Gold Enrichment Research Articles

Gold enrichment characteristics and exploration prospects in Zambia: Based on 1 ∶ 1000000 geochemical mapping

The 1∶1000000 geochemical mapping of Zambia provides catchment sediment geochemical data for 58 elements including Au from 746 sediment samples at 736 sampling sites, corresponding to a sampling density of about one site per 1000 km2. Under strict quality control using field duplicates, certified reference materials, and analytical replicate samples, the Au was determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The detection limit of Au was 0.20 × 10−9. The 95% range (2.5%–97.5%) of Au concentrations was from 0.24 × 10−9 to 1.36 × 10−9, and the median value was 0.40 × 10−9. The most noticeable Au distribution patterns shown on the map are mainly located between Lusaka and Ndola (Lufilian Arc Belt). In addition, several high Au value areas occurred in Mansa, Muyombe, Chipata, and Livingstone. The spatial distribution patterns of Au in tectonic units, drainage basins, and geomorphological landscapes could be related to the Lufilian Arc Belt and Bangweulu Block. The Au concentrations show metallogenic belts between Muyombe and Mbala areas, between Mansa and Ndola areas, and between Lusaka and Kasempa areas.© 2022 China Geology Editorial Office.

Gossan mineralogy, textures, and gold enrichment over the Au (As, Bi, Ag) deposit in the Buracão Area (Brasília Fold Belt, Brazil): Implications for gold prospecting in weathering profiles

The orogenic pyrite–rich Au (As, Bi, Ag) mineralization (up to 31 g/t) of the Buracão area is one of several orogenic deposits in the Brasilia Fold Belt (BFB; Central Brazil), exposed to tropical weathering that shows extensive oxidized profiles. The distribution and thickness of these gossan profiles lead to variable gold enrichments and uncertainties to gold prospecting in this region. In order to investigate physicochemical processes related to the evolution of the gossan cover and the supergene gold enrichment, we present mineralogical, textural and geochemical data from each zone of twenty gossan profiles in the Buracão area. These results show that the mineralogy and textures along the gossan profile reflect oxidation, hydrolysis, dissolution and precipitation processes controlled by different Eh–pH conditions, which are controlled by supergene conditions and hypogene ore composition. The geometry, distribution, and thickness of gossan profiles, as well as the style of gold occurrence, reflects textures of the hypogene orebodies, while the weathering has an important role in the gold chemical refining (i.e., increasing ca. 25% in gold fineness) and the distribution of gold grades along the profile.These points allowed us to identify that the leached spongy hematite–rich zone in the upper indigenous profile contains the higher fineness gold grades (i.e., the “bonanza level”), and Bi–minerals such as waylandite and bismutite that may be important pathfinders for high gold grades in these gossan profiles. Our results demonstrate that ore prospection in gossans over Au–As–Bi–Ag deposits should be done focusing on leached zones, different to that traditionally conducted in gossans related to base metal sulfide deposits that have focused on precipitation zones. The similarities between the hypogene mineralization of the Buracão area and many deposits in the BFB and worldwide encourage the application of this approach elsewhere, which improve gold prospecting in weathering profiles over orogenic deposits.

Lingbaoite, AgTe3, a new silver telluride from the Xiaoqinling gold district, central China

