown that pyrite phase 1 has the lowest Au concentration (mean 0.4 ppm) and is interpreted to have preceded the main Au mineralisation stage. Pyrite phases 2 and 3 are the main Au mineralisation stage with Au mean range from 1.5 to 4.5 ppm and contributed most of the invisible gold to the ore-forming system. Pyrite phase 4 has low gold concentrations (mean 0.5 ppm) and post-dated the main Au mineralisation stage recorded in pyrite phases 2 and 3. The trace element chemistry of pyrite indicates that Au mineralisation recorded in pyrite is likely a product of four stages ore forming system centred on the rhyolite corridor. In terms of gold exploration, gold has a positive correlation with As, Ag, Cu, Se, Sb, Pb and Tl in pyrite. These metals can be used as pathfinders to detect proximity to ore for orogenic gold deposit type in the central gold belt of Peninsular Malaysia.

Re-Os Systematics of Löllingite and Arsenopyrite In Granulite-Facies Garnet Rocks: Insights Into the Metamorphic History and Thermal Evolution of the Broken Hill Block During the Early Mesoproterozoic (New South Wales, Australia)

Abstract Lollingite and arsenopyrite aggregates occur in spessartine-almandine garnet rocks (garnetite) metamorphosed to granulite facies, which are spatially associated with Pb-Zn-Ag mineralization in the giant Broken Hill deposit, southern Curnamona Province, New South Wales, Australia. Sulfarsenide and sulfide minerals comprise lollingite and coexisting arsenopyrite ± galena ± tetrahedrite that occur interstitial to garnet crystals. Lollingite formed first while gold-bearing lollingite, which occurs as rare relicts in arsenopyrite, was destroyed to produce arsenopyrite ± detectable micro-inclusions of invisible gold. Standard mineral separation procedures produced pure separates of lollingite, arsenopyrite, and mixtures of arsenopyrite ± lollingite and lollingite ± arsenopyrite. In a plot of 187 Re/ 188 Os versus 187 Os/ 188 Os, samples of lollingite and lollingite ± arsenopyrite have 187 Re/ 188 Os ratios between 6.87 and 7.40 and 187 Os/ 188 Os ratios between 0.8506 and 0.8651, whereas arsenopyrite and arsenopyrite ± lollingite samples have higher 187 Re/ 188 Os ratios (7.14 to 11.32) and more radiogenic 187 Os/ 188 Os ratios (0.8828 to 0.9654). Thirteen analyses of arsenopyrite and arsenopyrite ± lollingite define a Model 1 isochron with an age of 1574 ± 38 Ma (2σ; MSWD = 1.4, initial 187 Os/ 188 Os ratio of 0.666 ± 0.006), whereas the five lollingite and lollingite ± arsenopyrite samples define a Model 1 isochron with an age of 1707 ± 290 Ma (2σ; MSWD = 0.32, initial 187 Os/ 188 Os ratio of 0.652 ± 0.036) that is indistinguishable from the arsenopyrite age. Rhenium and Os contents are extremely high for lollingite and arsenopyrite (Re = 120–475 ppb; Os = 65–345 ppb), likely as a result of concentration of Re and Os in these minerals during granulite-facies metamorphism from the inferred exhalite protolith. Petrographic observations combined with the Model 1 Re-Os ages and literature SHRIMP U-Pb ages of monazite in garnetite suggest that arsenopyrite formed on the retrograde path at the expense of lollingite. Cooling from peak Olarian P-T conditions (∼800 °C at 1602 Ma) to at least 550 °C (first temperature of stability of arsenopyrite) at ca . 1574 Ma occurred at a rate of ∼9 °C/Myr, which is similar to the rate of cooling determined for previously published SHRIMP U-Pb ages from successive monazite generations (McFarlane & Frost 2009). These results are consistent with the late phase of retrograde metamorphism that began between ca . 1590 and 1575 Ma.

