Remobilization Of Gold Research Articles

LA-ICP-MS Analyses of Sulfides from Gold-Bearing Zones at the Perron Deposit, Abitibi Belt, Canada: Implications for Gold Remobilization through Metamorphism from Volcanogenic Mineralizations to Orogenic Quartz–Carbonate Veins

The Perron deposit, located in the northern part of the Archean Abitibi belt, bears some of the highest gold-grade mineralization for orogenic-vein-type deposits worldwide (High-Grade Zone: HGZ). More than 13 gold-bearing zones with different sulfide assemblages, hydrothermal alterations, and gold grades have been recently outlined, and they range from volcanogenic to orogenic in origin. In addition, seven zones are hosted in a restricted volume of ~1 km3, which is called the Eastern Gold Zone. Pyrite, sphalerite, pyrrhotite, and chalcopyrite—each from a different gold-bearing zone—were analyzed with LA-ICP-MS to decipher their genetic links, mineralizing processes, and temperature of formation. The temperatures calculated with the sphalerite GGIMFis thermometer range from 348 to 398 °C. All gold-bearing zones recorded volcanogenic hydrothermal inputs at different intensities, manifested by pyrrhotite. Pyrite was late-metamorphic and related to the orogenic gold system induced by the contact metamorphism of amphibolite facies. The pyrrhotite grains had very homogeneous trace element signatures in all zones, which is a characteristic of metamorphic recrystallization, exhibiting a loss of mobile elements (Au, Te, Bi, Tl, Sn, W, In) but high concentrations of Ni, Co, and As. Conversely, the pyrite was systematically enriched with all elements depleted from pyrrhotite, bearing five specific signatures of element enrichments: W, Tl, Sn, In-Cd-Zn, and Bi-Te-Au. For gold-rich zones (e.g., the HGZ), gold was linked to the Bi-Te-Au signature of pyrite, with Bi enrichment occurring at up to 72,000 times the background level in Archean shale pyrite. It was concluded that gold was transported, at least in part, as Bi-Te melts in the previously documented non-aqueous orogenic fluids, hence accounting for the very-high-grade gold content of the HGZ. Genetically, the metamorphism of primary gold-bearing volcanogenic mineralizations was the main source of gold during the overprinting of amphibolite (600 °C) in a metamorphically induced orogenic mineralizing event. A strong volcanogenic pre-enrichment is considered the main factor accounting for the gold endowment of the Eastern Gold Zone.

Paragenetic and geological setting of the Starra iron oxide copper–gold deposits, Mount Isa Inlier, Queensland, Australia: constraints on IOCG deposit models

The five Starra iron oxide copper–gold (IOCG) deposits, which are located in the Cloncurry district of the Mount Isa Inlier, NW Queensland, are hosted in a N-trending zone of deformed iron-rich metasedimentary rocks that are underlain by two mineralogically distinct mafic–intermediate meta-igneous rocks. Copper–Au mineralisation is subdivided into two stages—an early main stage that transitioned from a magnetite-dominated Cu substage comprising chalcopyrite, pyrite, biotite and chlorite, to a hematite–muscovite-bearing Cu–Au substage with chalcopyrite, bornite and rare chalcocite, as well as Au–Ag-tellurides and native Au. This second stage was followed by a subordinate but nonetheless important carbonate–quartz–barite vein stage with minor but locally high-grade Cu–Au mineralisation characterised by abundant chalcocite and bornite, minor native bismuth and wittichenite (Cu3BiS3), together with minor Au–Ag telluride minerals. Bornite in these carbonate–quartz–barite veins contains considerable amounts of Au (>10 ppm), which appears to be deported either as lattice substitution in the bornite crystal structure or as homogeneously distributed Au0 nanoparticles. An earlier, coarse-grained mafic intrusion was affected by early Na–Ca alteration, whereas a second mafic intrusion lacks evidence of Na–Ca alteration but was exposed to K–Fe alteration and subsequent mineralisation at Starra 222. Mushketovite (magnetite after specular hematite) is only observed in ironstone lodes proximal to the mafic–intermediate rocks. The close spatial and mineralogical associations of the mafic–intermediate bodies and the ironstone lodes point to a stronger link between magmatic activity and IOCG mineralisation than has previously been recognised at Starra. Remobilisation of gold by late-stage hydrothermal fluids aided by the relatively low fluid temperatures during the main pulse of Cu–Au mineralisation refined gold grades at Starra, making it a relatively gold-rich IOCG deposit. We show that the Cu–Au deposits of the Starra trend feature mineralogical and geochemical elements common to both Cloncurry IOCG deposits (e.g. abundant magnetite) and IOCG deposits of the Tennant Creek district (e.g. Au–Cu–Bi mineralisation; hematite–bornite–chalcocite assemblages), making them a unique and important member of this complex family of deposits.

Dislocation-mediated interfacial re-equilibration of pyrite: An alternative model to interface-coupled dissolution-reprecipitation and gold remobilisation

