Orogenic Deposits Research Articles

Evolution of Pyrite Compositions at the Sizhuang Gold Deposit, Jiaodong Peninsula, Eastern China: Implications for the Genesis of Jiaodong-Type Orogenic Gold Mineralization

Gold deposits in the Jiaodong Peninsula represent a primary gold resource in China and mostly exhibit similar ore-forming features related to sericite-quartz-pyrite alteration and other controls from (micro-)structural deformation. This study investigates the pyrite textures and trace elements in the Sizhuang gold deposit (>100 t Au) to document the key factors impacting on the genesis of the Jiaodong-type orogenic deposits. Three main types of pyrite are identified: (1) the first generation of pyrite (Py1) occurs as disseminated euhedral to subhedral grains in K-feldspar-albite-rutile-hematite and sericite alteration (stage 1), (2) Py2 as aggregates in quartz-sericite-pyrite altered rocks or quartz-pyrite veins (stage 2) can be subdivided into Py2a as irregular cores, Py2b as a zoned overgrowth on Py2a, and Py2c as overgrowth on early pyrite, and (3) Py3 as fine-grained crystals in siderite-polymetallic veins (stage 3). Primary gold at the Sizhuang deposit is coevally or slightly later deposited with Py2b, Py2c, and Py3. Laser ablation–inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses show that the highest Co and Ni contents in Py1 and high but variable Co in Py2b favors the involvement of deep high-temperature magmatic waters at stage 1 and middle stage 2. The elevated As contents from Py2a to Py2c and depletion of trace elements (e.g., Co, Ni, As and Te) and high Au/Co, Cu/Ni, and As/Ni values in Py2a and Py3, combined with published H-O isotope data, imply a meteoric water ingress during stage 2–3. Thus, the fluid evolution at Sizhuang is a consequence of pulsed deep magmatic fluid release plus progressive meteoric fluid ingress. The rhythmic Co–As–Ni–Au bands of Py2b additionally suggest episodic changes in the composition of ore-forming fluids. Moreover, the sharp textural features (e.g., pyrite overgrowth on previously cataclastic crystals) of Py2 and As-Cu-rich and Co-poor bands in zoned Py2b probably also reflect rapid metal deposition and self-organization and subsequent mineral crystal growth due to the pressure release during phase separation in the Sizhuang deposit. Considering the significantly concentrated gold (>1300 t) in the regional Jiaojia fault zone and Au-bearing mineral formation related to phase separation (boiling) in the Sizhuang deposit, gold mineralization in the Sizhuang deposit was interpreted to be controlled by the pressure-driver owing to the seismic activities in the Jiaojia fault system.

Transport and coarsening of gold nanoparticles in an orogenic deposit by dissolution\u2013reprecipitation and Ostwald ripening

The role of nanoparticle gold in the formation of gold deposits has long been debated. Evidence in support of this process is generally limited to epithermal-, Carlin- and seafloor massive-sulfide-type deposits where gold nanoparticles are associated with primary fluid related processes. At the Kenty orogenic deposit in Ontario, Canada, gold has been remobilized from early pyrite through secondary coupled dissolution-reprecipitation processes to form high-grade gold external to pyrite. Here we report gold nanoparticles and related textures that help document this gold coarsening process. A combination of focused ion beam technology and transmission electron microscopy provides a rare glimpse of gold coarsening frozen in time, which includes nanoparticles trapped in iron oxides and rutile and non-oriented attachment of gold nanoparticles to bulk gold suggesting coarsening via Ostwald ripening. The processes documented are applicable to orogenic deposits formed through Earth’s history, and may explain the formation of ultra high-grade ore zones.

Revealing the multi-stage ore-forming history of a mineral deposit using pyrite geochemistry and machine learning-based data interpretation