Magmatic-hydrothermal origin of the early Triassic Laodou lode gold deposit in the Xiahe-Hezuo district, West Qinling orogen, China: implications for gold metallogeny

The Xiahe-Hezuo district in the West Qinling orogen contains numerous Au-(As-Sb) and Cu-Au-(W) deposits. The district is divided into eastern and western zones by the Xiahe-Hezuo Fault. The western zone is exposed at a shallow level and contains sediment-hosted disseminated Au-(As-Sb) deposits, whereas the eastern zone is exposed at a deeper level and contains Cu-Au-(W) skarn and lode gold deposits within or close to granitic intrusions. The Laodou gold deposit in the eastern zone consists of auriferous quartz-sulfide-tourmaline and minor quartz-stibnite veins that are structurally controlled by fault zones transecting the Laodou quartz diorite porphyry stock and enveloped by potassic and phyllic alteration. Both the veins and alteration halos commonly contain quartz, sericite, tourmaline, pyrite, and arsenopyrite, with minor galena, sphalerite, chalcopyrite, tetrahedrite, and enargite. Gold occurs mainly as invisible gold in pyrite or arsenopyrite and locally as inclusions less than 50 μm in diameter. The zircon U-Pb age of 247.6 ± 1.3 Ma (2σ) on the host quartz diorite porphyry and the sericite 40Ar/39Ar plateau ages of 249.1 ± 1.6 and 249.0 ± 1.5 Ma (2σ) on two ore-related hydrothermal sericite samples are within analytical errors of one another. At the formation temperature (275 °C) inferred from microthermometric measurements of fluid inclusion, sericite and tourmaline yield calculated δDH2O values of −70 to −45‰ and δ 18OH2O of 5.8 to 9.7‰, while quartz yields calculated δ 18OH2O values of 5.1∼5.7‰. Hydrothermal tourmaline in quartz-sulfide-tourmaline veins has δ 11B of −11.2 to −0.9‰ (mean of −6.3‰) that are similar to the values of magmatic tourmaline (−8.9 to −5.5‰ with a mean of −6.8‰) in the host quartz diorite porphyry. The δ 34S values of sulfide minerals range from −5.9 to +5.8‰ with a mean of −0.6‰ that is typical of magmatic sulfur. Pyrite from hydrothermally altered quartz diorite porphyry and quartz-sulfide-tourmaline veins have relatively homogeneous lead isotopic compositions, compatible with granitic intrusions in the district. The geochronological and isotopic data combined support a magmatic origin for the Laodou gold deposit, most likely formed from fluids exsolved from the Laodou quartz diorite porphyry or associated intrusive phases at deeper levels beneath the stock. Orogenic and Carlin-like gold deposits in the West Qinling orogen have been commonly thought to have formed from metamorphic fluids. This study, however, highlights the role of magmatic-derived fluids in the formation of lode gold deposits. Synthesis of geochronological, geological, and geochemical data on magmatic rocks and ore deposits in and surrounding the Xiahe-Hezuo district indicates that gold mineralization predominantly occurred within a subduction-related magmatic arc prior to collision between the Yangtze and North China cratons that produced the West Qinling orogen.

Multiple-stage mineralization in the Sawayaerdun orogenic gold deposit, western Tianshan, Xinjiang: Constraints from paragenesis, EMPA analyses, Re–Os dating of pyrite (arsenopyrite) and U–Pb dating of zircon from the host rocks

The Sawayaerdun deposit, located in the southern Tianshan orogenic belt in Xinjiang, NW China, is one of many orogenic gold deposits in the Central Asian Orogenic Belt (CAOB). The deposit is controlled by reverse faults and hosted in highly deformed carbonaceous turbidites that have undergone pyritic, arsenopyritic, silicification, carbonate and sericitic alteration. Gold occurs mainly in the form of electrum and native gold, and is also present in pyrite and arsenopyrite as “invisible gold”. Three generations of auriferous pyrite have been recognized through detailed petrographic studies and EMPA analyses. The pyrite with framboidal texture (Py0) is disseminated in the host rock, and is locally enriched in gold. The anhedral pyrite (Py1), associated with silicification and quartz veins that experienced strong deformation, has an average Au content of 0.021wt.%. The euhedral–subhedral pyrite (Py2), showing octahedron cube and pyritohedron habits, is associated with least deformed quartz veins that are best developed in the main orebodies, and has an average Au content of 0.023wt.%. The Py0 is interpreted to have formed contemporaneously with the ore-bearing rocks, with a maximum depositional age of 355±7.3Ma based on U–Pb dating of detrital zircons. Re–Os isotopic analyses of four Py1 samples yielded an isochron age of 324±4.8Ma, and those of five Py2 samples yielded an isochron age of 282±12Ma. Py1 is interpreted to have formed in the syn-tectonic stage, during the collision between the Tarim craton and the Central Tianshan terrane in Late Carboniferous, whereas Py2 was formed in a later mineralization event, during the late- to post-tectonic stage in Early Permian. These study results suggest that multiple stages of gold mineralization have developed in the Sawayaerdun deposit, and similar mineralization processes may have taken place in other parts of the Western Tianshan Orogen.

