Invisible Gold Research Articles

Origin of the Qiyugou gold deposit in the southern margin of the North China Craton: Insights from trace elements of pyrite and mineralogy of Bi-minerals

The Qiyugou gold deposit, located in the Xiong’ ershan area of the North China Craton, contains abundant bismuth-sulfosalts that are closely associated with gold mineralization. Pyrite is the dominant Au-hosted mineral, and has been formed in three generations (Py1, Py2, and Py3). Py1 grains, generally intergrown with milky quartz, are coarse (>1 mm), euhedral in shape, and Au-depleted in composition. In contrast, subhedral Py2 grains, associated with light gray quartz, are medium to coarse (0.2–3 mm) and are enriched in gold that is both invisible and visible. Py3 grains (0.1–0.5 mm), intergrown with abundant sulfide minerals, are relatively fine and Au-depleted. The time-resolved LA-ICP-MS depth profiles of the Py2 grains indicate that invisible gold occurs as either solid solution or nano-particles of native gold and electrum. Visible gold occurs as small blebs in the Py2 grains where inclusions of native bismuth, galenobismutite, lillianite homologs, tetradymite, and galena are also present. In addition, it is common that electrum in microfracture infillings or along grain boundaries of the Py1 and Py2, are intergrown with bismuthinite derivatives, Bi-Cu sulfosalts, emplectite, tetradymite, chalcopyrite, galena, and Py3.Based on textural relationships and mineral assemblages, calculation of physicochemical conditions show that gold was formed in conditions of fTe2 = ~10−11 and fS2 = ~10−11 to 10−12 for Py2, and fTe2 = ~10−9 to 10-11and fS2 = ~10−10 to 10−11 for Py3. We thus proposed that such physicochemical conditions may have triggered the precipitation of Bi melt, and sulfidation driven by cooling or increase in sulfur content results in the transformation of the Au-Bi liquid into a stable assemblage of native gold and bismuthinite. These bismuth minerals are associated with native gold/Au-bearing minerals, indicating that the Au mineralization of the Qiyugou gold deposit might be genetically associated with Bi melt. The present study highlights the role of Bi as important gold scavengers in arsenic-deficient ore-forming fluid.

Gold deportment and leaching study from a pressure oxidation residue of chalcopyrite concentrate

Pressure oxidation (POX) is an effective method for base metal extraction and a pretreatment of refractory gold ores. A residue from pressure leaching of chalcopyrite concentrate was collected and investigated for gold recovery and deportment of metals. A comprehensive mineralogical analysis and gold deportment study were conducted using techniques including X-ray diffraction (XRD), optical microscope, scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDX), and dynamic secondary ion mass spectrometry (D-SIMS). Gold leaching tests were performed using cyanide and glycine as lixiviants. The gold grade is 3.1 g/t, with 0.31% residual copper in the sample. Hematite was a dominant iron oxide (31.5%), and iron sulfate salts (45.4%) existed in various minerals phases. The gold deportment study revealed that 32.3% of the total gold was present as a visible gold, and the remainder was sub-microscopic gold. The visible gold was native gold with an average particle size of less than 20 μm. Colloidal gold, the dominant existence of invisible gold, was found in hematite, jarosite, and iron sulfate. Hematite was the major sub-microscopic gold carrier, forming a nonporous rimmed structure and locking gangue minerals randomly. Gold in arsenopyrite was found as solid-solution, accounting for 7.2%. Gold extraction of 84% was achieved by 500 mg/L NaCN, while only 35% of gold was extracted by 100 mg/L NaCN. A significant synergistic effect of cyanide and glycine was observed. The addition of glycine in cyanide leaching solution boosted the gold leaching kinetics and recovery. Gold recovery of 85% was observed with 100 mg/L NaCN and 0.3 M glycine in 24 h. In the hybrid leaching system, copper was stabilized by glycine and further utilized as a catalyst for gold leaching. To achieve higher gold extraction and lower lime consumption, a product with high sulfur oxidation degree and low iron sulfate content is preferable in pressure oxidation operation. Hematite is a stable product and does not impair gold leaching.

Quantitative XPS characterization of “invisible gold” in Carlin-type gold ores through controlled acid erosion

Quantitative XPS analysis of “invisible gold” in Carlin-type gold ores was accomplished, which revealed Au concentration, percentages of Au+ and Au0, and Au NP size. An acid etching step was demonstrated to be the key to enhancing Au signal in XPS.

