Gold In Pyrite Research Articles

Genesis of high-grade lode gold shoot dominated by ore fluid overprinting during a ductile-brittle shear event

Lode gold deposits hosted by ductile-brittle shear zones account for more than one-third of the world’s gold production. High-grade ore shoots from this type of deposit are the most critical exploration targets. The ore shoots can form through the post-depositional deformation of auriferous sulfides or overprinting of ore fluids accompanied by coupled dissolution-reprecipitation (CDR) reactions. However, the mechanism that dominates ore shoot genesis remains unknown, primarily due to the controversial single progressive or polyphase nature of ore-bearing shear zones. Here, we report on geological and geochemical analyses we conducted at the large Hetai goldfield, South China, to construct an accurate gold upgrading model for the formation of ore shoots. Stages 1−3 of mineralization at Hetai show features typical of ductile shearing, while Stage 4 is characterized by quartz-sulfide veinlets in brittle fractures. 40Ar/39Ar ages of ca. 184 Ma and 157 Ma for the mineralization of stages 1 and 4 overlap with the regionally dextral ductile-brittle shear that occurred during ca. 210−162 Ma. Thus, the gold event at Hetai should have been controlled by a single progressive ductile-brittle shear episode, rather than polyphase structural events. The auriferous fluids at Hetai precipitated minor invisible gold in pyrites (mean 0.173 ppm) produced during stages 1−4 through fluid-rock interaction. The systematic increase of elements Au, As, Sb, Bi, Ag, and Cu and δ34S values in ductile-deformed pyrites from stages 1−3 indicate that early invisible gold upgrading should be the result of the post-depositional remobilization of auriferous sulfides during the long-lived ductile-brittle transition. Cataclastic pyrites hosting invisible gold from Stage 4 have zoned and porous mantles with elevated invisible gold (mean 0.503 ppm) and Sb, Bi, Pb, Co, Ni, and Ti contents. These pyrites are further replaced by chalcopyrite and pyrrhotite with increasing invisible gold, Co, and Ni contents. In addition, numerous visible native gold grains in Stage 4 are included in sulfides formed by replacement and develop along the microfractures and grain boundaries of these sulfides. We suggest the late invisible and visible gold upgrading events in Stage 4 can be attributed to the auriferous fluid superposition and subsequent replacement of pyrite via CDR reactions in a brittle regime. Therefore, the gold upgrading process at Hetai is jointly caused by the early remobilization induced by ductile-brittle deformation and the late ore fluid superposition with accompanying CDR reactions within a brittle domain. As the ore fluid superposition and CDR reactions in Stage 4 produce a significant amount of visible gold, they exert a first-order control on the genesis of ore shoots at Hetai. The refined model may be widely applicable to lode gold deposits elsewhere and can be used to identify regions with promising exploration targets.

The Xilaokou carbonate-sulfide vein type gold deposit: A distinct mineralization in the giant Jiaodong gold province, North China

The Xilaokou gold deposit with ca. 50 t of gold reserve @ 2.7 g/t represents a novel type (carbonate-sulfide vein type) of gold mineralization within the Jiaodong gold province. However, its mineralization age and metallogenic mechanism remain poorly constrained, hindering a comprehensive understanding of the ore-forming processes in the Jiaodong gold province. In this study, we employ syn-ore stage hydrothermal monazite in situ U-Pb geochronology to determine the ore-forming age of the Xilaokou gold deposit. Additionally, we conduct in situ LA-(MC)-ICP-MS elemental mapping and sulfur isotope analysis in ore-related pyrite to unravel the sulfur source(s) and provide new insights into the ore-forming processes of the Xilaokou gold deposit. Our findings reveal the following key points: (1) U-Pb dating of hydrothermal monazite in Au-bearing pyrite yields an ore-forming age of119.9 ± 3.0 Ma. This age is consistent with the mineralization ages (around 120 ± 5 Ma) of other gold deposits in the region, including Liaoshang-, Jiaojia-, and Linglong-type deposits. (2) Gold in pyrite primarily occurs as micro-grains (5–20 μm) within pyrite fissures associated with sphalerite and galena. (3) Elemental mapping and sulfur isotope analysis indicate that major Au mineralization is linked to elevated concentrations of As, Sb, and Tl, along with heavy sulfur isotope values (δ34S∼24.7 ‰). (4) Early-stage Au mineralization is characterized by enrichment of As, Cu, and Bi, with normal sulfur isotopic composition (δ34S∼8 ‰). We propose that the carbonate-sulfide vein type gold deposits represented by the Liaoshang and Xilaokou gold deposits in the Jiaodong gold province are genetically linked to quartz-sulfide vein and disseminated type deposits. The major ore-forming stage involved the addition of S and Au from a metamorphic massif at slightly lower temperatures. These findings highlight a new exploration direction within the North China Craton. In summary, the Xilaokou gold deposit provides valuable insights into gold mineralization processes in Jiaodong, emphasizing the importance of considering diverse deposit types and their genetic relationships in the region.