Hydrothermal Evolution of the Çöpler Porphyry-Epithermal Au Deposit, Erzincan Province, Central Eastern Turkey

The Copler Au deposit (measured and indicated resource of 122.8 million tonnes at 1.7 g/t Au, 4.8 g/t Ag, and 0.1 wt % Cu) is centered around shallow-level dioritic to granodioritic intrusive rocks of the middle Eocene Copler-Kabatas magmatic complex, which have been emplaced into a succession of regionally metamorphosed late Paleozoic-Mesozoic sedimentary and carbonate rocks. The deposit comprises a centrally located subeconomic porphyry Cu-Au system characterized by a potassically altered (biotite-K-feldspar-magnetite) core overprinted by a more extensive phyllic (quartz-sericite) alteration zone. The potassic alteration zone is associated with early M-type hairline magnetite and crosscutting B-type quartz ± magnetite ± sulfide veinlets, whereas the enveloping phyllic-altered rocks contain abundant D-type quartz-pyrite and lesser polymetallic quartz-sulfide veinlets. Intermediate-sulfidation epithermal Au mineralization that overprints the porphyry occurred in two stages. Main-stage epithermal mineralization is characterized by carbonate sulfide veinlets consisting of manganocalcite, arsenical pyrite, arsenopyrite, marcasite, chalcopyrite, tennantite/tetrahedrite, galena, and sphalerite. Late-stage sooty pyrite veinlets contain some realgar and orpiment and are associated with zones of extensive carbonate alteration. In the carbonate sulfide veinlets, invisible gold is primarily hosted within arsenical pyrite and, to a lesser extent, by arsenopyrite, tetrahedrite, and tennantite. In sooty pyrite veinlets, invisible gold is associated with fine-grained arsenical pyrite. Manto-type carbonate-replacement zones occur in the distal portions of the porphyry system and constitute a significant gold resource. These carbonate-replacement bodies display a mineral paragenesis similar to that of the epithermal carbonate sulfide veinlets in that they contain abundant arsenical pyrite together with lesser chalcopyrite, arsenopyrite, and marcasite, and sparse sphalerite, galena, tennantite, and tetrahedrite. Invisible gold in these ores is contained mainly within pyrite and chalcopyrite and, to a lesser extent, in arsenopyrite, tetrahedrite, and tennantite. Polyphase brine inclusions (~47–62 wt % NaCl equiv) in early B-type quartz ± magnetite ± sulfide veinlets were trapped together with low-salinity (~3–5.5 wt % NaCl equiv), vapor-rich inclusions at temperatures ~390°C and at a depth of ~1.5 km under lithostatic conditions. Fluids associated with the overprinting phyllic alteration were slightly cooler (~370°C) and less saline (37–42 wt % NaCl equiv). Fluid inclusions in manganocalcite and sphalerite from epithermal carbonate sulfide veinlets trapped moderate-salinity (4–15 wt % NaCl equiv) fluids at ~290°C, whereas fluid inclusions hosted in barite and realgar from sooty pyrite veinlets were formed from low-temperature (~100°C) and low- to moderate-salinity (1–14 wt % NaCl equiv) fluids. These data indicate that the Au-mineralizing system at Copler progressed from a high-temperature porphyry system to a relatively low temperature, intermediate-sulfidation epithermal system. Deposition of gold in the early stages of epithermal mineralization resulted from cooling, sulfidation, and neutralization of predominantly magmatic sourced hydrothermal fluids, whereas a meteoric water component is evident in the latest stage of mineralization. Deposit-scale geologic observations combined with fluid inclusion and stable isotope evidence suggest that mineralization at Copler records activity of a relatively deep epithermal system and that its formation was structurally and lithologically controlled. Specifically, the thick, premineralization carbonate sequence once overlying the deposit acted as a pressure seal and also as a neutralizing agent during the build up of the magmatic-hydrothermal system. The weakening of this carbonate cover by igneous intrusion and subsequent hydrothermal activity may have contributed to later selective erosion of the alteration zone, thereby telescoping the intermediate-sulfidation epithermal system onto the earlier porphyry system and creating the Copler window.

