甘肃寨上金矿床矿物组成特征与矿质沉淀机理

位于西秦岭礼(县)—岷(县)成矿带西段的寨上金矿床,是近年发现的一个大型微细浸染型金矿。笔者通过显微镜观察、电子探针和扫描分析等综合分析技术,确认金矿床中矿物组成相当丰富,既有大量硫化物、硫盐、氧化物、硫酸盐、碳酸盐、钨酸盐,又有碲化物、自然金属及多金属互化物。除常见矿物为自然金、黄铁矿、黄铜矿、黝铜矿、方铅矿、闪锌矿、辉锑矿、石英、白钨矿、方解石、菱铁矿、铁白云石和重晶石外,笔者还鉴定出在卡林型金矿床较少见的一些矿物,如硫铜锑矿、车轮矿、辉锑铅矿、辉钼矿、碲汞矿、碲镍矿、Cu-Zn-Ni-Sn-Fe的金属互化物和白钨矿等。矿石中矿物种类较多,组成较复杂以及存在显微自然金,构成寨上金矿床的一大特色。赋矿围岩中含Fe碳酸盐矿物溶解释放Fe以及溶解Fe的大量硫化物化,是寨上金矿床中存在显微可见自然金的最重要因素和金沉淀富集的有利条件。     

甘肃礼县-岷县成矿带西段寨上金矿床中自然金的发现及成因意义

位于西秦岭礼(县)-岷(县)成矿带西段的寨上金矿床是近年发现的一个大型微细浸染型金矿。在研究甘肃寨上金矿床物质组分的过程中,采用显微镜观察、电子探针扫描、化学分析等综合分析技术发现了显微自然金颗粒。矿石中的自然金颗粒形态多样,粒度变化较大(5～150μm)。电子探针分析结果显示,自然金中Au=88.23%～92.73%,Ag=7.41%～9.08%,为含银自然金,成色905～926。金的载体矿物有砷黄铁矿、方铅矿、碲汞矿、铁白云石等。金的嵌布类型有粒间金、裂隙金和连生金3种。赋矿围岩中含Fe碳酸盐矿物溶解释放出Fe和溶解Fe的大量硫化物化,可能是寨上金矿床中存在显微可见自然金的最重要因素。     

The Vorontsovskoe Au-Hg-As ore deposit (Northern Urals,Russia): Geological setting,ore mineralogy,geochemistry, geochronology and genetic model

The large Vorontsovskoe Au-Hg-As deposit in the Urals is located in the exocontact of the Early Devonian Auerbah gabbro-diorite-granodiorite massif, which intrudes volcano-sedimentary rocks. The orebodies are confined to a tectonic contact of calcareous and tuffaceous rocks. They are composed of 6 types of disseminated ores, but the main reserves of gold are associated with the following ore types: gold-pyrite-arsenopyrite in altered tuffaceous rocks, pyrite-realgar ores in limestone breccia with a carbonate-volcanogenic cement, and gold-oxide-clay from regolith with residual gold. Early ore associations have been formed at 450–300 °C, whereas the late ores have been formed at lower temperature of 260–110 °C. We propose a model for the genesis of the Vorontsovskoe deposit based on synchronicity of mineralization with the formation of the Auerbah volcano-plutonic complex. The Ar-Ar age of hydromica from the gold-arsenopyrite association is 391.1 ± 4.9 Ma, which coincides with the age of igneous rocks of the Auerbah complex. The main sources of water and carbon dioxide were composed of the fluid derived from the magma chamber and the metamorphic water equilibrated with carbonate sedimentary rocks. Magmatic fluid dominated during the development of skarns, jasperoids and quartz veins, whereas metamorphic water was dominant during quartz-sericite alteration of volcano-sedimentary rocks and calcareous breccias. The bulk of the sulfur was supplied by a deep magma reservoir, however this source prevailed only during skarn ore formation. The mixing between deep-sourced sulfur and sedimentary or biogenic sulfur was established for other ore types. Gold and other ore components were possibly introduced during the volcanic and intrusive activity and also extracted from host sedimentary rocks.     

