Geochemistry and evolution of ore-forming fluids of the Yueshan Cu–Au skarn- and vein-type deposits, Anhui Province, South China

The Yueshan mineral belt is geotectonically located at the centre of the Changjiang deep fracture zone or depression of the lower Yangtze platform. Two main types of ore deposits occur in the Yueshan orefield: Cu–Au–(Fe) skarn deposits and Cu–Mo–Au–(Pb–Zn) hydrothermal vein-type deposits. Almost all deposits of economic interest are concentrated within and around the eastern and northern branches of the Yueshan dioritic intrusion. In the vicinity of the Zongpu and Wuhen intrusions, there are many Cu–Pb–Zn–Au–(S) vein-type and a few Cu–Fe–(Au) skarn-type occurrences.Fluid inclusion studies show that the ore-forming fluids are characterised by a Cl⁻(S)–Na⁺–K⁺ chemical association. Hydrothermal activity associated with the above two deposit types was related to the Yueshan intrusion. The fluid salinity was high during the mineralisation processes and the fluid also underwent boiling and mixed with meteoric water. In comparison, the hydrothermal activity related to the Zongpu and Wuhen intrusions was characterised by low salinity fluids. Chlorine and sulphur species played an important role in the transport of ore-forming components.Hydrogen- and oxygen-isotope data also suggest that the ore-forming fluids in the Yueshan mineral belt consisted of magmatic water, mixed in various proportions with meteoric water. The enrichment of ore-forming components in the magmatic waters resulted from fluid–melt partitioning. The ore fluids of magmatic origin formed large Cu–Au deposits, whereas ore fluids of mixed magmatic-meteoric origin formed small- to medium-sized deposits.The sulphur isotopic composition of the skarn- and vein-type deposits varies from − 11.3‰ to + 19.2‰ and from + 4.2‰ to + 10.0‰, respectively. These variations do not appear to have been resulted from changes of physicochemical conditions, rather due to compositional variation of sulphur at the source(s) and by water–rock interaction. Complex water–rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulphur mixing. Lead and silicon isotopic compositions of the two deposit types and host rocks provide similar indications for the sources and evolution of the ore-forming fluids.Hydrodynamic calculations show that magmatic ore-forming fluids were channelled upwards into faults, fractures and porous media with velocities of 1.4 m/s, 9.8 × 10^− 1 to 9.8 × 10^− 7 m/s and 3.6 × 10^− 7 to 4.6 × 10^− 7 m/s, respectively. A decrease of fluid migration velocity in porous media or tiny fractures in the contact zones between the intrusive rocks and the Triassic sedimentary rocks led to the deposition of the ore-forming components. The major species responsible for Cu transport are deduced to have been CuCl, CuCl₂⁻, CuCl₃²⁻ and CuClOH, whereas Au was transported as Au₂(HS)₂S²⁻, Au(HS)₂⁻, AuHS and AuH₃SiO₄ complexes. Cooling and a decrease in chloride ion concentration caused by fluid boiling and mixing were the principal causes of Cu deposition. Gold deposition was related to decrease of pH, total sulphur concentration and fO₂, which resulted from fluid boiling and mixing.Geological and geochemical characteristics of the two deposit types in the Yueshan mineral belt suggest that there is a close genetic relationship with the dioritic magmatism. Geochronological data show that the magmatic activity and the mineralisation took place between 130 and 136 Ma and represent a continuous process during the Yanshanian time. The cooling of the intrusions and the mineralisation event might have lasted about 6 Ma. The cooling rate of the magmatic intrusions was 80 to 120 °C my^− 1, which permitted sufficient heat supply by magma to the ore-forming system.     

The sources and evolution of mineralising fluids in iron oxide–copper–gold systems, Norrbotten, Sweden: Constraints from Br/Cl ratios and stable Cl isotopes of fluid inclusion leachates