ПРОСТРАНСТВЕННО-ВРЕМЕННЫЕ И ГЕНЕТИЧЕСКИЕ СООТНОШЕНИЯ ЗОЛОТОРУДНОЙ И РТУТНОСУРЬМЯНОЙ МИНЕРАЛИЗАЦИЙ НА HG-SB-ЗОЛОТОСОДЕРЖАЩЕМ МЕСТОРОЖДЕНИИ ЧАУВАЙ (КИРГИЗИЯ): ГЕОЛОГИЯ, МИНЕРАЛОГИЯ РУД И ОСОБЕННОСТИ ГИДРОТЕРМАЛЬНО-МЕТАСОМАТИЧЕСКИХ ПРОЦЕССОВ

The Chauvai Hg-Sb deposit is a striking example of combining two contrasting types of mineralization in space: mercury-antimony and gold ones. The article studies the spatial-temporal and genetic relationships of goldore and mercury-antimony mineralization based on a complex of both traditional geological and mineralogicalgeochemical methods, as well as modern instrumental methods for analyzing the mineral composition. Two types of ores with clear structural confinedness have been found at the deposit: a) mercury-antimonic (cinnabarantimonite) ores, associated with jasperoid breccias and manifested exclusively along the tectonic contact of limestone of the Alai section and terrigenous rocks of the Tolubai Formation, and b) gold- sulphide (arsenopyritepyritic) ores, localized in slightly modified carbonate-terrigenous rocks of the Tolubai Formation, overlying the plane of tectonic contact. Ore formation occurred during the following stages: in the late diagenetic, without interruption passing into the catagenetic-hydrothermal, characterized by the formation of gold mineralization, and then in the later hydrothermal-telethermal, characterized by the development of Hg-Sb mineralization. It is established that the main carrying agent of invisible gold (“invisible gold”) in ores is framboidal and idiomorphic pyrite and, especially, its high-arsenic varieties. A set of conducted studies has shown that the gold ore and mercury-antimony mineralization is broken in time and is genetically associated with various hydrothermalmetasomatic processes, and the Chauvai deposit can be classified as a Carlin-like type.

Paragenesis of an Orogenic Gold Deposit: New Insights on Mineralizing Processes at the Grass Valley District, California

Abstract The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, is the largest historical gold producer of the North American Cordillera. Gold mineralization is associated with shallowly dipping north-south veins hosted by the 160 Ma Grass Valley granodiorite to the southwest of the Grass Valley fault and steeply dipping east-west veins in accreted oceanic rocks to the northeast of this major fault. Quartz veins from both vein types show well-preserved primary textural relationships. Using a combination of petrographic and microanalytical techniques, the paragenetic sequence of minerals within the veins and the compositions of ore minerals were determined to constrain the mechanisms of quartz vein formation and gold deposition. The veins are composed of early quartz that formed through cooling of hydrothermal fluids derived from a geopressured reservoir at depth. The early quartz shows growth zoning in optical cathodoluminescence and contains abundant growth bands of primary inclusions. The primary inclusion assemblages and myriads of crosscutting secondary fluid inclusions have been affected by postentrapment modification, suggesting that early quartz formation was postdated by pronounced pressure fluctuations. These pressure fluctuations, presumably involving changes from lithostatic to hydrostatic conditions, may be related to fault failure of the host structure as predicted by the fault-valve model. Fluid flow associated with pressure cycling took place along microfractures and grain boundaries resulting in extensive recrystallization of the early quartz. Deposition of pyrite, arsenopyrite, and first-generation gold from these hydrothermal fluids causing recrystallization of the early quartz occurred as a result of wall-rock sulfidation. The gold forms invisible gold in the compositionally zoned pyrite or micron-sized inclusions within pyrite growth zones. The latest growth zones in euhedral quartz crystals that formed in association with this stage of the paragenesis contain very rare primary fluid inclusions that have not been affected by postentrapment modification. The hydrothermal system transitioned entirely to hydrostatic conditions immediately after formation of the latest quartz, explaining the preservation of the primary fluid inclusions. The formation of minor quartz in open spaces was followed by the deposition of second-generation native gold and telluride minerals that are commonly associated with base metal sulfides. Ore formation at this stage of the paragenesis is attributed to the rapid decompression of hydrothermal fluids escaping from the geopressured part of the crust into the overlying hydrostatic realm. There is no fluid inclusion evidence that this pressure drop resulted in fluid immiscibility of the hydrothermal fluids. Fluid inclusion evidence suggests that the north-south veins formed at a paleodepth of ~8 km, whereas the east-west veins appear to have formed at ~10 to 11 km below surface, confirming previous inferences that the NE-dipping Grass Valley reverse fault accommodated a large displacement. The findings of the study at Grass Valley have significant implications for the model for orogenic gold deposits, as the reconstruction of the paragenetic relationships provides evidence for the occurrence of two discrete events of gold introduction that occurred at different conditions during the evolution of the hydrothermal system.