Geology,geochemistry, and genesis of Axi: A Paleozoic low-sulfidation type epithermal gold deposit in Xinjiang,China

Axi is a low-sulfidation type epithermal gold deposit hosted in Paleozoic subaerial volcanic rocks in the western Tianshan orogenic belt, Xinjiang, China. The resource is more than 50 t gold at an average grade of > 4.4 ppm. The deposit occurs in the Tulasu volcanic fault-basin in the Paleozoic active continental margin on the northern side of the Yili-Central Tianshan plate. The host rocks are andesitic volcaniclastic rocks of the Paleozoic Dahalajunshan Formation, and the orebodies occur as veins in annular faults of a paleocaldera. Mineralization at Axi can be subdivided into five stages: quartz and/or chalcedony vein, quartz vein, quartz-carbonate vein, sulfide vein and carbonate vein. There are two types of ore host: quartz vein and altered rocks. Ore minerals are native gold, electrum, pyrite, marcasite, arsenopyrite, hematite, limonite, and trace amounts of pyrargyrite, polybasite, naumannite, cerargyrite, sphalerite, chalcopyrite, tetrahedrite, galena, pyrrhotite and clausthalite; gangue minerals are mainly quartz, chalcedony, illite, calcite, siderite, dolomite, adularia and laumontite. The main wall-rock alteration is silicification and phyllic alteration, carbonatization and propylitization. The deposit is characterized by an enrichment, relative to crustal abundance, of Au, Ag, As, Sb, Bi, Hg, Se, Te and Mo, depletion in base metals (Cu, Pb, and Zn), and a low Ag/Au ratio (0.5–3.7).Three types of fluid inclusions were recognized in quartz from the major mineralization stages: liquid aqueous inclusions, liquid-rich two-phase inclusions and small amounts of vapor-rich two-phase inclusions. Microthermometric measurements indicate that the final ice melting temperatures are − 0.3 to − 4.4 °C, corresponding to salinities of 0.5–6.9 wt.% NaCl equiv. (2.2 wt.% NaCl equiv. in average). The peak temperatures of ice melting varies from − 0.4 to − 1.9 °C, corresponding to salinities of 0.7–3.1 wt.% NaCl equiv. Homogenization temperatures range mainly between 120 and 240 °C, with an average of 190 °C and a maximum of 335 °C. The fluid density is 0.73 to 0.95 g/cm³ and thus the estimated maximum mineralization depth is about 700 m.Hydrogen and oxygen isotopic compositions of the ore fluids lie within a narrow range: δD_H2O is − 98 to − 116‰ and δ¹⁸O_H2O − 1.8 to 0.4‰. ³He/⁴He ranges from 0.0218 to 0.138 Ra, with an average of 0.044 Ra, indicating that He derived predominantly from crust with negligible mantle He in the ore fluids. By contrast, the ⁴⁰Ar/³⁶Ar ranges from 317.7 to 866.0, suggesting that crust-derived radioactive ⁴⁰Ar⁎ accounts for 7.0 to 66%, and atmospheric ⁴⁰Ar about 43 to 93% in the ore fluids. Hydrogen, oxygen, carbon, sulfur and noble gas isotopes indicate that the ore-forming fluids of the Axi gold deposit consisted predominantly of circulating meteoric water. Ore-forming metals may have derived mainly from the host volcaniclastic rocks of the Dahalajunshan Formation and basement rocks. The occurrence of adularia, platy calcite, and quartz or sulfide aggregates as pseudomorphs after bladed calcite in ore veins, and occurrence of aqueous liquid, and liquid-rich and vapor-rich two-phase inclusions, indicates that boiling of the ore-forming fluid have occurred, leading to supersaturation of the hydrothermal solution and deposition of ore metals. This is the main mineralization mechanism for quartz-vein type ores in Axi. The ore-forming fluid was buffered to a near-neutral pH in a reduced environment during mineralization. The preservation of this Paleozoic Axi deposit and its discovery required a rapid accumulation of sediments in the basin after formation of the deposit, and minimal amount of erosion after Late Cenozoic uplift.     

Chadormalu Kiruna-type magnetite-apatite deposit,Bafq district,Iran: Insights into hydrothermal alteration and petrogenesis from geochemical,fluid inclusion,and sulfur isotope data

