Characteristics and genesis of Gangdese porphyry copper deposits in the southern Tibetan Plateau: Preliminary geochemical and geochronological results

Porphyry-type Cu (Mo, Au) deposits have been discovered along the Gangdese magmatic arc in the southern Tibetan Plateau. Extensive field investigations and systematic studies of geochemistry, S–Pb isotopic tracing, together with Re–Os and ⁴⁰Ar/³⁹Ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post-collisional crustal relaxation period of the Late Himalayan epoch. These porphyries are petrochemically K-enriched and belong to shoshonitic to high-K calc-alkaline series. They display enrichment of large ion lithophile elements (LILE) Rb, K, U, Th, Sr, Pb and depletion of high field strength elements (HFSE) Nb, Ta, Ti and the heavy rare earth elements (HREE) and Y without Eu anomalies. These characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources. Pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic Pb in the eastern segment of the copper belt to mantle Pb in the western segment. These constitute a mixing line of the Indian Oceanic MORB with Indian Oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components.The Gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry-type copper deposits which include potassic alteration (K-feldspathisation and biotitisation), silicification, sericitisation, and propylitisation. Mineralisation mainly occurs in strongly altered granitic cataclasite at the exo-contact with veinlet-disseminated textures. The porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite. The copper deposits contain simple ore mineral associations consisting of chalcopyrite, pyrite, bornite, molybdenite, sphalerite and oxidised minerals of malachite, covellite and molybdite. During supergene oxidation, primary ores underwent secondary enrichment to form economic orebodies with Cu grade ranging from 1% to 5%.Ore sulphides of the copper belt display S and Pb isotopic compositions identical to the ore-bearing porphyries. Their δ³⁴S values vary between − 3.8‰ and + 2.4‰ and are typical of mantle sulphur. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios vary in the ranges: 18.106 to 18.752, 15.501 to 15.638, and 37.394 to 39.058, respectively, and yielded radiogenetic lead-enriched signatures. Twelve molybdenite samples from the copper belt yielded isochron ages of 14.76 ± 0.22 Ma and 13.99 ± 0.16 Ma for the Nanmu and Chongjiang deposits and model ages of 13.5 to 13.6 Ma for the Lakang'e deposit. Meanwhile, ⁴⁰Ar/³⁹Ar isotopic dating of two biotite phenocrysts from the Chongjiang and Lakang'e deposits give plateau ages of 13.5 ± 1.0 Ma and 13.42 ±0.10 Ma, respectively. During the geodynamic evolution of the Gangdese collision-orogenic belt, intrusion of the ore-bearing porphyries took place just before the rapid uplift and E–W extension of the southern Plateau. And the ore-forming process may have occurred simultaneously with the uplift and extension (14 ± 0.1 Ma).     

Metamorphic ore remobilization in the Hällefors district, Bergslagen, Sweden: constraints from mineralogical and small-scale sulphur isotope studies

The marble- and metavolcanic-hosted Pb–Zn–(Ag–Sb–As) deposits of the Hällefors district, located in the Palaeoproterozoic Bergslagen ore province, south central Sweden, comprise both stratabound sulphides and discordant, Ag-rich sulphide–sulphosalt veins. The complex sulphide–sulphosalt assemblages of the Alfrida-Jan Olof mines at Hällefors were investigated by a combination of ore microscopy, electron-microprobe analysis, and in situ laser sulphur isotope analysis. The massive ore is characterized by positive and homogeneous ³⁴S (+1.4 to +2.7 V-CDT), whereas vein-hosted sulphides and sulphosalts exhibit similar, but generally less positive to slightly negative ³⁴S (–0.6 to +2.0). Comparison of the observed ore mineral assemblages with calculated phase equilibria in the system Fe–As–S–O–H and isotopic fractionation as a function of temperature, oxygen fugacity and pH indicates that the vein-type mineralization was formed from relatively reduced and rather alkaline hydrothermal fluids. At these reduced conditions, fractionation of ³⁴S via changes of fO₂ is insignificant, and thus the isotopic signatures of the vein minerals directly reflect the composition of the sulphur source. We therefore conclude that the vein-type ore essentially inherited the sulphur isotope signature from the pre-existing massive sulphides via metamorphic remobilization at approximately 300–400°C and 2–3 kbar. Scales of remobilization observable are on the order of about 5 mm to 30 cm. Overall, the sulphide–sulphosalt assemblages from the Alfrida-Jan Olof mines exhibit ³⁴S values which are comparable to a majority of metasupracrustal-hosted deposits in the Bergslagen province, thereby suggesting a common origin from ca. 1.90–1.88 Ga volcanic-hydrothermal processes.Editorial handling: S. Nicolescu     