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

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

Textural, compositional and isotopic characteristics of pyrite from the Zaozigou gold deposit in West Qinling, China: Implications for gold metallogeny

The Zaozigou gold deposit (ZZGD) located within the West Qinling Orogen in China, with 142 t of Au at an average grade of 2.69 g/t, is hosted in a sequence of metasediments and altered dacitic to granodioritic rocks. The majority of gold is present as invisible gold in disseminated arsenian pyrite. Here we investigate the morphological and compositional evolution of the pyrite grains and constrain the ore-forming fluid source(s) and genetic models related to the ore-forming processes through the application of Scanning electron microscopy, electron probe microanalysis, in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and fsLA-MC-ICP-MS analyses. Based on texture and trace element concentrations, we identify five generations of pyrite (namely, Py1, Py2, Py3, Py4, and Py5). Among these, the pre-ore framboidal pyrite (Py1), which is characterized by relatively low concentrations of As, Au, Co, Ni, and Sb, has negative δ34S values of −31.07 to −20.79‰, which we correlate to bacterial reduction of marine sulfate during sedimentation and diagenesis. The auriferous Py2 and Py5, which occur as either porous aggregates or epitaxial overgrowth on earlier pristine pyrite, contain relatively high concentrations of As, Sb, Tl, and Au compared with pyrite in the other generations. Both (Py2 and Py5) display a relatively narrow range of negative δ34S values, ranging from −5.66 to −1.16‰, which are interpreted to result from a magmatic source. Gold-free Py3 and Py4 exhibit relatively variable δ34S values of −17.1 to −6.02‰, indicating that a small amount of sulfur in the sedimentary strata was added to the magmatic hydrothermal systems during fluid-rock interaction. Sulfur isotopes and LA-ICP-MS mapping of pyrite grains revealed that the gold endowment of the ZZGD is associated with multiple mineralizing events and multiple magmatic and hydrothermal activities. We propose that gold in the arsenian pyrite from the ZZGD was scavenged in ore-forming fluids by chemical adsorption onto the As-rich, Fe-deficient, and actively growing surfaces of pyrite. Our results indicate that the primary source of the sulfur, gold, and mineralizing fluids was likely fluids from magmatic hydrothermal systems in the West Qinling orogen.

Pyrite Textures and Trace Element Compositions from the Granodiorite-Hosted Gold Deposit at Jonnagiri, Eastern Dharwar Craton, India: Implications for Gold Mineralization Processes

Abstract The lone granodiorite-hosted gold deposit at Dona sector of Jonnagiri, eastern Dharwar craton, India, contains typical shear-hosted and vein-hosted alteration zones. Pyrite is the dominant sulfide mineral in these alteration zones. Texturally three varieties of pyrites were identified in these alteration zones: (1) early pyrite-I is coarse to medium grained and subhedral shaped and contains near margin-parallel silicate inclusions, (2) main (ore)-stage pyrite-II overgrows early pyrite-I and also occurs as discrete grains invariably associated with visible gold, and (3) late-stage pyrite-III is anhedral and coarse grained and contains randomly oriented inclusions of silicates, sulfides, and native gold grains. Electron microprobe analysis, coupled with X-ray element mapping and laser ablation-inductively coupled plasma-mass spectrometry, reveals that most early pyrites (pyrite-I) have higher concentrations of As and Au in both the zones. The shear-hosted main-stage pyrite-II can be divided into Ni-rich (median 211 ppm) pyrite-IIa and Co-rich (median 274 ppm) pyrite-IIb, respectively. While invisible gold content is higher in vein-hosted late-stage pyrite (pyrite-IIIa; ≤287 ppm) when compared to shear-hosted pyrites, native visible gold is associated with only vein-hosted main- and late-stage pyrites (pyrite-II and IIIa). Arsenic, Ni, Au, Se, Mo, and Te concentrations decrease from pyrite-I to pyrite-III, reflecting remobilization of trace elements during subsequent dissolution-reprecipitation of early formed pyrites. The oscillatory zoning of As, Co, and Ni and slight increase in Bi, Te, Se, Au, and Ag in pyrite-II and pyrite-IIIa represent pressure fluctuations and repeated local fluid phase separation in the ore-forming environment. A positive correlation of Au with Pb, Sb, Bi, and Te confirms the presence of nanoinclusions of mineral phases such as nagyagite, Pb-Sb-Bi tellurides, Au-Ag tellurides, tellurosulfides, and sulfosalts within pyrites, particularly in the vein-hosted zone. Based on several lines of evidence, the following paragenetic sequence is proposed for pyrite formation at Dona, Jonnagiri. Rapid crystallization of early (porous) pyrite-I was followed by its dissolution during ~E-W–trending Sh1 shearing. Crystallization of main-stage pyrite-II and the late-stage pyrite-IIIa is the product of dissolution-reprecipitation of early pyrite during ~N-S–trending Sh2 shearing. Changing fluid compositions caused by episodic fault-valve actions and associated boiling resulted in dissolution-reprecipitation of early formed pyrites and remobilization of trace elements. This further resulted in precipitation of the bulk of gold within the inner vein-hosted zone during the later Sh2 shearing event. At the culmination of shearing, late-stage pyrite-IIIb precipitation occurs with very low concentrations of all trace elements, including gold.