Microimpurities in pyrite of gold ore occurrences of the Manitanyrd ridge (the Polar Urals)

The microelement composition was studied of pyrite, the main concentrator of gold in sulfidization zones in ore occurrences of the Manitanyrd Ridge. The ore mineralization is assigned to the gold-sulfide-quartz vein and gold-sulfide types. We determined a wide range of ore-forming elements, associated with gold in pyrite. These elements correspond to geochemical features of the late galena-chalcopyrite-sphalerite mineral assemblage of the ore process, as well as to its final stage expressed in the appearance of elements reflecting the mineralogy of gray ores and bismuth sulfosalts. Pyrites from the vein and mineralized zones, from the ores of the deposit and from the secondary lithochemical anomalies, were found to contain the same set of the gold satellite elements: Ag, Cu, Zn, Pb, As. In pyrite of the gold-sulfide-quartz veins, Au has close correlations with the main elements of the ore association. In pyrite of the mineralized zones, all the correlations between Au and the ore-forming elements are absent. The pair-wise correlation between the ore-forming elements in these zones also changes completely. It is demonstrated that the identified differences in correlations between Au and the ore-forming elements in pyrite are a criterion for assessment of the lithochemical anomalies and allow one to propose a strategy of prospecting for new ore objects in the Manitanyrd region.

Release and re-enrichment of invisible gold in arsenian pyrite promoted by coupled dissolution-reprecipitation reactions

The mechanisms responsible for invisible gold enrichment driven by coupled dissolution-reprecipitation reaction (CDR) are debated. Here we report the micro- to nano-scale textures of arsenian pyrite in a high-grade (>10 g/t) gold ore from the Chang’an deposit to trace the gold enrichment process. Our study records a CDR-driven evolution of mineral growth from an As-rich, Au-poor pyrite core, with numerous fine arsenopyrite inclusions, to an inclusion-free, As-Au-rich oscillatory pyrite rim. The reaction occurred at ~260 °C under 4.7 to 5.8 pH and –36.6 to –32.9 logfO2 conditions. The elevated As but depleted S contents in the pyrite core indicate a combined elevation of S fugacity and solubility of Au. The coprecipitation of arsenopyrite inclusions in the core caused a depletion of S fugacity to –13.8 ~ –11.7, triggering Au enrichment in the rim. This non-unique process has the potential to explain the upgrade of invisible Au in arsenian sulfides, worldwide.

Gold source and deposition in the Sanakham gold deposit, SW Laos: Constrains from textures, trace element geochemistry and in-situ sulfur isotopes of pyrite

The Luang Prabang (Laos) – Loei (Thailand) metallogenetic belt is endowed with major porphyry-skarn, and epithermal gold deposits. The Sanakham gold deposit is a newly discovered hydrothermal vein-type orogenic gold deposits (10.6 t Au @ 3.05 g/t) in the belt, and consist of a series of quartz-sulfide veins that are controlled by secondary structures of the regional NNE-trending brittle-ductile shear zone. Its mineralization can be divided into four stages: (I) quartz–pyrite–arsenopyrite, (II) quartz–pyrite–pyrrhotite–chalcopyrite, (III) quartz–pyrite–base-metal sulfide, and (IV) quartz–carbonate. Three pyrite generations were identified: py1 (stage I) as disseminated euhedral to subhedral coarse grains; py2 (stage II) as subhedral to anhedral fine grains occur around the cataclastic and corrosion textures in py1; py3 (stage III) as subhedral to anhedral fine grains occurred in sulfide veins or veinlets that overprint the existing quartz-sulfides. All three generations of pyrite have low trace-element contents including very low lattice-bound gold (py1: 0.040 ppm; py2: 0.098 ppm; py3: 0.028 ppm) and arsenic (py1: 227 ppm; py2: 133 ppm; py3: 13.3 ppm) contents. Visible gold grains are sited in textures of pyrite grains that relate to brittle cataclastic deformation. Considering the relatively low contents of pyrite in the ores of Sanakham, it is impossible to derive the high-gold grade orebodies through local remobilization of originally lattice-bound gold in pyrite. The py2 might be formed during hydrothermal overprinting and dissolution of py1 led by continuous supply of ore fluids with relatively higher contents of Au, Ag, Cu, Sb, Te, Pb, and Bi. A reduction reaction related to reduced carbon species in wallrock is an assignable cause for gold deposition, even though the fluid-rock interaction and fluid immiscibility processes might be important. The Au- and As-poor pyrites have similar δ34S values (py1: –2.58 to –0.83 ‰; py2: –2.17 ‰ to + 0.25 ‰; py3: –1.24 to –0.60 ‰ ppm) that close to 0 ‰, indicating the sulfur may have been originated from magmatic or mantle sources. Therefore, we propose that the Sankham gold deposit is a member of the orogenic deposit class, but notably characterized by the low concentrations of invisible gold in the relatively As-poor pyrite lattice.