铂族元素矿床的主要类型、成矿作用及研究展望

铂族元素(PGE)矿床的研究在过去几十年取得了重要的进展.它可以赋存于不同的岩石类型、形成于不同的时代.内生PGE矿床与不同的岩浆类型及热液活动有关.由于铂族元素特殊的化学性质,比较稳定且难熔于普通的酸、碱等,故铂族元素成矿具有特殊性.PGE矿床可划分为岩浆型、热液型、火山块状硫化物型(VMS)和外生型四大类型.岩浆型又可分为铜镍硫化物型、铬铁矿型和磁铁矿型,热液型主要有斑岩型和夕卡岩型,外生型包括黑色页岩型和砂铂矿型.本文讨论了各岩浆演化过程中:(i)硅酸盐和氧化物的分异,(ii)富Fe矿物(橄榄石、辉石、磁铁矿、铬铁矿)的分异,(iii)岩浆的混染,(iv)不同成分、硫不饱和的岩浆的混合等,都可以导致岩浆中硫达到饱和,一旦形成不混熔硫化物熔体,硫化物富集,将形成有经济价值的PGE矿床.同时,成矿还受温度、Ni和Cu含量、体系中其它组分和硫逸度的控制.岩浆后期的热液蚀变会改变PGE的含量和品位,但典型的铂矿床一般没有遭受热液蚀变作用的显著影响.本文指出了铂族元素矿床研究存在的主要问题.如PGE矿床的物质来源、PGE演化过程中的分配规律、铂族元素矿物(PGM)的赋存状态,并对以后的发展前景做了展望,指出西藏(蛇绿岩套铬铁矿亚类和俯冲增生弧斑岩型Cu-Au矿)和新疆(碰撞后二叠纪岩浆Cu-Nj硫化物型和黑色页岩型)是我国寻找PGE矿床的最有利地区.     

Sulphur dioxide,particle and elemental emissions from Mount Etna,Italy during July 1987

Sulphur dioxide emission rates were measured at Mount Etna, Italy during July 1987 while the volcano was undergoing relatively quiet activity. The SO₂ flux averaged 930 ± 587 (1) Mg/d, excluding 19 July when the flux was 3200 ± 1730 (1) Mg/d. Rising magma and/or an influx of less degassed magma could explain the increased SO₂ flux. The high SO₂ flux did not correlate with changes in observed volcanic activity. This suggests that SO₂ monitoring may be useful as an indicator of shallow magmatic activity, but not as a predictor of future eruptions of Mount Etna.Particles emitted from the two active craters, Bocca Nuova and Southeast Crater (SE), were composed of silicates, sulphates and dithionites. Chloride species were only observed in particles from SE. Different eruptive styles probably produced the differences in particle compositions emitted from each crater.Vapour — magma enrichment factors were calculated for many elements from both craters. Cl, Br and S were the most enriched elements in the sampled fumes. Similar enrichment factors at both craters suggest a common magma supply.Cl, S and F have the largest elemental fluxes emitted from Mount Etna. During quiet activity, the Cl flux represents 27% of the global anthropogenic emissions, but its effect is limited to the local region due to atmospheric removal processes. Mount Etna also exhaled significant amounts of Zn, Br, Mo, F and Cu compared with regional anthropogenic emissions.     

Isotopic ratio and concentration of sulfur in the undersaturated alkaline magmas of Vulture Volcano (Italy)

Both the ³⁴ value and the total S content of products from Vulture Volcano, Italy are mainly controlled by the separation of S gases, predominantly SO₂, from high f _O2magmas containing S predominantly as SO _2- ⁴ . The addition of evaporites to such magmas appears to be a relatively uncommon and limited phenomenon. The total S content of the most primitive product of Vulture Volcano (5600 mg/kg) is very high. The high ³⁴S value of 4 indicates an origin through the partial melting of a mantle containing high S, enriched in ³⁴S of unknown origin.     

滇中铅锌矿床地球化学与成因研究

滇中地层铅锌丰度高,成矿元素对于地层有继承性和一致性;硫同位素分散,富重硫;铅同位素稳定,多为单阶段演化正常铅,模式年龄与地层时代大体一致,矿质来自地层,矿床为层控型。同生沉积形成矿源层,后期改造(叠加)形成矿体。为中低温度的地下水热液成矿。     