Minerals and/or their compositions (substituted minor elements) can become metastable in changing conditions or if formed outside of equilibrium. Unstable minerals undergo chemical and/or structural modifications at rates determined by re-equilibration processes, such as diffusion, coupled dissolution-reprecipitation and recrystallization. However, re-equilibrated domains with sharp contacts that lack porosity or deformation microstructures are difficult to reconcile with previously documented processes. In this study, we investigate the mechanism by which Au-rich pyrite re-equilibrates to Au-poor pyrite. Gold and As-rich {100} oscillatory bands are truncated by Au-As-poor pyrite along {100} re-equilibration interfaces. At the nanoscale, dislocations oriented consistently along <100>, are enriched in Ni, As, Cu, Sb, Pb, and Au. Dislocations are located at the re-equilibration interfaces between the Au-As-rich and Au-As-poor pyrite. Quantitative crystallographic orientation maps do not show the presence of deformation-related boundaries along the re-equilibration interfaces, indicating that the dislocations are not deformation-related but are misfit dislocations to accommodate for lattice stain between As-rich and As-poor pyrite. The co-location of steps along the re-equilibration interfaces and dislocations suggests that pyrite can re-equilibrate by the migration of dislocations. The process is likely driven by lattice strain minimisation induced by As impurities. Element transport is achieved by a two step process with (1) capture of impurities by dislocation-impurity pair diffusion during the migration of dislocations and (2) pipe diffusion along the dislocation network towards the exterior of the crystal. We propose that re-equilibration of Au-rich arsenian pyrite, and the resulting remobilisation of Au, can operate through a dislocation-mediated interfacial re-equilibration (DMIR) process. This new mechanism may be active in a range of mineral reactions, particularly in metamorphic settings where limited fluid availability precludes interface-coupled dissolution-reprecipitation processes.

Multistage evolution of gold mineralization in the Kibali gold district: Insights from pyrite analyses

The Kibali gold district, located on the Congo Craton, comprises a Neoarchean gold resource exceeding 28.7 Moz. The district is located within a westerly-verging stacked fold-and-thrust belt associated with a compressional tectonic regime active from ca. 2.64–2.60 Ga. Deposits are hosted by a major fault zone that consists of a complex series of smaller thrust faults, shear zones, and folds, which extend over 60 km. Gold deposits are primarily hosted in BIF and clastic sedimentary rocks. The gold and associated sulfides are in veins and disseminated in NE-to NNE-trending ore shoots within the deformed and metamorphosed volcanosedimentary rocks. Pyrite is the dominant sulfide mineral and hosts both solid solution gold and free gold. Because of its strong association with the gold mineralization, our study focusses on unravelling the metallogeny of the area by considering pyrite textural analyses, in situ trace element analyses using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), and S isotopic composition (δ34S) analyses using Secondary Ion Mass Spectrometry (SIMS).Eight distinct pyrite types were identified, including microcrystalline sooty (py-0a), small grains (<40 μm) (py-0b), aggregate recrystallized (py-1), core-rich inclusion (py-2), vein-like (py-3), non-zoned (py-4a), oscillatory As-bands zoned (py-4b), and cubic euhedral (py-5) pyrite. The ore-forming processes occurred over a polyphase evolution. Syn-diagenetic pyrite py-0a in volcaniclastic layers of older basin sediments served as the primary repository of Au (15.6 ppm median) with a near-zero δ34S value. Late diagenetic or early metamorphic pyrite (py-0b) was found in younger basin sediments. Py-1 resulted from solid-state recrystallization of py-0a and py-0b. During the early ore stage, porous py-2 with high Au concentrations (3.1 ppm) and various trace element enrichments precipitated due to fluid-rock interaction. Metal precipitation occurred through sulfidation and carbonation of wall rock with a relatively reduced fluid undergoing 34S-depletion. However, py-3 displayed notable trace element depletion and a complex mineral association. In a second ore stage, py-4a and zoned py-4b deposition occurred, dominated by an over-pressured fluid fracturing brittle rock. Py-4b, rich in Au (5.5 ppm) concentrated in oscillatory zoned As-rich bands, resulted from interactions during pulsed fluid flow. This process involved fluid phase separation, non-equilibrium conditions, 32S-depletion, and precipitation of py-4b with a high δ34S value (6.4 ‰ mean). Subsequently, pyrite (0–4) generations underwent deformation during a later ductile deformation event, leading to the remobilization and concentration of invisible gold into adjacent gold grains within the high-grade shoot plunging NE. This process involved shearing, boudinaging, and folding ore minerals in tightly folded structures. Late-stage py-5 formed under gold-poor fluid conditions or inefficient gold depositional conditions. Brittle fracturing of these grains later provided local sites for remobilized gold. The study contributes insights into the origin and evolution of gold mineralization in the Kibali gold district, supporting a multistage ore-forming fluid system.

The Snow Lake Deposits in Manitoba, Canada: Formation of Metamorphosed Amphibolite Facies Orogenic Gold Deposits During a Progressive and Prograde Orogenic Event

Abstract The Snow Lake camp is located in the ca. 1.89 Ga Flin Flon-Glennie Complex in the Paleoproterozoic Trans-Hudson orogen, Manitoba, Canada. The Flin Flon-Glennie Complex is bordered by the ca. 1.855 to 1.84 Ga metasedimentary Kisseynew domain to the north and by the Archean Superior craton to the east and is underlain by the Archean Sask microcraton. It hosts several orogenic gold deposits, including the No. 3 zone, Boundary zone, and New Britannia deposit (Toots, Dick, Hogg, Ruttan, and Mine East zones), which formed during thrusting of the Flin Flon-Glennie Complex and Kisseynew domain over the Sask microcraton. The New Britannia deposit produced 1.6 Moz Au and is the largest orogenic gold deposit in the Trans-Hudson orogen in Manitoba. The deposits consist of quartz ± carbonate veins surrounded by alteration zones of biotite, hornblende, plagioclase ± carbonate, diopside, orthoclase, and garnet. The veins and ore zones are folded within the hinge of the synthrusting Nor-Acme anticline, which has an axial plane cleavage defined by biotite and hornblende, but they also cut across the hinge of the anticline and contain foliated fragments of the wall rocks. These mutually overprinting relationships suggest that the veins and ore zones are synfolding. Garnet, diopside, and amphibole porphyroblasts grew during folding because they overgrow the foliation, which also wraps around them. Inclusions of gold and sulfide minerals within the porphyroblasts indicate that mineralization was emplaced early during folding at greenschist or lower amphibolite metamorphic conditions prior to the growth of the porphyroblasts at peak middle amphibolite metamorphic conditions. These observations are corroborated by laser ablation-inductively coupled plasma-mass spectrometry element maps of arsenopyrite grains. These maps reveal a primary internal enrichment of gold in arsenopyrite grains from the Toots, No. 3, and Boundary zones and a lack of primary lattice-bound gold in arsenopyrite grains of the Dick, Ruttan, and Mine East zones. The latter results from the deposition or remobilization of gold during a second hydrothermal event that occurred during shearing of these ore zones along a structure, the Howe Sound fault, which acted as a detachment surface during folding. The Snow Lake deposits are examples of orogenic gold deposits that formed early during a major thrusting and folding event and were later modified and metamorphosed at middle amphibolite facies conditions during the same progressive orogenic event.

