Geology,geochronology and fluid characteristics of the Pingqiu gold deposit,Southeastern Guizhou Province,China

The junction of the southeastern Guizhou, the southwestern Hunan, and the northern Guangxi regions is located within the southwestern Jiangnan orogen and forms a NE-trending ∼250 km gold belt containing more than 100 gold deposits and occurrences. The Pingqiu gold deposit is one of the numerous lode gold deposits in the southeastern Guizhou district. Gold mineralization is hosted in Neoproterozoic lower greenschist facies metamorphic rocks and controlled by fold-related structures. Vein types present at Pingqiu include bedding-parallel and discordant types, with saddle-reefs and their down limb extensions dominating but with lesser discordant types. The major sulfide minerals are arsenopyrite and pyrite, with minor sphalerite, galena, chalcopyrite, and rare pyrrhotite, marcasite, and tetrahedrite. Much of the gold is μm- to mm-sized grains, and occurs as fracture-controlled isolated grains or filaments in quartz, galena, sphalerite, pyrite, and wallrock.Three types of fluid inclusions are distinguished in hydrothermal minerals. Type 1 aqueous inclusions have homogenization temperatures of 171–396 °C and salinities of 1.4–9.8 wt% NaCl equiv. Type 2 aqueous-carbonic inclusions yield final homogenization temperatures of 187–350 °C, with salinities of 0.2–7.7 wt% NaCl equiv. Type 3 inclusions are carbonic inclusions with variable relative content of CO₂ and CH₄, and minor amounts of N₂ and H₂O. The close association of CO₂-rich inclusions and H₂O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. The calculated δ¹⁸O_H2O values range from 4.3‰ to 8.3‰ and δD_H2O values of fluid inclusions vary from −55.8‰ to −46.9‰. A metamorphic origin is preferred on the basis of geological background and analogies with other similar deposit types.Two ore-related sericite samples yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 425.7 ± 1.7 Ma and 425.2 ± 1.3 Ma, respectively. These data overlap the duration of the Caledonian gold mineralization along the Jiangnan orogen, and suggest that gold mineralization was post-peak regional metamorphism and occurred during the later stages of the Caledonian orogeny.Overall, the Pingqiu gold deposit displays many of the principal characteristics of the Bendigo gold mines in the western Lachlan Orogen (SE Australia) and the Dufferin gold deposit in the Meguma Terrane (Nova Scotia, Canada) but also some important differences, which may lead to the disparity in gold endowment. However, the structural make-up at deposit scale, and the shallow mining depth at present indicate that the Pingqiu gold deposit may have considerable gold potential at depth.     

Ar-Ar ages of gold deposits in the Song Hien domain (NE Vietnam): Tectonic settings and comparison with Golden Triangle in China in terms of a single metallogenic province

The Song Hien rift basin is an important metallogenic area in NE Vietnam. This domain consists mainly of Triassic sulfide-rich black shale beds, which play a role as a sedimentary host for various mineral systems such as antimony, mercury and gold-sulfide deposits. Most of gold deposits are hosted in carbonaceous sedimentary rocks, however some deposits, which have similar characteristics, are hosted in fine-grained mafic magmatic rocks. An Ar-Ar isotopic dating of hydrothermal sericite from the sedimentary hosted Bo Va and Khung Khoang gold deposits and intrusion hosted orogenic Hat Han gold deposit yields plateau ages of 184.8 ± 2.1 Ma, 211.63 ± 2.3 Ma, and 209.12 ± 2.3 Ma, respectively. The obtained Ar-Ar ages convincingly show that the orogenic gold deposits in the Song Hien domain were formed in Late Triassic to Early Jurassic, while the age of the Bo Va deposit is at least older than 184.8 ± 2.1 Ma. Loss of argon by volume diffusion, supported by previously reported mineralogical and isotopic features of the Bo Va deposit may suggest that the Jurassic-Cretaceous (Yanshanian) tectonothermal events overprinted some deposits in the Song Hien domain. Formation of gold deposits in the Song Hien domain is linked to the same tectonic event as the Carlin-like gold deposits in SW China and is associated with an extensional tectonic regime that followed continental collision between the Indochina and South China Blocks. The similarity in geology setting and mineral composition of gold deposits of the Song Hien domain and the Golden Triangle region, as well as timing and kinematics of deformation, magmatic features, and stratigraphic sequence and bulk architecture, lead to conclusion that NE Vietnam and SW China is a single metallogenic zone. The study of gold deposits in Vietnam will provide a new data on the metallogenic history of this important part of SE Asia.     