Classification algorithms were constructed based on pyrite trace elements using two machine learning methods, support vector machine (SVM) and artificial neural network (ANN), to discriminate the genesis of pyrites sampled from sedimentary rock, orogenic and volcanic hosted massive sulfide (VHMS) deposits. The classifiers were trained with a dataset including published trace element compositions of 2104 pyrite samples from 92 mineral deposits or stratigraphic units. Cross validations were conducted to evaluate the performances of the classifiers on unknown samples, using a variant of the k-fold method. Each time, all pyrite samples from one of the 92 deposits/strata were set aside as the testing set, and the classifiers were trained with samples of the remaining 91. Then, the performances of the classifiers were evaluated based on whether it can correctly determine the genesis of testing deposit/stratum. The circulation was repeated 92 times for each one of the deposits/strata in the dataset, and 91 of them were correctly classified by the SVM-based classifiers, and 90 by the ANN-based. The trained classifiers were then applied to reveal the genesis of the Jiashengpan Zn-Pb deposit, which is characterized by two stages of pyrite formation: early-stage fine-grained massive and late-stage coarse-grained hydrothermal pyrite. The trained algorithms show that the early-stage is compositionally similar to sedimentary pyrite, while the late-stage has affinity to those from orogenic deposits, consistent with geological and geochemical features revealed by previous studies. This study sheds light on the power of machine learning in decoding the geochemical data of pyrite, which can well record the history of ore formation.

A New Density Model of Quartz Solubility in H2O-CO2-NaCl Ternary Systems up to High Temperatures and High Pressures

A novel density model for computing quartz solubility in H2O-CO2-NaCl hydrothermal fluids applicable to wide ranges of temperature and pressure is proposed. Based on the models of Akinfiev and Diamond (2009) and Wei et al. (2012), the effective partial molar volume of water (VH2O∗) is replaced by the partial molar volume of water (V¯H2O) by implementing an empirical correction, and water molar fraction (xH2O) is modified with water activity (aH2O), in addition to a series of changes to the model coefficient forms. The absolute values of averaged relative deviation of this model compared to the experimental data sets in pure water, H2O-CO2, and H2O-NaCl solutions are 5.74%, 6.69%, and 7.09%, respectively, which are better than existing models in the literature. The model can be reliably used for computing quartz solubilities in pure water from 0°C to 1000°C, from 0 bar to 20,000 bar, and in CO2- and/or NaCl-bearing solutions from 0°C to 1000°C, from 0 bar to 10,000 bar (with slightly lower accuracy at 5000-10,000 bar in H2O-NaCl systems) in the single liquid region. Moreover, the trends and overall ranges of this model may probably be more accurate in the H2O-CO2-NaCl fluid mixtures compared to the limited experimental data. In addition, a bisection algorithm for deriving the isopleths of quartz solubilities based on this new model is first proposed, and application perspectives are discussed for various geologic settings including subduction zone, lower crust-upper mantle, migmatite, pegmatite, porphyry, and orogenic deposits.

Multivariate statistical analysis of trace element compositions of native gold from orogenic gold deposits: Implication for mineral exploration

The mineralogy and chemical composition of native gold from twenty-seven representative orogenic gold deposits were investigated using optical microscopy, electron probe microanalysis (EPMA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Minor elements, such as Ag and Cu, occur in solid solution, while trace elements, such as Fe, As, S, Hg, form impurities or micro inclusions in native gold. Partial least squares-discriminant analysis (PLS-DA) has identified compositional characteristics based on mineral association, gold texture and dominant country rocks. Gold grains are commonly associated with pyrite, arsenopyrite, chalcopyrite, pyrrhotite, and tourmaline in the studied orogenic deposits. Chemical variations in gold with different associated mineral assemblages are related to the partitioning of trace elements between co-crystalizing minerals and gold during precipitation. Gold inclusions in gangue minerals are discriminated from later gold in fractures based on contents of Ag, Fe, Pb, and Bi, which indicates that gold in different paragenetic stages of mineralization can be identified using its trace element signature. Gold hosted in different regional country rocks can be discriminated by Ag, Cu, Pd, Sb, and Hg, likely because of reactions of hydrothermal fluids with the regional country rocks along the fluid flow paths.