Invisible gold distribution on pyrite and ore-forming fluid process of the Huangshan orogenic-type gold deposit of Zhejiang, SE China: implications from mineralogy, trace elements, impurity and fluid inclusion studies

The Huangshan orogenic-type gold deposit in Zhejiang of SE China occurred in quartz–pyrite veins. It is hosted by phyllonite that underwent greenschist-facies metamorphism along a large Jiangshan–Shaoxing tectonic belt with a NE–SW direction. Trace elemental characteristics, ore-forming process and invisible gold on different forms of pyrite and quartz are studied. The Au associated pyrite can be classified into two categories; recrystallized pyrite and euhedral pyrite. The precipitation of invisible Au on pyrite is mainly derived by Co and Ni with AuHS2 − complex in the mineralizing fluids in different events. The XPS results revealed that valence states of Au3+ replaced 2Fe2+ in the pyrite and Au0 replaced Si4+ in the quartz structure. The electron paramagnetic resonance and trace elemental results suggested that the element pairs of Ge–Li–Al in quartz and Mn–Co–Ni in pyrite have distinct impurities as identified. A fluid inclusion study showed that the auriferous quartz is characterized by low-saline and CO2-rich fluids. Coexistence of the type I–type III inclusions and same range of homogenization temperature with different mode are evidences of immiscible fluid process. The temperature–pressure values of ca. 250 °C/1250 bar and ca. 220 °C/780 bar for gold precipitation have been calculated by intersection of coexisting fluids during the entrapment. The Huangshan orogenic-type gold deposit may be associated with the Wuyi–Yunkai orogeny during the early Paleozoic, including an upper–mid greenschist-facies metamorphism (450–420 Ma). All the features suggest that the Huangshan gold deposit is probably a product linking with the early Paleozoic orogeny in South China.

A review of the Birimian Supergroup- and Tarkwaian Group-hosted gold deposits of Ghana

Ghana is the largest producer of gold in West Africa, a region with over 2,500 years of history with regards to gold production and trade. Modern exploration for and mining of gold in Ghana dates from 1874 with the establishment of the British Gold Coast Colony, which was followed in 1957 by the independence of Ghana and increased gold production since the early 1980s through Ghana's Economic Recovery Plan. At the time of writing, gold production (108.2 tonnes or 3.48 million ounces [Moz] in 2014) accounted for approximately one-third of Ghana's export revenues, with 36% of gold production coming from small-scale mining. The majority of the gold occurs in two styles of mineralisation, namely mesothermal quartz vein-hosted and associated gold in metavolcanics and metasediments and modified palaeoplacer gold in conglomerates. These styles of mineralisation occur in the Palaeoproterozoic Birimian Supergroup and Tarkwaian Group that make up Ghana's mainly southwest to northeast trending Birimian belts. Significant gold resources also occur as hydrothermal mineralisation in basement-type granitoids which show some geological association with the Birimian Supergroup-hosted mesothermal mineralisation. The majority of the gold mineralisation is believed to have formed between approximately 2.15 and 2.06 Ga during the Eburnean orogeny. The mesothermal quartz vein gold mineralisation is usually confined to tectonic corridors within the Birimian belts and is strongly associated with shear zones and fault systems. The quartz veins show multiple stages of formation and are steeply dipping, with the gold mineralisation occurring either as free gold within fractures in the veins or as invisible gold within disseminated sulphides in the host rocks surrounding the veins. The vein- and sulphide-hosted gold is strongly associated with deformational fabrics formed by the Eburnean extensional and compressional events, respectively, suggesting that disseminated sulphide mineralisation predates quartz vein-hosted mineralisation. The fluid from which the gold precipitated is believed to have been of metamorphic origin and carbon dioxide (CO 2 ) dominated, with lesser water (H 2 O) and nitrogen (N 2 ) and minor methane (CH 4 ). Gold precipitation was probably caused by decrease in pressure, temperature and CO 2 -H 2 O immiscibility, at depths of between 7 and 11 km. The palaeoplacer gold mineralisation shows some hydrothermal modification and occurs mostly within the conglomerates and to a lesser extent within the interbedded quartzite of the Banket Series of the Tarkwaian Group. The gold occurs as free gold in the matrix of the conglomerates and as intergrowths and overgrowths on and inclusions in, other heavy mineral grains. Based on strong evidence for palaeoplacer mineralisation, the gold was likely deposited with the conglomerates in alluvial fans and braided tributary channels. Because the Eburnean compressional event associated with the Birimian Supergroup-hosted mesothermal gold mineralisation also overprinted the gold-bearing Banket conglomerates, the former gold occurrences could not have acted as the source for the palaeoplacer gold. Hydrothermal gold mineralisation occurs in the Palaeoproterozoic belts and basin granitoids that intrude the Birimian belts, as well as in the sedimentary basins occurring between the belts. Gold mineralisation within the granitoids occurs as micro-inclusions in sulphides in small, steeply dipping stockworks and as sulphide disseminations concordant with regional faults and shears. A gold-bearing fluid similar to that for the Birimian Supergroup-hosted quartz vein gold mineralisation, but with a larger H2O component, is proposed to have formed the granitoid-hosted gold mineralisation.