Delayed accumulation of placers during exhumation of orogenic gold in southern New Zealand

The giant gold placer system on the Otago Schist of southern New Zealand was derived from Mesozoic orogenic gold deposits in the underlying schist basement. The core of the schist basement was exhumed in the middle Cretaceous, coeval with the accumulation of the oldest preserved nonmarine sedimentary rocks in the area (ca 112 Ma). Those sedimentary rocks contain quartz clasts, with distinctive ductile deformation textures, that were derived from structural zones in, or adjacent to, major orogenic gold deposits. Quartz textures in these structural zones are readily distinguishable from the rest of the schist belt, and hence provide a fingerprint for erosion of gold. The earliest sedimentary rocks on the margins of the gold-bearing schist belt are immature, and were derived from unoxidised outcrops in areas of high relief. Gold was not liberated from unoxidised basement rocks during erosion, and was removed from the system without placer concentration. Placer concentration did not begin until about 20 million years later, when oxidative alteration of gold deposits had facilitated gold grain size enhancement from micron scale (primary) to millimetre scale (secondary). Subsequent erosion and recycling of gold in the early Cenozoic, and again in the late Cenozoic, caused additional concentration of gold in progressively younger deposits. The Klondike giant placer goldfield of Canada had a similar geological history to the Otago placer field, and Klondike placer accumulation occurred in the late Cenozoic, at least 70 million years after Mesozoic exhumation of orogenic gold. The giant placer deposit on the western slopes of the Sierra Nevada in California occurs in Eocene and younger sedimentary rocks, at least 40 million years younger than the timing of major exhumation of the source rocks. Circum-Pacific giant gold placers formed under entirely different tectonic regimes from the emplacement of their source orogenic deposits, and these giant placer deposits do not form in foreland basins associated with convergent orogens. Formation of giant placers requires less active erosion and more subdued topography than the collisional orogenic activity that accompanied emplacement of source gold deposits in basement rocks, as well as oxidative alteration of the primary deposits to liberate gold from sulfide minerals and enhance secondary gold grain size.     

Recognition of Yanshanian magmatic-hydrothermal gold and polymetallic gold mineralization in the Laowan gold metallogenic belt,Tongbai Mountains: New evidence from structural controls,geochronology and geochemistry

The Laowan metallogenic belt in China is an important metallogenic belt within the Tongbai orogenic belt, and contains the medium-sized Laowan and Shangshanghe gold deposits, the small Huangzhuyuan lead–zinc–silver–gold deposit and some gold and Cu–Pb occurrences. These deposits are hosted in Mesoproterozoic plagioclase amphibolite (or schist) and mica-quartz schist. The gold ores are mainly quartz veins and veinlets and disseminated altered ores. Subordinate ore types include massive sulfides and breccias. The Laowan gold deposit is characterized by three right-stepping en-echelon fracture-controlled alteration zones that dip gently to the south and includes disseminated, sheeted and stockwork ores. These lodes were formed by the interaction of ore-forming fluid with foliated-to laminated cataclasite within the transpressional faults. The Shangshanghe gold deposit is characterized by parallel ore lodes that dip steeply to the north, and includes quartz veins and breccias in addition to ores in altered wallrocks. These lodes were formed by focusing of fluids into transtensional faults. These ore controlling faults displaced early barren quartz veins 10 m horizontally with a dextral sense of motion. The ore-hosting structures at the Laowan and Shangshanghe deposits correspond to the P and R-type shears of a brittle dextral strike-slip fault system, respectively, which make angles of about 15° and − 15° to the Laowan and Songpa boundary faults. The ore-controlling fault system post-dated formation of a ductile shear zone, and peak regional metamorphism. This precludes a genetic relationship between hydrothermal mineralization and regional metamorphism and ductile shear deformation. These gold deposits are not typical orogenic gold deposits. The metallogenic belt displays district-scale-zoning of Mo → Cu–Pb–Zn–Ag → Au relative to Songpa granite porphyry dike zone, suggesting the mineralization may be closely related to the granite porphyry. Measured δ³⁴S of sulfides and δ¹⁸O and δD of fluid inclusion waters in auriferous quartz also are consistent with a magmatic source for sulfur and ore fluids. The similarity of Pb isotope ratios between the ores and Yanshanian granitoids suggests a similar source. As the age (139 ± 3 Ma) of granite porphyry obtained by zircon U–Pb isotope overlaps the mineralization age (138 ± 1 Ma: Zhang et al., 2008a), the gold and polymetallic metallogenesis of the Laowan gold belt has close spatial, temporal and possibly genetic relationships with Yanshanian high level magmatism.     

The Mokrsko-West gold deposit,Bohemian Massif,Czech Republic: Mineralogy,deposit setting and classification

The Mokrsko-West deposit is unique among European Variscan gold deposits from the points of view of both the structure (an approx. 200 m thick complex of sheeted, several mm-thick, densely spaced quartz veins) and the economic viability (gold reserves of about 100 t). The deposit is hosted mainly by tonalite of the calc-alkaline Sázava tonalite suite (ca. 354 Ma) of the Central Bohemian Plutonic Complex. Mineralization is characterized by quartz-dominated gangue, no visible hydrothermal alteration, low sulfide content, high fineness native gold accompanied by maldonite, aurostibite, native bismuth and numerous Bi–Te–(S) phases. Five mineralogical stages are described in great detail. Arsenopyrite and chlorite thermometers, mineral phase stabilities and published isotope and fluid inclusion data are used to reconstruct the temperature and compositional evolution of the system. The role of liquid bismuth in the sequestration of gold is also discussed.The deposit shares the features of both orogenic gold (ORG) and intrusion-related gold (IRG) deposits. The IRG model is advocated by close spatial association between the ore zone and the tonalite host-intrusion, by the absence/scarcity of hydrothermal alteration, by the Au–Bi–Te–As elemental association and by marked thermal gradients from the early to late mineralization stages. The ORG model is advocated by an approx. 15–10 Ma gap between the intrusion of the tonalite-host and the ore formation, by isotope and geochemical evidence for a key role of metamorphic fluids in the mobilization and transfer of many elements/species (inclusive S and Au). The apparently ambiguous classification of the deposit can most probably be attributed to deposit formation at a depth of ≥ 9 km and to setting of the deposit at/inside a large-scale plutonic complex with multiple and prolonged tectonic and intrusive activity.     