We have analysed the halogen concentrations and chlorine stable isotope composition of fluid inclusion leachates from three spatially associated Fe-oxide ± Cu ± Au mineralising systems in Norrbotten, Sweden. Fluid inclusions in late-stage veins in Fe-oxide–apatite deposits contain saline brines and have a wide range of Br/Cl molar ratios, from 0.2 to 1.1 × 10⁻³ and δ³⁷Cl values from −3.1‰ to −1.0‰. Leachates from saline fluid inclusions from the Greenstone and Porphyry hosted Cu–Au prospects have Br/Cl ratios that range from 0.2 to 0.5 × 10⁻³ and δ³⁷Cl values from −5.6‰ to −1.3‰. Finally, the Cu–Au deposits hosted by the Nautanen Deformation Zone (NDZ) have Br/Cl molar ratios from 0.4 to 1.1 × 10⁻³ and δ³⁷Cl values that range from −2.4‰ to +0.5‰, although the bulk of the data fall within 0‰ ± 0.5‰.The Br/Cl ratios of leachates are consistent with the derivation of salinity from magmatic sources or from the dissolution of halite. Most of the isotopic data from the Fe-oxide–apatite and Greenstone deposits are consistent with a mantle derived source of the chlorine, with the exception of the four samples with the most negative values. The origin of the low δ³⁷Cl values in these samples is unknown but we suggest that there may have been some modification of the Cl-isotope signature due to fractionation between the mineralising fluids and Cl-rich silicate assemblages found in the alteration haloes around the deposits. If such a process has occurred then a modified crustal source of the chlorine for all the samples cannot be ruled out although the amount of fractionation necessary to generate the low δ³⁷Cl values would be significantly larger.The source of Cl in the NDZ deposits has a crustal signature, which suggests the Cl in this system may be derived from (meta-) evaporites or from input from crustal melts such as granitic pegmatites of the Lina Suite.     

The aqueous geochemistry of Zr and the solubility of some Zr-bearing minerals

Literature data on the thermodynamics of complexation of Zr with inorganic species, at 25°C, have been critically reviewed. The preponderance of published complexation constants deal with F⁻ and OH⁻ ions. Stability constants for the complexation reactions are relatively independent of ionic strength and thus recomended values for each ligand type are averages of the most reliable data. Complexation constants under elevated conditions (T 250°C andP_v = P_H2O) have been predicted for various Zr complexes (F⁻, Cl⁻, SO₄²⁻ and OH⁻) using Helgeson's electrostatic approach. Predominance diagrams (calculated for simple systems with these constants) suggest that, over a wide range of pH conditions, Zr(OH)_4(aq) will dominate the aqueous geochemistry of Zr except under very high activities of competing ligands (e.g., F⁻, SO₄²⁻).The solubilities of vlasovite [Na₂ZrSi₄O₁₁] and weloganite [Sr₃Na₂Zr(CO₃)₆·3H₂O have been measured in KCI solutions (0.5–1.0 M) at 50°C. Weloganite dissolution is complicated by the predictable precipitation of strontianite (SrCO₃) whereas vlasovite dissolves incongruently. Solubility products for the dissolution of welonganite and vlasovite are determined to be −28.96±0.14 and −20.40±1.18, respectively. Concentrations of Zr up to 10⁻³ m were present in the experimental solutions; the presence of large amounts of Zr in aqueous solutions support the possibility of extensive remobilization of Zr during hydrothermal mineralization.     

Boron isotope composition of geothermal fluids and borate minerals from salar deposits (central Andes/NW Argentina)

We have measured the boron concentration and isotope composition of regionally expansive borate deposits and geothermal fluids from the Cenozoic geothermal system of the Argentine Puna Plateau in the central Andes. The borate minerals borax, colemanite, hydroboracite, inderite, inyoite, kernite, teruggite, tincalconite, and ulexite span a wide range of δ¹¹B values from −29.5 to −0.3‰, whereas fluids cover a range from −18.3 to 0.7‰. The data from recent coexisting borate minerals and fluids allow for the calculation of the isotope composition of the ancient mineralizing fluids and thus for the constraint of the isotope composition of the source rocks sampled by the fluids. The boron isotope composition of ancient mineralizing fluids appears uniform throughout the section of precipitates at a given locality and similar to values obtained from recent thermal fluids. These findings support models that suggest uniform and stable climatic, magmatic, and tectonic conditions during the past 8 million years in this part of the central Andes. Boron in fluids is derived from different sources, depending on the drainage system and local country rocks. One significant boron source is the Paleozoic basement, which has a whole-rock isotopic composition of δ¹¹B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites (δ¹¹B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones (δ¹¹B≤+8‰) provide a potential third boron source.     

Fluid inclusion and stable isotope study of the Cobre–Babilonia polymetallic epithermal vein system, Taxco district, Guerrero, Mexico

The Cobre–Babilonia vein system formed during a single major hydrothermal stage and is part of the Taxco district in Guerrero, southern Mexico. Homogenization and ice melting temperatures range from 160 to 290 °C and from − 11.6 to − 0.5 °C, respectively. We determined an approximate thermal gradient of 17 to 20 °C per 100 m using fluid inclusions. A thermal peak marked by the 290 °C isotherm is interpreted as a major feeder channel to the veins. The highest content of Zn + Pb in ore coincides with the 220 and 240 °C isotherms. Salinities of mineralizing fluids range from 0.8 to 15.6 wt.% NaCl equiv, and are distributed in two populations that can be related with barren or ore-bearing vein sections, with 0.8 to 6 wt.% NaCl equiv and 7 to 15.6 wt.% NaCl equiv, respectively. δ¹³C and δ¹⁸O water values from calcite from the Cobre–Babilonia vein system and the Esperanza Vieja and Guadalupe mantos range − 5.4‰ to − 10.4‰ and 9.9‰ to 13.4‰, respectively. δ³⁴S values range from 0‰ to 3.2‰ and − 0.7‰ to − 4.3‰ in sphalerite, − 4‰ to 0.9‰ in pyrite, and − 1.4‰ to − 5.5‰ in galena. Both fluid inclusion and stable isotope data are compatible with magmatic and meteoric sources for mineralizing fluids. Also, sulfur isotope compositions suggest both magmatic and sedimentary sources for sulfur.     