Forms of Gold and Some Typomorphic Characteristics of Native Gold of the Pavlik Orogenic Deposit (Magadan Oblast)

Abstract—The ratios of vein and ore minerals bearing native gold, the shape and size native gold particles, their surface features, variations in chemical composition, and the presence and distribution of impurities and inclusions in them, allow sufficiently substantiated assumptions about ore-forming settings and composition, phase state, and certain other parameters of ore-forming fluids that precipitated native gold. If these data are insufficient for a full-fledged judgment on the ore formation processes, they can at least be used to curtail various genetic speculations. Certain typomorphic features of native gold were studied from one of the largest orogenic gold deposits in northeastern Russia. New data on the grain size distriubtion and chemical composition of native gold and gold contents in sulfides of the Pavlik deposit were obtained. Optical and SEM images were used to determine the ore texture, morphology, and internal structure of native gold and auriferous pyrite and arsenopyrite. ICP-MS, EDS, EDX were used to assess the composition of gold grains, EPMA and LA-ICP-MS were used to qualitatively and quantitatively characterize gold-bearing grains of pyrite and arsenopyrite. It was established that microscopic (finely dispersed (>0.0005–0.01), dusty (>0.01–0.05), extremely fine (>0.05–0.1)) (peak size at 0.075 mm), visible (very fine (>0.1 –0.25)), and fine (>0.25–1.00)) (size peaks at 0.25, 0.55 and 0.85 mm) native gold in association with sphalerite, pyrrhotite, pyrite, arsenopyrite, quartz, carbonates, albite, and sericite is widespread at the Pavlik deposit. Gold particles with an average size of 0.3 mm predominate by weight. Comparison of grain size distribution data with the technological properties of ores and the results of mining in the Omchak ore–placer district made it possible to establish that the share of dusty and fine-dispersed size classes accounts for at least 15–20% of the gold mass at the deposit. ICP-MS analysis of a bulk sample of a monofraction of native gold, which included native gold grains from different parts of the deposit, showed that the composition of the monofraction includes Fe, Hg, As, Zn, Cu, and Pb impurities. The average gold fineness was 806 with additional peaks of 775, 855, and 985 (EPMA determinations). The heterogeneity of native gold is due to the redistribution of silver during late deformation events and, to a lesser extent, removal of silver from gold in the ore oxidation zone. Inclusions in gold are represented by microvoids, disseminated fragments of host metasomatites, and crystals or crystal fragments of potassium feldspar, albite, ankerite–dolomite, and arsenopyrite. The low gold content in pyrite and arsenopyrite, with a high (up to 287 g/t) gold content in arsenic-bearing pyrite and ratios of molar amounts of gold and arsenic in arsenopyrite, may indicate that gold is bound in the crystal structure of sulfides. The relationship between native gold and vein and ore minerals, the features of native gold, and the regularities of gold distribution in pyrite and arsenopyrite do not contradict the hypothesis that native gold was deposited at a single mineralization stage in a different substrate: (1) in the intergranular space (quartz–carbonate metasomatites), (2) in the interstitial space (in arsenopyrite and carbonates), (3) in microfissures in the host minerals. Obviously, the deposition of native gold is an independent process occurring against a background of metamorphogenic (quartz–carbonate veins) and magmatic–metamorphogenic (disseminated pyrite and arsenopyrite) mineralization. Comparison of native gold from ore and placer objects of the Omchak ore–placer district has culminated in methods for using the typomorphic features of native gold to search for placer sources. It seems that various inclusions in native gold with a porous surface are promising for identifying old sources and reconstructing its crystallization conditions.

Finding forms and Micro- and Nanoscale Assemblages of Gold as Indicators of Formation Conditions, Distribution, and Typification of Orogenic Deposits of Uzbekistan (South Tien Shan)

Abstract—Gold deposits of the Kyzylkum and Nurata regions of Uzbekistan are confined to the South Tien Shan orogenic belt. They are hosted in black shales (Muruntau and Myutenbai); carbonate, terrigenous, and volcanogenic rocks (Kokpatas and Balpantau); and in intrusive units (Zarmitan zone). The age of gold mineralization is 280–290 Ma, which corresponds to the age of postcollisional granitoid magmatism. Gold is observed as micro- and nanoscopic particles and is a part of various minerals (Au2Bi, AuTe2, AuAg2Te3, AuAgS, AuAg2Se3, AuSb2, etc.), which form inclusions in pyrite, arsenopyrite, antimonite, and quartz. These minerals form regular micro- and nanoscopic assemblages, which are direct signs of prospecting and evaluating gold ores of the distinguished mineral-geochemical types.

