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.     

Fluid evolution in the Rushan lode gold deposit of Jiaodong Peninsula, eastern China

The Rushan gold deposit in the Jiaodong Peninsula is currently the largest lode gold in China. Gold occurs mainly in pyrite- and polymetallic sulfide–quartz vein/veinlet stockworks. Fluid inclusions in the deposit are divided into three main types, namely CO₂–H₂O, H₂O–CO₂ ± CH₄ and aqueous ones. Microthermometric data show that the pre-gold fluids were CO₂-dominant (X_CO2 up to 0.53), and the total homogenization temperatures fall in the range of 298377 °C. These fluids, modified by fluid/wallrock reactions, gradually evolved into fluids with less CO₂ (X_CO2 = 0.010.19) in the main ore-forming stage, and the total homogenization temperatures range from 170 to 324 °C. Hydrogen and oxygen stable isotope data suggest that ore-forming fluids were mixture of magmatic and meteoritic origin. Co-occurrence of gold and sulfides implies that gold was most likely transported in the form of gold–sulfide complexes. The wide distribution of CO₂ inclusions means that the pH variation during gold transportation was controlled by CO₂ buffering.     

The Santa Rita gold deposit in the Proterozoic Paranoá Group, Goiás, Brazil: An example of fluid mixing during ore deposition

The Santa Rita gold deposit (Central Goiás, Brazil) is hosted by Middle to Upper Proterozoic carbonate-pelite sequences of the Paranoá Group that have been metamorphosed in the greenschist facies. The ore is contained in pyrite-bearing quartz-carbonate veins. The mineralization is structurally controlled by WNW-ESE high-angle faults and fractures resulting from the reactivation of older NE-SW lineaments. Pyrite is the sole sulphide and it shows growth zones enriched in Co, Ni and As (up to 4 wt%). Hydrothermal alteration zones are enriched in Co, Ni and As and are characterized by diffuse albitization, carbonatization, silicification and pyritization.A fluid inclusion study on quartz from pyrite-bearing quartz-carbonate veins led to the identification of two fluids: (1) a highly saline CO₂---N₂-rich aqueous fluid with halite and ± sylvite daughter minerals, and (2) a CO₂---N₂ rich aqueous fluid with moderate salinity. The two fluid types occur in the same quartz domain and display great variation in the degree of filling and notable dispersion of the microthermometric data. On heating, all the inclusions decrepitate between 200° and 300°C. Raman spectrometry detected high concentrations of N₂ in the gas phase, with a molar ratio between 1 and 19 and a small proportion of CH₄ (up to 2 mole %).The simultaneous entrapment of compositionally variable fluids in the system H₂O---CO₂---N₂---NaCl---KCl allow us to propose a mechanism of heterogeneous trapping. The entrapment may result from the mixing of a high-salinity fluid (H₂O---NaCl---KCl system) with a carbonic fluid (H₂O---CO₂---N₂ system) produced by the devolatilization process of carbonate and phyllitic host rocks. Considering the absence of spatially and temporally related igneous activity and the low P-T regional metamorphism in the Paranoá Group, the brines are inferred to result from leaching of evaporites occurring in the lower part of the Paranoá lithostratigraphic column.Gold was probably initially transported as an AuCl₂-complex (T>300°C, low pH, moderate ƒ_O2-pyrite field stability). As temperature decreased below 290°C, the “switch-over” process would lead to the predominance of Au(HS)₂⁻ in the fluid. Pyrite precipitated in this temperature interval. The oscillatory zoning of the As---Co---Ni-bearing pyrites indicates episodic fluctuation of the fluid composition. Such changes in fluid composition are favoured by a mechanism of fluid mixing by intermittent supplies in the hydrothermal system. The proposed mechanism of heterogeneous trapping of two separate fluids in the system H₂O---CO₂---N₂---NaCl---KCl and the resulting changes in the physicochemical conditions caused by the fluid mixing appears as a conspicuous process for the Santa Rita hydrothermal fluid evolution.A model based on the existence of a Proterozoic geothermal system involving the regional thermal gradient is proposed.     

Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central Eastern Desert of Egypt: Evidence from hydrothermal alteration, fluid inclusions and stable isotope data

The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO₂-rich, H₂O-rich and mixed H₂O–CO₂ fluid inclusions in the gold-bearing quartz veins. The H₂O–CO₂ inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ³⁴S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO₂.     

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

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

Age and sources of gold mineralization in the Marmato mining district, NW Colombia: A Miocene–Pliocene epizonal gold deposit

Epigenetic gold mineralization occurs in the Marmato mining district, within the Calima Terrain of the Setentrional Andes, Colombia. Regional rocks associated with this mineralization include: graphite- and chlorite-schists of the Arquia Complex; metamorphosed during the Cretaceous, Miocene sandstones, shales and conglomerates of the Amagá Formation; as well as pyroclastic rocks (clasts of basalt, andesites and mafic lavas) and subvolcanic andesitic/dacitic bodies of the Combia Formation (9 to 6 Ma). The subvolcanic Marmato stock hosts mesothermal and epithermal low-sulfidation Au–Ag ores in the form of distensional veins, stockwork, and quartz veinlets within brecciated zones. Ore minerals are pyrite, sphalerite and galena with subordinate chalcopyrite, arsenopyrite, pyrrhotite, argentite and native gold/electrum.Sericitized plagioclase from a porphyry dacite yielded a K–Ar age of 5.6 ± 0.6 Ma, interpreted as the age of ore deposition. This is in close agreement with the age of reactivation of the Cauca–Romeral Fault System (5.6 ± 0.4 Ma), which bounds the Calima Terrain. A porphyry andesite–dacite (6.7 ± 0.1 Ma), hosting the Au–Ag veins, shows a measured ⁸⁷Sr/⁸⁶Sr between 0.70440 and 0.70460, ε_Nd between + 2.2 and + 3.2 and ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.964 to 19.028; 15.561 to 15.570; and 38.640 to 38.745, respectively. The ⁸⁷Sr/⁸⁶Sr and ε_Nd values of rocks from the Arquia Group range from 0.70431 to 0.73511 and − 12.91 to + 10.0, respectively, whereas the corresponding Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb) range from 18.948 to 19.652; 15.564 to 15.702; and 38.640 to 38.885, respectively. ⁸⁷Sr/⁸⁶Sr and ε_Nd values obtained on sulfides from the gold quartz veins, which occur at shallow and intermediate levels, range from 0.70500 to 0.71210 and from − 1.11 to + 2.40. In the deepest veins, ε_Nd values lie between + 1.25 and + 3.28 and the ⁸⁷Sr/⁸⁶Sr of calcite and pyrite fall between 0.70444 and 0.70930. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of all mineralization are in the ranges 18.970 to 19.258; 15.605 to 15.726 and 38.813 to 39.208, respectively. Carbonates have an average ⁸⁷Sr/⁸⁶Sr ratio of 0.70445, which is within the range of values measured in the host dacite. The Sr isotopic data indicate that carbonic fluids have a restricted hydrothermal circulation within the host igneous body, while the Sr, Pb and Nd isotopic compositions of the sulfides suggest that the fluids not only circulated within the Marmato stock, but also throughout the Arquia Complex, inferring that these rocks offer a potential target for mineral exploration. Based on geological and geochronological evidence, the epizonal Marmato gold ores formed during the Miocene to Pliocene, as a result of cooling of the Marmato stock and reactivation along a crustal-scale fault zone related to thermal processes in an accretionary oceanic–continental plate orogen.     