Gold and Arsenic in Pyrite and Marcasite: Hydrothermal Experiment and Implications to Natural Ore-Stage Sulfides

Hydrothermal synthesis experiments were performed in order to quantify the states of Au and As in pyrite and marcasite. The experiments were performed at 350 °C/500 bar and 490 °C/1000 bar (pyrite–pyrrhotite buffer, C(NaCl) = 15 and 35 wt.%). The synthesis products were studied by EPMA, LA-ICP-MS, and EBSD. The EPMA was applied for simultaneous determinations of Au, As, Fe, and S, with a Au detection limit of 45–48 ppm (3σ). The analyses were performed along profiles across zonal grains. The concentrations of As and Au up to 5 wt.% and 8000 ppm, respectively, were determined in pyrite and up to 6 wt.% and 1300 ppm in marcasite. In pyrite, the Au concentration decreases with fluid salinity and temperature increases. Strong positive Au–As correlation and strong negative Au–Fe and As–S correlation were identified in pyrite. Comparison of the correlations with theoretical lines implies Au–As clustering. The cluster stoichiometry is inferred to be [AuAs10]. Most probably, As in pyrite presents in the form of clusters and in the As→S solid solution. Incorporation of Au in As-rich pyrite can be controlled by the reductive deposition mechanism. In marcasite, the concentrations of Au are not correlated with the As content. The [AuAs10] clusters enrich the {210}, {113}, and {111} pyrite faces, where the former exhibits the highest affinity to Au and As. The affinity of {110} and {100} forms to Au and As is lower. Implication of the experimental results to data for natural auriferous pyrite shows that the increase of Au content at C(As) > 0.5–1 wt.% is caused by the incorporation of the Au-As clusters, but not because of the formation of Au→Fe solid solution. Therefore, the concentration of “invisible” gold in pyrite is dictated solely by the hydrothermal fluid chemistry and subsequent ore transformations.

Sediment‐hosted disseminated gold mineralization at the Gegalaw deposit in Central Myanmar

AbstractGegalaw is one of the gold deposits situated in the Sagaing Fault zone, which marks the boundary between the Central Myanmar Basin to the west and the Mogok metamorphic belt to the east. The gold mineralization is mainly confined along northeast striking faults between carbonaceous silty limestone of the Kywethe Chaung Formation and carbonaceous and calcareous shale of the Eocene Male Formation. Hydrothermally induced dissolution breccia associated with progressive silica replacement increase near the northeast trending faults. The hydrothermal alteration associated with the mineralization is silicification in addition to illite‐kaolinite argillic alteration, of which illite is dated by the K‐Ar method as 40.26 ± 0.91 Ma. Sulfide minerals contained in the ores are mainly pyrite and arsenopyrite, with minor to trace amounts of marcasite, chalcopyrite, and sphalerite, among which pyrite and arsenopyrite are the hosts of gold. Pyrite and arsenopyrite are divided into diagenetic and hydrothermal stages, based on their mineral textures. Diagenetic stage pyrites are framboidal (Py1A) and coalescent framboidal (Py1B). Those of the hydrothermal stage are zoned pyrite with porous cores (Py2) and compact rims (Py4). In addition, euhedral homogeneous pyrites with or without compact rim are present (Py3). Some pyrites show pseudomorphs of ilmenite (Py3i) and carbonate minerals, possibly ankerite (Py3c). Arsenopyrites are also divided into porous, sooty arsenopyrite (Apy1), and compact arsenopyrite (Apy2) that co‐exists with Py3 and Py4 pyrites. Electron probe microanalysis for sulfides indicates that Py1A and Py1B are deficient in Fe while Py2, Py3 and Py4 are deficient in S compared to stoichiometry of pyrite. Such deficiencies are compensated by metal ions such as Co2+ and Ni2+ for Py1A, As2+ for Py1B, and As1− for Py2, Py3, and Py4. Arsenic equilibrium temperature of Apy2‐Py3 pairs shows a mode range of 250–270°C. Detected gold concentrations (~650 ppm) in arsenian pyrite (Py2, Py3, and Py4) indicate that gold in pyrite are below the solubility, suggesting that gold is present as solid‐solution in pyrite. Sulfur isotopic values of the pyrites in the unaltered host rocks are −18.9‰ and −8.9‰, and those of the pyrites from the hydrothermal altered rocks range from −17.3‰ to −0.5‰. These similar negative δ34S ranges of the pyrites suggest that the source of sulfur is dominated in biogenic reduction origin. Nevertheless, the two relatively heavier sulfur isotopic values (−4.9‰ and −0.5‰) suggest a contribution of an external hydrothermal fluid source. The research results indicate that gold mineralization occurred by infiltration of auriferous reduced hydrothermal fluids into the carbonaceous silty limestone and carbonaceous and calcareous shale through northeast trending secondary faults of the dextrally moving Sagaing Fault system and the fluids reacted with framboidal pyrite as well as iron‐bearing minerals in the host rocks to precipitate gold‐bearing arsenian pyrite. The age of hydrothermal illite suggests a link to the right‐lateral strike‐slip movement of the Sagaing Fault that is related to the collision of the Indian and Asian plates.