The genesis of the Baguamiao orogenic gold deposit linked to multistage ore-forming processes in Triassic tectonic transition of South Qinling, China

The Baguamiao gold deposit represents one of the largest Triassic orogenic gold deposits in the South Qinling metallogenic belt of central China. Here we investigate Ar-Ar dating of sericite and muscovite, fluid inclusion of quartz, the trace element and sulfur isotopic composition of pyrite from different mineralization stage in the Baguamiao gold deposit to gain insights into the ore genesis. The Ar-Ar dating results yield 40Ar/39Ar plateau ages of 217.5 ± 2.1 Ma, 209.2 ± 0.2 Ma and 208.8 ± 0.2 Ma, indicating a two-stage gold mineralization during the tectonic transition from the early metamorphic deformation (at ca. 218 Ma) to the late extension (ca. 209 Ma) in the post-collision environment. Fluid inclusion, trace element and sulfur isotopic analyses reveal that the two stages gold mineralization show different sources. The early mineralization period developed CO2-rich inclusions with medium temperature (317–413 °C) and medium–low salinity (4.62–18.81 wt% NaCleqv.), and is characterized by formation of Py1 and Py2 with depleted Au, Cu, Pb, Zn, Ag, Te and Bi. The sulfur isotope data of Py1 and Py2 are between 3.6 ∼ 10.5 ‰, suggesting that the sulfur source was mainly derived from the surrounding rock (Xinghongpu Group). The late mineralization period developed medium–high temperature (321–453 °C), medium–high salinity (3.3–19.62 wt% NaCleqv.) CO2-rich inclusions and high-temperature and high-salinity daughter crystal inclusions, and is characterized by formation of Py3 and Py4 with obviously higher arsenic and metal contents, such as Au, As, Cu, Pb, Zn, Ag, Cr, Ti, Mn, Sb. The δ34S values of Py3 and Py4 are 8.3 ∼ 18.8 ‰, suggesting a mixed fluid of the Xinghongpu Group and Gudaoling Group with deep magmatic-hydrothermal fluids. We propose that the two periods of gold mineralization in the Baguamiao deposit, in response to the collision assembly of Qinling Orogenic Belt, preserve evidence for diverse metal sources with the early mineralization period sourced from the metamorphism of the upper crust (Xinghongpu Group) in the compressional stage (ca. 218 Ma), and the late mineralization period characterized by the contribution of the Xinghongpu Group, Gudaoling Group and deep magmatic-hydrothermal fluids in a post-collisional extensional setting (ca. 209 Ma). The role of metamorphism and magmatic activity during the Triassic Qinling orogenic processes is considered to be important in the remobilization, transport and enrichment of sulfur, arsenic and gold (and potentially other trace elements) from the earlier formed seafloor exhalation sequence.

In situ trace element and sulfur isotopic analysis of multi-stage pyrite in Mingsai gold deposit, southern Tibet: Insights into mineralization and gold remobilization

Pyrite is the main gold-bearing mineral and can provide information about the formation of gold deposits. This study focuses on the Mingsai gold deposit, a newly discovered deposit in the east of the Tethyan Himalayan Sequence. We analyzed the texture, composition, and formation process of pyrite in the deposit to understand how gold was formed. We identified five types of pyrite: diagenetic pyrite (Py0), pre-ore stage pyrite (Py1), gold-rich pyrite (Py2a and Py2b), and post-stage pyrite (Py3). All of these types contain varying amounts of “invisible” gold. Py0 is framboidal pyrite enriched in certain elements but depleted in gold. It has high δ34S values (mean 10.75 ‰). Py1 has a porous structure and contains silicate inclusions. It has low gold content and high Co/Ni ratios. The δ34S values of Py1 cluster around ∼ 0 ‰. Py2a is subhedral-euhedral pyrite found in the middle of disseminated pyrite and quartz-carbonate-sulfide veins. It has abundant gold (0.16 – 103 ppm) and relatively low δ34S (1.17 ‰ – 3.50 ‰). Py2b occurs as the rim of disseminated pyrite and sulfide inclusions at the interface with Py2a. It has lower gold content (0.90 – 9.85 ppm) and distinctly high δ34S (6.58 ‰ – 8.87 ‰) compared to Py2a. Py3 is vein-like pyrite in metamorphic tuff, which does not contain significant amounts of gold (Au < 1 ppm) and has higher δ34S values (9.86 ‰ – 11.69 ‰) than Py2b. The concentrations of metal elements, including gold, in Py2b are intermediate between those of Py2a and Py3. The researchers propose that Py2a is the main source of gold, and Py2b is a mixture of Py2a and Py3. During the mixing process, gold is released from Py2a and re-precipitates in Py2b through dissolution-reprecipitation reactions. The sulfur isotope composition suggests that the materials that formed Py2a (∼2‰) may have originated from magmatic-hydrothermal sources and undergone transformation. The detailed geochemical evidence indicates a close connection between the Mingsai gold deposit and other gold-antimony deposits in the Zhaxikang ore district, which are likely related to early Miocene magmatism in the Cuonadong dome.