Abstract Lingbaoite, AgTe3, is a new silver telluride discovered in the S60 gold-bearing quartz vein, Xiaoqinling gold district, central China. The new mineral is named after Lingbao city, the municipality of which covers a major part of the Xiaoqinling gold district. Lingbaoite is only microscopically visible and occurs within pyrite as small composite inclusions (<50 μm) that commonly consist of lingbaoite, sylvanite, and chalcopyrite, and locally of bornite, galena, altaite, and stützite. The largest lingbaoite grain is about 30 × 12 μm in size. At least two stages of gold and telluride mineralization are recognized in the lingbaoite-bearing sample set. The first stage is characterized by the deposition of lingbaoite, native tellurium, and sylvanite, within the commonly observed mineral assemblages of lingbaoite + sylvanite + chalcopyrite and sylvanite + native tellurium + stützite. The second stage is characterized by the deposition of Bi-bearing minerals and native gold, within the commonly observed mineral assemblages of rucklidgeite + altaite + volynskite ± hessite ± petzite and rucklidgeite + gold ± altaite. Lingbaoite is opaque and exhibits no internal reflections. In plane-polarized reflected light, lingbaoite shows a creamy yellow reflection color. The calculated density is 7.06 g/cm3. Seventeen WDS spot analyses from 17 different lingbaoite grains gave an empirical formula of Ag0.946Fe0.134Cu0.008Pb0.003 Te2.841S0.067. When considering Ag and Te as the only two essential structural components, the empirical formula is Ag1.00Te3.00. The EBSD and SAED data confirm the structural identity of lingbaoite and synthetic AgTe3. Synthetic AgTe3 is trigonal, space group R3m, with a = 8.645 Å, c = 5.272 Å, V = 341.2 Å3, and Z = 3. The unit-cell parameters of lingbaoite are: a = 8.60 (5) Å, c = 5.40 (18) Å, V = 346 (9) Å3, and Z = 3. Synthetic AgTe3, and by analog lingbaoite, can be viewed as silver-stabilized cubic tellurium, which is an ordered (1:3 Ag:Te) analog of the α-polonium structure (i.e., simple cubic crystal structure). Synthetic AgTe3 becomes a stable phase at above 0.4 GPa, but can also occur in a metastable state at atmospheric pressure. Lingbaoite probably formed through the cooling of polymetallic melt droplets within the hydrothermal system. Lingbaoite and associated minerals (e.g., sylvanite, native tellurium) reveal a previously unrecognized but perhaps common magmatic-hydrothermal process in the Xiaoqinling gold district, which precedes the precipitation of native gold, suggesting that gold mineralization in the Xiaoqinling gold district involves multiple superimposed processes of gold enrichment.

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.

The efficacy of a new iodination roasting technology to recover gold and silver from refractory gold tailing

Gold tailing is residue waste, but it’s also a secondary resource. Thus far, little is known about the iodination roasting and it was first used to dispose gold tailing. Here, we show that iodination roasting can efficient recovery of gold and silver from the refractory gold tailing with higher recovery efficiency than chlorination roasting. Different factors affecting the recovery effectiveness of gold and silver were investigated to obtain the maximum gold and silver recovery efficiencies. We achieved recovery efficiencies of 99.92% and 87.78% for gold and silver by the iodination roasting, and thus can be applied to treat and recover various kinds of gold and silver residues from the solid phase. We also provide roasting principles of the iodination roasting process using experimental design method by changing the contact between Iodine Donor (KI) and gold tailing, and found that Iodine Donor decomposes to iodine gas and the iodine gas reacts with the Au and Ag, which is similar to the reported mechanism of chlorination roasting. Moreover, the behavior results of other elements during iodination roasting confirm that it could selective enrichment of gold and silver from gold tailing.

Gold in the lithosphere of the western South China Block, SW China: Insights from quartz porphyries from the giant Zhenyuan gold deposit

Numerous Au-rich deposits in the western South China Block, SW China, provide insights into the gold enrichment process at a lithospheric scale. Zhenyuan is a giant gold deposit in the western South China Block, but its genesis remains controversial with competing hypotheses of magmatic–hydrothermal and metamorphic–hydrothermal development. Here, we present new petrographic, major and trace element, and zircon U–Pb age data for quartz porphyry from the Zhenyuan deposit to evaluate the specific gold enrichment process. Zircon SHRIMP and LA–ICP–MS results demonstrate that the Zhenyuan quartz porphyries crystallized at ca. 255–247 Ma. This age is broadly consistent with the Re–Os age of gold-bearing pyrite measured in previous studies. The porphyries are characterized by high-K calc-alkaline compositions and exhibit significant enrichment in LILEs and LREEs, and strong depletion in HFSEs. They share the same source (Au-rich lower crust) with Cenozoic Au-bearing intrusions. The initial melts had a weak oxidation state (ΔFMQ = –0.15–0.55), which favored Au accumulation and migration. However, the gold was precipitated with the crystallization of Ti–Fe oxides in the middle crust, resulting in the emplacement of Au-poor magma in the upper crust. The gold orebodies in the Zhenyuan deposit formed due to Cenozoic hydrothermal processes, as evidenced by our observations of ore-bearing pyrite and Ar–Ar ages of phlogopite measured in previous studies. Combined with regional mineralization data, we propose lithosphere-scale processes for gold accumulation, migration, and precipitation from the Neoproterozoic to Cenozoic. Specifically, we predict an Au-rich middle crust beneath the western South China Block.