Sn-polymetallic greisen-type deposits associated with late-stage rapakivi granites, Brazil: fluid inclusion and stable isotope characteristics

Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondônia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondônia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Bárbara deposit (Rondônia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sn (±W, ±Ta, ±Nb), and base-metal suite (Zn–Cu–Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0–19 wt.% NaCl eq.) CO₂-bearing aqueous fluids homogenizing at 245–450 °C, and (2) aqueous solutions with low CO₂, low to moderate salinity (0–14 wt.% NaCl eq.), which homogenize between 100 and 340 °C. In the Santa Bárbara deposit, the early inclusions are represented by (1) low-salinity (5–12 wt.% NaCl eq.) aqueous fluids with variable CO₂ contents, homogenizing at 340 to 390 °C, and (2) low-salinity (0–3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320–380 °C. Cassiterite, wolframite, columbite–tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0–6 wt.% NaCl eq.) CO₂-free aqueous solution, which homogenizes at (100–260 °C) and characterizes the sulfide–fluorite–sericite association in the Correas deposit. The late fluid in the Santa Bárbara deposit has lower salinity (0–3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240–450 °C, and 1.0–2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids (δ¹⁸O_quartz from 9.9‰ to 10.9‰, δD_H2O from 4.13‰ to 6.95‰) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 °C. In the Santa Bárbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 °C, respectively), and that for the cassiterite-quartz-veins is 415 °C. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids (δ¹⁸O_qtz-H2O=5.5–6.1‰) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica (δ¹⁸O_mica-H2O=3.3–9.8‰) suggest mixing with meteoric water. Late muscovite veins (δ¹⁸O_qtz-H2O=−6.4‰) and late quartz (δ¹⁸O_mica-H2O=−3.8‰) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor columbite–tantalite precipitation. Change in the redox conditions related to mixing of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit.     

Sediment hosted lead–zinc deposits of the Neoproterozoic Bambuí Group and correlative sequences, São Francisco Craton, Brazil: A review and a possible metallogenic evolution model

The Proterozoic sediment-hosted Zn–(Pb) sulfide and non-sulfide deposits of the São Francisco Craton, Brazil, are partially syn-diagenetic and epigenetic and were probably formed during extensional events. The majority of the deposits occur within shallow water dolomites. The Pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the Plumbotectonic model. Some of the deposits are characterized by highly radiogenic lead (²⁰⁶Pb/²⁰⁴Pb ≥ 21) originating from the highly radioactive crust of the São Francisco Craton. Pb and S isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments, respectively. The relatively high temperatures of formation (100 to 250 °C) and moderate salinity (3% to 20% NaCl equiv.) of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation. The steep paleo-geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids.     

Geochemistry of selenium: formation of ferroselite and selenium behavior in the vicinity of oxidizing sulfide and uranium deposits

The geochemistry of Se is largely controlled by that of iron, with which Se is closely affiliated in both oxidizing and reducing environments. In aerated waters the Se(IV) oxyanions, HSeO^?₃ and SeO^2?₃, are strongly adsorbed by hydrated surfaces of ferric oxides over the pH range 2–8; above pH8 adsorption decreases to complete desorption at pH 11. This adsorption immobilizes Se(IV) in neutral-to-acid waters and increases the range of oxidation potential over which Se(IV) is stable. During experimental aeration of aqueous Fe-S-Se systems, the stability field of Se(IV) is attained and elemental Se is slowly oxidized to this higher valence; oxidation potentials of the Se(VI) stability field were never reached, however, even by continued aeration of an alkaline system. Under reducing conditions, elemental Se either is incorporated within pyrite or forms the mineral ferroselite (FeSe₂.Selenium geochemistry is summarized on an Eh-pH diagram, synthesized from equilibrium calculations, experimental work and reported geologic occurrences. A stability field for ferroselite, constructed for a Gibbs free-energy value of ?23.2 kcal/mole, is in accord both with its geologic occurrence and behavior and with conditions under which ferroselite has been synthesized. Traces on this diagram of Eh-pH variation show the behavior of selenium during oxidation of associated iron-sulfide minerals. Such considerations also demonstrate the manner in which selenium migrates, is deposited and is increasingly concentrated in roll-type sandstone uranium deposits, as well as the relative positions of the several forms of selenium within the deposit.     