40Ar/39Ar geochronological constraints on the formation of the Dayingezhuang gold deposit: New implications for timing and duration of hydrothermal activity in the Jiaodong gold province,China

China's largest gold resource is located in the highly endowed northwestern part of the Jiaodong gold province. Most gold deposits in this area are associated with the NE- to NNE-trending shear zones on the margins of the 130–126 Ma Guojialing granite. These deposits collectively formed at ca. 120 ± 5 Ma during rapid uplift of the granite. The Dayingezhuang deposit is a large (> 120 t Au) orogenic gold deposit in the same area, but located along the eastern margin of the Late Jurassic Linglong Metamorphic Core Complex. New ⁴⁰Ar/³⁹Ar geochronology on hydrothermal sericite and muscovite from the Dayingezhuang deposit indicate the gold event is related to evolution of the core complex at 130 ± 4 Ma and is the earliest important gold event that is well-documented in the province. The Dayingezhuang deposit occurs along the Linglong detachment fault, which defines the eastern edge of the ca. 160–150 Ma Linglong granite–granodiorite massif. The anatectic rocks of the massif were rapidly uplifted, at rates of at least 1 km/m.y. from depths of 25–30 km, to form the metamorphic core complex. The detachment fault, with Precambrian metamorphic basement rocks in the hangingwall and the Linglong granitoids and migmatites in the footwall, is characterized by early mylonitization and a local brittle overprinting in the footwall. Gold is associated with quartz–sericite–pyrite–K-feldspar altered footwall cataclasites at the southernmost area of the brittle deformation along the detachment fault. Our results indicate that there were two successive, yet distinct gold-forming tectonic episodes in northwestern Jiaodong. One event first reactivated the detachment fault along the edge of the Linglong massif between 134 and 126 Ma, and then a second reactivated the shears along the margins of the Guojialing granite. Both events may relate to a component of northwest compression after a middle Early Cretaceous shift from regional NW–SE extension to a NE–SW extensional regime.     

Geodynamic settings for Paleoproterozoic gold mineralization in the Svecofennian domain: A tectonic model for the Fäboliden orogenic gold deposit,northern Sweden

Northern Sweden is currently experiencing active exploration within a new gold ore province, the so called Gold Line, situated southwest of the well-known Skellefte VMS District. The largest known deposit in the Gold Line is the hypozonal Fäboliden orogenic gold deposit. Mineralization at Fäboliden is hosted by arsenopyrite-rich quartz veins, in a reverse, mainly dip-slip, high-angle shear zone, in amphibolite facies supracrustal host rocks. The timing of mineralization is estimated, from field relationships, at ca. 1.8 Ga.The gold mineralization is hosted by two sets of mineralized quartz veins, one steep fault–fill vein set and one relatively flat-lying extensional vein set. Ore shoots occur at the intersections between the two vein sets, and both sets could have been generated from the same stress field, during the late stages of the Svecofennian orogen.The tectonic evolution during the 1.9–1.8 Ga Svecofennian orogen is complex, as features typical of both internal and external orogens are indicated. The similarity in geodynamic setting between the contemporary Svecofennian and Trans-Hudson orogens indicates a potential for world-class orogenic gold provinces also in the Svecofennian domain.The Swedish deposits discussed in this paper are all structurally associated with roughly N–S striking shear zones that were active at around 1.8 Ga, when gold-bearing fluids infiltrated structures related to conditions of E–W shortening.     

The Dewulu reduced Au-Cu skarn deposit in the Xiahe-Hezuo district,West Qinling orogen,China: Implications for an intrusion-related gold system

