Gold-quartz Veins Research Articles

Metallogenic model of the Lykling ophiolite-hosted lode Au deposit, Scandinavian Caledonides: Insight from fluid inclusions, mineral chemistry and stable isotope geochemistry

The Lykling lode Au deposit represents a unique example of gold mineralization in the Upper Allochthone of the Scandinavian Caledonides. The mineralization is hosted by the Early Ordovician Lykling Ophiolite Complex and the intruding trondhjemite unit and spatially associated with two generations of mafic dykes that crosscut both the ophiolitic complex and trondhjemite. Field observations indicate that the mafic dykes did not play an active role in the emplacement of Au at Lykling, however their contacts with the immediate host rocks (i.e., gabbro and trondhjemite) may have focused ore-forming fluids.Three main stages in the evolution of the hydrothermal system in the Lykling area are documented by two generations of auriferous quartz veins: 1) older quartz-carbonate veins hosted by moderately angled brittle-ductile shear zones and 2) younger quartz-sulfide veins that fill steeply dipping brittle faults. Stage 1 resulted in deposition of barren quartz and gold-free pyrite from moderately saline NaCl-CaCl2-H2O ± CO2 fluids at temperatures between ∼310–330 °C and pressures in the range from 2.7 to 3.5 kbars. Under the given physicochemical conditions Au was mobile in the form of its chloride complexes. Stage 2 is associated with multiple episodes of ductile deformation punctuated by concomitant brecciation reflecting brittle-ductile transition processes. Fluid inclusion data reveals a decrease in temperature and pressure that may result in the formation of stable Au-bisulfide complexes and making the hydrothermal mobility of Au sensitive to changes in pressure. Consequently, the fluctuation in pressure controlled by the development of brittle-ductile structures likely resulted in the precipitation of native gold in quartz veins. Stage 3 represents a pure brittle event which is accompanied by a significant decrease in the fCO2/fS2 ratio. Mixing of higher-temperature and moderate-salinity fluids with cooler fluids has been recognized as the principal trigger for deposition of sulfides and a paragenetically late phase of native gold in the Lykling hydrothermal system.The Lykling ophiolite-hosted lode Au deposit shows numerous similarities with orogenic gold deposits elsewhere, including structural controls during mineralizing events, deposition from moderately to low salinity CO2-bearing aqueous solutions, deposition from a focused fluid flow along trans-crustal fault zones with a mixed brittle-ductile character and the spatial association with regionally metamorphosed terranes. In contrast to the great majority of the known orogenic gold deposits, the Lykling ophiolite-hosted lode Au deposit records a magmatic origin of volatiles. Therefore, taking into consideration field relationships and the regional setting of the Lykling deposit, we argue that its formation is concomitant with emplacement of the Sunnhordland Batholith during post-collisional thinning of the crust and associated localized uplift within the Lykling Ophiolite Complex.

Genetic implications of vein texture and mineralogy at the Au-bearing El Morado mining district, Sierras Pampeanas, San Juan, Argentina

El Morado mining district is located in Sierra de La Huerta, in the Sierras Pampeanas of San Juan province, Argentina. It comprises numerous Au-bearing quartz veins hosted by intensely deformed igneous-metamorphic units of lower Paleozoic age. In the El Morado mining district, Maira mine is the only deposit producing gold at the present time. Quartz veins in this mine and surroundings, occur in concordance with their host rocks foliation and show numerous evidences of brittle-ductile deformation, mainly represented by a pinch-and-swell structure and extended development of breccia bodies.The application of scanning electron microscopy cathodoluminiscence in quartz samples lead to the recognition of homogeneous texture, which is evidence of recrystallization, commonly observed in orogenic gold quartz veins. Intracrystalline deformation is ubiquitous and represented by undulose and patchy extinction in relicts of large old grains where bulging and sub-grain rotation recrystallization microstructures are well represented. These dynamic quartz deformation processes indicate temperatures going up to ∼350 °C. Fluid inclusion post-entrapment modifications were recognized in quartz veins as numerous collapsed, small-sized fluid inclusions.Ore mineralization at Maira mine occurred after quartz veins emplacement and deformation in the upper crustal brittle-ductile transition zone. Galena, chalcopyrite and lesser pyrite and bornite, together with gold and tetrahedrite were identified infilling fractures in quartz veins. Gold grains were also identified closely related to malachite, cuprite, goethite and lepidocrocite in oxidizing horizons.