The Daduhe gold field at the eastern margin of the Tibetan Plateau: He, Ar, S, O, and H isotopic data and their metallogenic implications

The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have ³He/⁴He ratios of 0.16 to 0.86 R_a, whereas their ⁴⁰Ar/³⁶Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ³⁴S values of pyrite are of 0.7–4.2‰ (n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ¹⁸O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.     

Geology and geochemistry of the adjacent Changkeng gold and Fuwang silver deposits, Guangdong Province, South China

The Changkeng Au and Fuwang Ag deposits represent an economically significant and distinct member of the Au–Ag deposit association in China. The two deposits are immediately adjacent, but the Au and Ag orebodies separated from each other. Ores in the Au deposit, located at the upper stratigraphic section and in the southern parts of the orefield, contain low Ag contents (< 11 ppm); the Ag orebodies, in the lower stratigraphic section, are Au-poor (< 0.2 ppm). Changkeng is hosted in brecciated cherts and jasperoidal quartz and is characterized by disseminated ore minerals. Fuwang, hosted in the Lower Carboniferous Zimenqiao group bioclastic limestone, has vein and veinlet mineralization associated with alteration comprised of quartz, carbonate, sericite, and sulfides. Homogenization temperatures of fluid inclusions from quartz veinlets in the Changkeng and Fuwang deposits are in the range of 210 ± 80 °C and 230 ± 50 °C, respectively. Salinities of fluid inclusions from the two deposits range from 1.6 to 7.3 wt.% and 1.6 to 2.6 wt.% equiv. NaCl, respectively. The δD_H2O, δ¹⁸O_H2O, δ¹³C_CO2 and ³He/⁴He values of the fluid inclusions from the Changkeng deposit range from − 80‰ to − 30‰, − 7.8‰ to − 3.0‰, − 16.6‰ to − 17.0‰ and 0.0100 to 0.0054 Ra, respectively. The δD_H2O, δ¹⁸O_H2O, δ¹³C_CO2 and ³He/⁴He values of fluid inclusions from the Fuwang deposit range from − 59‰ to − 45‰, − 0.9‰ to 4.1‰, − 6.7‰ to − 0.6‰ and 0.5930 to 0.8357 Ra, respectively. The δD_H2O, δ¹⁸O_H2O, δ¹³C_CO2 and ³He/⁴He values of the fluid inclusions suggest the ore fluids of the Changkeng Au-ore come from the meteoric water and the ore fluids of the Fuwang Ag-ore are derived from mixing of magmatic water and meteoric water. The two deposits also show different Pb-isotopic signatures. The Changkeng deposit has Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb: 18.580 to 19.251, ²⁰⁷Pb/²⁰⁴Pb: 15.672 to 15.801, ²⁰⁸Pb/²⁰⁴Pb: 38.700 to 39.104) similar to those (²⁰⁶Pb/²⁰⁴Pb: 18.578 to 19.433, ²⁰⁷Pb/²⁰⁴Pb: 15.640 to 15.775, ²⁰⁸Pb/²⁰⁴Pb: 38.925 to 39.920) of its host rocks and different from those (²⁰⁶Pb/²⁰⁴Pb: 18.820 to 18.891, ²⁰⁷Pb/²⁰⁴Pb: 15.848 to 15.914, ²⁰⁸Pb/²⁰⁴Pb: 39.579 to 39.786) of the Fuwang deposit. The different signatures indicate different sources of ore-forming material. Rb–Sr isochron age (68 ± 6 Ma) and ⁴⁰Ar–³⁹Ar age (64.3 ± 0.1 Ma) of the ore-related quartz veins from the Ag deposit indicate that the Fuwang deposit formed during the Cenozoic Himalayan tectonomagmatic event. Crosscutting relationships suggests that Au-ore predates Ag-ore. The adjacent Changkeng and Fuwang deposits could, however, represent a single evolved hydrothermal system. The ore fluids initially deposited Au in the brecciated siliceous rocks, and then mixing with the magmatic water resulted in Ag deposition within fracture zones in the limestone. The deposits are alternatively the product of the superposition of two different geological events. Age evidence for the Fuwang deposit, together with the Xiqiaoshan Tertiary volcanic-hosted Ag deposit in the same area, indicates that the Pacific Coastal Volcanic Belt in the South China Fold Belt has greater potential for Himalayan precious metal mineralization than previous realized.     