Genesis of Carbonate Breccia Containing Invisible Gold in Taebaeksan Basin, South Korea

The Yemi breccia developed and is distributed within the Paleozoic carbonate rock (Maggol Formation) in the central part of the Taebaeksan Basin, South Korea. Explanation for the genesis of the Yemi breccia has been controversial. We investigated the petrological and mineralogical properties of the breccia and the matrix materials at 60 outcrops. The Yemi breccia is divided into crackle, mosaic, and chaotic breccias based on morphology. In addition, these are divided into blackish, reddish, grayish, and white to pinkish matrix breccias according to the materials of the matrix. Quartz, calcite, pyrite, hematite (after pyrite), and minor epidote, chlorite, and opaque materials mainly comprise the matrix materials. The pyrite grains from the Yemi breccia can be divided into two types based on the mineral texture: diagenetic and hydrothermal. We analyzed the chemistry of pyrite and hematite (after pyrite) from the Yemi breccia with an electron probe X-ray microanalyzer (EPMA). Invisible gold was detected within the pyrite grains by EPMA and disseminated micron-sized isolated gold particles were discovered by backscattered electron (BSE) images. The texture of Au-bearing pyrite and gold particles in the Yemi breccia is especially well matched with pyrite and gold from the Shuiyindong Carlin-type hydrothermal gold deposits, China. Therefore, we suggest an important role of hydrothermal fluid in karstification within the Paleozoic carbonate rock.

Epithermal mineralization of the Bonanza-Sandy vein system, Masara Gold District, Mindanao, Philippines

The Masara Gold District in southeastern Mindanao island is an area of prolific hydrothermal copper and gold mineralization. This study documents the mineralization characteristics of the NW-trending Bonanza-Sandy epithermal veins to constrain possible hydrothermal fluid sources and ore-forming mechanisms. Epithermal mineralization in the NW veins is divided into three main stages: Stage 1 - massive quartz-sulfide; Stage 2 - massive to amorphous quartz-carbonate (calcite); and Stage 3 - colloform-cockade quartz-carbonate (bladed rhodochrosite). Stage 1 is the main gold mineralization phase, with chalcopyrite, pyrite, sphalerite and galena occurring with native gold and tellurides. Stages 2 and 3 contain invisible gold in the sphalerite, galena, pyrite and chalcopyrite. The deposit exhibits mineralization characteristics typical of intermediate sulfidation epithermal deposits based on the dominant chalcopyrite-pyrite mineral assemblage; illite-muscovite-chlorite alteration mineralogy that point to neutral pH conditions; and sphalerite composition of 2.26 to 8.72 mol% FeS in Stage 1 and 0.55 to 1.13 mol% FeS in Stage 2. The K-Ar age date of illite separates from highly altered diorite porphyry of the Lamingag Intrusive Complex yielded an Early Pliocene age (5.12 ± 0.16 Ma). Hydrothermal fluid exsolved from the magma that formed the Lamingag Intrusive Complex probably formed the ore-forming Stage 1 veins. Stages 2 and 3 involved the deposition of quartz and carbonate veins possibly by boiling hydrothermal fluids. Precious and base metal deposition was controlled by the Masara Fault Zone. Exploration markers for gold mineralization in the Masara Gold District and vicinity include the presence of Lamingag Intrusive Complex and massive sulfide veins.