The Chadormalu is one of the largest known iron deposits in the Bafq metallogenic province in the Kashmar-Kerman belt, Central Iran. The deposit is hosted in Precambrian-Cambrian igneous rocks, represented by rhyolite, rhyodacite, granite, diorite, and diabasic dikes, as well as metamorphic rocks consisting of various schists. The host rocks experienced Na (albite), calcic (actinolite), and potassic (K-feldspar and biotite) hydrothermal alteration associated with the formation of magnetite–(apatite) bodies, which are characteristic of iron oxide copper-gold (IOCG) and iron oxide-apatite (IOA) systems. Iron ores, occurring as massive-type and vein-type bodies, consist of three main generations of magnetite, including primary, secondary, and recrystallized, which are chemically different. Apatite occurs as scattered irregular veinlets in various parts of the main massive ore-body, as well as apatite-magnetite veins and disseminated apatite grains in marginal parts of the deposit and in the immediate wall rocks. Minor pyrite occurs as a late phase in the iron ores. Chemical composition of magnetite is representative of an IOA or Kiruna-type deposit, which is consistent with other evidence.Whole rock geochemical data from various host rocks confirm the occurrence of Na, Ca, and K alteration consistent with the formation of albite, actinolite, and K-feldspar, respectively. The geochemical investigation also includes the nature of calc-alkaline igneous rocks, and helps elaborating on the spatial and temporal association, and possible contribution of mafic to felsic magmas to the evolution of ore-bearing hydrothermal fluids.Fluid inclusion studies on apatites from massive- and vein-type ores show a range of homogenization temperatures from 266 to 580 °C and 208–406 °C, and salinities from 0.5 to 10.7 wt.% and 0.3–24.4 wt.% NaCl equiv., respectively. The fluid inclusion data suggest the involvement of evolving fluids, from low salinity-high temperature, to high salinity-low temperature, in the formation of the massive- and vein-type ores, respectively. The δ³⁴S values obtained for pyrite from various parts of the deposit range between +8.9 and +14.4‰ for massive ore and +18.7 to +21.5‰ for vein-type ore. A possible source of sulfur for the ³⁴S-enriched pyrite would be originated from late Precambrian-early Cambrian marine sulfate, or fluids equilibrated with evaporitic sulfates.Field observations, ore mineral and alteration assemblages, coupled with lithogeochemical, fluid inclusion, and sulfur isotopic data suggest that an evolving fluid from magmatic dominated to surficial brine-rich fluid has contributed to the formation of the Chadormalu deposit. In the first stages of mineralization, magmatic derived fluids had a dominant role in the formation of the massive-type ores, whereas a later brine with higher δ³⁴S contributed to the formation of the vein-type ores.     

A comparison of Jiaojia- and Linglong-type gold deposit ore-forming fluids: Do they differ?

The Jiaodong peninsula contains the most important concentration of gold deposits in China, which can be divided into Jiaojia-type and Linglong-type deposits based on mineralization style. The former is characterized by disseminated- and stockwork-style mineralization hosted in first-order regional faults, with relatively larger tonnages and lower gold grades. The latter is characterized by massive auriferous quartz veins commonly hosted in subsidiary second- or third-order faults, with smaller tonnage but higher grade orebodies. Despite these differences, both groups of deposits have the same alteration assemblages, mineral paragenesis, element concentrations, and ore-forming ages.The mainly Jiaojia-type Luoshan gold deposit and the mainly Linglong-type Fushan gold deposit are characterized by H-O-S-Pb isotope data that indicate the ore-forming fluids have a dominantly metamorphic source. The fluids were derived during the Yanshanian orogenic event, and were most likely associated with dehydration and decarbonization processes near the top of the subducting paleo-Pacific plate. The Linglong-type ores have relatively lighter calculated δ¹⁸O compositions (−3.9 to −2.3‰) than the Jiaojia-type ores (0.3–8.0‰), possibly because of a greater degree of mixing with meteoric water. Petrographic, cathodoluminescence, microthermometric, and laser Raman spectroscopic analyses of fluid-inclusion assemblages in quartz from the two types of ores indicate fluids were similar, in both cases characterized by medium–high homogenization temperatures (211–393 °C), significant CO₂ (∼15% mol), minor CH₄ (⩽18% in the carbonic phase), and low salinity (⩽11.2 wt% NaCl eq.). The Linglong-type ores, however, have a wider range of CO₂ and CH₄ concentration and salinity than the Jiaojia-type ores. Fluid immiscibility, occurred in main ore stage of both ore types, with the trapping conditions of 77–185 MPa and 284–328 °C, although the unmixing is more intense and widespread in the Linglong-type ores. Both fluid-wallrock interaction and fluid immiscibility are important gold-deposition processes in the two types, but immiscibility is more important in the Linglong-type ores and that has led to the typical higher gold grade.In general, there is little geochemical differences between the ore-forming fluids for Jiaojia- and Linglong-type gold deposits. Both Jiaojia- and Linglong-type ores can exist in a single deposit and form in the same metallogenic event. The Linglong-type ores developed as more massive veins, because of their location in zones of more extensive extension and they lack significant post-ore cataclastic deformation.     

Ore petrography and chemistry of the tellurides from the Dongping gold deposit,Hebei Province,China