As–(Ag) sulphide veins in the Spanish Central System: further evidence for a hydrothermal event of Permian age

The Spanish Central System (SCS) has been subjected to repeated deformation and fluid flow events which have produced both sulphide-bearing and barren vein systems. Although several hydrothermal episodes took place between 300 and 100 Ma, fluid circulation during the Permian was especially important, giving rise to a range of different types of deposits. This study presents a multidisciplinary approach leading to the characterisation of the chemistry and age of the hydrothermal fluids that produced the As–(Ag) mineralised stockwork of Mónica mine (Bustaviejo, Madrid). Fluid inclusion data indicate the presence of two different fluids. An initial ore stage (I) formed from a low- to moderate salinity (3–8 wt.% eq. NaCl) H₂O–NaCl–CO₂–CH₄ fluid, at minimum trapping temperature of 350±15 °C and 0.3 kbar. A second H₂O–NaCl fluid is found in three types of fluid inclusions: a high temperature and low salinity type (340±20 °C; 0.8–3.1 wt.% eq. NaCl) also associated to ore stage I, a moderate temperature and very low salinity type (160–255 °C; 0–1.5 wt.% eq. NaCl) represented in ore stage III, and a very low temperature and hypersaline type (60–70 °C; 30–35 wt.% NaCl), unrelated to the mineralising stages and clearly postdating the previous types. ⁴⁰Ar–³⁹Ar dating on muscovite from the early As–Fe stage (I) has provided an age of 286±4 Ma, synchronous with the late emplacement phases of La Cabrera plutonic massif (288±5 Ma) and with other Permian hydrothermal events like Sn–W skarns and W–(Sn) sulphide veins. δ¹⁸O of water in equilibrium with stage I quartz (5.3–7.7‰), δD of water in equilibrium with coexisting muscovite (−16.0‰ to −2.0‰), and sulphide δ³⁴S (1.5–3.6‰) values are compatible with waters that leached metamorphic rocks. The dominant mechanism of the As–(Ag) deposition was mixing and dilution processes between aqueous–carbonic and aqueous fluids for stage I (As–Fe), and nearly isobaric cooling processes for stages II (Zn–Cu–Sn) and III (Pb–Ag). The origin and hydrothermal evolution of As–(Ag) veins is comparable to other hydrothermal Permian events in the Spanish Central System.     

Simultaneous determination of sulfide, polysulfides and thiosulfate as an aid to ore exploration

The interaction of water and sulfide minerals yields dissolved species which can be utilized to trace back the presence of sulfide minerals and associated minerals. Computer modeling and laboratory and field results show that the most characteristic dissolved species are hydrogen sulfide (H₂S, HS⁻), polysulfide ions (S_n²⁻) and thiosulfate (S₂O₃²⁻), derived from the hydrolysis of sulfide minerals. Typical concentration ranges are: 10⁻⁵ – 10⁻⁷ mole/l for hydrogen sulfide, 10⁻⁶ – 10⁻⁹ mole/l for polysulfides and 10⁻⁵ – 10⁻⁸ mole/l for thiosulfate. The chemical reactivity of these species at contact with air makes them difficult to assess unless determined immediately after sampling.These sulfur species can be determined rapidly and accurately in field conditions by simultaneous titration with mercuric chloride employing an Ag/Ag₂S electrode for the determination of the end points.The application to ore exploration is exemplified by the results of the research on roll-type uranium deposits in the southwest of France.     

A comparative study on the dissolution and solubility of hydroxylapatite and fluorapatite at 25 °C and 45 °C

Dissolution of the synthetic hydroxylapatite (HAP) and fluorapatite (FAP) in pure water was studied at 25 °C and 45 °C in a series of batch experiments. The XRD, FT-IR and SEM analyses indicated that the synthetic, microcrystalline HAP and FAP with apatite structure used in the experiments were found to have no obvious variation after dissolution except that the existence of OH groups in FT-IR spectra for FAP after 2880 h dissolution was observed. During the HAP dissolution (0–4320 h), the aqueous calcium and phosphate concentrations reached the maxima after 120 h and then decreased slowly with time. For the FAP dissolution in pure water, after a transient time of 1440 h (< 60 d), element concentrations and pH became constant suggesting attainment of a steady-state between the solution and solid. During early stages of the FAP dissolution reaction (< 72–120 h), mineral components were released in non-stoichiometric ratios with reacted solution ratios of dissolved Ca:P, Ca:F and P:F being lower than mineral stoichiometric ratios of Ca₅(PO₄)₃F, i.e., 1.67, 5.0 and 3.0, respectively. This indicated that F were preferentially released compared to Ca from the mineral structure. The mean K_sp values were calculated by using PHREEQC for HAP of 10^− 53.28 (10^− 53.02–10^− 53.51) and for FAP of 10^− 55.71 (10^− 55.18–10^− 56.13) at 25 °C, the free energies of formation ΔG_f^o[HAP] and ΔG_f^o[FAP] were calculated to be − 6282.82 kJ/mol and − 6415.87 kJ/mol, respectively.     