Combined Effect of Organic Carbon and Arsenic on the Formation of Sediment-Hosted Gold Deposits: A Case Study of the Shahuindo Epithermal Deposit, Peru

Abstract Sediment-hosted gold deposits represent a significant portion of the world’s gold resources. They are characterized by the ubiquitous presence of organic carbon (Corg; or its metamorphosed product, graphite) and the systematic occurrence of invisible gold-bearing arsenian pyrite. Yet the role played by these features on ore formation and the distribution of gold remains a long-standing debate. Here, we attempt to clarify this question via an integrated structural, mineralogical, geochemical, and modeling study of the Shahuindo deposit in northern Peru, representative of an epithermal gold deposit contained in a sedimentary basin. The Shahuindo deposit is hosted within Lower Cretaceous fluvio-deltaic carbon-bearing sandstone, siltstone, and black shale of the Marañón fold-and-thrust belt, where intrusions of Miocene age are also exposed. The emplacement of the auriferous orebodies is constrained by structural (thrust faults, transverse faults) as well as lithological (intrusion contacts, permeable layers, anticlinal hinge in sandstone) features. The defined gold reserves (59 tons; t) are located in the supergene zone in the form of native gold grains. However, a primary mineralization, underneath the oxidized zone, occurs in the form of invisible gold in arsenian pyrite and arsenopyrite. Here, four subsequent pyrite generations were identified—namely, pyI, pyII, pyIII, and pyIV. PyI has mean Au concentrations of 0.3 ppm, contains arsenic that is not detectable, and is enriched in V, Co, Ni, Zn, Ag, and Pb compared to the other pyrite generations. This trace element distribution suggests a diagenetic origin in an anoxic to euxinic sedimentary basin for pyI. Pyrite II and pyIV have comparable mean Au (1.1 and 0.7 ppm, respectively) and As (2.4 and 2.9 wt %, respectively) concentrations and precipitated under conditions evolving from lower (pyrrhotite, chalcopyrite, sphalerite) to higher (enargite, digenite, chalcocite) sulfidation, respectively. The pyIII generation is the major gold event in the primary mineralization, with pyrite reaching 110 ppm Au (mean ~7 ppm) and 5.6 wt % As (mean ~1.8 wt %), while coeval arsenopyrite attains 460 ppm Au. Pyrite III is also enriched in other trace elements such as Se, Ge, Mo, In, Ga, and Bi compared to the other pyrite generations, which is indicative of a magmatic source. Bulk analyses of the surrounding unmineralized rocks show only parts per billion levels of Au and less than 25 ppm As. These data, combined with mass balance considerations, demonstrate that the sedimentary rocks could not be the sole source of gold, as they could only contribute a minor portion of arsenic and sulfur (and iron) to the deposit. Conversely, fluids exsolved from a pluton crystallizing at depth likely provided the great part of the gold endowment. Equilibrium thermodynamics simulations, using geochemical constraints established in this study, demonstrate that interaction between Au-As-S-Fe–bearing fluids and organic carbon-bearing rocks strongly enhanced the fluid ability to transport gold by maximizing its solubility as AuI hydrosulfide complexes via a combined increase of pH and aqueous sulfide concentration. This finding challenges the traditional qualitative view of organic matter acting exclusively as a reducing agent for AuI that should promote gold deposition in its native state (Au0) rather than enhance its solubility in the fluid. Our results have significant implications for the exploration of carbonaceous sedimentary environments. Such settings may provide a very effective mechanism for focusing gold transport. Subsequent scavenging of AuI from solution in a chemically bound form is promoted by the precipitation of arsenian pyrite in permeable structural and lithologic traps, bound by more impermeable units, similar to what occurs in petroleum systems. Our integrated study underlines the important potential of sedimentary Corg-bearing rocks in the formation and distribution of gold and associated metal resources.

Ore Mineralogy, Fluid Inclusion Geochemistry, and Zircon U-Pb Geochronology of the Nanmingshui Gold Deposit in East Junggar, Xinjiang, Northwest China: Implications for Ore Genesis