Fluid geochemistry in the Ivigtut cryolite deposit, South Greenland

The 1.27 Ga old Ivigtut (Ivittuut) intrusion in South Greenland is world-famous for its hydrothermal cryolite deposit [Na₃AlF₆] situated within a strongly metasomatised A-type granite stock. This detailed fluid inclusion study characterises the fluid present during the formation of the cryolite deposit and thermodynamic modelling allows to constrain its formation conditions.Microthermometry revealed three different types of inclusions: (1) pure CO₂, (2) aqueous-carbonic and (3) saline-aqueous inclusions. Melting temperatures range between − 23 and − 15 °C for type 2 and from − 15 to − 10 °C for type 3 inclusions. Most inclusions homogenise between 110 and 150 °C into the liquid.Stable isotope compositions of CO₂ and H₂O were measured from crushed inclusions in quartz, cryolite, fluorite and siderite. The δ¹³C values of about − 5‰ PDB are typical of mantle-derived magmas. The differences between δ¹⁸O of CO₂ (+ 21 to + 42‰ VSMOW) and δ¹⁸O of H₂O (− 1 to − 21.7‰ VSMOW) suggest low-temperature isotope exchange. δD (H₂O) ranges from − 19 to − 144‰ VSMOW. The isotopic composition of inclusion water closely follows the meteoric water line and is comparable to Canadian Shield brines. Ion chromatography revealed the fluid's predominance in Na, Cl and F. Cl/Br ratios range between 56 and 110 and may imply intensive fluid–rock interaction with the host granite.Isochores deduced from microthermometry in conjunction with estimates for the solidification of the Ivigtut granite suggest a formation pressure of approximately 1–1.5 kbar for the fluid inclusions. Formation temperatures of different types of fluid inclusions vary between 100 and 400 °C. Thermodynamic modelling of phase assemblages and the extraordinary high concentration in F (and Na) may indicate that the cryolite body and its associated fluid inclusions could have formed during the continuous transition from a volatile-rich melt to a solute-rich fluid.     

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.     

Ore-forming fluids associated with granite-hosted gold mineralization at the Sanshandao deposit, Jiaodong gold province, China

The Sanshandao gold deposit, with total resources of more than 60 t of gold, is located in the Jiaodong gold province, the most important gold province of China. The deposit is a typical highly fractured and altered, disseminated gold system, with high-grade, quartz-sulphide vein/veinlet stockworks that cut Mesozoic granodiorite. There are four stages of veins that developed in the following sequence: (1) quartz-K-feldspar-sericite; (2) quartz-pyrite±arsenopyrite; (3) quartz-base metal sulfide; and (4) quartz-carbonate. Fluid inclusions in quartz and calcite in vein/veinlet stockworks contain C-O-H fluids of three main types. The first type consists of dilute CO₂–H₂O fluids coeval with the early vein stage. Molar volumes of these CO₂–H₂O fluid inclusions, ranging from 50–60 cm³/mol, yield estimated minimum trapping pressures of 3 kbar. Homogenization temperatures, obtained mainly from CO₂–H₂O inclusions with lower CO₂ concentration, range from 267–375 °C. The second inclusion type, with a CO₂–H₂O±CH₄ composition, was trapped during the main mineralizing stages. These fluids may reflect the CO₂–H₂O fluids that were modified by fluid/rock reactions with altered wallrocks. Isochores for CO₂-H₂O±CH₄ inclusions, with homogenization temperatures ranging from 204–325 °C and molar volumes from 55 to 70 cm³/mol, provide an estimated minimum trapping pressure of 1.2 kbar. The third inclusion type, aqueous inclusions, trapped in cross-cutting microfractures in quartz and randomly in calcite, are post-mineralization, and have homogenization temperatures between 143–228 °C and salinities from 0.71–7.86 wt% NaCl equiv. Stable isotope data show that the metamorphic fluid contribution is minimal and that ore fluids are of magmatic origin, most likely sourced from 120–126 Ma mafic to intermediate dikes. This is consistent with the carbonic nature of the fluid, and the cross-cutting nature of those deposits relative to the host Mesozoic granitoid.Editorial handling: R.J. Goldfarb     

Copper–gold occurrences in the Palaeoproterozoic Inglefield mobile belt, northwest Greenland: a new mineralisation style?