Relationship between Se/S and sulfur isotope ratios of hydrothermal sulfide minerals

The pH and f_{O 2} dependences of the [Se^2–]/[S^2–] ratio in chloride solutions at 100°, 200° and 300°C are predicted thermodynamically. Under the high f_{O 2} conditions where sulfate species are dominant in solution, the [Se^2–]/[S^2–] ratio always increases with increasing pH and/or f_{O 2}. Under the low f_{O 2} conditions where sulfide species are dominant in solution, the pH and f_{O 2} dependences of the [Se^2–]/[S^2–] ratio are seriously affected by the presence of native selenium. With native selenium present, the [Se^2–]/[S^2–] ratio decreases with increasing f_{O 2}, but almost independent of pH in geologically important pH regions. When native selenium is absent, the [Se^2–]/[S^2–] ratio is solely a function of pH and independent of f_{O 2}. Combining the above with the pH and f_{O 2} dependences of ³⁴S value of aqueous sulfur species, we discuss the possible influences of the pH and f_{O 2} of ore-forming solutions on the relationship between the Se/S ratio and ³⁴S value of hydrothermal sulfide minerals. The results are applied to some Japanese sulfide ore deposits.     

Siderite mineralization of the Gemericum superunit (Western Carpathians, Slovakia): review and a revised genetic model

The Gemericum is a segment of the Variscan orogen subsequently deformed by the Alpine–Carpathian orogeny. The unit contains abundant siderite–sulphide and quartz–antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano-sedimentary sequences. The siderite–sulphide veins and siderite replacement deposits of the Gemericum represent one of the largest accumulations of siderite in the world, with about 160 million tonnes of mineable FeCO₃. More than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern, reflecting the distribution of regional metamorphic zones. Siderite–sulphide veins are typically contained in low-grade (chlorite zone) sedimentary, volcano-sedimentary or volcanic Lower and Upper Paleozoic rocks. Quartz–antimony veins are hosted by higher-grade units (biotite zone). Siderite–sulphide veins are dominated by early siderite followed by a complex set of stages, including quartz–sulphide (chalcopyrite, tetrahedrite), barite, tourmaline–quartz, and sulphide-remobilization stages. The temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes, within-vein replacement and recrystallization. Siderite–sulphide veins show considerable vertical (up to 1200 m) and lateral (up to 15 km) extent, and a thickness typically reaching several metres. Carbonate-replacement siderite deposits of the Gemericum are hosted by a Silurian limestone belt and are similar to stratabound siderite deposits of the Eastern Alps (e.g., Erzberg, Austria).Based on a review of geological, petrological and geochronological data for the Gemericum, and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals, the siderite–sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense. The deposits were formed during several stages of regional crustal-scale fluid flow. Isotope (S, C, Sr, Pb) fingerprinting identifies the metamorphosed rock complexes of the Gemericum as a source of most components of hydrothermal fluids. Fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types, and the existence of contrasting P–T conditions during vein evolution. A high-δ¹⁸O, medium- to high-salinity, H₂O-type fluid is the most important component during siderite deposition, whereas H₂O–CO₂-type fluid inclusion containing dense liquid CO₂ and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline–quartz stage. Younger quartz–ankerite(±siderite)–sulphide stages are characterized by high-salinity (17 to 35 wt.% NaCl equivalent) and low-temperature (T_h=90 to 180 °C) H₂O-type fluids.The vein deposits are interpreted as a result of multistage hydrothermal circulation, with Variscan and Alpine mineralization phases. Based on available indirect data, the most important mineralization phase was related to regional fluid flow during the uplift of a Variscan metamorphic core complex, producing siderite–sulphide (±barite) mineralization, while tourmaline–quartz stage and sulphide remobilization stages are related to Alpine processes. Two phases of vein evolution are evident from two groups of ⁸⁷Sr/⁸⁶Sr isotope ratios of Sr-rich, Rb-poor hydrothermal minerals: 0.71042–0.71541 in older barite and 0.7190–0.7220 in late-stage celestine and strontianite.     