Characteristics and nature of gold-bearing fluids in Fatira area, North Eastern Desert of Egypt: possible transition from intrusion-related to orogenic deposits

The gold lode in Fatira area is represented in three modes of occurrence: (1) altered felsite dykes, (2) massive quartz veins that are cutting through the hosting granitic rocks, and (3) siliceous-hybrid shear zone which is trending in NNW-SSE direction and separating between metavolcanic and granitic rocks. Metavolcanic rocks are described by meta-andesite and meta-dolerite varieties; they exhibit calc-alkaline magma type related to island arc system. However, granitic rocks are of I-type granodiorite composition with calc-alkaline affinity developed in convergent plate environment (island arc/active continental margins) during the late stage of orogeny. The wallrock alteration around the ore body includes propylitization, sericitization, and silicification facies. The ore minerals were observed as disseminated sulfides (pyrite, sphalerite, chalcopyrite, and covellite). Goethite, jarosite, and ilmenite are the main iron-rich mineral phase as well as malachite and rutile. Gold was detected by microprobe analysis within the internal composition of pyrite and/or jarosite. Two groups of fluid inclusions could be recognized in samples from different mineralized zones: (a) aqueous fluid inclusions (H2O–NaCl/KCl) and (b) aqueous-carbonic fluid inclusions (H2O–CO2–NaCl/KCl). Isochore construction from fluid inclusion data, combined with chlorite thermometry from alteration zone, revealed that Fatira gold deposits were formed at temperature range of 275–297 °C and pressure between 0.2 and 1.1 kb). The Fatira gold deposits exhibit similar features of both intrusion-related and orogenic deposits. Hence, the initial ore-bearing fluid was proposed to have originated from magmatic source at deeper levels related to the granitic emplacement (5–10 kb); moreover, the granitic batholith acted as an important vehicle for driving of the metamorphic hydrothermal currents and probably as a cause for the metamorphic process itself. Then, the migration of gold was taken place in form of bi-sulfide complexes and the final deposition occurred through fluid uplifting consequent with a possibility of mixing with meteoric water at shallow depths, pressure/temperature drop, and fluid/wallrock interactions.

Towards an integrated tectonic model for the interaction between the Bambuí basin and the adjoining orogenic belts: Evidences from the detrital zircon record of syn-orogenic units

The São Francisco craton and its surrounding orogens are some of the most important tectonic elements of South America, and key regions to understanding the assembly of Western Gondwana in the Proterozoic-Palaeozoic transition. During this major tectonic event, diachronic collisions between small continents developed an intricated orogenic system, where several orogens evolved in unique paths through time and space. In such collisional settings, there are some tectonic processes that promote subsidence, thus controlling the formation of orogenic-related sedimentary basins. Furthermore, the tectonic activity in collisional orogens generates dynamic landscapes that usually favor increased erosion and sediment generation to feed these basins, making them key places to seek for clues about the tectonic evolution of their surroundings. On the São Francisco craton, the Bambuí Group records a complex foreland system, which evolved in response to the lithospheric overload exerted by the uplift of both Brasília belt and Araçuaí orogen. In turn, both Salinas Formation and Ibiá Group comprise orogenic deposits resting within the Araçuaí orogen and Brasília belt, respectively, whose tectonic significance is still under debate. Here we present new U–Pb (LA-ICP-MS and SHRIMP) and Lu–Hf isotopic analyses on a great amount of detrital zircon grains extracted from the rocks of the Bambuí Group and Salinas Formation, together with a thorough data compilation from the literature. The two units present similar provenance patterns, sharing the major detrital zircon age peaks (550–650 Ma, 950–1050 Ma, 1750–2000 Ma, 2600–2800 Ma) and maximum depositional age in c. 550 Ma. The Ediacaran zircons recovered from the Bambuí Group show a wide range of the εHf(t), ranging from c. −17 to +15, which suggest the existence of multiple late Neoproterozoic sources, some of them juvenile and some with a long crustal residence. The variation in detrital zircon age patterns and εHf(t) values from different units within the Bambuí Group provided additional clues of provenance changes occurred during the evolution of the basin. The new data obtained for the Salinas Formation constrain its deposition between 548 and 500 Ma, which have an important implication on its tectonic significance. We propose that in both Brasília belt and Araçuaí orogen sides, the early foredeep deposits of the Bambuí basin should have been incorporated to the orogenic domains, which could explain the apparent lack of deposits recording the climax of the Brasilia belt uplift (c. 630 Ma) within the cratonic area. In this same direction, we consider that both Salinas Formation and Ibiá Group could represent remnants of these early foreland deposits related to the uplift of Brasília belt and Araçuaí orogen, respectively, incorporated to the orogenic wedges due to the advance of the deformational fronts. Therefore, what we know as Bambuí Group is in fact the remaining record of an advanced stage of the foreland system, when subsidence was already influenced by the two evolving orogens. Altogether, the analyses of the stratigraphic, structural and geochronological data converge towards an integrated tectonic model for the interaction between the Bambuí basin and the surrounding orogens during West Gondwana amalgamation.