Fluid evolution and gold precipitation in the Muping gold deposit (Jiaodong, China): Insights from in-situ trace elements and sulfur isotope of sulfides

The Early Cretaceous Muping gold deposit is hosted in the Late Jurassic monzogranite. Crosscutting relations of ore veins implied a presence of multiple fluid pulses, and the mineralization process comprises four mineralization stages. Three generations/types of pyrite (Py1, Py2, and Py3) have been discriminated. Coarse-grained and euhedral Py1 pertains to pyrite-quartz veins. Coarse-fine grained and subhedral Py2 comes from quartz-pyrite veins. Fine-grained and anhedral Py3 is from quartz-polymetallic sulfides veins. Py3 further can be divided into three sub-types (Py3a, Py3b, and Py3c), in which Py3a did not have a core-rim texture, Py3b is the core and Py3c is the rim of pyrite with core-rim texture.Py1 and Py2 contain relatively higher Co (738 and 29 ppm on median) and Ni (30 and 7.6 ppm on median), but lower As (754 and 15 ppm on median) and Au (median < 0.1 ppm) content. However, Py3 has extremely low Co and Ni (medians below detection), but elevated As (from 16,290 to 206 ppm on median) and Au (from 36 to 0.4 ppm on median) content. Co and Ni contents of pyrite gradually decreased but As content increased from Py1 to Py3, implied that the temperature of ore-forming fluids progressively decreases. Extremely higher As content and high δ34S of Py3c could be associated with an incursion of As-rich and heavy δ34S meteoric water which has flowed through meta-sedimentary rocks. Sulfur isotopes revealed a gradual decrease from Py1 (+11.6‰) to Py3b (+7.5‰). As for the pyrite with core-rim texture, the sulfur isotope progressively increased from the core (Py3b, +6.0‰) and the rim (Py3c, +11.8‰). The high δ34S could be derived from the metasomatic and fertile lithospheric mantle, and the difference between Py1 and Py2 attributed to suffering from degassing processes with different condition. Rayleigh fractionation process caused a “reservoir effect” can explain the inside variations of Py2, while the increase of fO2 caused by phase separation is the main factor leading to δ34S decrease for Py3. It is the fluid mixture that resulted in the inter-crystalline sulfur isotope variation of pyrite with core-rim texture. Gold and As contents for all types of pyrite plot well below the solubility limit for gold in arsenian pyrite, indicating that invisible gold occurs as Au1+ in the pyrite lattice. The extent of invisible gold was incorporated into pyrite depending on the arsenic content, but visible gold mineralization mainly was controlled by vapor phase escape and fO2 increase which attributed to phase separation.

The Varvarinskoye Complex Copper–Gold Deposit in Northern Kazakhstan: Mineral Types and Ore Composition

The Varvarinskoye copper–gold deposit, large in terms of Au and small in terms of Cu, was discovered in 1979 in the western part of the Torgai trough in Kazakhstan. It is composed of Devonian volcanosedimentary and intrusive rocks. Ore zones with a thickness from a few meters to a few tens of meters have been identified against a background of disseminated ore mineralization. They are composed of densely disseminated (porphyry) up to massive ores. The ore zones are localized in the thermal metamorphism band of volcanosedimentary and intrusive rocks, while disseminated mineralization is widespread beyond it. Ten ore mineralization types can be distinguished. The Au–Cu mineralization type is common in volcanosedimentary rocks and diorites, while the Au–Cu–Ni ores occur in serpentinites formed after thermally metamorphized intrusive rocks. Densely disseminated and massive magnetite ores are confined to skarns. Sulfide and sulfide–arsenide mineralization can also be observed. Each type of Au–Cu mineralization (pyrrhotite, chalcopyrite–marcasite–pyrite, pyrite, and gold–sulfide–arsenopyrite) can be correlated to an analog of Au–Cu–Ni mineralization (pentlandite–pyrrhotite, chalcopyrite–pentlandite–pyrite, pentlandite–pyrite, and gold–nickeline–gersdorffite). This indicates the uniform ore-forming process in serpentinites and volcanosedimentary rocks with diorites and borrowing of ore material from host rocks, at least, partially. The gold–chalcopyrite mineralization type, synchronous to the thermal metamorphism and analogous to mineralization of the alkali stage at porphyry copper deposits, has no analog in the Au–Cu–Ni mineralization. Gold is present in all mineralization types, but gold–chalcopyrite and sulfide–arsenide are the most enriched in Au. Eighty ore minerals have been found at the studied deposit. Pyrrhotite disulfidization and chalcopyrite pyritization are universal in occurrence. These processes likely led to the formation of chalcopyrite–marcasite–pyrite and chalcopyrite–pentlandite–pyrite mineralization after pyrrhotite and pentlandite–pyrrhotite types, respectively. Invisible gold present in chalcopyrite was enlarged during these processes.