Rare earth element geochemistry of the Woxi W–Sb–Au deposit, Hunan Province, South China

The Woxi W–Sb–Au deposit in Hunan, South China, is hosted by Proterozoic metasedimentary rocks, a turbiditic sequence of slightly metamorphosed (greenschist facies), gray-green and purplish red graywacke, siltstone, sandy slate, and slate. The mineralization occurs predominantly (> 70%) as stratabound/stratiform ore layers and subordinately as stringer stockworks. The former consists of rhythmically interbedded, banded to finely laminated stibnite, scheelite, quartz, pyrite and silty clays, whereas the latter occurs immediately beneath the stratabound ore layers and is characterized by numerous quartz + pyrite + gold + scheelite stringer veins or veinlets that are typically either subparallel or subvertical to the overlying stratabound ore layers. The deposit has been the subject of continued debate in regard to its genesis. Rare earth element geochemistry is used here to support a sedimentary exhalative (sedex) origin for the Woxi deposit. The REE signatures of the metasedimentary rocks and associated ores from the Woxi W–Sb–Au deposit remained unchanged during post-depositional processes and were mainly controlled by their provenance. The original ore-forming hydrothermal fluids, as demonstrated by fluid inclusions in quartz from the banded ores, are characterized by variable total REE concentrations (3.5 to 136 ppm), marked LREE enrichment (La_N/Yb_N = 28–248, ∑LREE/∑HREE = 16 to 34) and no significant Eu-anomalies (Eu/Eu = 0.83 to 1.18). They were most probably derived from evolved seawater that circulated in the clastic sediment pile and subsequently erupted on the seafloor. The bulk banded ores are enriched in HREE (La_N/Yb_N = 4.6–11.4, ∑LREE/∑HREE = 3 to 14) and slightly depleted in Eu (Eu/Eu = 0.63 to 1.14) relative to their parent fluids. This is interpreted as indicating the influence of seawater rather than a crystallographic control on REE content of the ores. Within a single ore layer, the degree of HREE enrichment tends to increase upward while the total REE concentrations decrease, reflecting greater influence and dilution of seawater. There is a broad similarity in chondrite-normalized REE patterns and the amount of REE fractionation of the banded ores in this study and exhalites from other sedex-type polymetallic ore deposits, suggesting a similar genesis for these deposits. This conclusion is in agreement with geologic evidence supporting a syngenetic (sedex) model for the Woxi deposit.     

Geology, mineralogy and stable isotope geochemistry of the Kabwe carbonate-hosted Pb–Zn deposit, Central Zambia

The carbonate-hosted Kabwe Pb–Zn deposit, Central Zambia, has produced at least 2.6 Mt of Zn and Pb metal as well as minor amounts of V, Cd, Ag and Cu. The deposit consists of four main epigenetic, pipe-like orebodies, structurally controlled along NE–SW faults. Sphalerite, galena, pyrite, minor chalcopyrite, and accessory Ge-sulphides of briartite and renierite constitute the primary ore mineral assemblage. Cores of massive sulphide orebodies are surrounded by oxide zones of silicate ore (willemite) and mineralized jasperoid that consists largely of quartz, willemite, cerussite, smithsonite, goethite and hematite, as well as numerous other secondary minerals, including vanadates, phosphates and carbonates of Zn, Pb, V and Cu.Galena, sphalerite and pyrite from the Pb–Zn rich massive orebodies have homogeneous, negative sulphur isotope ratios with mean δ³⁴S_CDT permil (‰) values of − 17.75 ± 0.28 (1σ), − 16.54 ± 0.0.27 and − 15.82 ± 0.25, respectively. The Zn-rich and Pb-poor No. 2 orebody shows slightly heavier ratios of − 11.70 ± 0.5‰ δ³⁴S for sphalerite and of − 11.91 ± 0.71‰ δ³⁴S for pyrite. The negative sulphur isotope ratios are considered to be typical of sedimentary sulphides produced through bacterial reduction of seawater sulphate and suggest a sedimentary source for the sulphur.Carbon and oxygen isotope ratios of the host dolomite have mean δ¹³C_PDB and δ¹⁸O_SMOW values of 2.89‰ and 27.68‰, respectively, which are typical of marine carbonates. The oxygen isotope ratios of dolomite correlate negatively to the SiO₂ content introduced during silicification of the host dolomite. The depletion in ¹⁸O in dolomite indicates high temperature fluid/rock interaction, involving a silica- and ¹⁸O-rich hydrothermal solution.Two types of secondary fluid inclusions in dolomite, both of which are thought to be related to ore deposition, indicate temperatures of ore deposition in the range of 257 to 385 and 98 to 178 °C, respectively. The high temperature fluid inclusions contain liquid + vapour + solid phases and have salinities of 15 to 31 eq. wt.% NaCl, whereas the low temperature inclusions consist of liquid + vapour with a salinity of 11.5 eq. wt.% NaCl.Fluid transport may have been caused by tectonic movements associated with the early stages of the Pan-African Lufilian orogeny, whereas ore deposition within favourable structures occurred due to changes in pressure, temperature and pH in the ore solution during metasomatic replacement of the host dolomite. The termination of the Kabwe orebodies at the Mine Club fault zone and observed deformation textures of the ore sulphides as well as analysis of joint structures in the host dolomite, indicate that ore emplacement occurred prior to the latest deformation phase of the Neoproterozoic Lufilian orogeny.     