The Great Dun Fell Experiment 1995: an overview

During March and April of 1995 a major international field project was conducted at the UMIST field station site on Great Dun Fell in Cumbria, Northern England. The hill cap cloud which frequently envelopes this site was used as a natural flow through reactor to examine the sensitivity of the cloud microphysics to the aerosol entering the cloud and also to investigate the effects of the cloud in changing the aerosol size distribution, chemical composition and associated optical properties. To investigate these processes, detailed measurements of the cloud water chemistry (including the chemistry of sulphur compounds, organic and inorganic oxidised nitrogen and ammonia), cloud microphysics and properties of the aerosol and trace gas concentrations upwind and downwind of the cap cloud were undertaken. It was found that the cloud droplet number was generally strongly correlated to aerosol number concentration, with up to 2000 activated droplets cm⁻³ being observed in the most polluted conditions. In such conditions it was inferred that hygroscopic organic compounds were important in the activation process. Often, the size distribution of the aerosol was substantially modified by the cloud processing, largely due to the aqueous phase oxidation of S(IV) to sulphate by hydrogen peroxide, but also through the uptake and fixing of gas phase nitric acid as nitrate, increasing the calculated optical scattering of the aerosol substantially (by up to 24%). New particle formation was also observed in the ultrafine aerosol mode (at about 5 nm) downwind of the cap cloud, particularly in conditions of low total aerosol surface area and in the presence of ammonia and HCl gases. This was seen to occur at night as well as during the day via a mechanism which is not yet understood. The implications of these results for parameterising aerosol growth in Global Climate Models are explored.     

Supergene sulphides and related minerals in the supergene profiles of VHMS deposits from the South Urals

The mineralogy and structure of the supergene profile in recently-exploited volcaniс hosted massive sulphide (VHMS) deposits of Cyprus, Uralian and Kuroko type in the South Urals, Russia, have been studied. Specific subzones enriched in secondary sulphides and associated minerals have been distinguished in residual pyrite and quartz–pyrite sands at the Gayskoye, Zapadno-Ozernoye, Dzhusinskoye and Alexandrinskoye deposits. Besides minerals which are common to the cementation subzones (covellite, chalcocite and acanthite), non-stoichiometric colloform and framboidal pyrite, pyrite–dzharkenite, pyrrhotite-like and jordanite-like minerals, metacinnabar, sphalerite, selenium-enriched tetrahedrite and unidentified As-, Sb sulphosalts of Pb or Hg and Ag, sulphur-bearing clausthalite, naumannite and tiemannite were also found. Secondary sulphide minerals in VHMS deposits of the South Urals region are characterized by light sulphur isotope compositions (− 8.1 to − 17.2‰). Superposition of the advanced oxidation of colloform pyrite, an enrichment in impurities (sphalerite, galena, and tennantite) from the primary ores, stagnant water conditions, an elevation of the water table during oxidation, and bacterial activity led to supergene concentrations of the base metals as sulphide, selenides or sulphosalts.     

Optimization and modeling of boric acid extraction from colemanite in water saturated with carbon dioxide and sulphur dioxide gases

In this study, the boric acid extraction from colemanite ore in aqueous media saturated by CO₂ and SO₂ gases was studied and the effects of relevant parameters, namely; reaction temperature, solid-to-liquid ratio, mean particle size, stirring speed and reaction time have been investigated on the boric acid extraction from colemanite ore by using the fractional factorial design and central composite design methods. The chosen experimental parameter levels were as follows: reaction temperature, 11.4–58.6 °C; solid-to-liquid ratio, 0.0685–0.1315 g/mL; mean particle size, 0.2835–3 mm; stirring speed, 107–893 rpm; reaction time, 7.125–22.875 min. A model has been developed between the boric acid extraction efficiency from colemanite ore and relevant parameters by means of variance analysis by using the matlab computer software and the obtained model was used to determine optimum conditions. The optimum conditions were found to be as follows: reaction temperature, 41 °C; solid-to-liquid ratio, 0.0685 g/mL; mean particle size, 0.2835 mm; stirring speed, 266 rpm; reaction time, 7 min. The calculated boric acid extraction efficiency from colemanite ore was approximately 99.9% under the optimum conditions.     

Using dual-isotope data to trace the origin and processes of dissolved sulphate: a case study in Calders stream (Llobregat basin,Spain)

Whereas most of the reported δ³⁴S values of dissolved sulphate are positive in the Llobregat basin, Calders stream, which is a tributary of the Llobregat River, is characterised by negative values. Stream waters, sampled monthly between 1997 and 1998, and quarterly in 1999, show an overall increase in δ³⁴S from −10‰ to 0‰, coupled with an increase in Na and Cl concentration. On the other hand, the oxygen isotopic composition of dissolved sulphate, δ¹⁸O, displayed an opposite trend with a slight decrease, from +9‰ to +6‰. Detailed sampling up stream in November 2000 indicated that, contrary to most of the surficial waters of the Llobregat basin with a δ³⁴S_SO4 mainly controlled by evaporites, in Calders stream, sulphate is derived from pyrite oxidation. The dual-isotope approach, coupled with chemical data, allowed us to identify the contribution of ³⁴S-rich sulphate effluents from anthropogenic sources, while mixing models, calculated between natural and anthropogenic sources, enabled us to estimate their contribution. Sudden increases of δ³⁴S and δ¹⁸O of dissolved sulphate in stream waters are believed to be caused by a sulphate reduction process related to oil spillage. The long-term enrichment in δ³⁴S, coupled with a decrease in δ¹⁸O_SO4, from Jan-97 to Aug-99, is interpreted as a progressive increase in the contribution of pig manure.