Geochemical studies of the gold mineralization of Kodiarani-(Doko permit). Prefecture of Siguiri, Administrative region of Kankan, Republic of Guinea

West Africa is now a major player in world gold production with numerous gold deposits hosted in Birimien green belts (2.2-2.0 Ga). However, these belts and their geological formations remain unexplored, as does their evolution during the Eburnean orogeny. This is why, after acquiring its exploration Licence, TOROGOLD focused its work on Air Core drilling activities in the Doko-Kodiarani Licence in north-eastern Guinea. During these activities, geochemical studies will be carried out on rock samples taken from the six (6) boreholes pre-selected for this purpose. The aim of this research work is to carry out a geological study in order to determine the chemical associations between the mineralization, the nature of the host rock and others for future work. To achieve this objective, we used a number of methods, including a statistical approach, field reconnaissance, and macroscopic and lithogeochemical analyses. Geochemical studies reveal that the rocks studied are meta-sediments. They belong to the schist groups. Their protholites are moderately altered (60 &lt; CIA &lt; 80) and the source is thought to be andesitic and basaltic, emplaced in a geotectonic oceanic island arc. As for the various AC drilling sections, our observations reveal that the gold mineralization is found in two types of deposit: disseminated, present in all lithology’s, and vein-type, resulting from the filling of fracture zones, associated with the highest concentrations. We noted a strong discontinuity of mineralization in these veins, due to a tectonic phase that may have played an important role in the remobilization of gold in these structures.

A modified paleoplacer model for the metaconglomerate-hosted gold deposits at Jacobina, Brazil

Evidence is presented that the metaconglomerate-hosted Jacobina gold deposits in Brazil represent paleoplacers that became partly remobilized during later metamorphic overprint analogous to Witwatersrand-type deposits elsewhere, notably in South Africa. This includes strong lithological and sedimentological control on the gold, presence of detrital minerals with gold inclusions, and detrital gold particles. Detrital, synsedimentary, and post-depositional pyrite types can be differentiated. Whereas the first two types can be linked to gold accumulation, the latter was associated with gold dispersion. Synsedimentary pyrite has the highest Au content, from which elevated Au concentrations in Archean rivers can be inferred. The nature and extent of post-depositional alteration, mainly in the course of the Paleoproterozoic Orogeny, distinguishes the Jacobina deposits from other Witwatersrand-type gold deposits. Phase equilibria and Zr-in-rutile thermometry indicate peak metamorphic temperatures of ca. 600 °C. Both Mg-chlorite and Fe-chlorite formed in disequilibrium at approximately 280–340 °C during retrograde metamorphism. An igneous signature in the chemistry of some of the tourmaline and the remobilization of gold associated with Fe-oxides, near intrusive rocks, point at a local magmatic influence on the post-depositional mineralization stage. Whether magmatic hydrothermal fluids added Au to the system at that stage remains to be determined. Remobilization during regional metamorphism was insufficient to form substantial ore bodies but led to purification of the initially detrital gold particles that now contain relatively little Ag and Cu and lack Hg.

Gold remobilization of the Sanshandao gold deposit, Jiaodong Peninsula, Eastern China: Perspective from in-situ sulfide trace elements and sulfur isotopes

The Sanshandao is a typical altered-rock-style gold deposit (>260 t @ 3.96 g/t). The deposit is located at the northernmost end of the NE-trending Sanshandao fault in the northwestern Jiaodong Peninsula. The orebodies are mainly hosted by the footwall (minor in hanging-wall) of the fault, which cut Jurassic granites. According to the mineral assemblage and crosscutting relationship, five hydrothermal/mineralization stages were identified: (I) veinlet-disseminated quartz + pyrite ± chalcopyrite, (II) quartz + pyrite + chalcopyrite + native Au veining, (III) quartz + pyrite + galena + sphalerite + native Au + chalcopyrite veining, (IV-a) veinlet-disseminated siderite + pyrite + galena + sphalerite, (IV-b) ore-barren siderite, and (V) ore-barren calcite veining. Stages II and III auriferous pyrite contains visible native gold, and can be divided into four generations (Py2a, Py2b, Py3a, and Py3b). Py2a contains little native gold, whereas Py2b, Py3a, and Py3b contain much coarse-grained native gold. Gold in these pyrite generations correlates positively with the As content. Typical Py2a and Py3a porous/sieve-like texture indicates that Au remobilization has occurred, which may have controlled mainly by coupled dissolution-reprecipitation (CDR). Relative Sb-Te enrichments in Py2b and Py3a imply that a melt with low-melting point chalcophile elements (LMCEs) also facilitated Au remobilization by efficiently scavenging Au from preexisting minerals. Combined with the trace element compositions, we show that CDR may have liberated Au and LMCEs, and the subsequent fluid and LMCEs melt have altogether caused Au remobilization. The narrow δ34SV-CDT range of the pyrite from stages II-III (11.75–12.22‰ (Py2a), 10.83–12.39‰ (Py2b), 11.63–12.88‰ (Py3a), 11.91–12.67‰ (Py3b)), together with the compiled S-isotope dataset (from literature data) (δ34SV-CDT = mostly 7–10‰), indicate that reduction of marine sulfates represents the main sulfur source.