The Nanmingshui gold deposit, located in the eastern segment of the Kalamaili gold belt (KGB), is hosted by the sub-greenschist facies rocks of the Lower Carboniferous Jiangbasitao Formation. The genesis of this deposit, however, has been debated for decades because of controversial constraints on the P-T-X conditions and origins of hydrothermal fluid and mineralization age. In this study, we present gold-bearing sulfide compositions, fluid inclusions, H-O isotopes, and the results of hydrothermal zircon U-Pb dating to provide new insights into the genesis of the gold deposit. Three gold mineralization stages are recognized: quartz–pyrite–minor native gold veins (early), quartz–tourmaline–arsenopyrite–pyrite–gold–polymetallic sulfide veins (middle), and quartz–calcite veinlets (late). Gold predominantly occurs as native gold with high fineness ranging from 941 to 944 in sulfides and quartz, and some as solid solutions (Au+) within the lattice of pyrite and arsenopyrite. Three types of primary fluid inclusions are identified in hydrothermal quartz: CO2-H2O (C-type), aqueous (W-type), and pure CO2 (PC-type) inclusions. The early-stage quartz mainly contains C-type and minor W-type inclusions, with total homogenization temperatures (Th) of 220–339 °C, salinities of 0.4–3.7 wt.% NaCl eqv., and bulk densities of 0.66–1.01 g/cm3. All three types of inclusions are observed in the middle-stage quartz, of which the C- and W-type inclusions yield Th values of 190–361 °C, with salinities of 0.4–6.0 wt.% NaCl eqv. and bulk densities of 0.69–0.99 g/cm3. The late-stage quartz contains only W-type inclusions that have lower Th values of 172–287 °C, higher salinities of 1.4–6.9 wt.% NaCl eqv., and bulk densities of 0.79–0.95 g/cm3. Trapping pressures estimated from C-type inclusions in the early and middle stages cluster at 280–340 MPa and 220–310 MPa, respectively, corresponding to metallogenic depths of 10–13 km and 8–11 km. The H-O isotopic compositions (δ18Owater = 1.8–10.9‰, δD = −99 to −62.9‰) and microthermometric data indicate that the ore-forming fluids belong to medium–high-temperature, low-salinity, medium-density, and CO2-rich-H2O-NaCl ± CH4 ± N2 systems, probably originating from metamorphic water. Fluid immiscibility is a crucial mechanism for gold precipitation. Additionally, the U-Pb dating of hydrothermal zircons, from the auriferous quartz–tourmaline vein, yield a weighted mean 206Pb/238U age of 314.6 ± 9.6 Ma. Taking all of the above, the Nanmingshui deposit can be reasonably classed as a typical mesozonal orogenic gold deposit in the KGB, which was formed in a Late Carboniferous tectonic transition from syn-collision between the Jiangjunmiao accretionary complex and Yemaquan arc to post-collision in the East Junggar Orogen. Our results serve to better understand the gold mineralization and genesis of the Late Paleozoic orogenic system in the Kalamaili area, Xinjiang.

Geochronology and ore-forming fluid source of the Buzhu orogenic gold deposit in southern Tibet

The Buzhu gold deposit is a newly discovered and representative gold deposit in the Indus-Yarlung Zangbo suture zone. At Buzhu, auriferous quartz veins with paragenetic sequences of quartz-pyrite-sericite hosted by Early Jurassic greenschist facies rocks, and gold occurs mainly as invisible lattice gold in pyrite as well as minor as visible micrograins inside (arseno)pyrite fractures. We have recognized two auriferous pyrite generations formed in the syn-ore stage (Py2a and Py2b). Py2a is euhedral-subhedral granular with cavities, whilst Py2b mainly grew colloidally around Py2a, indicating that pre-existing pyrite may have been dissolved to form the porous Py2a and Py2b rim. Py2a and Py2b have similar δ34SV-CDT values of 6.98‰ to 7.54‰ and 6.18‰ to 6.85‰, respectively, which are also similar to those of the ore-hosting sediments (6.08‰ to 7.08‰). In-situ trace element analysis shows that Py2b (avg. 1.63% Au) contains more gold than Py2a (avg. 0.42% Au). Combined with petrological and geochemical data, we hypothesized that the gold was derived from the metamorphic devolatilization of the sedimentary wallrocks, and pyrite dissolution-reprecipitation has driven the gold enrichment from Py2a to Py2b. The sericite Ar-Ar date of 19.60 ± 0.66 Ma likely represents the gold mineralization age, as the dated sericite coexists with the auriferous pyrite. This age is much younger than most orogenic gold mineralization (ca. 60–41 Ma) in southern Tibet and indicates an orogenic gold mineralization event (ca. 20–15 Ma) related to extensional tectonics during the late-stage India-Asia continental collision.

Nitric acid treatment of a pyrite rougher concentrate to oxidise differentially the pyrite types and modify the Au:S ratio in a resulting cleaner concentrate

The Lihir ore deposit is a sulfide associated refractory gold ore of epithermal origin containing different types of pyrite with a range of levels for gold. The presence of the low gold pyrite types decreases the Au:S value of the Lihir ores and its concentrates resulting in higher consumption of oxygen in the autoclave and thereby affecting the overall gold production at Lihir. This research, deriving from the fundamental principles of hydrometallurgy, is an attempt to develop a manipulative framework to modify the flotation response of gold-rich pyrite and low gold pyrite so that the Au:S ratio of the final pyrite concentrate from flotation can be upgraded. Metallurgical results confirmed that it is possible to upgrade the Au:S values of a pyrite flotation concentrate by using a hydrometallurgical scheme for preparation before flotation. A 144% increase in Au:S value was obtained with 25% oxidation of the pyrite concentrate with a gold recovery of 56%.

Trace Element Composition of Pyrite from Selected Black Shale and Chert Exposures in the Central Belt of Peninsular Malaysia: Implications for Mineral Exploration