МИНЕРАЛОГИЧЕСКИЕ ОСОБЕННОСТИ РУДНЫХ ПРОЯВЛЕНИЙ ЗОЛОТА СОЛОЛИЙСКОГО ПОДНЯТИЯ ОЛЕНЕКСКОГО СВОДА

The paper deals with studied mineralogical and geochemical features of gold-sulphide manifestations of the Sololi uplift of the Olenek arch (northeast of the Siberian Platform) in order to identify their mineralogical features and prospects for their further research. The identical mineral composition of gold-sulphide formations developed by Early Proterozoic rocks of the Eekitian series and rhyolites of the same age gave reason to assume the simultaneity of their formation and to refer them to the general stage of ore formation. According to the established mineral paragenesis, described gold sulphide manifestations are attributed to the gold-quartzsulphide formation type. The discovery of gold sulphide mineralization in the zone of tectonic fracturing of Early-Middle Permian sandstones indicates the formation of ore occurrences in the area associated with the processes of tectonomagmatic activation of the Mesozoic. The identification of indicators of low-temperature hydrothermal processes as mineral paragenesis – mercury-containing gold, cinnabar and barite, testifies to shallow and low-temperature conditions for the formation of this ore manifestation. It was found that according to the Co/Ni ratio, 28% of pyrites (including gold-bearing ones) are associated with hydrothermal processes. In general, the detection of native gold in quartz veins in free form and as microinclusions and impurities in pyrite, as well as the discovery of relatively large poorly rounded attachments of gold to quartz in stream flows dewatering the studied ore formations, allows researchers to positively assess the prospects for further study of ore manifestations of the Sololi uplift of the Olenek arch.

Gold source and ore-forming process of the Linglong gold deposit, Jiaodong gold province, China: Evidence from textures, mineral chemical compositions and sulfur isotopes of pyrite

The Linglong gold orefield is located northwest of the Jiaodong gold province, in the eastern North China Craton (NCC) area. It consists of over 10 gold mines with a total gold resource of approximately 1000 t, and gold mineralization occurs in gold-quartz veins. The Linglong gold deposit underwent four stages of ore-forming processes namely: Pyrite-quartz (stage I) – milky-white quartz vein with euhedral pyrites, Quartz-pyrite (stage II) – smoky quartz veins with euhedral-subhedral pyrites, Polymetallic sulfides (stage III) – quartz veins with large amounts of polymetallic sulfides (pyrite, pyrrhotite, sphalerite, galena), hematite, and native gold, and the Quartz–carbonate (stage IV) – calcite-quartz veins. This study focuses on the investigation of pyrites from three main mineralization stages (I-III), exploring their textures, in-situ major and trace element concentrations, and in-situ sulfur isotopic compositions.The study examined pyrite through various methods including BSE and EMPA major element, LA-ICP-MS trace element, and LA-MC-ICP-MS sulfur isotope analyses. Pyrite from three distinct stages of ore-forming processes were analyzed. Pyrites from stage I (Py-I) exhibited euhedral crystals with homogeneous brightness in the BSE, lacked mineral inclusions, porous microtextures, and oscillatory zones. Py-I possessed low contents of Au, Ag, As, Bi, Cu, Sb, Zn, Co, and Ni. Pyrites from stage II (Py-II) showed fine-coarse grains and euhedral-subhedral features with slight fractures, similar to Py-I, however, had the higher concentrations of As and Sb. Pyrites from stage III (Py-III) were found in the quartz-polymetallic sulfide veins and exhibited subhedral to anhedral features and more significant concentrations of metallic elements, particularly Au and As. The δ34S values of Py-I, Py-II and Py-III were in the ranges of 6.76‰–7.47‰, 6.11‰–7.27‰ and 6.07‰–8.33 ‰, respectively. Analysis of the texture and geochemical composition of pyrite suggested that invisible gold was primarily present in pyrite in the form of solid solution (Au+), accompanied by a small amount of nano-form gold. The ore-forming fluids that led to this mineralization were mostly derived from mantle-derived mafic magma mixed with some minor metamorphic sedimentary material. A decrease in fluids temperature, water–rock interactions and fluid boiling led to the depositions of sulfides and gold. In the Late Mesozoic, lithospheric delamination of the eastern block of the NCC triggered decompression and melting of the upwelling asthenospheric mantle resulting in partial melting of the lithospheric mantle. This lithospheric mantle-derived mafic magma provided the ore-forming fluids for large-scale gold mineralization in the Jiaodong gold province.