The Triassic West Qinling orogen, which formed by the continental collision between the South China block and North China craton following the subduction and closure of the paleo-Tethyan ocean, hosts numerous gold deposits that have commonly considered as products of metamorphic dehydration during the oceanic subduction and subsequent continental collision. However, whether or not there are gold deposits that are genetically related to regional magmatism remains poorly understood. Here we present mineralogical, compositional, and geochronological data of the Dewulu Au-Cu skarn deposit in the Xiahe-Hezuo district to understand the ore genesis and its genetic link to many coeval sediment-hosted disseminated and magmatic-hosted vein gold deposits in the same district, which collectively reveal a possible intrusion-related gold system. The Dewulu Au-Cu skarn deposit in the eastern zone of the Xiahe-Hezuo district is associated with the early Triassic I-type, ilmenite-series Dewulu quartz diorite pluton that intrudes Permian marine clastic and carbonate rocks. Ore-related hydrothermal biotite separates yield a well-defined ⁴⁰Ar/³⁹Ar plateau age of 239.9 ± 1.4 Ma (2σ), which agrees with a previous zircon U-Pb age (238.6 ± 1.5 Ma at 2σ) of the Dewulu quartz diorite and thus demonstrates a temporal and likely genetic link between the two. Both prograde and retrograde skarn assemblages are well developed in the Dewulu Au-Cu deposit. Prograde garnet and pyroxene are compositionally dominated by grossularite and hedenbergite, respectively, indicating a reduced skarn system. Au-Cu mineralization is largely represented by the arsenopyrite-loellingite-chalcopyrite-pyrrhotite-bornite assemblage and is associated with retrograde skarns. Gold is mostly hosted in arsenopyrite, loellingite and chalcopyrite, and has close textural relations with native bismuth or bismuthides. Mineral stability relationships and fluid inclusion microthermometric data confirm that the prograde skarns formed at a low oxygen fugacity, high temperature (> 630 °C), and a depth range of 2.9 to 6.5 km. Arsenopyrite and chlorite geothermometers indicate that the retrograde skarns and late quartz-sulfide-calcite assemblages formed at 550 to 345 °C and < 350 °C, respectively. The oxygen and sulfur fugacities of ore fluids were below the pyrite-pyrrhotite buffer during the retrograde stage, confirming a reduced fluid system. The reduced nature of ore-forming fluids is inherited from the reduced Dewulu quartz diorite, which was likely caused by significant assimilation of Permian to early Paleozoic carbonaceous and pyritic sedimentary sequences into the parental magmas that originally had a high oxidation state. The Xiahe-Hezuo district also hosts several major sediment-hosted disseminated and magmatic-hosted vein gold deposits, which are broadly coeval with the Dewulu Au-Cu skarn deposit. All those disseminated and vein gold deposits have also been recognized to be genetically related to reduced granitoid intrusions in the Xiahe-Hezuo district. These deposits combined constitute the first intrusion-related gold system in the West Qinling orogen.     

Typomorphic features of placer gold of Vagran cluster (the Northern Urals) and search indicators for primary bedrock gold deposits

The Vagran placer cluster is located on the eastern slope of Northern Urals. During > 100 years of gold mining history approximately 40 tons of gold have been extracted from the placer deposits.Bedrocks of the region consist of high metamorphic Upper Proterozoic and Paleozoic terrigeneous, terrigeneous-volcanogenic and igneous rocks. Gold placer deposits are mostly alluvial genesis deposits and of Quaternary to Oligocene (?) age. The alluvial deposits consist of gravel with pebbles, boulders, and sandy clay covered by sandy silt and a soil layer. The thickness of the alluvial sequence is usually 5–10 m and reaches 18 m in the main watercourses of the third order. Nearly all of the alluvial sediments are gold bearing but concentrations of economic importance prevail in the bottom part of the sequence above the bedrock.There are four different types of gold particles: (I) rounded and well-rounded particles of high fineness and homogeneous inner structure, (II) rounded to sub-rounded high fineness particles with a pure gold rim developed over a core, (III) crystallomorphic (idiomorphic) high fineness with a homogeneous inner structure, and (IV) irregular angular and subangular particles of medium fineness with a significant content of Ag (10–40 wt.%) and elevated Hg (up to 1.15 wt.%).The first type is prevalent and comprises up to 65% of the total gold particles; it is uniformly distributed throughout the territory. There are features with initially complicated dendritic and laminar shaped particles which were rounded during transportation. The second and third types have a propensity for zones of the inherited erosion–tectonic depressions. Apparently, types I, II and III are related with orogenic mesothermal gold-sulfide-quartz mineralization; the differences of these types depend on the primary zonation of ore bodies and supergenic transformation of the alloys. They were connected with middle-depth ore bodies of an orogenic gold-sulfide-quartz formation. The fourth type is evident of nearby transportation from primary sources and a short duration of supergenic influence. It is controlled by a zone of NW-SE orientation, diagonal to the main structures of Ural Fold Belt.The plot of Au content vs coefficient of heterogeneity (ratio of the Au content in the core and in the rim of the grains) is the distinguishing factor between the four types of gold grains both by primary hypogenetic characteristics and supergenetic features.No corresponding lode occurrence of gold-sulfide-quartz mineralization has been identified to date in this region. Placer gold concentrations are related to the intermediate hosts of the Mesozoic-Cenozoic surfaces of the Ural peneplain uplift in the Oligocene and eroded in Miocene-Quaternary time. This factor determines the widespread distribution of placer gold in the territory of the Vagran cluster.The large, Carlin-type Vorontsovsk gold deposit is located 60 km south-east from the Vagran area. It has a shallow erosional level, small size of native gold, and its distal location from the placer deposits makes it an unlikely primary source for the Vagran placers. However, mineralization of this type of deposit is noted within the cluster.Gold of the fourth type nearly resembles the gold of the Vorontsovsk deposit and, apparently, the source is related to the same hydrothermal mineralization event. ICP MS analyses of the quartz-sulfide lodes in the floor of gold-bearing valleys revealed a gold content of 2.0–6.9 g/t in the zone of type IV distribution. Therefore, gold of the fourth type can be used as an indicator for the exploration of primary bedrock mineralization. The geological setting and typomorphic features of this placer gold shows that the primary gold mineralization is similar to the Vorontsovsk deposit and within the zone of distribution of the placer gold of the fourth type.     