Sedimentary and hydrothermal pyrites contained in selected Malaysian black shale and cherts have been analysed using laser ablation inductively coupled plasma (LA ICP-MS) and electron probe microanalysis (EPMA) at the University of Tasmania, Australia. This study shows that gold is concentrated in sedimentary and hydrothermal pyrite in the Middle Permian to Late Triassic black shales and Devonian cherts. According to LA ICP-MS analysis, gold contents in pyrite varied from 0.5 to 0.8 ppm Au in the Permo-Triassic black shale and between 0.2 and 0.8 ppm Au in the Devonian cherts. The lowest level of gold (0.3 ppm Au) was observed in the Permo-Triassic black shale that crops out at the Selinsing gold mine. In the Permo-Triassic period, the selenium contents display one peak (average range: 63.4–103.4 ppm Se) that is far from any gold deposit and one lowest point (average: 5.3 ppm Se) at the Selinsing gold deposit. In the Devonian period, the selenium content in sedimentary pyrite shows a peak (72.6–243.8 ppm Se) in the cherts. EPMA and LA ICP-MS data show consistent Se content variation in the Devonian and Permo-Triassic periods. Using selenium as a proxy for atmospheric oxygenation, the lowest level of Se content in the Permo-Triassic period is believed to decrease atmospheric oxygenation, as recorded in sedimentary pyrite found in black shale from the Selinsing gold deposit. The two peaks of selenium contents are interpreted as periods of increased atmospheric oxygenation. From an exploration perspective, the concentration of gold in sedimentary pyrites makes them sources for gold in the central sedimentary basin of Peninsular Malaysia. Therefore, the two maximum levels of Se and gold content during Permo-Triassic and Devonian times correspond to two stratigraphic levels of potential for orogenic gold mineralisation in the district. The EPMA data show significant values of Co over Ni in pyrite from the Gua Musang, Semantan, and Karak formation black shales, indicating a volcanic contribution of Co during the formation of sedimentary pyrite. Based on the current study’s findings, gold exploration should not be restricted to areas in and around the Selinsing gold mine, Buffalo Reef, Penjom mine, Tersang mine, and Bukit Koman mine but can be extended to BRSZ Units 1 and 2, Gua Musang, and Karak formations in the central belt of Peninsular Malaysia.

Gold–Sulfide Mineralization in the Manitanyrd Region, Polar Urals, Russia

This article describes the characteristics of gold–sulfide–quartz and gold–sulfide (gold–arsenic) ore occurrences in the Manitanyrd region of the Polar Urals. Ore occurrences are confined to NE-trending shear zones and have the common features of a geological structure. The host rocks are metamorphosed volcanic and volcanic–sedimentary rocks. We analyzed the mineral and chemical composition of the ore mineralization in all studied ore occurrences, showing that they belong to the same mineral type—pyrite–arsenopyrite, with a variable ratio of the main minerals. Arsenic pyrite is present in all ore occurrences. Two stages of ore formation were distinguished: early gold–pyrite–arsenopyrite with finely dispersed gold and late gold–galena–chalcopyrite–sphalerite with coarse gold, fahlore, and sulfosalts Pb, Cu, Bi, Sb. Native gold of the first generation, finely dispersed in arsenopyrite and pyrite, had an average to high fineness (800‰–1000‰) with a relatively low dispersion. Native gold of the second generation was larger, and its fineness in ore occurrences varied; in one of them, it varied from 300‰ to 950‰, while in others, it varied from 800‰ to 950‰. The isotope composition of sulfur in sulfides (δ34S) ranged between −0.2‰ and −8.0‰. δ34S values of sulfides in the range of −0.2‰ to −3.5‰ were similar to meteorite, indicating the participation of a single deep magmatic source of sulfur in the ore formation. According to the study of fluid inclusions, the formation of ore quartz veins occurs in the temperature range of 467–109 °C. The similarity of the geological–structural, mineralogical–geochemical, and isotope–geochemical features of the gold–sulfide–quartz and gold–sulfide occurrences in the area suggest their formation in a single hydrothermal system.

Effects of lead nitrate and pre-aeration on the deportment of base/precious metals in cyanide leaching of a pyritic refractory gold concentrate

This study was undertaken to improve gold and silver extraction from a pyritic gold flotation concentrate, which assayed 11 g/t Au, 42 g/t Ag, 0.21% Cu, 3.57% Zn, and 31% Fe. Direct cyanide leaching of the concentrate at 1.5 g/L NaCN yielded a low gold extraction (37%), confirming its refractory nature. Effects of lead nitrate (200-500 g/t Pb(NO3)2) and pre-aeration (24 h) before cyanide leaching at 1.5-3.5 g/L NaCN were investigated. Earlier studies have focused on the impact of these parameters on gold leaching. Besides gold, this study demonstrated the behaviour of silver and base metals (copper and zinc) from the pyritic gold concentrate. Adding lead nitrate had a negligible effect on gold extraction whilst improving silver extraction. Dissolution of copper was substantially suppressed by adding lead nitrate, i.e., from 23% (no Pb(NO3)2) to 4% (500 g/t Pb(NO3)2) over 24 h. Zinc dissolution was negligible (≤0.01%). Pre-aeration of the concentrate improved the gold and silver extractions by 4-14% and 23-44% at the subsequent cyanide leaching (1.5-3.5 g/L NaCN). However, it did not affect the leaching of copper. Only negligible leaching of zinc (≤0.6%) occurred during cyanide leaching. Pre-aeration also reduced cyanide consumption in subsequent cyanide leaching (1.5 g/L NaCN), i.e., from 2.83 kg/t to 2.03 kg/t NaCN per solids. These results suggested that lead nitrate can improve silver extraction while suppressing copper dissolution, which would be advantageous in the leaching-adsorption circuit (CIP), mitigating the dissolved copper-associated problems. Pre-aeration can also be suitable for improved gold/silver extractions and reduced reagent consumption.