Origin of Disseminated Gold-Sulfide Mineralization from Proximal Alteration in Orogenic Gold Deposits in the Central Sector of the Yana–Kolyma Metallogenic Belt, NE Russia

The Yana–Kolyma metallogenic belt, NE Russia, is a world-class gold belt with resources numbering ~8300 tons of gold. The belt is localized in the central part of the Verkhoyansk–Kolyma orogen, formed by a collage of diverse terranes. The Tithonian-to-Early-Cretaceous orogenic gold deposits are hosted in a sequence of Permian–Triassic and Jurassic clastic rocks and altered Late Jurassic andesite, dacite, granodiorite, trachyandesite, and trachybasalt dykes. High-fineness gold (800–900‰) in quartz veins and invisible gold in disseminated arsenian pyrite-3 (Py3) and arsenopyrite-1 (Apy1) are present in ores. Here, we present new data about microtextures; the chemical composition and stable sulfur isotopes of auriferous pyrite-3 and arsenopyrite-1 from proximal alterations in sediment-hosted (Malo–Taryn, Badran, Khangalas); and intrusion-hosted (V’yun, Shumniy) orogenic Au deposits in the central sector of the Yana–Kolyma metallogenic belt to better constrain the ore-forming process and tracking their evolution. Detailed petrography defined the following generations of pyrite: syn-sedimentary/diagenetic Py1, metamorphic Py2 and hydrothermal Py3, and Apy1. Hydrothermal Py3 and Apy1 are localized in the proximal pyrite–arsenopyrite–sericite–carbonate–quartz alteration in ore zones and make a major contribution to the economic value of the veinlet-disseminated mineralization with “invisible” gold in the orogenic deposits of the Yana–Kolyma metallogenic belt. Electron microprobe analysis (EMPA) of Py3 in both types of deposits shows concentrations of As (up to 3.16 wt%), Co, Ni, Cu, Sb, and Pb. Py3 in intrusion-hosted orogenic gold deposits reveals elevated concentrations of Co (up to 0.87 wt%), Ni (up to 3.52 wt%), and Cu (up to 2.31 wt%). The identified negative correlation between S and As indicates an isomorphic substitution of sulfur by As1−. Py3 from igneous rocks is characterized by a high degree of correlation for the pairs Fe2+→ Co2+ and Fe2+→ Ni2+. For hydrothermal Apy1, Co (up to 0.27 wt%), Ni (up to 0.30 wt%), Cu (up to 0.04 wt%), and Sb (up to 0.76 wt%) are typomorphic. According to atomic absorption spectrometry, the concentration of Au in Py3 reaches 159.5 ppm; in Apy1, it reaches 168.5 ppm. The determination of the precise site of the invisible gold within Py3 and Apy1 showed the predominance of solid-solution Au+ in the crystal lattice. The values of δ34S in Py3 and Apy1 (from −6.4 to +5.6‰, mean value of about +0.6‰), both from sediment-hosted and from intrusion-hosted deposits, display a relatively narrow range and are characteristic of the hydrothermal ore stage. Our analytical results showed no systematic differences between the chemical and stable sulfur isotope compositions of both auriferous pyrite-3 and arsenopyrite-1 from the proximal alteration in sediment-hosted (Malo–Taryn, Badran, Khangalas) and intrusion-hosted (V’yun, Shumniy) orogenic Au deposits, indicating that the primary source of sulfur, gold, and mineralizing fluids was likely from subcrustal and metamorphic systems in the Late-Jurassic-to-Early-Cretaceous Verkhoyansk–Kolyma orogen.

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

AbstractThe Sukari gold deposit (>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.

Geochronology (U-Pb and Re-Os) and H[sbnd]O[sbnd]S isotopes reveal ore fluid origin and crucial role of meteoric water for Laozuoshan quartz-vein-hosted gold metallogeny in Jiamusi, NE China