Mechanism and age of gold remobilization in orogenic gold deposit from primary laminated ore to the high-grade ore: A case study from Cenozoic Bangbu gold deposit, Tethyan Himalaya

The formation of high-grade gold ore has always been attractive, mostly located in Precambrian gold deposits with various mechanisms. Yet, in the orogenic Bangbu gold deposit, Tibet, we discovered that the coupled dissolution-reprecipitation (CDR) process and low melting point chalcophile elements (LMCE) -related melting are responsible for the formation of high-grade ore at ca. 38 Ma constrained by in-situ U-Th-Pb dating of hydrothermal monazite. Three generations of pyrite have been recognized, metamorphic barren pyrite (Py I) in the host schist, hydrothermal pyrite (Py II) in the preceding primary laminated ore with an Au grade of ∼8 g/t, and the hydrothermal coarse pyrite (Py III) in later massive ore with a grade of up to 29 g/t. In the primary laminated ore, the Py II in the sulfide ribbons is intergrown with native gold and colloform quartz and carries abundant trace element contents, such as Au, As, Cu, Pb, Zn, Te, Se, and Bi. These suggest that the rapid precipitation and local supersaturation occurred during the episodic crack-seal process. In the later high-grade massive ore, Py III exhibits distinct zonation, ranging from Py III-1 in the core to Py III-3 in the rim. The Py III-1, porous and rich in inclusions, has sharp boundaries and successive lattice orientation with the mantle Py III-2 suggesting a CDR process. Higher Au and As in Py III-2 than the Py III-1, and the close affinity between visible gold and LMCE melt inclusions at the boundary suggest that gold was remobilized and facilitated by the LMCE during the CDR process. The electron backscatter diffraction (EBSD) mapping result shows little crystallographic misorientations of Py III, excluding the deformation process that has significantly contributed to the remobilization of gold. The decrease of the δ34S ratio of pyrite between Py II (δ34S = 2.0‰ to 4.3‰, mean 3.4‰) and Py III (δ34S = −2.5‰ to 3.8‰, mean −0.2‰) most likely reveals a more oxidized condition in the open space which may trigger the CDR process and gold remobilization of the pyrite in the primary ore. Within the zonation of Py III, an increasing trend of the δ34S ratio represent the evolution of the redox state of fluids towards relatively reducing conditions during the healing of quartz veins. The newly obtained ca. 38 Ma age is comparable to regional amphibolite- to granulite-facies metamorphism during the continental subduction, which was considered to trigger the modification of primary gold orebody to form high-grade ore.

Mineralogical and sulfur isotopic characteristics of Archean greenstone belt‐hosted gold mineralization at the Tau deposit of the Mupane gold mine, Botswana

AbstractThe Mupane gold deposit, which is one of the numerous gold occurrences in the Tati Greenstone Belt in the northeastern part of Botswana, consists of four orebodies, namely Tau, Tawana, Kwena, and Tholo deposits. The present research, which focuses on the genesis of the Tau deposit, was based on ore petrography, mineral chemistry of sulfides, and sulfur isotope data. Mineralogical characteristics of the host rocks indicate that banded iron formation at the Tau deposit includes iron oxides (magnetite), carbonates (siderite and ankerite), silicates (chlorite and amphibole), and sulfides (arsenopyrite and pyrrhotite). The deposit features arsenopyrite‐rich zones associated with biotite‐chlorite veins, which are indicative of the precipitation of arsenopyrite concomitant with potassic alteration. The replacement of magnetite by pyrrhotite in some samples suggests that sulfidation was likely the dominant gold precipitation mechanism because it is considered to have destabilized gold‐thiocomplexes in the ore‐forming fluids. Based on textural relationships and chemical composition, arsenopyrite is interpreted to reflect two generations. Arsenopyrite 1 is possibly early in origin, sieve textured with abundant inclusions of pyrrhotite. Arsenopyrite 1 was then overgrown by late arsenopyrite 2 with no porous textures and rare inclusions of pyrrhotite. Gold mineralization was initiated by focused fluid flow and sulfidation of the oxide facies banded iron formation, leading to an epigenetic gold mineralization. The mineralogical assemblages, textures, and mineral chemistry data at the Tau gold deposit revealed two‐stage gold mineralizations commencing with the deposition of invisible gold in arsenopyrite 1 followed by the later formation of native gold during hydrothermal alteration and post‐depositional recrystallization of arsenopyrite 1. Laser ablation inductively coupled plasma mass spectrometric analysis of arsenopyrite from the Tau deposit revealed that the hydrothermal event responsible for the formation of late native gold also affected the distribution of other trace elements within the grains as evidenced by varying trace elements contents in arsenopyrite 1 and arsenopyrite 2. The range of δ34S of gold‐bearing assemblages from the Tau deposit is restricted from +1.6 to +3.9‰, which is typical of Archean orogenic gold deposits and indicates that overall reduced hydrothermal conditions prevailed during the gold mineralization process at the Tau deposit. The results from this study suggest that gold mineralization involved multi‐processes such as sulfidation, metamorphism, deformation, hydrothermal alteration, and gold remobilization.