Tectonic controls on gold deposits and their environmental impact, New Zealand

New Zealand is an active orogenic belt which varies along its length from continental collision to continent–ocean subduction tectonics. Mesothermal gold deposits, with rare mercury, have formed in collisional settings from late Mesozoic to Pleistocene, with youngest deposits along the axis of the actively rising Southern Alps mountain chain. Epithermal gold and mercury deposits have formed, and are still forming associated with calcalkaline volcanism above the subduction zone, and with basaltic volcanism associated with extensional faulting. The environmental impact of mineral deposits is related to tectonic setting, mineralogy, and climate, all of which are governed by the geometry of the orogen. Mesothermal mineralisation added carbonates to calcite-bearing host rocks in what is now cool semiarid or extreme Alpine settings, and oxidation of sulphides does not result in significant acidification of the environment. Arsenic is the principal metal of environmental significance, and is readily mobilised from these deposits at neutral to alkaline pH. Mercury may be leached from cinnabar and/or gold on geological time scales. In contrast, epithermal mineral deposits have hydrothermal clay alteration, locally further clay-altered by deep temperate to subtropical weathering, and calcite is generally subordinate to sulphides. Acidification accompanies oxidation of these deposits, and copper, cadmium, lead and zinc are readily mobilised into the environment. Mercury can be mobilised by these acid solutions as well, where mercury occurs in minerals other than cinnabar.     

The optimal windows for seismically-enhanced gold precipitation in the epithermal environment

Epithermal gold (Au) deposits result from the combination of a sustained flux of metal-rich fluids and an efficient precipitation mechanism. Earthquakes may trigger gold precipitation by rapid loss of fluid pressure but their efficiency and time-integrated contribution to gold endowment are poorly constrained. In order to quantify the feedbacks between earthquake-driven fracturing and gold precipitation in the shallow crust, we studied the gold-rich fluids in the active Tolhuaca geothermal system, located in the highly seismic Southern Andes of Chile. We combined temperature measurements in the deep wells with fluid inclusion data, geochemical analyses of borehole fluids and numerical simulations of coupled heat and fluid flow to reconstruct the physical and chemical evolution of the hydrothermal reservoir. The effect of seismic perturbations on fluid parameters was constrained using a thermo-mechanical piston model that simulates the suction pump mechanism occurring in dilational jogs. Furthermore, we evaluated the impact of fluid parameters on gold precipitation by calculating the solubility of gold in pressure (P)–enthalpy (H) space. The reconstructed fluid conditions at Tolhuaca indicate that single-phase convective fluids feeding the hydrothermal reservoir reach the two-phase boundary with a high gold budget (~ 1–5 ppb) at saturated liquid pressures between 20 and 100 bar (210 °C < T_sat < 310 °C). We show that if hydrothermal fluids reach this optimal threshold for gold precipitation at a temperature near 250 °C, small adiabatic pressure drops (~ 10 bar) triggered by transient fault-rupture can produce precipitation of 95% of the dissolved gold. Our results at the active Tolhuaca geothermal system indicate that subtle, externally-forced perturbations – equivalent to low magnitude earthquakes (Mw < 2) of a hydrothermal reservoir under optimal conditions – may significantly enhance gold precipitation rates in the shallow crust and lead to overall increases in metal endowment over time.     

Boron,sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt,eastern Finland

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73 Ga volcanic and sedimentary sequences, as well as 2.75–2.72 Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70 Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5 kbar pressures and at 500–600 °C temperatures affected the belt at around 2.70 Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe^3 +–Al^3 +-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ¹¹B data for this kind of tourmaline is from − 17.2‰ to − 12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe^3 +–Al^3 + substitution) and δ¹¹B values between − 19.0‰ and − 9.6‰. The uvite–foitite compositional trend and δ¹¹B ‰ values between − 24.1% and − 13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ¹¹B values between − 14.5‰ and − 10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ³⁴S_VCDT values measured in pyrite, chalcopyrite and pyrrhotite is from − 9.1 to + 8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ³⁴S_VCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ³⁴S_VCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ³⁴S_VCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ⁶⁵Cu_NBS978 values from chalcopyrite is from − 1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ⁶⁵Cu_NBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ³⁴S_VCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.     

Archean turbidite hosted orogenic gold mineralization in the Gadag greenstone belt,Western Dharwar Craton,Peninsular India