The Dongping gold deposit is a mesothermal lode gold deposit hosted in syenite. The ore petrography and chemistry of the tellurides from the alteration zone of the deposit have been studied in detail using optical microscopy, scanning electron microscopy, electron probe micro-beam and X-ray diffraction facilities. The tellurides, consisting mostly of calaverite, altaite, petzite tellurobismuthite and tetradymite, are hosted irregularly in pyrite fractures and voids. In the ore bodies, the species and quantity of tellurides decrease from the top downwards, accompanied with lowering of gold fineness, and the existence of tellurides exhibits a positive correlation with gold enrichment. Mineral paragenesis and chemical variations suggest that during evolution of the ore-forming fluids Te preferably incorporated with Pb to form altaite, followed in sequence by precipitation of petzite, and calaverite when Ag has been exhausted, and the residue fluids were enriched in Au, giving rise to formation of native gold. Calculation with reference of the fineness of native gold coexisting with the tellurides indicates that at 300 °C, log f (Te₂) varied between − 8.650 and − 7.625. Taking account of the Au–Ag–Te mineral paragenesis, it is inferred that log ƒ (Te₂) varies from − 9.12 to − 6.43, log ƒ (S₂) − 11.47 to − 8.86. In consideration of the physicochemical conditions for formation of tellurides, with comparison to some known telluride deposits, it is suggested that high log ƒ (Te₂) is a key factor for high fineness of native gold as well as precipitation of abundant tellurides.     

Fluid inclusion,rare earth element geochemistry,and isotopic characteristics of the eastern ore zone of the Baiyangping polymetallic Ore district,northwestern Yunnan Province,China

The Baiyangping Cu–Ag polymetallic ore district is located in the northern part of the Lanping–Simao foreland fold belt, which lies between the Jinshajiang–Ailaoshan and Lancangjiang faults in western Yunnan Province, China. The source of ore-forming fluids and materials within the eastern ore zone were investigated using fluid inclusion, rare earth element (REE), and isotopic (C, O, and S) analyses undertaken on sulfides, gangue minerals, wall rocks, and ores formed during the hydrothermal stage of mineralization. These analyses indicate: (1) The presence of five types of fluid inclusion, which contain various combinations of liquid (l) and vapor (v) phases at room temperature: (a) H₂O (l), (b) H₂O (l) + H₂O (v), (c) H₂O (v), (d) C_mH_n (v), and (e) H₂O (l) + CO₂ (l), sometimes with CO₂ (v). These inclusions have salinities of 1.4–19.9 wt.% NaCl equivalents, with two modes at approximately 5–10 and 16–21 wt.% NaCl equivalent, and homogenization temperatures between 101 °C and 295 °C. Five components were identified in fluid inclusions using Raman microspectrometry: H₂O, dolomite, calcite, CH₄, and N₂. (2) Calcite, dolomitized limestone, and dolomite contain total REE concentrations of 3.10–38.93 ppm, whereas wall rocks and ores contain REE concentrations of 1.21–196 ppm. Dolomitized limestone, dolomite, wall rock, and ore samples have similar chondrite-normalized REE patterns, with ores in the Huachangshan, Xiaquwu, and Dongzhiyan ore blocks having large negative δCe and δEu anomalies, which may be indicative of a change in redox conditions during fluid ascent, migration, and/or cooling. (3) δ³⁴S values for sphalerite, galena, pyrite, and tetrahedrite sulfide samples range from −7.3‰ to 2.1‰, a wide range that indicates multiple sulfur sources. The basin contains numerous sources of S, and deriving S from a mixture of these sources could have yielded these near-zero values, either by mixing of S from different sources, or by changes in the geological conditions of seawater sulfate reduction to sulfur. (4) The C–O isotopic analyses yield δ¹³C values from ca. zero to −10‰, and a wider range of δ¹⁸O values from ca. +6 to +24‰, suggestive of mixing between mantle-derived magma and marine carbonate sources during the evolution of ore-forming fluids, although potential contributions from organic carbon and basinal brine sources should also be considered. These data indicate that ore-forming fluids were derived from a mixture of organism, basinal brine, and mantle-derived magma sources, and as such, the eastern ore zone of the Baiyangping polymetallic ore deposit should be classified as a “Lanping-type” ore deposit.     

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.     

Geology and genesis of Kafang Cu–Sn deposit,Gejiu district,SW China

Kafang is one of the main ore deposits in the world-class Gejiu polymetallic tin district, SW China. There are three main mineralization types in the Kafang deposit, i.e., skarn Cu–Sn ores, stratiform Cu ores hosted by basalt and stratiform Cu–Sn ores hosted by carbonate. The skarn mainly consists of garnet and pyroxene, and retrograde altered rocks. These retrograde altered rocks are superimposed on the skarn and are composed of actinolite, chlorite, epidote and phlogopite. Major ore minerals are chalcopyrite, pyrrhotite, cassiterite, pyrite and scheelite. Sulfur and Pb isotopic components hint that the sources of different types of mineralization are distinctive, and indicate that the skarn ore mainly originated from granitic magma, whereas the basalt-hosted Cu ores mainly derived from basalt. Microthermometry results of fluid inclusions display a gradual change during the ore-forming process. The homogenization temperature of different types of inclusions continuously decreases from early to late mineralization stages. The salinities and freezing temperatures exhibit similar evolutionary tendencies with the T _{homogenization}, while the densities of the different types keep constant, the majority being less than 1. Oxygen and hydrogen isotopic values (δ¹⁸O and δD) of the hydrothermal fluids fall within ranges of 3.1 to 7.7‰ with an average of 6.15‰, calculated at the corresponding homogenization temperature, and − 73 and − 98‰ with an average of − 86.5‰, respectively. Microthermometry data and H–O isotopes indicate that the ore-forming fluid of the Kafang deposit is mainly derived from magma in the early stage and a mixture of meteoric and magmatic water in late stage. Molybdenite Re–Os age of the skarn type mineralization is 83.4 ± 2.1 Ma, and the stratiform ores hosted by basalt is 84.2 ± 7.3 Ma, which are consistent with the LA-ICP-MS zircon age of the Xinshan granite intrusion (83.1 ± 0.4 Ma). The evidence listed above reflects the fact that different ore styles in the Kafang deposit belong to the same mineralization system.     