Fluid signature of the shear zone–controlled Veio de Quartzo ore body in the world-class BIF-hosted Cuiabá gold deposit, Archaean Rio das Velhas greenstone belt, Brazil: a fluid inclusion study

The world-class Cuiaba gold deposit of the Archaean Rio das Velhas greenstone belt in Brazil is hosted in banded iron formation containing carbonaceous matter and carbonate, within the reclined, isoclinal Cuiaba fold. Mineralised quartz veins are hosted in andesite in the stratigraphic footwall of the banded iron ores and form some of the more recently discovered ore bodies. Fluid inclusion data of the quartz vein–associated “Veio de Quartzo” ore body are obtained from four quartz types (Qz1, Qz2, Qz3, Qz5) in gold-mineralised V1 shear vein and V2 extensional veins, barren V3 extensional vein array, and V4 breccia-style veins, all developed during the Archaean D1 event. Three fluid types are distinguished: (i) aqueous fluids of low salinity (1.8–3.8 wt% NaCl equiv), homogenisation (into liquid) at 220 to 230 °C; (ii) aqueous fluids of moderate salinity (5.3–12.7 wt% NaCl equiv), and homogenisation at 250 to 290 °C; and (iii) aqueous-carbonic fluids of moderate salinity (6.0–15.1 wt% NaCl equiv), with 30–91 mol% CO2, 8–41 mol% CH4 and up to 28 mol% N2 and decrepitation (into vapour) at 280 to 310 °C. Based on an independent pressure estimate, a pressure correction was applied to aqueous fluid inclusions, resulting in minimum trapping temperatures at 360 °C for V1 veins, 330 °C for V2 veins, 300 °C for V3 veins and 270 °C for the late-stage V4 veins. Ion chromatography analyses reveal a Br/Cl ratio of 0.7 × 10−3 in Qz1-V1, from 1.4 to 1.5 × 10−3 in Qz2-V2, 0.3 to 0.4 × 10−3 in Qz3-V3 and 0.7 to 0.9 × 10−3in Qz5-V4 veins. Zinc, Pb and Cu are relatively enriched with ~ 100 to 1000 ppm in aqueous and aqueous-carbonic fluid inclusion assemblages in all vein and quartz types, which is similar to other orogenic gold deposits hosted in the Rio das Velhas greenstone belt. The fluid inclusion data are consistent with a model invoking a metamorphic origin for the mineralising fluid. A two-step model of hydrothermal fluid flow and gold enrichment is suggested to have developed during the Archaean D1 event, with an early, aqueous-carbonic fluid pulse of relatively high temperature (from V1 up to V3) and an evolved, aqueous-carbonic fluid pulse of lower temperature (V4, breccia-style veins). The Rio das Velhas greenstone belt is dominated by regionally metamorphosed metasedimentary rocks, resulting in a complex hydrothermal fluid evolution and related gold mineralisation such as the shear zone–controlled Veio de Quartzo ore body.