Multistage gold mineralization in the Wa-Lawra greenstone belt, NW Ghana: The Bepkong deposit

The Bepkong gold deposit is one of several gold camps in the Paleoproterozoic Wa-Lawra greenstone belt in northwest Ghana. These deposits lay along the Kunche-Atikpi shear zone, which is part of the larger transcurrent Jirapa shear zone. The formation of these shear zones can be attributed to the general ESE-WNW major shortening that took place in the Wa-Lawra belt. Gold mineralization in the Bepkong deposit mainly occurs within graphitic shales and volcaniclastic rocks. The ore consists of four N-S trending lenticular bodies, plunging steeply to the south, that are lithologically and structurally controlled. Their shape and thickness are variable, though a general strike length of 560 m and an overall thickness of 300 m can be defined. An alteration mineral assemblage characterises the ore, and consists of chlorite-calcite-sericite-quartz-arsenopyrite-pyrite. Pyrite, as distinct from arsenopyrite, is not limited to the altered rocks and occurs throughout the area. At Bepkong, gold is associated with arsenopyrite and pyrite, which occur disseminated in the mineralized wall rock, flanking Type-1 quartz veins, or within fractures crossing these veins. Textural observations indicate the early formation of abundant arsenopyrite, followed by pyrite, with chalcopyrite, galena, sphalerite and pyrrhotite occurring as inclusions within pyrite and altered arsenopyrite. Detailed petrography, coupled with SEM, LA-ICP-MS and EMP analyses, indicate that gold in the Bepkong deposit occurs in three distinct forms: (i) invisible gold, mostly in arsenopyrite (ii); visible gold as micron-size grains within fractures and altered rims of arsenopyrite, as well as at the interface of sulphide grains; (iii) free visible gold in fractures in quartz veins and their selvages. We interpret the invisible gold to have co-precipitated with the early-formed arsenopyrite. The small visible gold grains observed within the sulphide interfaces, altered arsenopyrite, fractures and grain boundaries, are interpreted to have formed as a result of the dissolution and redistribution of the invisible gold during later alteration of arsenopyrite, which took place at lower temperatures during crenulation and fracturing accompanying late deformation, and was accompanied by pervasive pyritization of the wall rock.

Comparison of the native antimony-bearing Paiting gold deposit, Guizhou Province, China, with Carlin-type gold deposits, Nevada, USA

The Paiting gold deposit, Guizhou Province, China, has been regarded as a Carlin-type gold deposit by several researchers. Alteration and ore-related minerals from the Paiting deposit were examined, and results were compared with the Cortez Hills Carlin-type gold deposit, Nevada, USA. Similarities include the structural and stratigraphic controls on the orebodies in both deposits and the occurrence of invisible gold ionically bound in arsenian pyrite. Significant differences include the following: (1) The gold-bearing mineral in Nevada is arsenian pyrite. However, gold-bearing minerals in the Paiting deposit include arsenopyrite, arsenian pyrite, and trace pyrrhotite. Also, euhedral or subhedral gold-bearing arsenian pyrite at Paiting contains significantly less As, Cu, and Hg than gold-bearing pyrite from Nevada. (2) Alteration in the Paiting deposit displays significantly less decarbonatization. Instead, dolomite precipitation, which has not been described in Nevada deposits, is associated with deposition of gold-bearing sulfide minerals. (3) Stibnite and minor native antimony typify Paiting late-ore-stage minerals, whereas in Nevada, realgar, orpiment, and calcite are common late-ore-stage minerals. Precipitation of native antimony in the Paiting deposit reflects the evolution of a late-ore fluid with unusually low sulfur and oxygen fugacities. Some characteristics of the Paiting gold deposit, including formation of ore-stage dolomite and precipitation from CO2-rich ore fluids at temperatures in excess of 250 °C, are more typical of orogenic deposits than Nevada Carlin deposits. The presence of similarities in the Paiting deposit to both Carlin type and orogenic deposits is consistent with formation conditions intermediate to those typical of Carlin type and orogenic systems.