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.

Geology and pyrite sulfur isotopes of the Suoluogou gold deposit: Implication for crustal continuum model of orogenic gold deposit in northwestern margin of Yangtze Craton, SW China

Since the crustal continuum model was proposed mainly based on the variation of mineralization and alteration features of hypozonal, mesozonal, to epizonal deposits, genetic relationship between the different types in one ore belt still requires research. This issue was addressed by the research on the Suoluogou mesozonal orogenic gold deposit and its comparison with the hypozonal orogenic gold deposit in northwestern margin of Yangtze craton. Gold mineralization in Suoluogou is controlled by the bends of roughly EW-trending brittle fault developed in the Late Triassic strata, and it is associated with quartz, sericite, chlorite, carbonate, and sulfide alteration. Pyrite and arsenopyrite are the predominant sulfide mineral, with minor amounts of chalcopyrite, sphalerite, and galena. Four paragenetic stages are divided based on crosscutting relationship and mineral assemblage, including the pre-mineralization stage (stage Ⅰ), the hydrothermal quartz-sulfide stage (stage Ⅱ) and carbonate-quartz-sulfide stage (stage III), and the post-mineralization carbonate-pyrite stage (stage Ⅳ). The δ34S values for pyrite in stage Ⅱ vary from 6.01% to 8.59‰, and those in stage III are from 0.96% to 4.62‰. The decrease of δ34S value from stage Ⅱ to III is explained to the result of the isotope fractionation during separation of oxidised sulfur species in the brecciated space. The sulfide δ34S values in the initial gold mineralization stage of the Suoluogou deposit are similar to those of regional hypozonal orogenic deposit with derivation from metasomatized mantle in the craton margin. This similarity suggests that the mesozonal and hypozonal gold deposits formed in different depths of a crustal continuum model for the deposits sharing similar subcrustal source.

Physicochemical Parameters and Geochemical Features of Ore-Forming Fluids for Orogenic Gold Deposits Throughout Geological Time

This paper reviews data from numerous publications focused on the physicochemical parameters and chemical composition of ore-forming fluids from orogenic gold deposits formed during various geological epochs. The paper presents analysis of the distribution of the principal parameters of mineralizing fluids depending on the age of the mineralization. Some parameters of the fluids (their salinity and pressure) at orogenic gold deposits are demonstrated to systematically vary from older (median salinity 6.1 wt.%, median pressure 1680 bar) to younger (median salinity 3.6 wt.%, median pressure 1305 bar) deposits. The detected statistically significant differences between some parameters of mineralizing fluids at orogenic gold deposits are principally new information. The parameters at which mineralization of various age was formed are demonstrated to pertain to different depth levels of similar mineralization-forming systems. The fluid parameters of the most ancient deposits (which are mostly deeply eroded) correspond to the deepest levels of orogenic fluid systems. Hence, the detected differences in the salinity and pressure of the mineralizing fluids at orogenic deposits of different age reflect the vertical zoning of the mineralizing fluid systems.

Direct imaging of alteration with high-resolution hard-rock 3D seismic data at the Darlot Gold Mine

SummaryThe Darlot gold mine is an Archean orogenic deposit located in the world-class gold and nickel terrain of the Yandal granite-greenstone belt within the Yilgarn Craton in Western Australia.In 2016-2017, HiSeis designed, acquired and processed a high-resolution 3D seismic survey centred on the Darlot-Centenary mineralised system with the objective of improving lithological and structural interpretation of the project area and to support targeting of mineralisation.The capability of modern 3D seismic surveys to image formational contacts and structures in hardrock environments has had a game-changing impact on the effectiveness of brownfields exploration programs because the geometry of mineralised systems can now be directly imaged over large volumes of ground. More specifically, partially-preserved relative seismic amplitudes can allow the direct imaging of alteration associated with gold mineralisation in fertile structures, allowing discrimination between these and barren structures and dramatically improving drilling success rates.We present the results of a structural interpretation of the seismic data which focused on identifying drilling targets prioritised with seismic amplitude variation.