Inglefield Land in northwest Greenland is an ice-free 7000 km² region underlain by the Palaeoproterozoic Inglefield mobile belt, composed of quartzo-feldspathic gneisses, meta-igneous and supracrustal rocks. These rocks are unconformably overlain by an unmetamorphosed cover of sedimentary and igneous rocks of the Mesoproterozoic Thule Basin and the Lower Palaeozoic Franklinian Basin. Mineralisation in Inglefield Land is characterised by a copper–gold metal association that can be classified in terms of the hosting rocks, namely: garnet–sillimanite paragneiss-hosted, orthogneiss-hosted and mafic–ultramafic-hosted. The paragneiss-hosted mineralisation, the topic of this paper, is essentially confined within a NE-trending structural corridor and consists of bands of sulphide±graphite-bearing, hydrothermally altered, quartzo-feldspathic gneiss, called “rust zones”. These are commonly parallel to the paragneiss main foliation, suggesting a close relationship. The rust zones have strike lengths from a few metres to more than 5 km, and widths ranging from a few centimetres to 200 m. Sulphides mainly include pyrrhotite, pyrite and chalcopyrite. The sulphides form disseminations, up to 30% by volume, but in places they form massive pods or lenses up to 20–30 m, and about 0.1–0.5 m wide. Graphite contents are up to 5 vol.%. Rust zones typically consist of a quartz–plagioclase mosaic associated with a late generation of red-brown biotite, sericite, chlorite and epidote. Mylonitic or cataclastic textures are locally recognisable. XRD analyses of graphite indicate temperatures of between 650 and 700 °C. Sulphur isotope analyses show δ³⁴S values ranging from −6.2‰ to +9.3‰.An ore genesis model is proposed in which the Palaeoproterozoic precursor sandstone–carbonaceous shale succession is polydeformed and polymetamorphosed to granulite facies quartzo-feldspathic and pelitic gneisses, with transposition of layering to axial plane of folds, followed by ductile shearing and mylonitisation, from which future rust zones were derived. The mylonitic zones were infiltrated by fluids, whose origin can be ascribed to deep-penetrating surface waters and/or external brines. In our ore genesis model, we envisage that brines derived from the overlying Lower Palaeozoic Franklinian succession infiltrated the basement into the structural channels provided by the shear/mylonitic zones. At the regional scale, this infiltration was facilitated by a NE-trending corridor, postulated to be a deep structure.     

Fluid inclusion and stable isotope (O, H, C, and S) constraints on the genesis of the Serrinha gold deposit, Gurupi Belt, northern Brazil

The Serrinha gold deposit of the Gurupi Belt, northern Brazil, belongs to the class of orogenic gold deposits. The deposit is hosted in highly strained graphitic schist belonging to a Paleoproterozoic (∼2,160 Ma) metavolcano-sedimentary sequence. The ore-zones are up to 11 m thick, parallel to the regional NW–SE schistosity, and characterized by quartz-carbonate-sulfide veinlets and minor disseminations. Textural and structural data indicate that mineralization was syn- to late-tectonic and postmetamorphic. Fluid inclusion studies identified early CO₂ (CH₄-N₂) and CO₂ (CH₄-N₂)-H₂O-NaCl inclusions that show highly variable phase ratios, CO₂ homogenization, and total homogenization temperatures both to liquid and vapor, interpreted as the product of fluid immiscibility under fluctuating pressure conditions, more or less associated with postentrapment modifications. The ore-bearing fluid typically has 18–33mol% of CO₂, up to 4mol% of N₂, and less than 2mol% of CH₄ and displays moderate to high densities with salinity around 4.5wt% NaCl equiv. Mineralization occurred around 310 to 335°C and 1.3 to 3.0 kbar, based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions. Stable isotope data on quartz, carbonate, and fluid inclusions suggest that veins formed from fluids with δ¹⁸O_H2O and δD_H2O (310–335°C) values of +6.2 to +8.4‰ and −19 to −80‰, respectively, which might be metamorphic and/or magmatic and/or mantle-derived. The carbon isotope composition (δ¹³C) varies from −14.2 to −15.7‰ in carbonates; it is −17.6‰ in fluid inclusion CO₂ and −23.6‰ in graphite from the host rock. The δ³⁴S values of pyrite are −2.6 to −7.9‰. The strongly to moderately negative carbon isotope composition of the carbonates and inclusion fluid CO₂ reflects variable contribution of organic carbon to an originally heavier fluid (magmatic, metamorphic, or mantle-derived) at the site of deposition and sulfur isotopes indicate some oxidation of the originally reduced fluid. The deposition of gold is interpreted to have occurred mainly in response to phase separation and fluid-rock interactions such as CO₂ removal and desulfidation reactions that provoked variations in the fluid pH and redox conditions.     