Turbidite hosted orogenic gold mineralization in the Archean Gadag greenstone belt of the Western Dharwar Craton, forms a major auriferous zone (Central Auriferous Zone) extending over a strike length of about 12 km in the Gadag duplex. The turbidite sequence comprises thick inter-bedded, medium to coarse grained lithic graywacke and thin laminated layers of fine grained carbonaceous phyllite. Gold bearing quartz veins impregnate preferentially along the en-echelon shear planes, fractures and schistosity planes. Auriferous quartz veins are enveloped by the altered wall rocks.Mineralogy of the auriferous zone is dominated by gangue minerals like quartz, ankerite, chlorite, sericite and carbonaceous matter, with subordinate plagioclase. Monazite and xenotime are the important accessory minerals. Arsenopyrite and pyrite are the major sulfide minerals, but pyrrhotite, chalcopyrite, sphalerite, galena and scheelite are also present. Gold in native state occurs within quartz, silicates and arsenopyrite.Notable distinctions in mineral assemblage, texture and in chemical compositions of altered wall rocks compared to the precursor host rock in the study area implies that the metasomatism and wall rock alterations are the results of pervasive infiltration and intense interaction between hydrothermal fluids and the surrounding host rocks over a prolonged period.Sulfides, carbonates, carbonaceous matter, K₂O, MgO, CaO, Cr, Ni, Cu, Pb, Zn, As and higher values of gold (0.98–4.72 ppm) are added into the altered wall rocks, immediately enveloping the auriferous quartz vein bodies. The chondrite normalized REE pattern of altered wall rocks exhibits enriched LREE (La_N/Yb_N = av. 9.54), with prominent negative Eu anomaly. The observed variation in geochemical characteristics and mineral assemblages in the alteration zones indicates differential response of the host rock and intensity of alteration depending on the composition of host rocks and hydrothermal fluids.The auriferous hydrothermal fluids were of low salinity (2.0 to 6.6 wt.% NaCl), dominated by CO₂–H₂O (about 30 mol% CO₂) with moderate densities (0.7 to 1.04 g/cm³), and gold deposition occurred over a wide temperature range between 175 °C and 325 °C. Gold deposition was influenced by fluid mixing, phase separation and redox reactions. Mixing between CO₂–H₂O fluids and more reduced fluids, which evolved during fluid reaction with adjacent carbonaceous wall rocks, was the key factor causing gold deposition.The formation of the Gadag duplex, deformation, folds and reverse strike slip faults (discontinuities) was caused by the compression associated with subduction related tectonic processes. During the initial period of intrusive magmatism (2,555 ± 6 Ma), regional metamorphism occurred in the entire greenstone belt, while during later period, hydrothermal fluids responsible for gold mineralization probably were derived from metamorphic processes as well as from intrusive granites. Such fluids channeled through the thrust in host turbidite sequence carrying dissolved gold, associated metals and sulfur, ultimately were precipitated in a reducing environment in the splays to the thrust in the Gadag duplex at about 2,522 ± 6 Ma, resulting in retrograde alteration assemblages.     

P–T conditions of mineralization in the Jonnagiri granitoid-hosted gold deposit,eastern Dharwar Craton,southern India: Constraints from fluid inclusions and chlorite thermometry

Gold mineralization at Jonnagiri, Dharwar Craton, southern India, is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units, typical of Archean greenstone–granite ensembles. The proximal alteration assemblage comprises of muscovite, plagioclase, and chlorite with minor biotite (and carbonate), which is distinctive of low- to mid-greenschist facies. The laminated quartz veins that constitute the inner alteration zone, contain muscovite, chlorite, albite and calcite. Using various calibrations, chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 °C and 268 to 324 °C, respectively. Gold occurs in the laminated quartz veins both as free-milling native metal and enclosed within sulfides. Fluid inclusion microthermometry and Raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous–gaseous (H₂O–CO₂–CH₄ + salt) fluid that underwent phase separation and gave rise to gaseous (CO₂–CH₄), low saline (~ 5 wt.% NaCl equiv.) aqueous fluids. Quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and P–T regime. Although the estimated P–T range (1.39 to 2.57 kbar at 263 to 323 °C) compare well with the published P–T values of other orogenic gold deposits in general, considerable pressure fluctuation characterize gold mineralization at Jonnagiri. Factors such as fluid phase separation and fluid–rock interaction, along with a decrease in f(O₂), were collectively responsible for gold precipitation, from an initial low-saline metamorphogenic fluid. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits in Australia and Canada confirms that fluids of low saline aqueous–carbonic composition with metamorphic parentage played the most dominant role in the formation of the Archean lode gold systems.     

The Youga gold deposit,Burkina Faso

The Youga gold deposits are located in southern Burkina Faso, close to the border with Ghana and classified as epigenetic mesothermal orogenic type gold deposits. They are hosted within or adjacent to Tarkwaian-type metasediments of the Youga Basin, composed of a succession of arkosic sandstones, conglomerates and subordinate chlorite schists. The Youga deposits are characterized by two distinct styles of mineralization; the moderately to weakly silicified host rock with quartz stockwork veining and pyrite as the predominant sulphide which generally grades between 0.5 and 2 g/t and the intensely silicified arkose with abundant quartz veins and more diverse sulphides (pyrite, arsenopyrite, chalcopyrite, pyrrhotite and galena) which generally grades > 3 g/t. The alteration paragenesis associated with the mineralized vein stockwork is characterized by quartz, ankerite, albite, chlorite and pyrite. The first mineralization episode occurred under brittle-ductile conditions during the D1_Y deformation event characterized by E–W trending penetrative to discrete structures. Gold is concentrated in zones affected by irregular fracturing, quartz veining and occasional brecciation. Reworking of these structures during D2_Y, by N–S to NE-trending sub-vertical shear zones, lead to further economic concentration of gold found in eight individual deposits, all localized in or immediately adjacent to Tarkwaian-type sediments (Main, East, West Zone 1, 2, and 3, A2NE, NTV and Zergoré). Absolute age of mineralization is unknown as well as that of the host sediments; however stratigraphic and structural craton-wide correlations suggest that the mineralization occurred after 2110 Ma if not much later. Commercial production was achieved at the Youga Gold Mine in 2008 and as of December 31st, 2014 has produced 537,621 oz of gold.     