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.     

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.     

Multi-isotope approach for the identification of metal and fluid sources of the Arroyo Rojo VMS deposit,Tierra del Fuego,Argentina

The Arroyo Rojo deposit, located in Tierra del Fuego, is the most important polymetallic, volcanic-hosted massive sulphide in the rhyolitic belt of the Fuegian Andes. The best intercepts in drill holes indicate a true thickness of 18.6 m and concentrations of 2.2% Cu, 3.9% Pb, 14.5% Zn, 140 g/t Ag, 1.1 g/t Au). This deposit, located near the town of Ushuaia, is hosted in a Middle Jurassic volcanic and volcanoclastic sequence. Massive and semimassive bodies display stacked lenticular morphologies with disseminated mineralization in both the footwall and hanging wall. The associated hydrothermal alteration system is partially conformable with the layering of the volcanic rocks. The ores and host rocks display a penetrative tectonic foliation and were metamorphosed to greenschist facies.Previous studies have not resulted in a consensus regarding the nature and the source of ore-forming fluids and the style of deposition of the sulphides at Arroyo Rojo. In this study, both stable and radiogenic isotopes were used develop a better understanding of these aspects of the deposit.Hydrogen and oxygen isotopes indicate that an evolved seawater mixed with significant contributions from other fluid reservoirs such as magmatic and/or metamorphic waters was the most likely source of the ore-forming fluids. These fluids underwent significant interaction with the underlying volcanic and sedimentary rocks, which promoted partial (Sr isotopes) or full (Pb isotopes) homogenization of radiogenic isotopes. δ³⁴S_CDT values suggest that the sulphur was derived from several sources: biogenic reduction of seawater sulphate (BSR) in a restricted to closed basin was mixed with a heavier component derived from inorganic reduction of seawater sulphate (TRS) and possibly from sulphur leached from igneous footwall rocks and/or direct contribution from magmatic fluids.Lateral infiltration of hydrothermal fluids resulted in the formation of a halo of semimassive to disseminated ore due to the replacement of porous, reactive glassy and breccia tuffs.As a result of the hydrothermal circulation, two styles of mineralization are observed in the Arroyo Rojo deposit: a stringer zone and a halo of semimassive to disseminated ore corresponding to sub-seafloor replacement, and syn-sedimentary mineralization consisting of massive sulphides.This model is consistent with the geodynamic context of the study area: a narrow, deep-marine volcano-tectonic rift parallel to the Andean side of South America and related to the initial break-up of Gondwana (ca. 145 Ma).     

The origin of the Dapingzhang volcanogenic Cu–Pb–Zn ore deposit,Yunnan province,SW China: Constraints from host rock geochemistry and ore Os–Pb–S–C–O–H isotopes