The Laozuoshan auriferous sulfide-quartz veins, which are hosted in the granodiorite, monzogranite, and quartz diorite, are the only Early Cretaceous gold deposit discovered in the central Jiamusi Massif, but its origin and ore-forming processes remain unclear. Here, we integrate mineralogy, HOS isotopes, and U-Pb and Re-Os geochronology to unravel the formation of the Laozuoshan quartz-vein gold mineralization. Zircon and molybdenite from the monzogranite wallrock and gold ores yield weighted mean U-Pb and isochron Re-Os dates of 259.9 ± 2.1 Ma and 103.3 ± 2.3 Ma, respectively. Our radiometric dating suggests that the Laozuoshan quartz-vein gold mineralization was coeval with the quartz diorite (105–103 Ma) but after the monzogranite emplacement. Quartz grains from the auriferous veins of different stages yielded scattered δ18O values (5.4 to 13.0 ‰), with the δD values (fluid inclusions) ranging from –99.9 to –74.9 ‰. The δ18O values of quartz (10.9–13.3 ‰) and feldspar (9.4–11.0 ‰) from the unaltered wallrocks are similar to those (δ18Oquartz = 11.2–13.2 ‰ and δ18Ofeldspar = 10.0–10.4 ‰) from the potassic alteration halos. Meanwhile, the sericite-altered rocks have δ18O values of 12.4–12.9 ‰ (quartz) and 3.4–5.2 ‰ (sericite). Sulfides from the gold ores yielded decreasing δ34S values from 5.3 to 6.9 ‰ (early stage), through 1.0–3.5 ‰ (intermediate stage), to –4.9 to –4.4 % (late stage). The isotope data reflect a largely magmatic source for the early ore fluids, whilst the sulfur was possibly derived from the Mashan Group migmatite. The results, in combination with geologic evidence, indicate that the Laozuoshan quartz-vein gold mineralization was spatially, temporally, and genetically related to the Early Cretaceous quartz diorite, and thus can be classified as intrusion-related, magmatic-hydrothermal-vein type. In the ore-forming processes, the ascending Early Cretaceous magma may have acquired most of their sulfur and gold through assimilation of the Mashan Group migmatite. The early ore fluids derived from the quartz diorite magma may have initially focused into narrow cracks/fractures in the quartz diorite margin and then flow along the fracture system. Subsequently, meteoric water incursion led to temperature drop, boiling, oxygen fugacity increase, and chemical exchange of the resultant fluids, which facilitated gold scavenging by bismuth-rich melts and rapid precipitation as arsenopyrite-hosted electrum and gold-bismuth mineral eutectic assemblage. This study highlights the crucial role of meteoric water in formation of the magmatic-hydrothermal-vein type gold deposit.

Structure of the Muruntau gold ore region in the Kyzyl-Kum desert (Central Asia)

The giant gold deposit Muruntau in most works belongs to the “orogenic type”. However, in the description of the geology of the deposit, there is usually no information about the stages (deformation, metamorphism) and the role of regional processes in the formation of host and mineralized rocks of the deposit. Our fieldwork provides unique geological evidence and previously unpublished data that are not reported in any other publications to date. This article presents the results of a detailed structural study of the terrane hosting the Muruntau gold deposit on the example of four sites and the results of deep drilling of two wells. The conducted studies confirm that the lowest Muruntau megaterrane consists of multi-stage metamorphosed Neoproterozoic–Early Paleozoic sedimentary-volcanic rocks and Late Paleozoic carbonate sediments overlain by Late Carboniferous flysch and olistostrome in the top. Two main stages (D1 and D2) of rock deformation in the lower part of the megaterrane associated with the Early Paleozoic accretion and two subsequent stages (D3 and D4) associated with the Late Paleozoic accretion and orogeny are described. Thrust processes D2 and D3 caused the formation of recumbent folds of different orientation and vergence. The normal antiform (D4) extending W–E is an orogenic fold. The Muruntau gold deposit is located in a regional synmetamorphic thrust zone called the Main Muruntau Thrust (MMT), which formed during the Early Paleozoic D2 stage. The MMT is responsible for the formation of secondary rock anisotropy along the cleavage of the D2 recumbent folds and conformal lens-like body of mineralized rocks and gently sloping veins with an area of 7.0 × 2.6 km. The internal structure of the deposit retains fragments of deformed gold quartz veins into recumbent folds. These structural data make it possible to link low-grade gold mineralized zones with Early Paleozoic deformations. We suggest that rock anisotropy within the MMT controlled mineralization twice: in the Early Paleozoic as a synmetamorphic flat high-permeable zone and in the Late Paleozoic as a relatively low-permeable zone for post-magmatic fluids.

Typomorphic Characteristics of Gold-Bearing Pyrite and Its Genetic Implications for the Fang’an Gold Deposit, the Bengbu Uplift, Eastern China

The Fang’an quartz-vein gold deposit is located in the eastern part of the Bengbu uplift. The eastern part of the Bengbu uplift is considered to be the western extension of the Zhaoyuan gold mineralization zone in the Jiaodong area of Shandong Province and has huge mineralization potential. The Fang’an deposit was a newly discovered, small-sized gold deposit, and the research in the area is relatively weak. In this study, samples of quartz-vein type ore were collected from the ZK141, ZK1549, and ZK1665 drill holes of the Fang’an gold deposit. Based on the study of the geological characteristics, the major and trace elements of pyrite in different stages were analyzed by electron probe microanalyzer (EPMA), to explore the compositional characteristics of pyrite, the occurrence of gold, and the source of ore-forming fluid. The studies indicate that the deposit experienced four ore-forming stages: the quartz stage, the quartz-pyrite stage, the polymetallic sulfide stage, and the carbonate stage. The pyrites are grouped into three stages, corresponding to the first three ore-forming stages. The EPMA analyses showed that the major elements of pyrite were high Fe and low S, indicating that the formation was hydrothermal. The high content of Ni indicated that the metallogenic materials were derived from between the mantle and the lower crust. The general Co/Ni ratio of >1, with an average of 5.34, indicated that the ore-forming fluid was derived from the magmatic–hydrothermal fluid and wall rock. The Fang’an gold deposit mainly contains nano-gold.