Gold occurrences of the Archean North Atlantic craton,southwestern Greenland: A comprehensive genetic model

The North Atlantic craton of southwestern Greenland hosts several orogenic gold occurrences, although, to date, none is in production. Four gold provinces are distinguished and include Godthåbsfjord, Tasiusarsuaq, Paamiut, and Tartoq. In the Godthåbsfjord gold province, the hypozonal gold occurrences are aligned along the major ca. 2660–2600 Ma Ivinnguit fault. Orogenic gold mineralization correlates temporally with, and is related to, ductile deformation along this first-order structure. The northern part of the Tasiusarsuaq gold province is characterized by small hypozonal gold occurrences that are controlled by 2670–2610 Ma folds and shear zones. Auriferous fluids were focused into the structures in both gold provinces during west-directed accretion of the Kapisilik terrane (2650–2580 Ma) to the already amalgamated terranes of the North Atlantic craton. In the southern part of the Tasiusarsuaq gold province, hypozonal gold mineralization is hosted in back-thrusts (Sermilik prospect) and thrusts (Bjørnesund prospect) that formed at 2740 Ma and 2860–2830 Ma, respectively. The deformation is related to the ca. 2850 Ma accretion of the Sioraq block and the Tasiusarsuaq terrane, and the 2800–2700 Ma accretion of the Tasiusarsuaq terrane and the Færingehavn and Tre Brødre terranes.Mesozonal orogenic gold mineralization is hosted in an accretionary complex in the Paamiut and Tartoq gold provinces. Gold occurrences cluster over a strike extent of approx. 40 km in thrusts and complex strike-slip settings in lateral ramps. The timing of the E-vergent terrane accretion in both areas is unknown, and could either be at ca. 2850 Ma or 2740 Ma. In the eastern part of the Paamiut gold province, quartz veins and associated alteration zones were overprinted by granulite facies metamorphism and show evidence for partial melting. These outermost parts of the accretionary complex were involved in burial-exhumation tectonics during crustal accretion.Mainly three different orogenic stages related to gold mineralization are distinguished in the North Atlantic craton between ca. 2850 Ma and 2610 Ma. These are generally accretionary tectonic episodes, and gold mineralization is hosted either in reactivated fault systems between terranes or accretionary complex structures along the deformed cratonic margin. The larger orogenic gold occurrences formed at ca. 2740–2600 Ma that appears to be a period of orogenic gold mineralization globally, although significant gold resources in the North Atlantic craton have yet to be identified.     

Geology,geochronology, geochemistry and ore genesis of the Wangu gold deposit in northeastern Hunan Province,Jiangnan Orogen,South China

The Wangu gold deposit in northeastern Hunan, South China, is one of many structurally controlled gold deposits in the Jiangnan Orogen. The host rocks (slates of the Lengjiaxi Group) are of Neoproterozoic age, but the area is characterized by a number of Late Jurassic–Cretaceous granites and NE-trending faults. The timing of mineralization, tectonic setting and ore genesis of this deposit and many similar deposits in the Jiangnan Orogen are not well understood. The orebodies in the Wangu deposit include quartz veins and altered slates and breccias, and are controlled by WNW-trending faults. The principal ore minerals are arsenopyrite and pyrite, and the major gangue minerals are quartz and calcite. Alteration is developed around the auriferous veins, including silicification, pyritic, arsenopyritic and carbonate alterations. Field work and thin section observations indicate that the hydrothermal processes related to the Wangu gold mineralization can be divided into five stages: 1) quartz, 2) scheelite–quartz, 3) arsenopyrite–pyrite–quartz, 4) poly-sulfides–quartz, and, 5) quartz–calcite. The Lianyunshan S-type granite, which is in an emplacement contact with the NE-trending Changsha-Pingjiang fracture zone, has a zircon LA-ICPMS U–Pb age of 142 ± 2 Ma. The Dayan gold occurrence in the Changsha-Pingjiang fracture zone, which shares similar mineral assemblages with the Wangu deposit, is crosscut by a silicified rock that contains muscovite with a ca. 130 Ma ⁴⁰Ar–³⁹Ar age. The gold mineralization age of the Wangu deposit is thus confined between 142 Ma and 130 Ma. This age of mineralization suggests that the deposit was formed simultaneously with or subsequently to the development of NE-trending extensional faults, the emplacement of Late Jurassic–Cretaceous granites and the formation of Cretaceous basins filled with red-bed clastic rocks in northeastern Hunan, which forms part of the Basin and Range-like province in South China. EMPA analysis shows that the average As content in arsenopyrite is 28.7 atom %, and the mineralization temperature of the arsenopyrite–pyrite–quartz stage is estimated to be 245 ± 20 °C from arsenopyrite thermometry. The high but variable Au/As molar ratios (>0.02) of pyrite suggest that there are nanoparticles of native Au in the sulfides. An integration of S–Pb–H–O–He–Ar isotope systematics suggests that the ore fluids are mainly metamorphic fluids originated from host rocks, possibly driven by hydraulic potential gradient created by reactivation of the WNW-trending faults initially formed in Paleozoic, with possible involvement of magmatic and mantle components channeled through regional fault networks. The Wangu gold deposit shares many geological and geochemical similarities as well as differences with typical orogenic, epithermal and Carlin-type gold deposits, and may be better classified as an “intracontinental reactivation” type as proposed for many other gold deposits in the Jiangnan Orogen.     