Textures, trace elements, sulfur, lead and He-Ar isotope analyses of pyrite: Implications for ore-forming processes and the origin of the Shenjiayao gold deposit, southern margin of the North China Craton

The Shenjiayao gold deposit (8.50 t Au, average grade of 2.21 g/t), hosted in a Precambrian metamorphic terrane, is located in the Xiaoshan region on the southern margin of the North China Craton. The gold mineralization is characterized by vein ores comprising native gold and gold-bearing pyrite. Paragenesis of the Shenjiayao deposit can be divided into four stages. Stage I is pyrite – milky quartz veins; Stage II consists of grey quartz – pyrite veins; Stage III is characterized by quartz – polymetallic sulfide veins; and Stage IV minor pyrite – carbonate – quartz veins. Native gold and electrum grains are mainly distributed in the fractures of quartz, pyrite, chalcopyrite, and other sulfides. In this study, three types of pyrite were distinguished based on textural and geochemical results. Py1 has silicate inclusions and mainly occurs as euhedral–subhedral grains formed in the Stage I. Py2 within quartz veins from Stage II shows oscillatory zoning and can be distinguished into two subtypes by their distinct textures (Py2a and Py2b). Py3 coexisting with native gold, chalcopyrite, sphalerite, and galena is characterized by sulfide and silicate inclusions and is divided into two subtypes (Py3a and Py3b). The porosity and heterogeneous textures of Au-bearing pyrite indicate a process of dissolution and reprecipitation. The replacement was likely the result of dissolution–reprecipitation. However, Py1 has very low lattice-bound gold contents, indicating that obtaining high-grade gold orebodies through remobilization of lattice-bound gold is unreasonable. We suggest that the deposition of abundant native gold was induced by a new volume of Au-rich ore-forming fluids and triggered by extensive fluid phase separation during Stage II and Stage III. The δ34S values for the sulfides are similar, ranging from +0.9 ‰ to +5.8 ‰, and the lead isotope compositions of Stage II and Stage III sulfides are consistent. Fluid inclusions extracted from gold-bearing pyrite have elevated 3He/4He ratios (0.18–1.13 Ra) and 40Ar/36Ar (329.5–848.5), indicating the presence of mantle-derived fluids in the ore system.

Gold and aurostibite from the metaturbidite-hosted Au–Zn–Pb–Ag Hera deposit, southern Cobar Basin, central NSW, Australia: geochemical and textural evidence for gold remobilisation

AbstractThe Devonian Hera metaturbidite-hosted polymetallic Au–Zn–Pb–Ag deposit of central NSW, Australia, contained a total undepleted resource of 3.6 Mt @ 3.3 g/t Au, 25 g/t Ag, 2.6% Pb and 3.8% Zn. The deposit comprises a number of distinctive lodes with each containing a distinctive ore and alteration/gangue mineralogy, though generally the sulfide ore comprises various mixtures of sphalerite, galena, chalcopyrite, pyrrhotite and relatively common visible gold–electrum. The North Pod and Far West lodes are distinctly Sb rich and contain a more diverse ore mineralogy with arsenopyrite, native silver, native antimony, gudmundite, tetrahedrite-(Fe), argentotetrahedrite-(Fe), acanthite, dyscrasite, nisbite and breithauptite. From analysis of 52,760 assays from across the deposit it was found that there was a very poor correlation between gold and each of Fe, Zn, S, Pb, Cu, As and Ag, whereas Ag correlated reasonably well with both Pb and Zn. Results from EPMA shows that gold varies widely in composition from host-rock associated gold (96 wt.% Au) through more intermediate compositions (88–73 wt.% Au) to electrum (46–27 wt.% Au), commonly associated with Sb-phases and containing significant Sb within the gold itself (1.05–2.58 wt.% Sb). From the Far West lense, aurostibite occurs as distinctive rims around gold. Although aurostibite associated with gold contains no silver, the gold itself contains constant moderate amounts (10.87–12.27 wt.% Ag). We suggest that the aurostibite and other Sb phases formed from a late-stage Sb-rich hydrothermal during low-temperature retrograde skarn alteration. There is abundant evidence for both chemical and physical remobilisation at Hera and this remobilisation is largely responsible for the spectrum of gold compositions observed. The source for these fluids may be an underlying magmatic body, evidence for which occurs as granite pegmatite dykes in various locations throughout the deposit. Furthermore, gold with a moderate to high Sb content may be indicative of a low temperature of formation.

El Álamo district (Baja California, México): A hint of a new Cordilleran orogenic gold belt?

The El Álamo is one of the most important historic gold mining districts in the Peninsular Ranges batholith. Gold mineralizations at El Álamo consist in fault-fill laminated quartz veins with small amounts of sulfides and oxides, and lesser amounts of gold. Veins are mainly emplaced along compressional brittle-plastic shear zones within the El Álamo quartz diorite (146.6 ± 1.6 Ma, U–Pb zircon), which is intruded by the San Marcos dike swarm (126.8 ± 1.7 Ma, U–Pb zircon). Several lines of evidence indicate that the dike swarm is older than quartz veins, and the formation of the later is related to the Peninsular Ranges orogeny (∼110−98 Ma). Gold bearing quartz veins formed during regional prograde metamorphism (greenschist facies), and the veins subsequently underwent plastic deformation, dynamic recrystallization and metamorphism. Gold remobilization and concentration seems to have been largely enhanced by metamorphism, probably accounting for very high grade, albeit sporadic, bonanza ores, typical of the district. 40Ar/39Ar dating of hydrothermal white micas from the auriferous veins yielded ages of 97.86 ± 0.17 Ma at the Ulises mine and 97.46 ± 0.17 Ma at the La Americana mine. These ages are interpreted as the minimum age of the regional metamorphism in the area. Based on their primary characteristics, the gold deposits from El Álamo district can be ascribed to the orogenic gold type. These deposits are similar to other known deposits in the same orogenic context (e.g., Julian district; southern California), and they probably conform a new Cordilleran orogenic gold belt that could extend over much of the exhumed axis of the Peninsular Ranges batholith.

Micro-fractures in the Ngubevu Gold and Base Metal Mines (Natal Thrust Front, Tugela Terrane, South Africa): Are they consistent with macro-scale deformation, and what do they tell us?