Base metal-enriched gold-quartz veins in the eastern Cameroon goldfields, West-Central Africa

Base metal-enriched gold-quartz veins in the eastern Cameroon goldfields, West-Central Africa

Typomorphism of Native Gold (Geological-Industrial Types of Gold Deposits in the North-East of Russia)

This study presents the typomorphic features of native gold grains from three different geological-industrial types (GIT) of gold deposits in the North-East of Russia: (1) gold–arsenic-sulfide in black shale strata (Natalka, Degdekan, Karalveem, Maldyak deposits), (2) gold–quartz veins in granitoids (Dorozhnoye, Butarnoye, Shkolnoye, Maltan deposits), and (3) gold–silver adularia in volcanogenic strata (Kupol, Olcha, Kubaka, Burgali, Primorskoe, Dalnee deposits). The reliability of the geological interpretation is directly related to mineral associations, fineness variations, its internal structure and the content of microimpurities. Native gold is a reliable indicator for identifying various GIT of gold deposits at the early geological-prospecting stages of studying gold-bearing areas. Typomorphic features of native gold for each of the considered GIT are stable and do not depend on the age and scale of mineralization. It is shown that using an integrated approach obtains genetic information about a particular ore object, which makes it possible to predict the vertical range of mineralization and outline the technology for processing ores. The information obtained can also be effectively used in the search for placer deposits in nearby watercourses. Identification of typomorphic features of ore and placer native gold opens up wide opportunities for delineating the distribution areas of placer deposits.

Srednekanskaya dike gold deposit in the northeast of Russia

This paper presents the results of a comprehensive material composition study of the host rocks and ores of the Srednekanskaya dike deposit. It has been found that the ores had been formed in several stages, namely, the metamorphogenic, hydrothermal-metasomatic (productive), and hypergene stages. At the same time, a decisive role in the formation of the gold mineralization was played by structural controls that are present in the form of breccias and hydrothermalmetasomatic alterations of rocks with gold deposition in the weakened zones. The prevailing features of gold mineralization of the Srednekanskaya dike are as follows: concentration of the bulk of gold in the vein material; predominant coarse gold occurrence as massive plates, lumps, and open-worked films; filling of interstitial openings in quartz, arsenopyrite, and pyrite with fine gold; native gold in quartz veins and quartz-carbonate-feldspar breccias localized in microdiorites confined to both arsenopyrite and pyrite and native gold associated with diorite-porphyry confined only to arsenopyrite, as supported by the geochemical data. Process studies have been carried out and the efficiency of Srednekanskaya dike ore processing by gravity methods has been established, with the gold recovery reaching 90 %. A recommended process flow sheet is presented.

AGE CONSTRAINTS AND METALLOGENIC PREDICTION OF GOLD DEPOSITS IN THE AKZHAL-BOKO-ASHALIN ORE ZONE (ALTAI ACCRETION-COLLISION SYSTEM)

We present new age constraints for igneous rocks and ore-metasomatic formations of the gold deposits in the Akzhal-Boko-Ashalin ore zone. In terms of their ore formation, these deposits correspond mainly to the orogenic type, which generally reflects specific metallogeny of the West Kalba gold-bearing belt in East Kazakhstan. Gold-quartz veins and mineralized zones of the gold-sulphide formation are confined to fractures feathering regional NW-striking and sublatitudinal faults. Their common features include the following: gold-bearing veinlet-disseminated pyrite-arsenopyrite ores that are localized in carbonaceous-sandy-schist and turbidite strata of different ages; structural-tectonic control of mineralization, numerous dikes of medium-basic compositions in ore-control zones; and the presence of post-orogenic heterochronous granite-granodiorite rocks, although their relation to gold-ore mineralization is not obvious. Igneous rocks of the study area have similar ages in a narrow range from 309.1±4.1 to 298.7±3.2 Ma, which is generally consistent with the previously determined age of granitoid massifs of gold-ore fields in East Kazakhstan. A younger age (292.9±1.3 to 296.7±1.6 Ma) is estimated for felsic rocks of the dyke complex. For the ore mineralization, the 40Ar/39Ar dating of sericite from near-ore metasomatites yields two age intervals, 300.4±3.4 Ma and 279.8±4.3 Ma. A gap between of the ages of the ore mineralization and the igneous rocks is almost 20 Ma, which may indicate that the processes of ore formation in the ore field continued in an impulse-like pattern for at least 20 Ma. Nevertheless, this confirms a relationship between the hydrothermal activity in the study area and the formation and evolution of silicic igneous rocks of the given age interval, which belong to the Kunush complex, according to previous studies. This interpretation is supported by reconstructed tectonic paleostress fields, showing that directions of the main normal stress axes changed during the ore mineralization stage, which is why the ore bodies significantly differ in their orientations. The above-mentioned data are the first age constraints for the study area. Additional age determinations are needed to further improve understanding of the chronology of ore-forming processes. Actually, all the features characterizing the gold mineralization of the Akzhal, Ashalin and Dauba ore fields, including the data on lithology, stratigraphy, structural tectonics, magmatism, isotope geochronology, mineralogy and geochemistry, can be used as criteria when searching for similar ore fields in East Kazakhstan.