Genesis of the Bangbu gold deposit in the southern Tibet: Evidenced from in-situ sulfur isotopes and trace element compositions of pyrite

The Bangbu orogenic gold deposit in the North Himalaya of the southern Tibet contains more than 40 t Au at an average grade of 7.0 g/t. In this deposits, gold-bearing quartz veins were controlled by nearly E-trending Qusong-Cuogu-Zhemulang shear zone and occurred within the secondary faults which crosscut Late Triassic greenschist- facies rocks. To further understand the sulfur source and ore-forming process, we have conducted a compressive study of in-situ SIMS sulfur isotopes and LA-ICP-MS trace element compositions of two stages of pyrite at Bangbu. Early-stage pyrite (Py1) is coarse-grained (mostly 0.2–2 mm) and euhedral, and has gold concentrations of less than 0.3–54 ppm (mean of 20 ppm) and δ34S values of 1.6–5.1‰ (mean of 3.2‰). Late-stage pyrite (Py2) is generally fine-grained (mostly <50 μm to 1 mm) and subhedral to anhedral, and has gold concentrations of 3.6–115 ppm (mean of 30 ppm) and δ34S values of 0.9–5.2‰ (mean of 2.7‰). Gold occurs mainly as invisible refractory within Py1 and Py2, and to a lesser extent as native gold within quartz, pyrite, arsenopyrite and sphalerite. Sulfur for Bangbu gold mineralization was probably sourced from the Greater Himalayan crystalline complex. Release of ore-forming fluids was likely related to amphibolite-facies metamorphism during ~50–45 Ma. Ore fluids deposited Au-rich pyrite during early and late mineralization stage and precipitation of native gold was probably related to fluid boiling and/or remobilization of invisible gold within pyrite.

Petrogenesis of the Bakoudou TTGs and high-K biotite granites and associated gold mineralization, southern Gabon: An Archean example of a porphyry–epithermal transitional gold system

The Bakoudou gold deposit, southern Gabon, is hosted in the Archean basement granitoids of the Chaillu Massif. These granitoids of the Bakoudou deposit are subdivided into two main groups: TTGs and porphyritic biotite granites. The TTGs show geochemical characteristics similar to those of other TTGs worldwide, such as high Al2O3 (13.49–18.93 wt%) and Na2O (3.27–4.66 wt%) and low K2O (1.01–1.98 wt%), with low K/Na (0.15–0.32) ratios. Their Mg# (Mg/(Mg + Fe)*100) values vary from 34.81 to 50.76, with low to moderate Nb/Ta and Zr/Sm ratios (7.03–23.43 and 19.13–75.25, respectively). They are characterized by moderate (La/Yb)N (6.70–41.71) and Sr/Y (20.80–72.13) ratios similar to those of adakites, negative Nb, Ta and Ti anomalies and LREE-enriched and HREE-depleted patterns, without significant Eu anomalies (Eu/Eu* = 0.93–1.11, average = 1.00). These geochemical characteristics suggest that the TTG rocks were formed by partial melting of hydrous basaltic crust, such as garnet-rich amphibolite. The porphyritic biotite granites show high-K calc-alkaline affinity. They are characterized by high SiO2 (71.01–74.86 wt%) and K2O (3.29–4.92 wt%) but relatively low MgO (0.38–0.92 wt%), with Mg# values varying between 31.78 and 40.27. The rocks show low to moderate Sr/Y (13.75–26.66) ratios and LREE-enriched and HREE-depleted patterns, with negative Eu anomalies (Eu/Eu* = 0.36–0.58, average = 0.47). These geochemical features indicate that the porphyritic biotite granites originated from partial melting of TTG lithologies.The orebody of the Bakoudou gold deposit occurs as quartz veins hosted in the TTG shear zones. Quartz is the main gangue mineral, with minor sulfides such as pyrite and chalcopyrite. Gold is present in solid solution (Au+1) and as nanoparticles of native gold (Au0) in pyrite and mostly as free gold. Pyrite samples analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) show high Ni (3.24–3600 ppm) and Co (4.68–4140 ppm) and low As (<34.50 ppm) and Au concentrations (<2.90 ppm) compared to Cu, which has rather high concentration of <3570 ppm. Moreover, the Au–As relationships of pyrites show a porphyry–epithermal transitional signature. Therefore, the Bakoudou gold deposit can be considered an example of Archean porphyry–epithermal transitional gold system, formed from magmatic fluids originating from a hidden porphyry system at depth during cooling and emplacement of the potassic granites. The disseminated free gold grains in the quartz veins may have originated from boiling of the mineralizing fluid (phase separation) and/or from remobilization of invisible gold in pyrite by later tectonic events and surficial weathering.