Current issues of search of gold placers in the south of Ukraine

The main gold ore territory of Ukraine is Ukrainian shield, mainly its central and eastern parts. Currently, the potential of Ukrainian shield is estimated at 2.4 thousand tons of gold. Gold ore deposits are insufficiently studied and not brought to development. The same can be said about the development of gold placers. But in the case of shallow deposits its study and development may be economically justified, it does not require large capital investments and allows to recoup costs in a short time. This is especially true given the extremely favorable situation on the world gold market in recent years. Erosion and denudation of native deposits certainly led to the formation of deposits of native placer gold in the sedimentary cover on the territory of the Ukrainian shield and its slopes. Significant amounts of geological information on the widespread distribution of placer gold in loose sediments, in the modern basins of the Dnieper, Southern Bug, Don, Black and Azov Sea shelf, have been accumulated. This corresponds to the location of indigenous sources of gold and the general direction of unloading of river systems. Previous geological studies in this area have revealed numerous manifestations of placer gold of various scales, but no industrial facilities have been recorded. Placer gold in the south of Ukraine is characterized by a high mass fraction of gold of small and thin size classes and significantly aleurite-clay composition of gold-bearing deposits. Such characteristics complicate the geological search and real assessment of gold-bearing objects and, provided that standard sampling and processing methods are used in geological works, lead to underestimation of the real gold content. Studies conducted on the most studied placer sites convincingly show that gold is lost when washing samples, even under careful control. And this is a typical situation for many objects (not only Ukrainian), which were studied using the standard method of extracting free gold by gravity. That is why there are reasonable doubts about the reliability and quality of the current assessment of placer gold deposits. In our opinion, the prospects for the search for loose gold on the Ukrainian shield and its slopes are significantly underestimated. The presence of indigenous sources with great resource potential and at the same time numerous known manifestations of different ages and scales of placer gold clearly indicate the possibility of the formation of placers with industrial characteristics in the sedimentary stratum of southern Ukraine. The use of modern technologies for the gold enrichment and extraction, high-precision analytical research and the favorable state of the world gold market force to urgently review the results of previous geological work. The problem of conducting new comprehensive research aimed at auditing the accumulated data, identifying, and studying of placer deposits namely with gold of small and thin size classes is becoming urgent. The result of such work should be the discovery and industrial development of gold placers, which can be the basis of gold production in Ukraine.

Gold enrichment in hydrothermal sulfides from the Okinawa Trough: An in situ LA-ICP-MS study

We assess the distribution of gold and investigate the physicochemical controls on gold enrichment in the Okinawa Trough, an immature back-arc basin. We conducted a systematic study of gold and other trace-element concentrations using in situ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) for various minerals in hydrothermal sulfide samples from the Iheya North Knoll and the Yonaguni Knoll IV hydrothermal fields. Orpiment, colloform pyrite, and colloform sphalerite are relatively enriched in Au (mean contents of 30.60, 17.48, and 8.68 ppm, respectively) compared with gold-poor massive chalcopyrite, sphalerite, and pyrite. Based on gold budget estimation, colloform pyrite is the most important Au host, followed by colloform sphalerite and orpiment, with these three minerals containing the majority of gold in hydrothermal sulfides. Gold occurs as “invisible” in host minerals: as lattice-bound and submicron inclusions in colloform pyrite, as homogeneously distributed lattice-bound or nano-inclusions in colloform sphalerite, and generally as lattice-bound in orpiment and tetrahedrite. Three sets of gold host minerals are identified with distinct ore-formation temperatures. Colloform sulfides and orpiment incorporate the majority of gold at low temperatures, whereas tetrahedrite and massive sulfides incorporate only minor or trace amounts of gold at medium and high temperatures, respectively. Our results show that gold enrichment in Si–Ba–Fe-rich hydrothermal sulfides at low temperatures is a significant contributor to gold enrichment in hydrothermal sulfide deposits in the Okinawa Trough, along with the more common Zn- and Cu-rich hydrothermal sulfides. Thermodynamic modeling calculations suggest that the near-neutral pH of hydrothermal fluids and the dominant transport species, namely Au–HS complexes rather than Au–Cl complexes, are likely responsible for gold enrichment in low-temperature mineral assemblages in hydrothermal precipitates from the Okinawa Trough.