The Dapingzhang volcanogenic Cu–Pb–Zn sulfide deposit is located in the Lancangjiang tectonic zone within the Sanjiang region, Yunnan province of southwestern China. The deposit occurs within a felsic volcanic dome belonging to a mid-Silurian volcanic belt stretching for more than 100 km from Dapingzhang to Sandashan. The mineralized volcanic rocks are predominantly keratophyre and quartz keratophyre with subordinate spilite. The Dapingzhang deposit is characterized by well-developed vertical zonation with stockwork ores in the bottom, disseminated sulfide ores in the middle, and massive sulfide ores in the top, overlain by a thin layer of chemical sedimentary exhalative rocks (chert and barite). The Re–Os age of the pyrites from the deposit is 417 ± 23 Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb–Ta anomalies and positive ε_Nd(t) values (+ 4.4–+6.5), similar to the coeval calc-alkaline volcanic rocks in the region. This observation supports the interpretation that the felsic volcanic rocks associated with the Dapingzhang deposit are the derivatives of arc basaltic magma by extensive fractional crystallization. The δ³⁴S values of the sulfides from the deposit vary from − 1.24 to + 4.32‰, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.310–18.656, ²⁰⁷Pb/²⁰⁴Pb = 15.489–15.643 and ²⁰⁸Pb/²⁰⁴Pb = 37.811–38.662), similar to the Pb isotopic compositions of the associated volcanic rocks. The Pb isotopic data indicate that mantle-derived Pb is more prevalent than crust-derived Pb in the deposit. The S–Pb isotopic data indicate that the important ore-forming materials were mainly derived from the associated volcanic rocks. The δ¹³C_PDB and δ¹⁸O_SMOW values of the associated hydrothermal calcite crystals vary from − 2.3‰ to + 0.27‰ and from + 14.6 to + 24.4‰, respectively. These values are between the mantle and marine carbonate values. The narrow range of the δ¹³C_PDB values for the calcite indicates that carbon-bearing species in the hydrothermal fluids were primarily derived from marine carbonates. The δ¹⁸O values for the hydrothermal fluids, calculated from the measured values for quartz, are between − 2.1‰ and + 3.5‰. The corresponding δD values for the fluids range from − 59‰ to − 84‰. The O–H isotopic data indicate mixing between magmatic fluids and seawater in the ore-forming hydrothermal system. Similar to a typical volcanogenic massive sulfide (VMS) deposit, the ore-forming fluids contained both magmatic fluids and heated seawater; the ore metals and regents were derived from the underlying magma as well as felsic country rocks.     

Relationship of antimony with gold mineralization in the ore districts of Eastern Kazakhstan

In Eastern Kazakhstan, Sb mineralization is the most widespread in the Irtysh and Bakyrchik ore districts of the West Kalba gold-bearing belt. It is spatially related to disseminated gold-sulfide ores at some deposits and is structurally and spatially isolated at others. Disseminated gold-sulfide mineralization is localized in Carboniferous carbonaceous-terrigenous carbonate rocks. It is marked off by zones of dynamic metamorphism and foliation and is characterized by the ribbon-like-lenticular morphology of ore deposits. Later Sb (predominantly, quartz-antimonite) mineralization is formed in an extension setting as brecciated/veined ores. In combination with gold-sulfide ores, Sb mineralization is more diverse. For example, microparageneses with berthierite, native Sb, aurostibite, ullmannite, jamesonite, and tetrahedrite coexist with pocket-vein quartz-carbonate-antimonite mineralization in the gold-sulfide ores of the Suzdal’skoe deposit. Also, Sb-containing minerals such as arsenopyrite and pyrite are observed. Two temperature regimes of mineralization are established here: 418-300 °C for gold-polysulfide mineralization and 280-200 °C for later Sb mineralization. The isotopic composition of antimonite sulfur at the Suzdal’skoe, Zherek, Zhanan, Bakyrchik, and Dal’ny I deposits shows close values within the interval 5³⁴S of -3.8 to 2.5%c, suggesting its great-depth origin. No visible gold is found in the antimonite of the quartz-antimonite veins, but atomic-absorption analysis reveals few ppm or more gold. Point X-ray analysis indicates the possible presence of the so-called “invisible” gold. Microstructural observations, temporal relationships of the parageneses, and studies of gas-liquid inclusions at the Suzdal’skoe deposit permit assigning Sb mineralization to the second productive gold-polysulfide stage of the ore deposition. The late antimonite stage of mineralization is separated from the gold-polysulfide stage by 7 Myr long intramineralization tectonic shifts. Gold-polysulfide mineralization (248.3 ± 3.4 Ma) was synchronous with Triassic tectonomagmatic activity.     

Timing and mechanism of gold mineralization at the Wang'ershan gold deposit,Jiaodong Peninsula,eastern China