Origin and evolution of the calcic and magnesian skarns hosting the El Valle-Boinás copper–gold deposit, Asturias (Spain)

The El Valle-Boinás copper–gold deposit is located in the southern part of the Rio Narcea Gold Belt 65 km west of Oviedo (NW Spain), within the Cantabrian Zone (Iberian Hercynian Massif). The deposit is related to the Boinás stock, which ranges from quartz-monzonite to monzogranite and intruded (303 Ma) the carbonated Láncara Formation (early Cambrian) and the siliciclastic Oville Formation (middle-late Cambrian).A copper–gold skarn was developed along the contact between the igneous rock and the carbonated sedimentary rocks. The skarn distribution and mineralogy reflects both structural and lithologic controls. Two types of skarn exists: a calcic skarn mainly developed in the upper calcic member of the Láncara Formation, and a magnesian skarn developed in the lower dolomitic and organic-rich member. The former mainly consists of garnet, pyroxene and wollastonite. Retrograde alteration consists of K-feldspar, epidote, quartz, calcite, magnetite, ferroactinolite, titanite, apatite, chlorite and sulfides. Magnesian skarn mainly consists of diopside with interbedded forsterite zones. Pyroxene skarn is mainly altered to tremolite, with minor phlogopite and serpentine. Olivine skarn is pervasively altered to serpentine and magnetite, and is commonly accompanied by high sulfide and gold concentrations. This altered skarn results in a very dark rock, referred to as “black skarn”, which has great importance in gold reserves. Sulfide mineralization mainly consists of chalcopyrite, bornite, arsenopyrite, pyrrhotite and pyrite, while wittichenite, sphalerite, digenite, bismuthinite, native bismuth and electrum occur as accessory minerals.After extensive erosion, reactivation of the northeast-trending fracture zone provided conduits for the subsequent emplacement of porphyritic dikes (285±4 Ma) and diabasic dikes (255±5 Ma). Alteration, characterized by sericitization, silicification, carbonatization and hypogene oxidation took place, as did sulfide mineralization (pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, bournonite, and Fe–Pb–Sb sulfosalts). Veins with quartz, carbonate, adularia and sulfide minerals crosscut all previous lithologies. Jasper and jasperoid breccias developed at the upper parts of the deposits.The fluid inclusion and stable isotope studies suggest a predominantly magmatic prograde-skarn fluid characterized by high-salinity (26–28 wt.% KCl and 32–36 wt.% NaCl) and high temperature, above 580°C. This fluid evolved into two immiscible fluids: a CO₂- and/or CH₄-rich, high-salinity aqueous fluid. Temperatures for the first retrograde-stage are between 350 and 425°C. A second stage is related to a more diluted aqueous fluid (3–6.2 wt.% NaCl eq.) and temperatures from 280 to 325°C. The fluid inclusion study performed on quartz from low-temperature mineralization indicates a very low salinity (0.2–6.2 wt.% NaCl eq.), low-temperature aqueous fluid (from 150 to 250°C), and trapping pressure conditions less than 0.2 kbar. In addition, the stable isotope study suggests that an influx of metamorphic waters derived from the country rocks produced these lower temperature fluids. The last control for the Au mineralization is the Alpine tectonism, which developed fault breccias (cataclasites to, locally, protomylonites) and gold remobilization from previous mineralization.     

High CO2 content of fluid inclusions in gold mineralisations in the Ashanti Belt, Ghana: a new category of ore forming fluids?