Metasomatized lithospheric mantle for Mesozoic giant gold deposits in the North China craton

Abstract The origin of giant lode gold deposits of Mesozoic age in the North China craton (NCC) is enigmatic because high-grade metamorphic ancient crust would be highly depleted in gold. Instead, lithospheric mantle beneath the crust is the likely source of the gold, which may have been anomalously enriched by metasomatic processes. However, the role of gold enrichment and metasomatism in the lithospheric mantle remains unclear. Here, we present comprehensive data on gold and platinum group element contents of mantle xenoliths (n = 28) and basalts (n = 47) representing the temporal evolution of the eastern NCC. The results indicate that extensive mantle metasomatism and hydration introduced some gold (<1–2 ppb) but did not lead to a gold-enriched mantle. However, volatile-rich basalts formed mainly from the metasomatized lithospheric mantle display noticeably elevated gold contents as compared to those from the asthenosphere. Combined with the significant inheritance of mantle-derived volatiles in auriferous fluids of ore bodies, the new data reveal that the mechanism for the formation of the lode gold deposits was related to the volatile-rich components that accumulated during metasomatism and facilitated the release of gold during extensional craton destruction and mantle melting. Gold-bearing, hydrous magmas ascended rapidly along translithospheric fault zones and evolved auriferous fluids to form the giant deposits in the crust.

New insights on chimney growth model and native gold enrichment in modern seafloor hydrothermal chimneys

Seafloor hydrothermal chimneys from back-arc basins are important hosts for metals, e.g. Cu, Zn, Pb, Ag and Au, and bear potential for deep-sea mining. A solid understanding of the distribution of ...

Genesis and evolution of the Hamama volcanic massive sulphide deposits, Egypt

Hamama Zn-Pb-Cu VMS deposits are hosted along the contact of bimodal metavolcanic sequence and their volcaniclastics in association with quartz-carbonate matrix. Ore forming process include multi-stage steps. 3D interpolation of drill hole database shows that the ore body was affected by multidirectional faults and fracture zones, that later were occupied by carbonates. The origin of carbonates is approached using a C and O isotopic compositions in whole-rock samples. The isotopic data revealed signs of both hydrothermal-sedimentary and hydrothermal-metasomatic processes. The primary gold and sulphide minerals were deposited from exhalations, forming massive sulphide ore then the tectonic processes due to the emergence of two successive phases of granitoids followed by the of post-tectonic dykes provided war to the introduction of carbonates that changed the configuration of the ore deposit and affected its grade. Later diagenetic and metasomatic processes resulted in enrichment of gold in the oxidation zone.

Spatial and temporal constraints of leached Cu-Au porphyry shoulder high-sulfidation epithermal deposit: insight from new discovered Kumbokarno Prospect, Trenggalek District, East Java

We conduct microscopic and microprobe quantitative analyses to confine characteristics of the newly found Cu-Au high sulfidation deposit which interestingly seems to highly associate with the presence of iron oxide-hydroxide complex. The resulting paragenesis shows four generation phases, which are magmatic, deep hypogene, shallow hypogene, and supergene. The alteration pattern suggests presence of a concealed intrusion which ascends the hypogene mineralization forming-fluid. Quantitative microprobe analysis shows enrichment of copper and gold in various types of iron oxide-hydroxide texture. A unique repetitive pattern of high and low copper concentration is found overlaps with goethite-hematite band of colloform texture. An interesting pattern between gold and titanium is also found as gold is almost exclusively located inside or near Fe-rich leucoxene. We propose a model of leached Cu-Au porphyry shoulder high-sulfidation epithermal deposit system to explain the prevalent condition of Kumbokarno Prospect where intensive late oxidation acts as the main process of metal concentration.

Redistribution and speciation of elements in gold-bearing sulfide mine tailings interbedded with natural organic matter: case study of Novo-Ursk deposit, Kemerovo Region, Siberia

The paper presents data on two cores from the dispersion train of gold-bearing sulfide tailings in the Novo-Ursk deposit (Kemerovo region, Russia), where organic-bearing mud and natural organic matter are interbedded with cyanide tailings and acid mine drainage (AMD). The study focuses on speciation of Au, Ag and potentially toxic elements (PTE), including Fe, Cu, Zn, As, Se and Pb according to selective sequential extractions. Since large contents of elements are found in water solution and bound to Fe(III) compounds, sulfides and/or organic matter in both cores, the organic material behaves as a natural biogeochemical barrier for potentially toxic elements and precious metals released from AMD. The interaction with organic matter results in biogeochemical sorption and immobilization of elements and is favourable for secondary enrichment in gold and silver. It leads to immobilization of elements that further form new mineral phases or become partitioned between existing phases. On the other hand, very high concentrations of water-soluble and oxidizable species of PTE and Au, which are unstable in surface conditions and may become remobilized when the environment changes, make the organic material less workable and even potentially hazardous. Thus, both positive (secondary enrichment) and negative (pollution) effects should be taken into account in the planning and design of systems for purification, disposal of cyanide tailings and recovery of useful components from tailings using natural organic matter as a geochemical barrier.