Invisible Gold Paragenesis and Geochemistry in Pyrite from Orogenic and Sediment-Hosted Gold Deposits

LA-ICPMS analysis of pyrite in ten gold deposits is used to determine the precise siting of invisible gold within pyrite, and thus the timing of gold introduction relative to the growth of pyrite and related orogenic events. A spectrum of invisible gold relationships in pyrite has been observed which suggests that, relative to orogenic pyrite growth, gold introduction in some deposits is early at the start of pyrite growth; in other deposits, it is late toward the end of pyrite growth and in a third case, it may be introduced at the intermediate stage of orogenic pyrite growth. In addition, we report a distinct chemical association of invisible gold in pyrite in the deposits studied. For example, in the Gold Quarry (Carlin type), Mt Olympus, Macraes and Konkera, the invisible gold is principally related to the arsenic content of pyrite. In contrast, in Kumtor and Geita Hill, the invisible gold is principally related to the tellurium content of pyrite. Other deposits (Golden Mile, Bendigo, Spanish Mountain, Witwatersrand Carbon Leader Reef (CLR)) exhibit both the Au-As and Au-Te association in pyrite. Some deposits of the Au-As association have late orogenic Au-As-rich rims on pyrite, which substantially increase the value of the ore. In contrast, deposits of the Au-Te association are not known to have Au-rich rims on pyrite but contain nano- to micro-inclusions of Au-Ag-(Pb-Bi) tellurides.

Application of Biological Oxidation for the Grade Improvement of Au and Ag in An Invisible Gold Concentrate

Application of Biological Oxidation for the Grade Improvement of Au and Ag in An Invisible Gold Concentrate

The characteristic of microstructural deformation of gold bearing pyrite in Jiaodong: The links between nanoscale gold enrichment and crystal distortion

Auriferous pyrite can host a large proportion of gold resource at giant gold deposits all around the world so that these sulfides minerals become key factors to reveal the genesis of ore deposits. In this paper, the focused ion beam-scanning electron microscope (FIB-SEM) method were conducted to present the in-situ transmission electron microscopy (TEM) high resolution images for dislocation ultrastructure study in auriferous pyrite. The TEM diffraction images evidences were provided to prove the evidence that gold cluster at the nanoscale could be existed among crystal interface or even crystal lattice (0.22 nm ~ 0.54 nm). Finally, we have proposed a model that link these nanostructures with gold concentration, gold aggregate in pyrite. The model illustrates the formation pathway of the of discrete invisible gold in pyrite grains which were controlled by shape and ultrastructure of pyrite crystalline, with ion-by-ion rapid diffusion on the surface or lattice. It’s well interpreting for inhomogeneous distribution of nanogold in pyrite resulted from combining with many of these basic pyrite units. In addition, it also reveals unique in-situ measurement result about crystal-fluid interface dynamic that identify the various lattice spacing with possibility for nanoparticle incorporation into deformed pyrite. This study not only provides a new insight into the interpretation of gold distribution and aggregation in pyrite that has significant indication for gold deposit formation in the world, but also provides important evidences for geochemical exploration for concealed deposits based on nanoscale gold.

In-situ trace elements on pyrite and arsenopyrite of the Zhengchong gold deposit, Jiangnan Orogen: Insights for the mineralization mechanism

The Zhengchong gold deposit, with a gold reserve of 19 t, is located in the Liling Goldfield, central Jiangnan Orogen, Southern China. The orebodies are structurally bound by NW- and NNE-NE-trending faults, hosted in the Neoproterozoic slates. The ore styles primarily comprise auriferous pyrite-arsenopyrite-quartz vein, disseminated pyrite-arsenopyrite-sericite-quartz mineralized slate, and intense mineralized slate breccia. The gold-bearing sulfide-quartz veins have average Au grades up to 13.5 g/t, generally higher than that of mineralized rock in all orebodies of 0.6–2.0 g/t and intense mineralized breccia within quartz veins of 2.5–4.0 g/t. Three primary and successive paragenetic sequences are identified: (i) Barren quartz – muscovite; (ii) Quartz – polymetallic sulfides – native gold – muscovite – minor chlorite; (iii) Barren quartz – calcite veins. Native gold has only been observed in the fractures of early barren quartz, pyrite and arsenopyrite in NW-trending auriferous veins. According to textural differences, five types of pyrite and four types of arsenopyrite are recognized from NW- and NNE-NE-trending orebodies.Based on LA-ICPMS spot and imaging analyses of different types of pyrites and arsenopyrites, the refractory, invisible gold in pyrites and arsenopyrites range from < 1 to >500 ppm and was incorporated in these sulfides through substitution of As1− for S2− as the Au and As data plotting below the both gold solubility lines for Carlin and orogenic gold deposits. The invisible gold as the form of Au+ concentrates in sulfides. Besides, minor micro-scale gold with Cu-, Pb-, Sb-, Ag-, and Bi-minerals occur in the pores of pyrite and arsenopyrite. The initial ore-bearing fluid with abundant Si, S, Fe, As and minor Au, Co, Ni, Cu, Zn, Sb, Te, Pb, Bi reacting with Neoproterozoic slates to generate low-grade pyrite (Py-I and Py-III) in both-trending orebodies. For NNE-trending sulfide-quartz veins, cyclic fluid pressure dropping resulted fluid phase separation and destabilization of bisulphide-gold. Due to the small fluid flux in narrow NNE-trending faults, Au+ exhausted after the precipitation of high-grade Py-II, Apy-II and Au-rich rim of Py-I with invisible gold. In NW-trending auriferous quartz veins, hydrofracturing under high fluid pressure in thrust shear-faults, generated massive slate breccias. The sharply pressure decrease in dilations along the ore-controlling faults resulted in significant auriferous fluid phase separation and the solubility reducing of trace elements and gold, which in turn led in the deposition of native gold with chalcopyrite, sphalerite and tetrahedrite in fractures of Py-IV and Apy-III.