The Wang'ershan gold deposit, located in the southern Jiaojia goldfield, is currently the largest gold deposit hosted within the subsidiary faults in Jiaodong Peninsula, with a gold reserve of > 60 t gold at a grade of 4.07 g/t Au. It is hosted in the Late Jurassic Linglong biotite granites and controlled by the second-order, N- to NNE-trending Wang'ershan Fault (and its subsidiary faults) which is broadly parallel to the first-order Jiaojia Fault in the goldfield. Gold mineralization occurs as both disseminated- and stockwork-style and quartz–sulfide vein-style ores, mainly within altered cataclasites and breccias, and sericite–quartz and potassic alteration zones, respectively. Mineralization stages can be divided into (1) the pyrite–quartz–sericite stage, (2) the quartz–pyrite stage, (3) the quartz–sulfide stage, and (4) the quartz–carbonate stage.Two sericite samples associated with the main ore-stage pyrites from pyritic phyllic ores of the deposit with weighted mean plateau ⁴⁰Ar/³⁹Ar age of 120.7 ± 0.6 Ma and 119.2 ± 0.5 Ma, respectively, were selected for ⁴⁰Ar/³⁹Ar geochronology. On the basis of petrography and microthermometry, three types of primary fluid inclusions related to the ore forming event were identified: type 1 H₂O–CO₂–NaCl, type 2 aqueous, and type 3 CO₂ fluid inclusions (in decreasing abundance). Stage 1 quartz contains all three primary fluid inclusions, while stages 2 and 3 quartz contain both type 1 and 2 inclusions, and stage 4 quartz contains only type 2 inclusions. The contemporaneous trapping, similar salinities and total homogenization temperature ranges, and different homogenization phases of type 1 and type 2 inclusions indicate that fluid immiscibility did take place in stages 1, 2 and 3 ores, with P–T conditions of 190 to 85 MPa and 334 to 300 °C for stage 1 and 200 to 40 MPa and 288 to 230 °C for stages 2 and 3. Combined with the H–O–C–S–Pb isotopic compositions, ore-forming fluids may have a metamorphic-dominant mixed source, which could be associated with the dehydration and decarbonisation of a subducting paleo-Pacific plate and characterized by medium–high temperature (285–350 °C), CO₂-bearing (~ 8 mol%) with minor CH₄ (1–4% in carbonic phase), and low salinity (3.38–8.45 eq. wt.% NaCl). During mineralization, the fluid finally evolved into a medium–low temperature NaCl–H₂O system. Au(HS)₂⁻ was the most probable gold-transporting complex at Wang'ershan, due to the low temperature (157–350 °C) and near-neutral to weakly acidic ore fluids. The reaction between gold-bearing fluids and iron-bearing wall-rocks, and fluid-immiscibility processes caused via fluid–pressure cycling during seismic movement along fault zones that host lode-gold orebodies, which led to breakdown of Au(HS)₂⁻, are interpreted as the two main precipitation mechanisms of gold deposition.In general, the Wang'ershan deposit and other deposits in the Jiaojia camp have concordant structural system and wall-rock alteration assemblages, nature of orebodies and gold occurrence conditions, as well as the similar geochronology, ore-forming fluids system and stable isotope compositions. Thus gold mineralization in the Jiaojia goldfield was a large-scale unified event, with consistent timing, origin, process and mechanism.     

The pilot knob magnetite deposit in the Proterozoic St. Francois Mountains Terrane,southeast Missouri,USA: A magmatic and hydrothermal replacement iron deposit

The Pilot Knob magnetite deposit is located in southeast Missouri within the 1380–1480 Ma St. Francois Mountains terrane rhyolitic/trachytic volcanic rocks. The deposit is tabular, dips about 45°, and is sill-like in nature, being approximately parallel to the bedding in the host tuffs. The deposit was uncovered by erosion and exposed to weathering in the late Proterozoic, and is overlain in angular unconformity by the Cambrian Lamotte Sandstone. This Proterozoic weathering cycle apparently had little effect on the deposit with only the updip edge being converted to hematite. Textural and mineralogical features of the deposit suggest a combined magmatic and hydrothermal replacement origin. The magnetite-rich ores that make up the bulk of the deposit are interpreted as having crystallized from an iron-rich magma, and a surrounding envelope of lower- to moderate-grade ores where magnetite has clearly replaced the tuffaceous host rocks are interpreted as hydrothermal in origin. After the development of the higher-grade magnetite ores and the envelope of lower- to moderate-grade ores, late hydrothermal minerals were deposited as cross-cutting veins and breccia fill. The two most abundant minerals in the higher-grade portions of the deposit are magnetite and albitic plagioclase, and petrologically the higher-grade ores could be described as a magnetite sodic syenite. The most abundant gangue mineral within the lower-grade impregnated envelope of ores around the higher-grade ores is K-feldspar, apparently relict from the rhyolites/trachytes. Thin lenses within the higher-grade ores contain calcite as a matrix mineral to the magnetite and are considered to indicate carbonatitic affinity. Rare earth elements are elevated in one of five whole rock chemical analyses of the ores and the REE-bearing mineral ferriallanite has been identified. Minor portions of the deposit, below the Proterozoic weathering cap, consist of high-grade hematite ores having equilibrium textures. At depth, the deposit is intruded by the Shepherd Mountain gabbro, a 120 m thick, near-horizontal dike, which resulted in minor contact metamorphism of the ore.     