Magnetite as an indicator of granite fertility and gold mineralization: A case study from the Xiaoqinling gold province, North China Craton

The Xiaoqinling district in the southern margin of the North China Craton is the second largest gold enriched area in China. The Huashan and Wenyu plutons represent two typical Mesozoic magmatic intrusions in this region and were coeval with the gold mineralization. Here we report the geochemistry and Mössbauer spectral characteristics of magnetite from the two plutons with a view to explore the role of the granitic magmatism in the gold enrichment. The results of EPMA and Mössbauer spectroscopy suggest that the average values of Fe2+/Fe3+ in the magnetite from the two intrusions are lower than the standard value of 0.50, indicating an oxidizing environment. The estimated oxygen fugacity is higher for the Wenyu pluton than that of the Huashan pluton, which is also supported by the occurrence of hematite in the former, and the distinct δEu-δCe anomalies for magnetite from the two intrusions. The Sn content in magnetite indicates lower formation temperatures of the Wenyu pluton as compared to that of the Huashan pluton. The results of LA-ICP-MS analysis demonstrate similar multi-element variation patterns for the two intrusions. Titanium vs. Ni/Cr relationship suggests that the magnetite from the Huashan pluton is of typical magmatic origin, whereas that from the Wenyu pluton shows both magmatic and hydrothermal signature. The Ni/Co ratio of magnetite from the Wenyu granite is higher than that of the Huashan granite, which indicates a deeper source for the Wenyu pluton. The comparatively more favorable physico-chemical conditions, such as higher oxygen fugacity, lower temperature, as well as the involvement of deep-sourced material are envisaged to have provided better metallogenic environment for generating the gold mineralization around the Wenyu pluton.

LA-ICP-MS trace element analysis of arsenopyrite from the Samgwang gold deposit, South Korea, and its genetic implications

The Samgwang deposit, previously classified as an orogenic gold deposit, consists of several structurally controlled auriferous quartz veins hosted by metasedimentary rocks of the Precambrian Gyeonggi massif, South Korea. The gold at Samgwang occurs as electrum commonly associated with arsenopyrite. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), coupled with ore microscopy, was conducted on Au and trace elements distributed in arsenopyrite grains contained in quartz vein and wall rock samples. Resulting profiles for Au, Co, Ni, and Sb were relatively flat and showed little variability, indicating that the elements were relatively homogeneously distributed and exchanged via substitution in arsenopyrite, while variability and noisy depth profiles inferred that Pb was present as tiny inclusions or discrete particles. Furthermore, measured gold concentrations were typically higher in arsenopyrite grains from laminated quartz veins (avg. 2.07 ppm) than in grains from massive quartz veins (avg. 0.26 ppm) and wall rock samples (avg. 0.33 ppm). In the case of wall rock and massive quartz vein samples, arsenopyrite grains of euhedral shape illustrate an element distribution pattern whereby Au concentrations tend to decrease towards the rims of the analyzed grains. In contrast, the rims of arsenopyrite grains in laminated quartz vein samples recorded much higher Au concentrations of up to 20.1 ppm, with increasing Pb, Bi, and Ag, particularly within domains of fractured or recrystallized arsenopyrite grains. The newly acquired LA-ICP-MS data, along with textural evidence, may be interpreted in terms of the release and remobilization of invisible gold that was originally locked up in the arsenopyrite. This release is linked to late deformation and hydrothermal event during which the remobilized invisible gold was redeposited as electrum. In addition, there is evidence for the introduction of additional gold and other precious and base metals by the late-stage hydrothermal fluids. Based on the above and in contrast to previous genetic models, we propose a more complex metallogenic concept for Samgwang in which the economically important gold enrichment process was genetically related to intrusive activity during the waning stages of Jurassic to Early Cretaceous Daebo orogenesis.