Coupling and decoupling of Au and As in pyrite from Carlin-type Au deposits, southwest China

The low-temperature geochemistry of gold (Au) and arsenic (As) in pyrite from Carlin-type Au deposits is significant for understanding the enrichment mechanisms of Au in pyrite, but remains poorly understood due to the complex processes that lead to the precipitation of micron-scale pyrite grains. This paper concerns the geochemical behaviors of Au and As during the precipitation of pyrite from six Carlin-type Au deposits (the Nibao, Shuiyindong, Banqi, Jinya, Yata, and Zimudang deposits), southwest China. These deposits are located in different locations and ore-bearing horizons of the Youjiang Basin. Arsenian pyrite and/or arsenopyrite are the dominant Au-bearing minerals. Gold-bearing pyrite grains are typically zoned (core, intermediate, and rim), with multiple, oscillatory banded sub-zones. In situ analysis demonstrates that invisible Au exists mainly as cationic Au+ in the growth zones of pyrite. Gold and As are positively correlated (i.e., coupled) in the Nibao and Shuiyindong deposits, negatively correlated in the Banqi, Jinya, and Yata deposits, and poorly correlated in the Zimudang deposit. Arsenic and S substitution in pyrite and the presence of a reduced fluid promoted the incorporation of Au, resulting in the coupling of Au and As. The Au precipitation was affected by changes in the physicochemical conditions of the hydrothermal systems, independent of the As concentration, resulting in decoupling of Au and As. As such, invisible Au precipitation is controlled by the extrinsic physicochemical conditions of the ore-forming fluids and microscopic reactions at the fluid–mineral interface, and also the episodic incursion of Au-rich hydrothermal fluids. High As contents in pyrite favor Au incorporation, but are not solely responsible for Au enrichment. Gold and As contents in pyrite in Carlin-type Au deposits may be coupled or decoupled.

Genesis of the Heihaibei gold deposit in the East Kunlun Orogenic Belt, China: Evidence from in situ trace elements of gold-bearing sulfides and stable isotopes

The Heihaibei gold deposit, located in the south part of the East Kunlun orogenic belt (EKOB), is a newly discovered gold deposit, with similar tectonic setting and geological characteristics with typical orogenic gold deposits in the orogenic belt. The aim of this study is to enrich the understanding of metallogeny of the orogenic gold deposit in the EKOB by studying the genesis of the Heihaibei gold deposit. Pyrite and arsenopyrite, as the predominant Au-bearing sulfides, were investigated using a combination of laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) and laser ablation–multi-collector–inductivity coupled plasma–mass spectrometry (LA–MC–ICP–MS) for in situ trace elements and sulfur isotope. The analyses results provided significant information about genesis for gold mineralization. There are two types of smoky quartz–sulfide veins (barren and mineralized) in this deposit. Three types of pyrite based on host rock can be distinguished, including barren smoky quartz–sulfide vein- (Py0), mineralized altered monzogranite- (Py1) and mineralized smoky quartz–sulfide vein-hosted pyrite (Py2, including Py2a and Py2b). Py1 coexists with arsenopyrite. Py2a and Py2b occurs as disseminated crystals and veinlet assemblage, respectively. Petrographic observations and LA–ICP–MS analyses show that invisible gold in pyrite and arsenopyrite occurs dominantly as solid solution (Au+). Co/Ni ratios of pyrite and S–H–O isotopic compositions exhibit that pyrites (Co/Ni > 1, δ34SV-CDT = −5.28 to 5.08‰) hosted by barren smoky quartz–sulfide vein are consistent with a magmatic origin, whereas those (Co/Ni < 1, δ34SV-CDT = 7.37 to 9.48‰, δ18OH2O = 3.5 to 4.9‰; δDV-SMOW = −97.9‰ to −93.7‰) hosted by mineralized altered monzogranite and mineralized smoky quartz–sulfide vein were derived from a metamorphic source. Combined with regional tectonic evolution, the gold mineralization in the Heihaibei gold deposit may be related to the metamorphism of the regional strata, which was associated with closure of the Paleotethys Ocean during the early Mesozoic. In the following study, we will pay more attention to the metallogenic regularity of orogenic gold deposits in the EKOB.

Geologic and Geochemical Features of the World-Class Archean Windfall Intrusion-Related Au Deposit, Abitibi Subprovince, Canada