AbstractIn the Natal Thrust Front (NTF) of the Tugela Terrane, there is a lack of multi-scale characterisation of structural controls on shear-zone hosted precious and base metal mineralisation. This paper contributes to the current understanding of the structural controls on shear-zone-hosted precious and base metal mineralisation in the NTF with insight derived from the analyses of micro-fractures at and around the known shear-zone-hosted lode gold and base metal mineralisation in the Ngubevu area of the NTF. Based on the notion that the spatial distribution of stresses that control geological structures is fractal (i.e., possessing scale-invariance or self-similarity properties) across a range of geographic scales, this study (a) determines whether the stresses inferred from micro-fractures in the gold mines in Ngubevu are consistent with the stresses associated with regional- to local-scale geological structures in the NTF, and (b) interprets the mineralisation controls implied by these inferred and known stresses. From rose diagrams showing trends of micro-fractures in oriented vein material samples from Ngubevu, the relative micro-scale stress field per sample based on the maximum horizontal stress (SHmax) and the minimum horizontal stress (SHmin) were inferred. These were then compared to the local- to regional-scale stress field orientations derived from existing studies in the NTF.The north-northeast–south-southwest orientations of SHmax of micro-fractures in samples from Ngubevu are consistent with the north-northwest–south-southeast orientations of SHmax of regional- to district-scale thrust faults in the NTF of the Tugela Terrane. Likewise, the west-northwest–east-southeast orientations of micro-fractures in samples from the eastern part of Ngubevu are consistent with the general east–west orientations of local-scale S1 foliation. These orientations are associated with a D1 thrusting event, which was due to north to north-northeast-directed regional thrust tectonics. The clockwise change from the north-northwest–south-southeast orientations of SHmax of regional- to district-scale thrust faults to the north-northeast–south-southwest orientations of SHmax of micro-fractures in vein material attests to an earlier posited notion of clockwise rotation of regional- to district-scale thrust vergence. These suggest that V1 veins in Ngubevu were likely formed late in the D1 event. The orientations of micro-fractures in samples, mostly from the western part of Ngubevu, depict a strike-slip regime, which is consistent with D2 sinistral transpression that affected the earlier-formed east–west-trending structures. This suggests that some V1 veins in Ngubevu, particularly those in its western part, were affected by the D2 sinistral transpression (e.g., by remobilisation of gold in some of the V1 veins). The novel findings in this study are crucial to geological mapping, recognition of mineralisation controls and mineral prospecting in poorly-explored regions where the geology is permissive (i.e., represents processes that are favourable) for the formation of certain type(s) of mineralisation.

Mineral Paragenesis, formation stages and trace elements in sulfides of the Olympiada gold deposit (Yenisei Ridge, Russia)

The mineral sequence and LA-ICP-MS study of the trace-elements distribution in chalcogenides (arsenopyrite, pyrite, pyrrhotite, sphalerite, chalcopyrite, ullmannite, tetrahedrite, berthierite, stibnite, gudmundite, jamesonite) of the Olympiada gold ore deposit (Yenisei Ridge, Russia) are presented. The deposit was formed in three stages, separated by tectonic breaks. Early (I) corresponds to the paragenesis of acicular arsenopyrite + pyrite + pyrrhotite. At the stage of base-metal sulfides (II), the deposition of the bulk of chalcopyrite, sphalerite and galena occurred. Mineral associations of these stages determined the gold-arsenic (Au-As) type of ores. The stage of late sulfides (III) is characterized by the paragenesis of antimony minerals (stibnite + berthierite + gudmundite) and corresponds to the gold-antimony (Au-Sb) ore type. Commercial concentrations of gold were formed at the early sulfides stage in the lattice bonded and nanosize mode in acicular arsenopyrite (12.5–1,512 ppm). The progress of the ore deposition process with the formation of late polymetallic and stibnite-berthierite mineralization led to recrystallization of early sulfides (prismatic arsenopyrite Au ∼ 36.4 ppm → dipyramidal arsenopyrite Au ∼ 0.5 ppm), remobilization and re-deposition of gold in a native form. Pyrite of the deposit does not contain significant amounts of gold (∼0.4 ppm). Silver is absent in early sulfides, but is concentrated in tetrahedrite (348.0–3,811 ppm), jamesonite (0.1–7.7 ppm), berthierite (0.1–2.3 ppm) and stibnite (0.2–2.0 ppm) of the later stage (III). Early sulfides (stage I) are characterized by the As-Au-W-Se-Te geochemical association; polymetallic sulfides (stage II) — by the Cu-Zn-Pb-Cd-In association; and late sulfides (stage III) — by the Sb-Co-Ni-Te-Bi-Pb-Hg-Ag association.

Deciphering multiple ore-forming processes of the Shuangqishan orogenic gold deposit, Southeast China by in situ analysis of pyrite