3D interpretation of geological, 3D seismic and conventional geophysical data from the Darlot Gold Mine

The Darlot gold mine is an Archean orogenic deposit located in the world-class gold and nickel terrain of the Yandal granite-greenstone belt, part of the Yilgarn Craton in Western Australia.A 3D se...

Formation of the Tocantinzinho 1989 Ma gold-only deposit (Pará State, northern Brazil), based on sulfur, oxygen, hydrogen, and carbon isotope data

The Tocantinzinho gold deposit in the Brazilian Amazon region, approximately 1989 Ma in age, contains 16.7 Mt of ore in syenogranite and monzogranite breccias and microbreccias, most with gold in pyrite, silicates and quartz, and a small part in quartz vein pyrites. Three hydrothermal phases are recognised, H1, H2, and H3, where H1, the first hydrothermal phase, is the oldest. The H1 transitional, late-magmatic hydrothermal phase altered feldspar to sericite and mafic minerals to chlorite. During the H2 hydrothermal phase, pyrite, gold, and minor galena were precipitated, followed by intense sericite alteration. The last hydrothermal phase, H3, was coeval with a large number of andesite intrusions that generated high-pressure fluid-driven cataclasis. This event opened veins, tension gashes, and pull aparts that were filled with carbonate, worm-like chlorite, prismatic quartz, galena, pyrite, and rutile, which likely displaced and concentrated the gold near the carbonate-filled structures.The minerals sulfide δ34S values vary between −4.4 and + 5.6‰, which is compatible with a sulfur magmatic source, and indicate that the sulfur and likely the gold are both magmatic. The δ18OSMOW values of carbonates vary between +7.63 and + 21.23‰ in a narrow range of δ13CPDB values between −3.94 and −1.69‰. Two explanations are possible for these values. If that the carbonate fluids were genetically related to the andesite magmatism, this fluid mixed with meteoric water, and degassing of this mixture occurred in the near-surface to produce dissolved inorganic CO2 with high δ18O value. Alternatively, when fluid mixing occurred, the oxidation state of the environment increased, and the δ13C values of the carbonates remained relatively uniform, whereas the δ34S values of the sulfides varied. The δ18O values of the quartz, sericite, and chlorite of the mineralized and barren rocks are consistent with δ18O values > +8 for the fluids that crystallized the chlorites and δ18O > + 4‰ for those that crystallized the muscovite. High δD values of these fluids, estimated from mineral δD values range between −2 and −25‰, and differentiate them from the fluids that form porphyry-type Cu–Au and orogenic deposits. The isotope data for Tocantinzinho present characteristics of an intrusion-related gold deposit that combines the features of equigranular, intrusion-hosted, mesozonal deposits, such as those hosted in reduced granite, with Au-rich porphyry deposits hosted in oxidized granites.

In situ Rb-Sr dating of K-bearing minerals from the orogenic Akçaabat gold deposit in the Menderes Massif, Western Anatolia, Turkey

The Akçaabat gold deposit is mainly composed of massive arsenopyrite veins in strongly foliated augen gneisses of the Çine Sub-massif in western Turkey. K-bearing minerals from orogenic gold veins were dated by the in situ Rb-Sr method in order to determine the formation age of the Akçaabat gold deposit. Textural relationships between the vein minerals suggest that arsenopyrite, K-feldspar, quartz, and muscovite formed in one paragenetic stage. Native gold, native bismuth, scorodite, and galena were introduced in a later paragenetic stage. The isochron derived from K-feldspar and muscovite occurring in the veins yielded 31.3 ± 4.7 Ma. In situ Rb-Sr dating of the same assemblage (K-feldspar and muscovite) in the host rock gave an isochron age of 40.8 ± 3.8 Ma, 10 Ma older than the vein ages, and corresponding to peak metamorphism of the regional main Menderes metamorphism during the Alpine-Himalayan orogenic event. Biotite from the host rock gave an isochron age of 28.1 ± 2.2 Ma and probably reflects the cooling age. Homogenization temperatures from fluid inclusions in quartz show that quartz formed at 280–390 ℃ but has a dominant mode in the 350–360 ℃ temperature interval. The age of vein formation was successfully distinguished from the age of the Alpine-Himalayan orogenic event associated with main Menderes metamorphism. This study demonstrates that in situ Rb-Sr isotopic study can be applied to resolve absolute dating of orogenic deposits in metamorphic complexes.