Abstract The Neoarchean Windfall gold deposit, hosted in the Urban-Barry greenstone belt of the Abitibi subprovince (Quebec, Canada), represents an emerging and significant Au deposit with a resource of 7.4 Moz of Au. It is hosted in 2717 Ma bimodal volcanic rocks that are cut by several generations of calc-alkaline quartz-feldspar porphyry dikes separated into (1) a 2697.6 ± 2.6 Ma group spatially related to Au mineralization and (2) a 2697.6 ± 0.4 Ma group that truncates the earlier dikes and the Au mineralization. The Au zones are structurally controlled and localized to faults and fractures proximal to the contacts of the early quartz-feldspar porphyry dikes; these zones form thin, subvertical, and elongate lenses plunging 35° east-northeast. Gold mineralization, present as both free gold and inclusions in pyrite, occurs (1) in gray quartz veins and stockworks with pyrite and subordinate carbonate and tourmaline and (2) in pervasive to patchy sericite-silica-pyrite-carbonate ± tourmaline ± fuchsite alteration zones. The Au mineralization and associated hydrothermal alteration, along with all the host rocks that include postmineralization intrusions, are overprinted by D2 deformational features that include a penetrative fabric, shear zones, and associated folds. The spatial and temporal association of the quartz-feldspar porphyry intrusions with the Au mineralizing event at the Windfall gold deposit, along with its elemental association (Ag, As, Sb, S, Se, Bi, Te, ± Zn, Cu, Pb, Mo, W), suggests an intrusion-related model and contrasts with the more abundant orogenic gold deposits in the Abitibi greenstone belt. This interpretation has important implications both locally and regionally for Au exploration in Archean greenstone terranes.

Analyses under the curve, identifying how invisible gold is held in pyrite

AbstractWhen laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of pyrite plot below the gold solubility line on a gold vs. arsenic plot and have relatively flat counts on laser ablation time-resolved output graphs, it is often interpreted that the gold is held within the pyrite structure. The study by Ehrig et al. (2023, this issue) shows, using a combination of LA-ICP-MS spot analyses of gold in pyrite, transmission electron microscopy, and electron backscatter diffraction that this is not necessarily the case. Furthermore, they use these same techniques to identify how trace elements, including gold, are remobilized in pyrite during deformation and metamorphism.

The Sukari Gold Deposit, Egypt: Geochemical and Geochronological Constraints on the Ore Genesis and Implications for Regional Exploration

AbstractThe Sukari gold deposit (&amp;gt;15 Moz Au) in the Eastern Desert of Egypt is hosted by a deformed granitoid stock (Sukari tonalite-trondhjemite intrusion) and mainly occurs as a network of crosscutting sulfide-bearing quartz (± carbonate) veins and intensely sulfidized-silicified-sericitized wall rock. Emplacement of the Sukari intrusion into a tectonized Neoproterozoic accretionary complex was controlled by a system of NE- to NNE-trending oblique faults that are related to a deep-seated positive flower structure. A robust genetic model has been hampered by the poorly understood relationships between gold mineralization and host rocks. In this study, zircon U-Pb ages of three samples from the Sukari intrusion define a crystallization age of ~695 ± 2 Ma. In contrast, hydrothermal sericite from the ore zone yields an 40Ar/39Ar age of ~625 ± 3 Ma, which coincides with the onset of major sinistral transpression in the region.Features including sigmoidal morphology of gold quartz veins and abundant subhorizontal tension gashes alongside widespread brecciation and recrystallization suggest that quartz veining occurred during renewed shortening and exhumation through the brittle-ductile transition. Petrographic and micro-X-ray fluorescence (µXRF) studies indicate that disseminated gold and sulfides, commonly associated with sericite and carbonate alteration, are mostly confined to stylolitic bands in the quartz veins. Oscillatory and sector zoning patterns, irregular As-rich bands, and truncations between early- and late-genetic pyrites reflect variations in temperature and mechanism of ore deposition, demonstrated by variable As/S and Co/Ni ratios in the different pyrite generations. Laser ablation-inductively coupled plasma-mass spectrometry analysis pinpoints the covariance of gold and arsenic contents in pyrite, but free milling gold inclusions in microfractures consistently have a mercury-bearing electrum composition, depicting different ore formation stages.Ore fluids with δ34SH2s values of –1.9 to –3.0‰, modeled from gold-associated pyrite and arsenopyrite assemblages with nearly identical δ34S values, suggest a likely single source of sulfur. Alternatively, multisourced sulfur could have extensively mixed and equilibrated by fluid reaction with carbonaceous wall rock. Gold deposition was triggered by abrupt changes in fluid pH and fO2. As an implication for future exploration, sites of maximized strain gradients adjacent to granitoid bodies along extensive transpression zones in the district could be highly prospective targets, particularly where imprinted by sericite-carbonate ± graphite alteration.

Hyperenrichment of gold in pyrite induced by solid-state transportation

The hyperenrichment of gold is critical for the formation of high-grade gold deposits in orogenic systems, where gold enrichment can reach several percent. However, many details on the nature of the hyperenrichment mechanism remain unclear. Here we show a “self-purification” model that could explain the hyperenrichment of gold from nanoparticles to veinlets in deformed pyrite. The three-dimensional spatial relationship and coexistence of gold nanoparticles and veinlets indicate a direct transformation from the former to the latter in solid-state. Theoretical evaluation reveals that this transformation involves a thermodynamically favorable self-purification process. We thus propose that deformation/thermo drives the transport of gold nanoparticles towards veinlets in pyrite through atomic diffusion. This deformation/thermo-driven model is applicable to any deformed nano-gold-bearing pyrite and explains how solid-state transport can effectively concentrate gold to a very high grade. Similar mechanisms may have broad implications in both ore-forming and beneficiation processes.