西秦岭金龙山卡林型金矿床地质-地球化学及矿床成因研究

金龙山金矿床位于南秦岭造山带的复理石褶冲带中,赋矿围岩为碎屑岩-碳酸盐建造,矿化明显受地层岩性与韧-脆性构造发育程度的控制。矿床稀土元素地球化学研究表明,地层岩石、矿石和热液矿物的轻重稀土分异程度和特征参数基本一致,表明成矿流体应主要来自于赋矿地层。铅同位素研究表明,地层岩石、矿石和热液矿物均具有较高的放射性成因的铅同位素组成,且均落入南秦岭造山带的泥盆系范围内,暗示铅也主要来自赋矿地层。对前人已有的碳-氧-硫-氢同位素组成和流体包裹体数据综合分析表明,碳和氧应主要来自海相碳酸盐的溶解作用,硫主要来自海相硫酸盐的热化学还原反应;从成矿早阶段到晚阶段,成矿流体的δ18O及δD值向大气降水线＂漂移＂,指示成矿流体以盆地建造水和变质水为主,在晚阶段有大气降水加入。金龙山金矿床与卡林型金矿床的矿床地质-地球化学特征相似,应属于卡林型金矿床,其形成于秦岭造山带陆内造山作用过程中,多层次陆壳叠置加厚的地球动力学背景,是陆内碰撞造山作用的产物。     

Geological characteristics and metallogenesis of iron deposits in western Tianshan,China

The western Tianshan metallogenic belt is one of the most significant polymetallic iron metallogenic belts in China. Important advances have been achieved recently in iron exploration in the Awulale Mountain in western Tianshan, China. These newly-discovered iron deposits are mainly hosted in the basic-medium andesitic lavas and volcaniclastics, often comprising a number of high-grade ores. Magnetite is predominated in ore mineral assemblages, and pyrite, chalcopyrite, pyrrhotite or sphalerite increase in certain deposits. Wallrock alterations are intensively developed, exemplified as sodic–calcic and potassic alterations which display in different patterns as country rocks and ore-controlled structures vary. Skarn assemblages are commonly developed in ore districts like Beizhan, Dunde and Chagangnuoer, and pyroxene + albite + K-feldspar − epidote + actinolite alterations are dominated around ore bodies in Zhibo deposit, whereas the Shikebutai deposit develops alteration assemblages comprising of jasper, barite, sericite, and chlorite. Thus, iron deposits can be divided into three types including volcanic-sedimentary type, volcanic magmatic-hydrothermal type and iron skarn type. Our preliminary interpretation about the tectonic background of this iron mineralization in this area is in the late stage of a collisional–accretional orogenic belt around Carboniferous, with some extrusional–extensional tectonic transition locally. Iron mineralization is likely to have a close genetic relationship with volcanic–subvolcanic activity, syn- or slightly post- the volcanism which took place besides continental arc. Volcanic eruption contributes to majority of mineralizing iron, with minor extracted from hydrothermal replacement from wall rocks.     

Mineral and geochemical compositions,regularities of distribution,and specific formation of ore mineralization of the Rogovik gold-silver deposit (northeastern Russia)

New data on the mineral composition and the first data on the geochemical composition of ores of the Rogovik gold-silver deposit (Omsukchan ore district, northeastern Russia) have been obtained. Study of the regularities of the spatial distribution of ore mineralization shows that the deposit ores formed in two stages. Epithermal Au-Ag ores of typical poor mineral and elemental compositions were generated at the early volcanic stage. The major minerals are low-fineness native gold, electrum, acanthite, silver sulfosalts, kustelite, and pyrite. The typomorphic elemental composition of ores is as follows: Au, Ag, Sb, As, Se, and Hg. The content of S is low, mostly < 1%. Silver ores of more complex mineral and elemental compositions were produced under the impact of granitoid intrusion at the late volcanoplutonic stage. The major minerals are high-Hg kustelite and native silver, silver sulfosalts and selenides, fahlore, pyrite, chalcopyrite, galena, and sphalerite. The typomorphic elemental composition of ores is as follows: Ag, As, Sb, Se, Hg, Pb, Zn, Cu, and B. The content of S is much higher than 1%. The ores also have elevated contents of Mo, Ge, F, and LREE (La, Ce, and Nd). At the volcanoplutonic stage, polychronous Au-Ag ores formed at the sites of the coexistence of silver and epithermal gold-silver mineralization. Their specific feature is a multicomponent composition and a strong variability in chemical composition (both qualitative and quantitative). Along with the above minerals, the ores contain high-Hg gold, hessite, argyrodite, canfieldite, orthite, fluorapatite, and arsenopyrite. At the sites with strongly rejuvenated rocks, the ores are strongly enriched in Au, Ag, Hg, Cu, Pb, Zn, Ge, Se, La, Ce, Nd, S, and F and also contain Te and Bi. The hypothesis is put forward that the late silver ores belong to the Ag-complex-metal association widespread in the Omsukchan ore district. A close relationship between the ores of different types and their zonal spatial distribution have been established. In the central part of the Rogovik deposit, epithermal Au-Ag ores are widespread in the upper horizons, Ag ores are localized in the middle horizons, and rejuvenated polyassociation Au-Ag ores occur at the sites (mostly deep-seated) with ore-bearing structures of different ages.