Gold deposits are characteristically formed via multiple mineralization and remobilization events. Inferences on the complex formation history of such deposits from ore textures can be ambiguous in the lack of chemical and isotopic evidence as exemplified by the Shuangqishan orogenic gold deposit in Southeast China. We, therefore, conducted LA-ICP-MS spot analysis coupled with LA-MC-ICP-MS sulfur and lead isotope analysis on texturally complex gold-bearing pyrite from the Shuangqishan deposit. Gold distribution in different syn- to late-ore pyrite generations indicate initial evenly deposition of nano- to micro-inclusions of Au-Ag-Pb-Bi tellurides within pyrite grains and subsequent Au remobilization following brittle deformation and fluid-mediated coupled dissolution-reprecipitation (CDR) reactions. Gold dissolved from the pyrite during the reworking event became re-concentrated and re-precipitated as visible gold either as inclusions in pyrite or larger free gold grains within microfractures of pyrite. The distribution of Bi-Te-Pb-Ag assemblage is comparable to that of Au, illustrating that these trace metals became redistributed in a similar fashion. Early, undeformed pyrite has the least radiogenic Pb isotopic compositions. Late, porous and inclusion-rich pyrite, brecciated pyrite, and recrystallized Te-Bi minerals have variable but in general more radiogenic Pb isotope ratios (higher 206Pb/204Pb and 207Pb/204Pb ratios), which are attributed to modification and precipitation from a more evolved crustal fluid during the re-precipitation of native gold. The Pb isotopic signature of the younger pyrite generations was likely inherited from Pb released during modification of earlier proto-pyrite and a second, crustal lead source that was involved in the hydrothermal system during later remobilization. The δ34S values for the early proto-pyrite and the reworked pyrite generations within the individual vein stage are broadly similar. The variations in sulfur isotope compositions reflect the change in physicochemical conditions (temperature and oxygen fugacity) during proto-ore formation. Our study highlights the importance of metal recycling from early proto-auriferous pyrite during later reworking and gold remobilization, which led to visible gold and high-grade ore shoot formation. The extensive variations of trace element concentrations and S-Pb isotope compositions of proto and reworked pyrites record the complicated Au mobilization and deposition processes and help to understand how gold ores evolved.

In situ trace elements and sulfur isotopes of sulfides in the Dabaiyang Te-Au deposit, Hebei Province, China: Implications for Au remobilization from pyrite

Coupled dissolution–reprecipitation (CDR) reactions and low-melting point chalcophile elements (LMCE) melt scavenging are effective mechanisms for gold remobilization and are important for the formation of Te-Au deposits. The Dabaiyang deposit is a typical Te-Au deposit in the Zhangjiakou district and contains tellurides, including altaite, hessite, petzite, tetradymite and tellurobismuthite. Two generations of pyrite have been recognized in the deposit: early pyrite without fractures or alteration (Py1) and later pyrite with more porosity, inclusions and fractures (Py2), and Py2 could be separated into Py2a that containing many fractures without mineral inclusions and Py2b that containing microfractures, micropores and mineral inclusions. Pyrite containing large amounts of mineral inclusions and free of microfractures (Py-MI) has also been recognized, but timing relationships between Py-MI and Py1 (or Py2) is hard to establish. The Pyrite textures indicate that Py2a underwent brittle deformation and that Py2b underwent CDR reactions and Te-melt scavenging. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses show that Py2a contains trace elements similar to Py1, while Py2b contains higher Au, Ag, Te and other trace elements, indicating that brittle fracturing has no effect on element mobilization and reprecipitation, and porous pyrite has Au-Ag-Te concentrations. High Co/Ni ratios and Te contents and low Se contents show that the Dabaiyang deposit has a genetic relationship with magmatic activity. In situ δ34S values of sulfides range from −15.77‰ to −6.09‰ (mostly from −13.89 to −10.06‰), which is consistent with bulk sulfide δ34S values. The negative δ34S values more likely resulted from sulfur isotope fractionation in high fO2 hydrothermal fluids. CDR reactions liberated Au, Te, Ag and other trace elements from pyrite and formed porous textures. Then, Te-rich melts scavenged Au from fluid and wallrocks during migration. Finally, the melt precipitated as mineral inclusions in pores or fractures in pyrite. CDR reactions and LMCE melt scavenging played an important role in upgrading the ores and the occurrence of visible native gold in the Dabaiyang gold deposit, as well as other Te-Au deposits in the Zhangjiakou district.

Genesis of visible gold in pyrite in the Zhaoxian gold deposit, Jiaodong gold province, China: Constraints from EBSD micro-structural and LA-ICP-MS elemental analyses

Pyrite (FeS2) is one of the most common hosts for gold (Au) in different types of gold deposits, including visible and invisible gold. For visible gold in pyrite, it remains unclear whether it resulted from remobilization of invisible gold in the gold-hosting pyrite or from external input. This paper reports a case study from the Zhaoxian gold deposit, which is the deep extension of the well-known Jiaojia gold deposit, in the Jiaodong gold province. The texture and composition of Au-bearing pyrite from the Zhaoxian deposit were studied with petrographic, Electron Back-Scattered Diffraction (EBSD) mapping, and Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) methods. Four types of pyrite (Py-1 to Py-4), corresponding to four generations of veins, i.e., the pre-ore V1, syn-ore V2 and V3, and post-ore V4, were distinguished. Py-2 is volumetrically the most abundant among the four, and is the main host of visible gold. LA-ICP-MS analyses of pyrite indicate that gold contents in most analysis points are below the detection limit, with measured Au contents ranging from 0.02 to 0.17 ppm (median 0.09 ppm) in Py-1, and 0.03 to 2.45 ppm (median 0.07 ppm) in Py-2. EBSD analyses indicate that pyrite (Py-2) display brittle deformation features including microstructures, lattice dislocation, and grain boundary displacement. Visible gold in Py-2 occurs in micro-fractures, cavities, and grain boundaries. The generally low contents of Au in pyrite, including Py-2, and the brittle deformation features, suggest that the visible gold grains did not result from remobilization of invisible gold from pyrite. Rather, the majority of gold was introduced by fluids after the formation of the gold-hosting pyrite. These fluids were derived from the same sources as those precipitating the gold-hosting pyrite, but were more evolved and enriched in Au. Circulation of these auriferous ore-forming fluids in microfractures of pyrite and interstitial space resulted in precipitation of visible gold.