MỘT SỐ ĐẶC ĐIỂM HÌNH THÁI VÀ THÀNH PHẦN HÓA HỌC CỦA ARSENOPYRIT, PYRIT TRONG CÁC TỤ KHOÁNG VÀNG - SULFIDE BÓ VA VÀ NAM QUANG (ĐÔNG BẮC VIỆT NAM)

Crystal morphology and chemical composition of arsenopyrite and pyrite in the Nam Quang and Bo Va deposits of the Song Hien basin (Northeast Vietnam) are studied in detail. Arsenopyrite crystals often form elongated crystals. Their chemical composition are relatively poor in As, but rich in S with S/As ratios often &gt; 1.1, impurity elements such as Ni, Co, Sb, and sometimes containing gold (up to 0.02 wt.%). Pyrite is characterized by the stoichiometric composition, but contains relatively high arsenic and sometimes contain gold. The correlation between the concentration of As and Au in pyrite has been recorded. The morphology and chemical composition of arsenopyrite and pyrite of Bo Va and Nam Quang deposits can be correlated to those in the Au-sulfide deposits in black shale in Kazastan and southestern China.ReferencesTrần Trọng Hòa (chủ biên), 1996: Xây dựng mô hình điều tra, sử dụng hợp lý, bảo vệ môi trường trong quá trình khai thác tài nguyên khoáng sản quý hiếm tỉnh Cao Bằng. Lưu trữ Viện Địa chất, Hà Nội. Trần Trọng Hòa, Polyakov G.V., Trần Tuấn Anh, Borisenko A.S., Izokh A.E., Balyakin P.A., Ngô Thị Phượng, Phạm Thị Dung, 2011: Hoạt động magma và sinh khoáng nội mảng miền Bắc Việt Nam. Nxb. Khoa học Tự nhiên và Công nghệ, 368tr. Trần Trọng Hòa, Nevolko P., Ngô Thị Phượng, Trần Tuấn Anh, Phạm Thị Dung, Bùi Ấn Niên, Nguyễn Xuân Huyên, Nguyễn Thùy Dương, Vũ Hoàng Ly, Trần Văn Hiếu, 2013: Các tụ khoáng vàng-sulfide trong các thành tạo lục nguyên - carbonat rift Sông Hiến, ÐBVN. Tuyển tập Báo cáo KHCN toàn quốc năm 2013. ISBN: 978-604-59-0693-4; tr.131-140. Nông Thế Tàng (chủ biên), 1996: Báo cáo Khảo sát và tìm kiếm đánh giá vàng gốc Nam Quang, Bảo Lạc, Cao Bằng. Lưu trữ Trung tâm Thông tin lưu trữ Địa chất, Hà Nội. Chen M.H., Mao J.W., Bierlein F.P., Norman T., Uttley P.J., 2011: Structural features and metallogenesis of the Carlin-type Lannigou gold deposit, Guizhou Province, China. OreGeol. Rev.43 (1), 217-234. Fleet, M.E., Mumin, A.H., 1997: Gold-bearing pyrite and arsenopyrite from Carlin Trend gold deposits and laboratory synthesis: American Mineralogist, v. 82, p.182-193. Kovalev K.R., Kalinin Yu.A., Naumov E.A., Kolesnikova M.K., Korolyuk V.N., 2011: Gold-bearing arsenopyrite in eastern Kazakhstan gold-sulfide deposits. Russian Geology and Geophysics 52, 178-192. Kovalev K.R., Kalinin Yu.A., Polynov V.I., Kydyrbekov E.L., Borisenko A.S., Naumov E.A., Netesov M.I., Klimenko A.G., and Kolesnikova M.K., 2012: ISSN 1075-7015, Geology of Ore Deposits, vol. 54, No.4, 254-275. Peters S.G., HuangJ.Z., Li Z.P., JingC.G., 2007: Sedimentary rock-hosted Au deposits of the Dian-Qian-Guiarea, Guizhou, Yunnan Provinces, and Guangxi District, China. Ore Geol. Rev.31,170-204. Volkova A. V., Genkina A. D., and Academician of the RAS Goncharovb V. I., 2006. New data on invisible gold in disseminated sulfide ores of the Natalka deposit. Vol. 409A, No 6, pp. 879-883. Zhong H.R., Chao S.W., Wu B.X., Zhi T.G., Hofstra A.H., 2002: Geology and geochemistry of Carlin- type gold deposits in China Mineralium Deposita 37:378-39. Zhou T.H., Goldfarb R.J., Phillips G.N., 2002: Tectonics and metallogeny of gold deposits in China. Mineralium Deposita 37, 249-282.