Fluid Inclusions and H-O-C-S Isotopes of the Wushan Copper Polymetallic Deposit in the Suizao Area, Hubei Province: Implications for Ore Genesis

The Wushan copper polymetallic deposit is located in the Tongbai-Dabie orogenic belt in central China. Two small granitoid stocks (Donggushan and Xigushan) occur in the deposit, which is next to the largest Qijianfeng Granite Complex in the Suizao area. The mineralization of Wushan copper polymetallic deposit is mainly composed of ore-bearing quartz veins and quartz stockworks. Two hydrothermal stages are identified as the quartz-sulfide stage (early stage) and the barren quartz stage (late stage). A detailed petrographic study shows four types of fluid inclusions in quartz, including the aqueous fluid inclusions (L+V/V+L), the aqueous-carbonic fluid inclusions (L+V+CO2), the pure carbon dioxide fluid inclusions (pure CO2), and the daughter mineral-bearing multiphase fluid inclusions (S). The daughter mineral-bearing multiphase fluid inclusions (S) are further divided into three subclasses according to their different solid mineral assemblages, including (1) S1: L+V+Hal, (2) S2: L+V+CO2+S (chalcopyrite), and (3) S3: L+V+S (calcite, chalcopyrite, and hematite)±Hal. A laser Raman spectroscopic analysis shows that the main components of fluid inclusions are water and carbon dioxide. The solid minerals of the S-type fluid inclusions include halite, calcite, chalcopyrite, and hematite. The homogenization temperatures of fluid inclusions are 377 to 468°C for the early stage, with a salinity of 11.1 to 34.1 wt.% NaCl equivalent (11.1 to 17.4 wt.% NaCl equivalent and 28.4 to 34.1 wt.% NaCl equivalent, respectively) and an estimated pressure of 89 to 137 MPa. The homogenization temperatures of fluid inclusions in the late stage are 267 to 380°C with salinity of 7.0 to 12.1 wt.% NaCl equivalent and an estimated pressure of 46 to 115 MPa. Therefore, the temperature, salinity, and pressure of the fluid show a decreasing trend from the early to the late stage. In the early stage, the fluid is immiscible, which leads to the precipitation of sulfides. Pyrite shows a δ34S of approximately 0 (-1.8 to +3.4‰), and chalcopyrite also shows a similar δ34S of approximately 0 (+1.5 to +2.4‰), which indicates that the sulfur in the ore-forming fluid is mainly derived from deep-seated magma. Combined with C-H-O isotopic compositions, the initial ore-forming fluid is likely magmatic water, but with the addition of meteoric water in the late stage. By comparing with the typical characteristics of magmatic hydrothermal vein deposit and orogenic deposit related to shear zones, we suggest that the Wushan copper polymetallic deposit is most likely a magmatic hydrothermal vein deposit, which is of great significance for the further exploration work in the Wushan and surrounding areas. This new finding also fills the gap that no magmatic hydrothermal vein type Cu deposits have been found in the Suizao area or even in the Qinling-Dabie orogenic belt in central China.

Supergene remobilization of Au in Au-bearing regolith related to orogenic deposits: A case study from Kazakhstan

Our study of the primary and supergene gold from the Suzdal and Raigorodok deposits in Kazakhstan confirms that during the formation of the regolith in both deposits, invisible and fine gold is released from the host sulfide minerals during their oxidation and destruction; and transported in the form of soluble thiosulfate complexes by near-surface solutions. Upon contact with a geochemical barrier, gold can be deposited as crystalline aggregates. In the deposition of this gold the role of geochemical barriers, such as variations of redox, pH and sorption are very important.Evidence of the past processes of dissolution, redistribution and aggregation of gold in the regolith in these deposits is provided by the following features: 1) The presence of dissolution and growth structures on the surface of newly formed gold; 2) The size of the gold grains increases and newly formed gold microcrystals appear on the surface of the grains; 3) Changes in the composition of native gold (e.g. decrease of silver content); 4) The accumulation of gold in subhorizontal localized layers within the regolith; 5) Gold association with supergene minerals (limonite, jarosite, goethite, Mn-hydroxides, opal, gypsum, calcite, clay minerals, etc.).This detailed study of the specific compositional and structural features of supergene gold and the physicochemical conditions of regolith environment allowed the recognition of the releasing, dissolution, transfer and aggregation processes of gold in the regolith.