Fractionation of sulfur isotopes and selenium between coexisting sulfide minerals from the Besshi deposit,Central Shikoku,Japan

Fractionation of sulfur isotopes and selenium was measured between coexisting pyrite and chalcopyrite and between coexisting pyrrhotite and chalcopyrite from the Besshi deposit of Kieslager-type, Central Shikoku, Japan. In all the pyrite-chalcopyrite pairs studied, ³⁴S is enriched in pyrite relative to chalcopyrite, while selenium is enriched conversely in chalcopyrite relative to pyrite. The mean ³⁴S_py-cp value is +0.53±0.36 per mil, and the mean value of the distribution coefficient of selenium, D_cp-py, is 2.58±0.64. In all the pyrrhotite-chalcopyrite pairs studied, the two minerals are very close to each other both in sulfur isotope and Se/S ratios. The mean ³⁴S_po-cp value is –0.08±0.16 per mil and the mean D_cp-po value is 0.99±0.05. The results have been discussed in comparison with similar data obtained for the Hitachi deposits of Kieslager-type, Japan (Yamamoto et al. 1983).     

Reduced sulfur and iron species in anoxic water column of meromictic crater Lake Pavin (Massif Central, France)

The vertical distribution of reduced sulfur species (RSS including H₂S/HS⁻, S⁰, electroactive FeS) and dissolved Fe(II) was studied in the anoxic water column of meromictic Lake Pavin. Sulfide concentrations were determined by two different analytical techniques, i.e. spectophotometry (methylene blue technique) and voltammetry (HMDE electrode). Total sulfide concentrations determined with methylene blue method (∑H₂S_MBRS) were in the range from 0.6 µM to 16.7 µM and were substantially higher than total reduced sulfur species (RSS_V) concentrations determined by voltammetry, which ranged from 0.1 to 5.6 μM. The observed difference in the sulfide concentrations between the two methods can be assigned to the presence of FeS colloidal species.Dissolved Fe was high (> 1000 µM), whereas dissolved Mn was only 25 µM, in the anoxic water column. This indicates that Fe is the dominant metal involved in sulfur redox cycling and precipitation. Consequently, in the anoxic deep layer of Lake Pavin, “free” sulfide, H₂S/HS⁻, was low; and about 80% of total sulfide detected was in the electroactive FeS colloidal form. IAP calculations showed that the Lake Pavin water column is saturated with respect to FeS_am phase. The upper part of monimolimnion layer is characterized by higher concentrations of S(0) (up to 3.4 µM) in comparison to the bottom of the lake. This behavior is probably influenced by sulfide oxidation with Fe(III) oxyhydroxide species.     

Chemistry and sulfur isotopic composition of precipitation at Bologna, Italy

Individual and monthly precipitation samples from the polluted atmosphere of Bologna (Emilia-Romagna province) were collected during March 1996 to May 1997 and analyzed for major ions in solution and S isotopes in dissolved SO₄.Weighted mean enrichment factors relative to seawater are found to be 1.0 for Na, 15.2 for K, 105 for Ca, 3.3 for Mg, 17.3 for SO₄ and 663 for HCO⁻₃. Very good positive correlations are observed for the Ca²⁺–Mg²⁺–HCO⁻₃–SO²⁻₄–NO⁻₃ system, indicating that dissolution of Ca (±Mg)-carbonate particles by H₂SO₄ and HNO₃ from combustion of oil and gas is a major process controlling the chemical composition of rain and snow. Na⁺ and Cl⁻ in monthly precipitation derive essentially from sea spray, but the contribution of Na⁺ from continental sources is appreciable in a number of individual rains. NH⁺₄ appears to be on average more abundant in spring and summer precipitation, its main sources being microbial activity in soils and application of fertilizers. K⁺ is probably of continental origin from soil dust.The S isotopic composition of SO₄ is systematically positive, with mean δ³⁴S values of +3.2±1.6‰ (n=40) in individual precipitation and +2.8±1.4‰ (n=12) in monthly precipitation. These isotopic compositions are interpreted in terms of a dominant contribution of S from anthropogenic emissions and subordinate contributions from biogenic and marine sources. Pollutant SO₄ is estimated to have a δ³⁴S value in the range +2.5 to +4.5‰, whereas a distinctive δ³⁴S of −4.5‰ or lower indicates SO₄ from oxidation of biogenic gases.The isotopic and chemical compositions of SO₄ do not depend on wind direction, thus testifying to a mostly local source for pollutant S in the Bologna atmosphere.