Source of gold in Neoarchean orogenic-type deposits in the North Atlantic Craton, Greenland: Insights for a proto-source of gold in sub-seafloor hydrothermal arsenopyrite in the Mesoarchean

Given that gold (Au) mostly remained in the incipient Earth mantle until ca. 3.9–3.8 Ga, a “proto-source” of gold may have been present in the dominantly mafic crust precursor born through first-stage melting of the early Earth mantle. In south-westernmost Greenland, a fragment of the North Atlantic Craton is characterised by greenstone belts comprising mafic volcanic and magmatic rocks, and harzburgite cumulates that were emplaced at ca. <3.19–3.01 Ga (e.g., Tartoq greenstone belt). Here, combining detailed sulphide petrography with rhenium-osmium-sulphur (Re-Os-S) isotope geochemistry of individual mineral separates of arsenopyrite from gold-sulphide mineralised shear zones, we pinpoint the precipitation of ca. 3.18–3.13 Ga (Re-Os model ages) hydrothermal arsenopyrite associated and coeval with arc-related magmatism of the Tartoq Group. We consider sub-seafloor hydrothermal alteration of the oceanic crust and magmatic activity to have supplied arsenic (As), Re, and Au, to result in the precipitation of the ca. 3.18–3.13 Ga arsenopyrite with primary invisible gold. Additionally, in major shear zones in a rigid juvenile continental crust, retrograde greenschist-facies metamorphism overprinted the ca. >3.0 Ga prograde amphibolite-facies metamorphic assemblages and caused local dissolution of arsenopyrite. During this retrograde tectono-metamorphic stage, in gold-rich shear zones, the Re-Os geochronometer in arsenopyrite was reset to a Neoarchean age while invisible gold was liberated and deposited as free gold with 2.66 Ga pyrite (Re-Os isochron ages). The initial Os isotope ratios of Neoarchean arsenopyrite (187Os/188Osi = 0.13 ± 0.02) and gold-bearing pyrite (0.12 ± 0.02) overlap with the estimated 187Os/188Os ratio of the Mesoarchean mantle (0.11 ± 0.01) and preclude contribution of radiogenic crustal Os from evolved lithologies in the accretionary arc complex, but instead, favour a local contribution in Os from basaltic rocks and serpentinised harzburgite protoliths by metamorphic fluids. Thus, the ca. 2.66 Ga lode gold mineralisation identified in the North Atlantic Craton may illustrate a gold endowment in shear zones in Earth’s stabilizing continental crust at the time of the 2.75–2.55 Ga Global Gold Event, through metamorphic upgrading of bulk gold which had originally been extracted from the Mesoarchean mantle and concentrated in hydrothermal arsenopyrite deposits in oceanic crust beneath the overall reduced Mesoarchean ocean.

The interaction of grinding media and collector in pyrite flotation at alkaline pH

A significant proportion of invisible gold is hosted by pyrite and the successful recovery of this gold requires a high recovery of pyrite in flotation. In the industry, the flotation of gold-bearing pyrite is operated in a broad range of pH and collector concentration with different types of grinding media. Following the previous study at pH 5.0 and 7.0, the current study investigated the interaction of grinding media with collector and its effect on pyrite flotation at pH 8.5. It was interesting to find that at the same xanthate dosage the highest pyrite flotation was achieved when forged steel, instead of 30% chrome steel, was used as the grinding media although it produced the highest quantity of iron contamination. Pulp chemistry measurement, surface analysis and electrochemical study revealed that pyrite oxidation prevailed over xanthate oxidation at pH 8.5 and pulp potential played a dominant role in pyrite flotation. At this pH, the high pulp potential generated by high chrome steel grinding media facilitated pyrite oxidation instead of xanthate oxidation, but the low pulp potential generated by forged steel grinding media still allowed xanthate oxidation, thus promoting pyrite flotation.