Quantifying tectonic versus erosive denudation by the sediment budget: the Miocene core complexes of the Alps

The denudation budget of the Alps is quantified for the main period of lateral extrusion between 22 and 12 Ma. The relative importance of tectonic denudation increases from W to E from ∼70% in the Lepontine window in the Swiss Alps to ∼80% in the Tauern window and to more than 95% in the Rechnitz window. The driving mechanism of tectonic denudation was eastward extrusion due to an unconstrained orogenic margin in the Pannonian basin. Tectonic denudation in the Alps was responsible for about 30% of the total exhumation between 22 and 12 Ma.     

Structural evolution of the Mokrsko-West,Mokrsko-East and Čelina gold deposits,Bohemian Massif,Czech Republic: Role of fluid overpressure

The studied Mokrsko-West (90–100 t Au), Mokrsko-East (30 t Au) and Čelina (11 t Au) deposits represent three spatially and genetically interrelated deposits of supposed affiliation to the intrusion-related gold deposit type. The deposits differ in their dominant host rocks, which are represented by ca 354 Ma old biotite tonalite (Mokrsko-West) and Neoproterozoic volcanic and volcanosedimentary rocks (Mokrsko-East, Čelina). Another difference lies in the style of veining — densely spaced networks of 0.1–5 mm thin veins (Q₂) within the tonalite, compared to thick (usually 5–20 cm; Q_1–2) and widely spaced veins within the Neoproterozoic rocks.Five generations of quartz veins, referred to as Q₀ through Q₄ were distinguished: Q₀ veins are the oldest and ore-barren, Q₁ veins mark the onset of the Au-ore formation, Q₂ veins its culmination and Q₃ veins its fading. Late quartz gangue (Q₄) is associated with uneconomic Ag–Pb–Zn vein-type ores hosted by calcite–barite–(quartz) veins.Quartz vein thickness (~ 0.3 to ~ 300 mm), spacing (~ 3 mm to ~ 500 mm), distribution, and related extensional strain (ca. 3–25%) evolve systematically across the studied ore district, reflecting both the major host rock and other tectonic factors. Detailed study of vein dimension parameters (thickness, length, width, aspect ratios) allowed estimation of the probable depth of the fluid source reservoir (~ 2 km or ~ 4 km) below the present surface. The depth to the fluid source seems to increase through time, being the shallowest for the Q₀ veins and the deepest for the Q₂ veins. Two independent methods of estimating fluid overpressure are discussed in the paper. Fluid overpressure during vein formation decreases from the Q₀ through the Q₂ veins, from 10 to 4 MPa or from 26 to 10 MPa, depending on the assumed tensile strength of the tonalite (5.5 and 15 MPa, respectively).The origin of joints and veins is discussed in terms of the stress orientation and crack-seal and crack-jump mechanisms. Field relationships unambiguously indicate that the veins hosted by Neoproterozoic rocks originated by reopening of the pre-existing extension joints (J₁) due to fluid overpressure. The origin of the densely-spaced thin veins (Q₂) hosted by the tonalite at the Mokrsko-West deposit is, however, less certain. It is probable that the tonalite was already affected by microfracturing analogous to the J₁ joints prior to the formation of quartz veins.The formation of the Q_1–2 veins at the Mokrsko-East deposit was constrained by the Re–Os dating of molybdenite to 342.9 ± 1.4 Ma. The ore-bearing hydrothermal system is thus ca 12 Ma younger than the tonalite that hosts the Mokrsko-West deposit. A similar ca 15–2 Ma difference between the age of the host-intrusion and the age of the hydrothermal event was encountered in several other gold deposits in the vicinity of the Central Bohemian Plutonic Complex. Two hypotheses to explain this are discussed in the paper.     

40Ar/39Ar dating of the Jiehe gold deposit in the Jiaodong Peninsula,eastern North China Craton: Implications for regional gold metallogeny

The Jiehe gold deposit, containing a confirmed gold reserve of 34 tonnes (t), is a Jiaojia-type (disseminated/stockwork-style) gold deposit in Jiaodong Peninsula. Orebodies are hosted in the contact zone between the Jurassic Moshan biotite granite and the Cretaceous Shangzhuang porphyritic granodiorite, and are structurally controlled by the NNE- to NE-striking Wangershan-Hedong Fault. Sulphide minerals are composed predominantly of pyrite with lesser amounts of chalcopyrite, galena, and sphalerite. Hydrothermal alteration is strictly controlled by fracture zones, in which disseminated sulfides and native gold are spatially associated with pervasive sericitic alteration. Mineralogical, textural, and field relationships indicate four stages of alteration and mineralization, including pyrite-bearing milky and massive quartz (stage 1), light-gray granular quartz–pyrite (stage 2), quartz–polysulfide (stage 3) and quartz–carbonate (stage 4) stages. Economic gold is precipitated in stages 2 and 3.The Jiehe deposit was previously considered to form during the Eocene (46.5 ± 2.3 Ma), based on Rb-Sr dating of sericite. However, ⁴⁰Ar/³⁹Ar dating of sericite in this study yields well-defined, reproducible plateau ages between 118.8 ± 0.7 Ma and 120.7 ± 0.8 Ma. These ⁴⁰Ar/³⁹Ar ages are consistent with geochronological data from other gold deposits in the region, indicating that all gold deposits in Jiaodong formed in a short-term period around 120 Ma. The giant gold mineralization event has a tight relationship with the extensional tectonic regime, and is a shallow crustal metallogenic response of paleo-Pacific slab subduction and lithospheric destruction in the eastern NCC.