Коренное золотопроявление на «Территории» Олега Куваева (север Чукотского нагорья)

A sample of a gold-bearing quartz vein from the Upper-Ichuveem ore occurrence, discovered in the Triassic terrigenous complex in the north of the Chukotka Upland, the Oleg Kuvaev's “Territory”, had been comprehensively studied. The gross gold grade was estimated at 3–4 g / t. The vein includes quartz, native gold, sulfide-goldsilver solid solutions, alumoseladonite, aluminium-sulfate-phosphates Fe-Pb-Mg-Ca compound, apatite, pyrite, iron-titanium oxides, litharge, native phases composed of Fe (Ni), Ag-Pb-Bi, Fe-Al-PS-As, multicomponent ocher of hypergene origin. Native gold ranges from medium to fine. The vein contains particles of non-crystalline organic matter, which is close in composition of organic groups to polysaccharides. It is possible that the carbon particles found in the gold-quartz vein are of an abiogenic nature and can be compared with abiogenic condensed organelles in the products of modern volcanism. The data obtained make it possible to attribute the Upper- Ichuveem gold ore occurrence to a low-sulfide gold-quartz formation, but with additional signs of gold-silver and polymetallic formations, which can be regarded as a favorable prerequisite for prospecting and exploration in Oleg Kuvaev's «Territory» not only of gold-placer deposits, but also of lode gold deposits.

Geology of the Structurally Controlled Quartz Vein Gold Mineralization in the Dikko Area, Part of Zungeru—Minna—Birnin Gwari Schist Belt, NW Nigeria

Dikko area, Niger State, North Central Nigeria falls within the Pan-African thermo-tectonic event which occurred in Nigeria as in other parts of West Africa with widespread effect on the mineralization of rejuvenated basement and geosynclinal trough deposits. The Pan-African Belt in Nigeria forms part of the Benin-Nigeria (Dahomeyan) shield which is regarded as the southern prolongation of the circumcratonic Pan-African belt of the West African Craton. Structurally controlled, gold-bearing quartz veins were measured and studied in the area from an extensive geological ground investigation. Field and petrographical studies indicate that the Neoproterozoic rocks constitute gneisses, amphibolite schist, amphibolites and migmatites. The amphibolites are intruded by pegmatites. The associated quartz veins are generally concordant with the main N-S regional structural trend. The lithological and structural studies carried out in these areas have demonstrated the relationship between structures and mineralization and/or lithology. The main characteristics of the gold mineralization in the area appear to indicate hydrothermal mineralization in connection with shear zones. This study allows us to predict that quartz veins in the amphibolite can be mined at a profit and a detailed geochemical and resource valuation needs to be carried out for the Au and associated Galena and Ag mineralization to fully exploit their economic potential.

P-T-fluid-deformation regime of the Ediacaran Serra do Cavalo Magro orogenic gold deposit, Ribeira Belt, Brazil