Mineralogical constraints on the genesis of the Dahu quartz vein-style Au-Mo deposit, Xiaoqinling gold district, China: Implications for phase relationships and physicochemical conditions

The quartz vein-style Dahu Au-Mo deposit, with an average grade of 6.8 g/t for 38 tons Au, and an average grade of 0.24% for 100,000 tons Mo, is situated in the Xiaoqinling region in the northernmost part of Qinling orogen in China. It is characterized by enrichment of native gold, molybdenite, telluride and Bi-sulfosalt. The current understanding of telluride and Bi-sulfosalt mineralization in the Dahu Au-Mo deposit is very poor, and this paper is the first systematic study of these minerals in this deposit. The Au-Mo ores occur mainly as veins and stockworks as well as disseminated in quartz veins, and they are hosted by biotite-plagioclase gneiss, amphibole-plagioclase gneiss, and amphibolite of the Archaeozoic Taihua Group. Four paragenetic stages of mineralization are recognized that reflect early formation of sulfide minerals and subsequent precipitation of telluride minerals: (I) quartz-K-feldspar-pyrite-molybdenite, (II) quartz-pyrite-molybdenite, (III) sulfide-telluride-sulfosalt-gold, and (IV) carbonate-barite. The telluride minerals are mainly altaite, tellurobismuthite, and tetradymite, and to a minor extent calaverite, petzite, hessite, and stützite. Some Bi-sulfosalt minerals also exist in the deposit, including aikinite, lindströmite, saddlebackite, krupkaite, and hammarite. Of these minerals, saddlebackite, krupkaite and hammarite are reported in this deposit for the first time herein. The Au-Ag-Te-Bi-S minerals substantially formed in the Dahu Au-Mo deposit under conditions of T = 260 °C, P ≈ 100 MPa, pH = 3.9 to 7.9, logƒO2 = −40.9 to −33.8, logƒTe2 = −12.5 to −7.5, logƒS2 = −13.5 to −8.2, logƒH2S = −2.4 to −0.3 and logαAu+(aq)/αAg+(aq) = −6.7 to −6.2. The deposit formed from an ore-forming fluid dominated by HTe- or/and Te22−, in which Au was transported in the forms of Au(HS)2− and/or an unknown aqueous Te species (e.g., AuTe2−). Furthermore, during the mineralization stages of the Dahu deposit, the ore-forming fluid evolved from an initial oxidation type to a later relative reduction type, and the telluride minerals, Bi-sulfosalt minerals and native gold were formed in a relatively reduced condition. The Au-Ag-Te-Mo-Bi metal associations in the Dahu Au-Mo deposit were derived from a magmatic-hydrothermal system during the Late Triassic.

Crystal habit-directed gold deposition on pyrite: Surface chemical interpretation of the pyrite morphology indicative of gold enrichment

The occurrence of the {2 1 0} shape of pyrite crystals has been used as an empirical indicator for gold enrichment in ore deposits. Such indicating, also called ‘typomorphism’ in ore mineralogy, was conventionally attributed to environmental factors such as sulfur fugacity and temperature and pressure conditions in the ore-forming hydrothermal fluid. Here we show that the crystal-face specific redox reactivity of pyrite is also an important governing potency that drives gold enrichment in the pyrite crystals with particular shapes. This conclusion was drawn from data compiled from chemical analysis of field samples, laboratory testing of gold deposition on natural pyrite crystals, as well as theoretical calculations. Kinetic measurements of gold reductive deposition first revealed the highest precipitation rate on the {2 1 0} faces, particularly when pyrite crystals are enclosed by complex forms containing {2 1 0}. Corroborating with the experimental observations, density functional theory (DFT) calculations further indicated that the {2 1 0} possesses a high adsorption energy for gold, a high energy state of the highest occupied molecular orbital and consequently a low Eh. These results afford evidence supporting the view that the face specific reactivity, an intrinsic property of crystal, plays an important role in the gold concentration in pyrite and the occurrence of specific pyrite shapes. Our findings not only provide a surface chemical view for understanding the empirical prospecting method but also exemplified the significance of crystal face specific-reactivity in understanding crystallographic orientation related geochemical processes. The crystal face specific reactivity may be a critical factor controlling geochemical reactions at mineral-water interface, such as prebiotic synthesis on pyrite surfaces and mineral-water interface controlled kinetic isotope fractionation.