Preparation of pyrite-arsenopyrite concentrate for gold leaching through the use of polyreagent complexes

The article focuses on the causes of refractoriness of sulfide ore stock with superdispersed gold inclusions. In order to increase gold recovery, a hypothesis about effective (peroxide) preparation of refractory raw materials in the process of leaching has been described. The author focuses on theoretical justification of the physicalchemical model and the two-stage mechanism of oxidation (photoelectrochemical and bacterial) of material using microorganisms Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. A scheme of two-staged (physicochemical and bacterial) oxidation was developed. It is based on photoelectrochemical effects for the leaching of gold from refractory materials. It is shown that photoelectrochemical pre-oxidation makes it possible to increase development of the mineral matrix with its further bacterial oxidation. It also helps to increase the efficiency of invisible gold extraction from sulfide ores. Laboratory tests have been performed on the combined oxidation scheme on electroactivation carbon-in-leach of invisible gold from the Kokpatas deposit’s sulfide ores. The authors present the results of the laboratory tests of the activation methods for impacting upon the refractory ores before metal leaching, which prove the efficiency of such non-traditional methods for developing mineral matrix. Gold extraction using the combined technology of sulfide and arsenopyrite oxidation increases by 18%.

The Pampe gold deposit (Ghana): Constraints on sulfide evolution during gold mineralization

The Ashanti belt, a world-renowned gold producing region in southwest Ghana, has received renewed attention in recent years. Most studies, however, focused on the major deposits situated along the Ashanti shear zone and in the adjacent Tarkwa Basin to the east, neglecting smaller yet important occurrences, such as the Pampe deposit which occurs few kilometers to the west of this shear zone, on the Akropong Trend. Nevertheless, investigating such simpler smaller-scale mineralizing systems is attractive, in that this can help shedding light on the processes that control gold deposition at the regional scale.At Pampe, gold endowment has been estimated at approximately half a million ounces with an average gold grade of 2.8g/t. The mineralization is related to two sets of quartz veins; a first set (V1), which has a NE trend and is sub-parallel to the main foliation (S1), and a second set (V2), which crosscuts this foliation. The V2 veins have a NNW–SSE trend with local conjugate geometries indicating that they formed during NNW–SSE shortening, which, regionally, is linked to major orogenic gold deposition. Gold mineralization is systematically associated with sulfides, which occur disseminated in the vein walls and in the surrounding host rocks. The ore sulfide paragenesis consists of 1) a first generation of pyrite, which is associated with V1 veins; 2) a second generation of sulfides, consisting of an intergrowth of arsenopyrite and pyrite that crystallized contemporaneously with the formation of the second vein set; 3) a late phase of pyrite growth which occurs as overgrowths on phase-2 sulfides and formed during the waning stages of V2 emplacement. Invisible (sub-microscopic) gold was detected in all sulfide generations by LA-ICP-MS. The analytical profiles for the Au signal are mimicked by those for Pb, Cu, As, Ag, Te, and Bi. Invisible gold is thus interpreted to have precipitated within sulfides in the form of nanoparticles (colloidal gold alloys). Conversely, visible native gold grains were recognized exclusively in association with arsenopyrite from late V2 veins, either as inclusions or, more commonly, at the boundary with other sulfides, as well as in micro-fractures that crosscut the sulfides. Gold precipitation was likely induced by sulfidation of the wall rock during fluid–rock interaction. The Pampe deposit exemplifies the mineralization processes that took place at larger scale in neighboring world-class deposits such as Obuasi, Bogoso and Prestea.