Fluid inclusions were studied in samples from the Ashanti, Konongo-Southern Cross, Prestea, Abosso/Damang and Ayanfuri gold deposits in the Ashanti Belt, Ghana. Primary fluid inclusions in quartz from mineralised veins of the Ashanti, Prestea, Konongo-Southern Cross, and Abosso/Damang deposits contain almost exclusively volatile species. The primary setting of the gaseous (i.e. the fluid components CO₂, CH₄ and N₂) fluid inclusions in clusters and intragranular trails suggests that they represent the mineralising fluids. Microthermometric and Raman spectroscopic analyses of the inclusions revealed a CO₂ dominated fluid with variable contents of N₂ and traces of CH₄. Water content of most inclusions is below the detection limits of the respective methods used. Aqueous inclusions are rare in all samples with the exception of those from the granite-hosted Ayanfuri mineralisation. Here inclusions associated with the gold mineralisation contain a low salinity (<6 eq.wt.% NaCl) aqueous solution with variable quantities of CO₂. Microthermometric investigations revealed densities of the gaseous inclusions of 0.65 to 1.06 g/cm³ at Ashanti, 0.85 to 0.98 g/cm³ at Prestea, up to 1.02 g/cm³ at Konongo-Southern Cross, and 0.8 to 1.0 g/cm³ at Abosso/Damang. The fluid inclusion data are used to outline the PT ranges of gold mineralisation of the respective gold deposits. The high density gaseous inclusions found in the auriferous quartz at Ashanti and Prestea imply rather high pressure trapping conditions of up to 5.4 kbar. In contrast, mineralisation at Ayanfuri and Abosso/Damang is inferred to have occurred at lower pressures of only up to 2.2 kbar. Mesothermal gold mineralisation is generally regarded to have formed from fluids characterized by H₂O > CO₂ and low salinity ( ±  6 eq.wt.%NaCl). However, fluid inclusions in quartz from the gold mineralisations in the Ashanti belt point to distinctly different fluid compositions. Specifically, the predominance of CO₂ and CO₂ >> H₂O have to be emphasized. Fluid systems with this unique bulk composition were apparently active over more than 200␣km along strike of the Ashanti belt. Fluids rich in CO₂ may present a hitherto unrecognised new category of ore-forming fluids. Received: 30 May 1996 / Accepted: 8 October 1996     

Palaeomagnetic data from the precambrian Gwalior traps, Central India

From alternating-field and thermal demagnetization studies on two dolerite “Traps” in the Gwalior Series (Central India), dated at 1830 ±200 m.y., three different palaeomagnetic directions could be distinguished. The characteristic magnetization component, which is considered as the primary magnetization, has a mean direction: D=78°, I=+34.5°, α₉₅=5°, k=369, N=4 (Pole): 155.5°E19°N, dp=3°, dm=5.5°.A comparison of the presented data with other Precambrian and Phanerozoic data from the Indian subcontinent might suggest that the Indian subcontinent underwent a continuous anticlockwise rotational movement during the last 1800 m.y.     

The speciation of carbon dioxide in sodium aluminosilicate glasses

Infrared spectroscopy has been used to study the speciation of CO₂ in glasses near the NaAlO₂-SiO₂ join quenched from melts held at high temperatures and pressures. Absorption bands resulting from the antisymmetric stretches of both molecular CO₂ (2,352 cm^–1) and CO ₃ ^2– (1,610 cm^–1 and 1,375 cm^–1) are observed in these glasses. The latter are attributed to distorted Na-carbonate ionic-complexes. Molar absorptivities of 945 liters/mole-cm for the molecular CO₂ band, 200 liters/mole-cm for the 1,610 cm^–1 band, and 235 liters/mole-cm for the 1,375 cm^–1 band have been determined. These molar absorptivities allow the quantitative determination of species concentrations in the glasses with a precision on the order of several percent of the amount present. The accuracy of the method is estimated to be ±15–20% at present.The ratio of molecular CO₂ to CO ₃ ^2– in sodium aluminosilicate glasses varies little for each silicate composition over the range of total dissolved CO₂ content (0–2%), pressure (15–33 kbar) and temperature (1,400–1,560° C) that we have studied. This ratio is, however, a strong function of silicate composition, increasing both with decreasing Na₂O content along the NaAlO₂-SiO₂ join and with decreasing Na₂O content in peraluminous compositions off the join.Infrared spectroscopic measurements of species concentrations in glasses provide insights into the molecular level processes accompanying CO₂ solution in melts and can be used to test and constrain thermodynamic models of CO₂-bearing melts. CO₂ speciation in silicate melts can be modelled by equilibria between molecular CO₂, CO ₃ ^2– , and oxygen species in the melts. Consideration of the thermodynamics of such equilibria can account for the observed linear relationship between molecular CO₂ and carbonate concentrations in glasses, the proposed linear relationship between total dissolved CO₂ content and the activity of CO₂ in melts, and observed variations in CO₂ solubility in melts.     