The geochemistry of Au–Ag minerals and base‐metal sulphides as indicators for gold precipitation: Case study of the Shihu gold deposit, central North China Craton

The Shihu quartz vein‐type gold deposit, the largest gold deposit in the central North China Craton, occurs within wall rocks of Neoarchean TTG (tonalite–trondhjemite–granodiorite) gneiss and shows a close relationship with the early Cretaceous Mapeng granitoid pluton and various intermediate–mafic dykes. The occurrence, morphology, composition, and precipitation mechanism of the Au–Ag minerals are short of research, which is the purpose of this study. The primary mineral assemblage of orebody contains quartz, calcite, and base‐metal sulphides including pyrite, galena, sphalerite, chalcopyrite, and minor pyrrhotite and enargite. The Au–Ag minerals can be classified into three distinct types, namely, (a) quartz‐hosted, (b) pyrite‐hosted, and (c) base‐metal sulphidehosted, corresponding to formation sequence. Electron microprobe analysis indicates that these Au–Ag minerals show fineness (=Au/[Au + Ag] × 1,000) ranging from 976 to 366 with an average of 667 and are rich in Bi, Te, and Hg while deficient of Cu, As, and Se. Bismuth and tellurium in the Au–Ag minerals show a linear relationship with Au, whereas in the coexisted sulphides are almost below detection limits. We suggest that Bi and Te fundamentally influenced the transportation, enrichment, and precipitation of gold as scavengers to concentrate and deposit gold at the location with an “appropriate distance” from magma. Phase analyses of native bismuth and Bi–Te minerals included in pyrite, reported for the first time in Shihu Gold deposit, imply that the gold precipitation witnessed the ore‐forming fluid evolution with log fTe2 decreasing from approximately −11 to approximately −15 and log fS2 increasing from approximately −13 to more than −11. Mineralogical mapping with the fineness of Au–Ag minerals (fineness contours) suggest that the ore‐forming fluids came from the deep earth beneath the north of the mining area with excellent gold potential, which has been verified by later drilling.

Geochemistry, mineralogy and mineral chemistry of the Welatam Cu-Mo-Bi-Au prospect, Northeast Myanmar

The Welatam prospect, located in Kachin State, Northeast Myanmar, is the first discovered bismuth-gold locality within the Tengchong-Lianghe (Tengliang) metallogenic district. Two mineralization types, identified in this area, are related to three hydrothermal events. The massive sulfide mineralization was formed during the hydrothermal activities of stage I and II, producing mainly iron sulfides (e.g. pyrrhotite, chalcopyrite, pyrite, arsenopyrite). Whereas the activity of stage III resulted in quartz-molybdenite mineralization which is characterized by the deposition of molybdenite, galena, and Bi-Te-Au-bearing phases. The Bi-mineral assemblages suggest that at stage III, the prevailing temperatures were above 271 °C. Fluid-rock interaction that changed the pH, fO2, fS2, and, in particular, temperature in hydrothermal fluids is important in causing the precipitation of liquid bismuth from Bi3+. The high correlation between Bi and Au (R = 0.782) demonstrates that liquid bismuth may have played a crucial role in gold enrichment. Here, Bi-Au-bearing minerals commonly precipitate along the cleavages of molybdenite, forming a distinctive mineral assemblage. It may indicate that the cleavages, besides providing spaces for the precipitation of liquid bismuth, create local chemical conditions (e.g., reducing and catalytic surfaces) favorable for Bi-melt to scavenge Au from ambient fluid. The abundant Bi minerals, metal association (e.g., Cu, Bi, Mo, Au, Te, W), the restricted δ34S values (−7.3‰ to 1.9‰) of sulfides, and the proximity with the Welatam monzogranites suggest a magmatic contribution to the ore-forming fluids.