Geochronological and sulfur isotopic evidence for the genesis of the post-magmatic, deeply sourced, and anomalously gold-rich Daliuhang orogenic deposit, Jiaodong, China

The Daliuhang gold deposit (> 20 t gold) is located in the central Penglai-Qixia belt of the giant Jiaodong gold province, eastern China. The ore-hosting Guojialing granodiorite and pegmatite were formed at 129.0 ± 0.6 Ma and 126.2 ± 0.6 Ma, respectively. Syn-ore monazite, with a U–Pb age of 120.5 ± 1.7 Ma, represents the timing of gold mineralization. Given at least 5 m.y. between magmatism and mineralization, a genetic relationship to magmatic-hydrothermal activity is negated. Noble gas isotopes of pyrite have crust-mantle-mixed 3He/4He (1.13 to 1.50 Ra) and air-like 40Ar/36Ar (327–574). Together with the broadly positive correlation between 3He and 36Ar, it is inferred that the initial ore-forming fluids were deeply sourced from the sedimentary wedge overlying the subducted plate and overlying mantle during early Cretaceous paleo-Pacific plate subduction. The pre-ore and post-ore pyrites have low δ34S values (3.7–5.6‰ and 5.3–6.4‰, respectively), whereas ore-related pyrites have higher δ34S values, especially in As–Au-rich domains (7.8–8.3‰). These positive δ34S values also suggest that the initial ore fluid and some of the sulfur component were derived via subduction-related devolatilization. The elevated δ34S values of the ore-related pyrite are partly ascribed to mass fractionation and partly to a contribution from sulfur leached from crustal host rocks. This interpretation is also supported by neodymium isotope ratios of monazite (eNd (~ 120 Ma) = − 13.7 to − 11.6), which correlate well with the ore-hosting Guojialing granodiorite. This study highlights the combined roles of deeply derived fluids and intense interaction with upper crustal rocks in the formation of Jiaodong gold deposits.

Fluid Inclusion and H-O Isotope Geochemistry of the Phapon Gold Deposit, NW Laos: Implications for Fluid Source and Ore Genesis

The Phapon gold deposit, located in northern Laos, is a unique large-scale gold deposit in Luang Prabang-Loei metallogenic belt. It is hosted in the Lower Permian limestone and controlled by a NE-trending ductile-brittle fault system. There are three types of primary ore including auriferous calcite vein type, disseminated type, and breccia type, and the first two are important in the Phapon gold deposit. Based on fluid inclusion petrography and microthermometry, three types of primary fluid inclusions including type 1 liquid-rich aqueous, type 2 vapor-rich aqueous and type 3 daughter mineral-bearing aqueous were identified in hydrothermal calcite grains. The ore-forming fluids are normally homogeneous, as indicated by the widespread type 1 inclusions with identical composition. The coexistence of type 1 and type 2 inclusions, showing similar final homogenization temperature but different compositions, indicate that fluid immiscibility did locally take place in both two types of ores. The results of microthermometry and H-O isotopes geochemistry indicate that there are little differences on ore-fluid geochemistry between the auriferous calcite vein-type and disseminated type ores. The oreforming fluids are characterized by medium-low temperatures (157—268 °C) and low salinity (1.6 wt.%-9.9 wt.% NaCl eq.). It is likely to have a metamorphic-dominant mixed source, which could be associated with dehydration and decarbonisation of Lower Permian limestone and Middle-Upper Triassic sandstones during the dynamic metamorphism. The fluid-wallrock interaction played a major role, and the locally occurred fluid-immiscible processes played a subordinate role in gold precipitation. Combined with the regional and ore deposit geology, and ore-fluid geochemistry, we suggest that the Phapon gold deposit is best considered to be a member of the epizonal orogenic deposit class.