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.     

Calcareous nannoplankton, planktonic foraminiferal, and carbonate carbon isotope stratigraphy of the Cenomanian–Turonian boundary section in the Ultrahelvetic Zone (Eastern Alps, Upper Austria)

Ultrahelvetic units of the Eastern Alps were deposited on the distal European continental margin of the (Alpine) Tethys. The Rehkogelgraben section (“Buntmergelserie”, Ultrahelvetic unit, Upper Austria) comprises a 5 m thick succession of upper Cenomanian marl-limestone cycles overlain by a black shale interval composed of three black shale layers and carbonate-free claystones, followed by lower Turonian white to light grey marly limestones with thin marl layers. The main biostratigraphic events in the section are the last occurrence of Rotalipora and the first occurrences of Helvetoglobotruncana helvetica and Quadrum gartneri. The thickest black shale horizon has a TOC content of about 5%, with predominantly marine organic matter of kerogen type II. Vitrinite reflectance and Rock-Eval parameter T_max (<424 °C) indicate low maturity. HI values range from 261 to 362 mg HC/g TOC. δ¹³C values of bulk rock carbonates display the well documented positive shift around the black shale interval, allowing correlation of the Rehkogelgraben section with other sections such as the Global Boundary Stratotype Section and Point (GSSP) succession at Pueblo, USA, and reference sections at Eastbourne, UK, and Gubbio, Italy. Sediment accumulation rates at Rehkogelgraben (average 2.5 mm/ka) are significantly lower than those at Pueblo and Eastbourne.     

Surface carbonate and land-derived clastic marine sediments from southern Chile: mineralogical and geochemical investigation

Surface carbonate and land-derived deposits in the sea off southern Chile were investigated for their mineralogical and geochemical composition. The data were related to environmental features and compared with those of similar temperate and polar carbonate deposits from Tasmania, New Zealand, Arctica, and Antarctica. The mineralogy of the siliciclastic fraction is typical of cold areas and is mainly composed of chlorite, mica, quartz, feldspars and amphibole. The CaCO₃ content varies from 30 to 90%; carbonate mineralogy is made up of low-Mg calcite, high-Mg calcite and minor amounts of aragonite. The Ca, Mg, Sr, Fe, and Mn contents of bulk carbonates and some selected skeletal hard parts are comparable to those of carbonates from Tasmania. The elemental composition is mainly related to carbonate mineralogy, skeletal components, and seawater conditions. The δ¹³C and δ¹⁸O values of carbonates are positive, and their field falls between the “seafloor diagenesis” and “upwelling water” trend lines, because the sediments are likely to be in equilibrium with waters of Antarctic origin. The mineralogical, elemental, and isotopic compositions of carbonates from southern Chile show better similarities with the “temperate” carbonates from Tasmania and New Zealand than with the “polar” carbonates from Arctica and Antarctica. Carbonate deposition is allowed by the low terrigenous input, the low SPM concentration and, probably, the upwelling of seawater from Antarctica.     