The Serra do Cavalo Magro orogenic gold deposit is located in the southern Ribeira Belt, Brazil. Gold-quartz veins are hosted in Calymmian (1500–1450 Ma) metasedimentary and metabasic rocks and Ediacaran (610–600 Ma) granitic rocks. Geochronological data available in previous literature and structural-petrological relationships with host rocks and shear zones indicate that the gold mineralization was formed within 580–540 Ma during the late-stage evolution of the Brasiliano-Pan African Orogeny, related with Gondwana supercontinent assembly. Petrological modeling indicates peak metamorphic conditions from 560 °C and 7 kbar (chlorite-biotite phyllite) to 625 °C and 6.8 kbar (garnet-biotite phyllite) recorded in immediate host rocks. Gold mineralization occurs in quartz shear and extensional veins, structurally controlled by NE-trending, second- and third-order sinistral, transcurrent shear zones. The NNW-trending subvertical and subhorizontal extensional veins are oriented 55–85° in relation to the fault planes, indicating the shear zones were severely misoriented to frictional reactivation. Deformation within the shear zones was accommodated by bulging recrystallization of quartz aggregates, while feldspar aggregates from the granitic protolith underwent cataclastic flow or were replaced by sericite-epidote, producing phyllonites. Gold precipitated during stages of vein deformation within microfractures in shear and extensional veins with quartz previously subjected to bulging recrystallization. Metamorphic ore-bearing fluids are recorded in distinct assemblages of low to moderate salinities (1–18 wt.% NaCleq.) H2O–NaCl–CaCl2 and H2O–CO2–N2–NaCl–CaCl2 fluid inclusions and CO2–N2 inclusions. Microthermometric data indicate entrapment conditions of 250–350 °C and 0.3–2.3 kbar, recording hydrostatic to supralithostatic fluid pressures in post-peak metamorphic stages. Sharp fluid pressure fluctuations (0.4–1.6 kbar) resulted from earthquake rupture events and the fault-valve behavior induced circulation of fluids from distinct sources with limited partial fluid mixing, favoring the gold transportation and precipitation.

Vein Mineralogy and Hydrothermal Gold Mineralization at the Kyaw Soe Thu Prospect in the Wabo Deposit, Mandalay Region, Central Myanmar

The Kyaw Soe Thu Prospect is a part of the Wabo deposit which is situated along Mogok Metamorphic Belt (MMB) in Central Myanmar. Gold bearing veins are hosted in the banded garnet-biotite gneiss, calc-silicate, marble and granitoid of the Wabo area. The granitic rocks intruded into banded garnet-biotite gneiss. The MMB was intensely deformed and it is a highly fractured metamorphic terrain in which metamorphic facies ranges from amphibolite to granulite facies. Gold mineralization at the Kyaw Soe Thu Prospect in the Wabo deposit occurred in the forms of gold-quartz veins. The quartz veins are a few centimeters in width. The color of the quartz vein at the Kyaw Soe Thu Prospect is commonly smoky and milky. The quartz vein exhibits chalcedonic, saccharoidal and granular textures. Three mineralizations were indicated. Stage I is the thin layered crustiform chalcedonic quartz. Stage II is chalcedonic quartz vein and stage III is Ankerite-quartz-sericite vein. Stage II and stage III are gold rich hydrothermal stages, and Ankerite-quartz-sericite veins were formed with a small amount of sericites in the Kyaw Soe Thu Prospect. The Ankerite-quartz-sericite veinlets range from a centimeter up to three centimeter in width within the stage II chalcedonic quartz vein. Textures of mineralized quartz vein samples were petrographically examined. The textures include a feathery texture that is most closely associated with electrum in both stage II chalcedonic quartz veins and stage III Ankerite-quartz-sericite veins. Ankerite varies in color from pink, white to buff. Among them, pink color is more common in the Kyaw Soe Thu Prospect. Ankerite in the Kyaw Soe Thu Prospect exhibits rhombohedral idiomorphic, which exhibits growth patterns. Ore minerals of the veins are electrum, pyrite, galena, and trace amount of very fine-grained chalcopyrite. On the basis of textures of the chalcedonic quartz veins, the Kyaw Soe Thu Prospect is ascribed to be a low-sulfidation epithermal deposit.

Trace element associations in magnetite and hydrothermal pyrite from the Geita Hill gold deposit, Tanzania