Nature and origin of the BIF-hosted São Bento gold deposit, Quadrilátero Ferrífero, Brazil, with special emphasis on structural controls

The orogenic banded iron formation (BIF)-hosted Au mineralization at São Bento is a structurally-controlled, hydrothermal deposit hosted by Archean rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero region, Brazil. The deposit has reserves of 14.3 t Au and historical (underground) production of 44.6 t Au between 1987 and 2001. The oxide-facies São Bento BIF is mineralized at its lower portion, where in contact with carbonaceous, pelitic schists, particularly in the proximity of sulfide-bearing quartz veins. Shear-related Au deposition is associated with the pervasive, hydrothermal sulfidation (mainly arsenopyrite) of the Fe-rich bands of the São Bento BIF. Auriferous, sulfide- and quartz-rich zones represent proximal alteration zones. They are enveloped by ankerite-dominated haloes, which reflect progressive substitution of siderite and magnetite within the BIF by ankerite and pyrrhotite, respectively. The São Bento BIF was intensely and extensively deformed, first into open, upright folds that evolved into tight, asymmetric, isoclinal folds. The inverse limb of these folds attenuated and gave way to sheath folds and the establishment of ductile thrusts. Mineralized horizons at São Bento result from early structural modifications imposed by major transcurrent and thrusts faults, comprising the Conceição, Barão de Cocais and São Bento shear zones. Dextral movement on the SW–NE-directed Conceição shear zone may have generated splays at a compressional side-stepping zone, such as the São Bento shear zone, which is the structural locus for the São Bento gold mineralization. Relaxation of the Conceição shear zone under more brittle conditions resulted in the development of dilatational zones where gold–sulfide–quartz veins formed. These structures are considered to have been generated in the Archean. Geochronological data are scarce, with Pb–Pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single-stage growth curve at 2.65 Ga. A later compressional, ductile deformation of unknown age overprinted, rotated and flattened the original, N60E-directed structure of the whole rock succession, with development of planar and linear fabrics that appear similar to Proterozoic-aged structures. Fluid inclusion studies indicate low salinity, aqueous fluids, with or without CO₂ and/or CH₄, with extremely variable CO₂/CH₄ ratios, of probable metamorphic origin. Fluid evolution shows a paragenetic decrease in the carbonic phase from 10–15% to 5%, and increase in the H₂O/(CO₂ + CH₄) and CO₂/CH₄ ratios, suggesting important interaction with carbonaceous sediment. Trapping conditions indicate a temperature of 300 °C at 3.2 kbar.     

Structural controls on gold mineralisation and the nature of related fluids of the Paiol gold deposit, Almas Greenstone Belt, Brazil

Chemical analyses suggest that the metavolcanic rocks of the Almas Greenstone Belt (AGB), Tocantins State, Brazil have a continental affinity, possibly related to a continental rift environment. They were metamorphosed to amphibolite facies during a regional tectono-metamorphic event (D_n), retrogressed to greenschist facies assemblages and then hydrothermally altered within dextral strike–slip shear zones (D_n+1). Fracture sets related to D_n+2 intersect S_n+1.The Paiol Gold Mine is one of several mineralised zones within metabasic and meta-intermediate rocks of the AGB. It exploits shoots of sulphide–Au–quartz mineralisation that occupy dilational zones approximately perpendicular to an elongation lineation (L_n+1) within mylonitic foliation S_n+1 (S_n+1=S within the S–C fabric). The dilational zones probably formed due to dextral displacement on sinistrally en echelon C surfaces. Minor amounts of gold may have been introduced or remobilised during D_n+2.Coexisting primary and pseudosecondary fluid inclusions in mineralised quartz veins from ore shoots comprise a high-salinity three-phase type (Type II) and a lower salinity two-phase type (Type I). Homogenisation temperatures for Type II inclusions range from 200 to 410 °C and Type I from 90 to 320 °C. The inclusions and their temperature ranges are believed to reflect heat exchange and some mixing between the two fluid types under relatively constant ambient temperatures, but variable (though broadly declining) fluid temperatures. This took place late in D_n+1 in conjunction with greenschist facies retrogression and localised hydrothermally induced metasomatism.