Gold mineralization in the Geita Hill deposit is associated with pyrite formed along microfracture networks and sulfidation fronts together with K-feldspar and biotite. The sulfidation fronts are best developed in magnetite-bearing ironstone. The gold is present mainly as electrum and gold tellurides along grain boundaries, and as inclusions in pyrite, quartz, biotite and K-feldspar. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of pyrite and magnetite grains reflect complex fluid-host rock interactions. Magnetite textures and chemistry change with alteration intensity, indicating the progression of the alteration front into the host rock. Pyrite textures are uniform across all rock types and reflect late-tectonic growth linked to multi-staged infiltration of hydrothermal fluids. Trace element distribution patterns in pyrite are locally complex and influenced by host rock chemistry.Gold distribution patterns in pyrite correlate closely with Te, Ag, Bi and Pb, indicating that gold occurs in micro- and nano-inclusions of telluride minerals. This is especially so for gold in quartz veins, whereas gold in ironstone and diorite also occurs as electrum with an average Au/Ag ratio of 0.41. As, Co and Ni in pyrite are lattice bound and occur in high concentrations in ironstone and diorite where they show characteristic growth zoning patterns. Pyrite in quartz veins has As, Co and Ni concentrations that are low and variable. Cr, Cu, Mo, Mn and Zn are present in all rock types in isolated inclusions in pyrite grains, whilst Pb, Bi and Sb occur in more dispersed patches of fine clustered inclusions. The Se content in pyrite is typical for Archean gold deposits, and reflects an average temperature of ~340 °C for the mineralizing fluid. The Co/Ni ratio of pyrite grains varies between 0 and 5.2 in ironstone and diorite, and most likely reflects the equilibration Co/Ni ratio of the host rock. The Co/Ni ratio of pyrite grains in quartz veins varies between 1 and 12, and is consistent with a magmatic-hydrothermal origin for the ore fluid. Trace element distribution patterns in magnetite and pyrite indicate that As, Ni, Co, Cr, Mn and Cu were mostly locally derived, and remobilised into the pyrite during sulfidation of the host rock. The concentrations of these elements are strongly lithologically controlled, and they are not consistently incorporated into the pyrite after initial stages of growth. Au, Ag, Te, Bi and Pb were externally derived, and closely correlate in all varieties of pyrite as well as strongly altered magnetite. The alteration footprint of the Geita Hill deposit is limited in extent, and does not involve As and Sb that are typically enriched in Archaean lode-gold systems. Instead, Te and Bi are most characteristic for the deposit and could be of use as path finder elements together with altered magnetite grains.

Orogenic Gold in Transpression and Transtension Zones: Field and Remote Sensing Studies of the Barramiya–Mueilha Sector, Egypt

Multi-sensor satellite imagery data promote fast, cost-efficient regional geological mapping that constantly forms a criterion for successful gold exploration programs in harsh and inaccessible regions. The Barramiya–Mueilha sector in the Central Eastern Desert of Egypt contains several occurrences of shear/fault-associated gold-bearing quartz veins with consistently simple mineralogy and narrow hydrothermal alteration haloes. Gold-quartz veins and zones of carbonate alteration and listvenitization are widespread along the ENE–WSW Barramiya–Um Salatit and Dungash–Mueilha shear belts. These belts are characterized by heterogeneous shear fabrics and asymmetrical or overturned folds. Sentinel-1, Phased Array type L-band Synthetic Aperture Radar (PALSAR), Advanced Space borne Thermal Emission and Reflection Radiometer (ASTER), and Sentinel-2 are used herein to explicate the regional structural control of gold mineralization in the Barramiya–Mueilha sector. Feature-oriented Principal Components Selection (FPCS) applied to polarized backscatter ratio images of Sentinel-1 and PALSAR datasets show appreciable capability in tracing along the strike of regional structures and identification of potential dilation loci. The principal component analysis (PCA), band combination and band ratioing techniques are applied to the multispectral ASTER and Sentinel-2 datasets for lithological and hydrothermal alteration mapping. Ophiolites, island arc rocks, and Fe-oxides/hydroxides (ferrugination) and carbonate alteration zones are discriminated by using the PCA technique. Results of the band ratioing technique showed gossan, carbonate, and hydroxyl mineral assemblages in ductile shear zones, whereas irregular ferrugination zones are locally identified in the brittle shear zones. Gold occurrences are confined to major zones of fold superimposition and transpression along flexural planes in the foliated ophiolite-island arc belts. In the granitoid-gabbroid terranes, gold-quartz veins are rather controlled by fault and brittle shear zones. The uneven distribution of gold occurrences coupled with the variable recrystallization of the auriferous quartz veins suggests multistage gold mineralization in the area. Analysis of the host structures assessed by the remote sensing results denotes vein formation spanning the time–space from early transpression to late orogen collapse during the protracted tectonic evolution of the belt.

Physicochemical Conditions of Quartz Formation at the Bazov Gold-Ore Deposit (East Yakutia, Russia)

The evolution of the hydrothermal system forming the Bazov gold-ore deposit is traced by the data on the composition and properties of the fluid inclusions within the veined quartz. The gold-bearing quartz veins were formed under a moderate decrease of temperature from 290 to 230°C and considerable decrease of pressure from 0.75 to 0.25 kbar. The pressure drop was accompanied by the volume crystallization of quartz from SiO2-saturated hydrothermal springs with the appearance of a residual fluid phase. In addition, the fluid composition was transformed from hydrocarbonate into hydrocarbonate–methane with the increase in the concentrations of chlorides of alkali and alkaline-earth metals (from 1 to 8 wt % of NaCl-equiv) along with the ore-forming elements (As, Sb, Fe, Zn, etc.). The pressure under which the formation of the gold–quartz veins occurred (about 0.25 kbar) limited the formation depth of the deposit to the range from 1 to 2.5 km (for the lithostatic and hydrostatic gradient, respectively).