ALBERT

Notes: Abstract The Rosita Hills volcanic centre is an alkalicalcic, mid-Tertiary complex overlying orthoand paragneissic basement, on the eastern margin of the Rio Grande Rift in south central Colorado, USA. The centre contains vein-hosted, adularia-sericite type, epithermal Ag and base-metal mineralisation with minor Au. Stable isotope studies (O and H) of whole rock and mineral separate (quartz and sericite) samples from veins and hydrothermal eruption breccias show that the hydrothermal fluid had both magmatic and meteoric components. The δD and δ18O values of the hydrothermal fluid, calculated from mineral values, range from -22‰ to -103‰ and 0.5‰ to 5.9‰ respectively. Fluid inclusion data from vein minerals (quartz, baryte and sphalerite) and from an advanced argillic lithocap overlying the veins again show that the hydrothermal system had more than one component fluid. Fluid inclusions have salinities which range from 1.7 to 25.1 wt% NaCl equivalent and show evidence of boiling in the advanced argillic lithocap. Homogenisation temperatures range from 135°C to 298°C. Liquid CO2 is present in some inclusions. These data indicate that a saline, isotopically heavy fluid mixed with a dilute, isotopically light fluid to precipitate the ore. We argue that the saline, isotopically heavy fluid is magmatic and derived from a resurgent rhyolitic magma below the mineralisation.

Notes: Abstract Emerald deposits in Swat, northwestern Pakistan, occurring in talc-magnesite and quartz-magnesite assemblages, have been investigated through stable isotope studies. Isotopic analyses were performed on a total of seven emeralds, associated quartz (seven samples), fuchsite (three samples) and tourmaline (two samples) from the Mingora emerald mines. The oxygen isotopic composition (δ 18O SMOW) of emeralds shows a strong enrichment in18O and is remarkably uniform at + 15.6 ± 0.4‰ (1σ,n = 7). Each of the two components of water in emerald (channel and inclusion) has a different range of hydrogen isotopic composition: the channel waters being distinctly isotopically heavier (δD = −51 to −32‰ SMOW) than the other inclusion waters (δD = −96 to −70‰ SMOW). Similarly the oxygen isotopic compositions of tourmaline and fuchsite are relatively constant (δ 18O = + 13 to + 14‰ SMOW) and show enrichment in18O. Theδ 18O values of quartz, ranging from + 15.1 to + 19.1‰ SMOW, are also high (+ 16.9 ± 1.4‰ 1σ, n = 7). The meanδD of channel waters measured from emerald (−42 ± 6.6‰ SMOW) and that of fluid calculated from hydrous mineralsδDcalculated (−47 ± 7.1‰ SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measuredδD values of the hydroxyl hydrogen in fuchsite (−74 to −6‰ SMOW) and tourmaline (−84 and −69‰ SMOW) with pegmatitic muscovite and tourmaline suggests that the mineralization was probably caused by modified (18O-enriched) hydrothermal solutions derived from an S-type granitic magma. The variation in the carbon and oxygen isotopic composition of magnesite, locally associated with emerald mineralization, is also very restricted (δ 13 ∼ −3.2 ± 0.7%, PDB;δ 18O ∼ + 17.9 ± 1.27‰ SMOW). On the basis of the isotopic composition of fluid (δ 13C ≈ −1.8 ± 0.7‰ PDB;δ 18O ≈ + 13.6 ± 1.2‰ SMOW calculated for the 250-550 °C temperature), it is proposed that the Swat magnesites formed due to the carbonation of previously serpentinized ultramafic rocks by a CO2-bearing fluid of metamorphic origin.

Notes: Abstract The Björkdal gold deposit is located in the eastern part of the Early Proterozoic Skellefte district in northern Sweden. The ore zone is hosted by a granitoid which intrudes a 1.9 Ga old supracrustal sequence and consists of a network of quartz veins between two shear zones. The ore mineralogy, alteration assemblages, ore fluid characteristics and general setting of the Björkdal deposit reveal many similarities with mesothermal Archean systems. Three types of fluids are represented by fluid inclusions observed in quartz, scheelite and calcite. The first type consists of a CO2-rich fluid which is syngenetic with the formation of the quartz veins. These inclusions occur in quartz and scheelite. Isotopic equilibrium temperatures derived from quartz-scheelite pairs reflect depositional temperatures around 375 °C. Molar volumes of the carbonic fluid inclusions, ranging down to 55 cm3mole, indicate a maximum lithostatic trapping pressure of 1.8 kbar. These fluids were generated at depth in conjunction with early orogenic magma-forming processes. The gold was introduced to the vein system by the carbonic fluid but the gold was deposited after reactions between this fluid and the wall-rock, producing a slightly alkaline, more CH4-rich aqueous type 2 fluid. Fluid inclusions of this chemically modified fluid indicate that the precipitation of the gold, together with pyrrhotite, pyrite and chalcopyrite, occurred under heterogenous conditions at a temperature of 220 °C and a hydrostatic pressure of 0.5 kbar. The gold deposition occurred from fluids with a δ 18O signature of around +8‰ and δD values close to zero per mil. Any metamorphic influence on the stable isotopic signatures is regarded as minimal. The isotope data suggest rather that a surface-derived fluid component had access to the vein system during this process. At a post-vein forming stage (metamorphic stage ?) a secondary episode of gold mobilization occurred as suggested by the aqueous type 3 inclusions trapped in cross-cutting microfractures in quartz and randomly in calcite, and with homogenization temperatures between 145–220 °C and a salinity up to 11eq. wt.% NaCl. The Skellefte district is a major ore province, which forms a 200 by 50 km area in northern Sweden (Fig. 1), comprising numerous stratabound massive sulfide ore deposits. During the last decade epigenetic gold deposits have received increasing interest from a prospecting point of view. The Björkdal deposit is one of several epigenetic gold discoveries made recently in the Skellefte district. In 1985 a geochemical survey, designed on a grid-pattern basis, revealed a gold anomaly about 12 km north-east of the Boliden community and three years later the Björkdal gold mine was in operation. The annual production is about 960 000 metric tons of ore (1992) and the total reserves are estimated at a minimum of 7 Mton of ore with a gold grade of 2.9 ppm. This paper reports on the geological features of the Björkdal deposit and discusses the genesis of the deposit on basis of fluid inclusions and distribution of oxygen and hydrogen isotopes.

Notes: Abstract Mo mineralization within the Galway Granite at Mace Head and Murvey, Connemara, western Ireland, has many features of classic porphyry Mo deposits including a chemically evolved I-type granite host, associated K- and Si-rich alteration, quartz vein(Mace Head) and granite-hosted (Murvey) molybdenite, chalcopyrite, pyrite and magnetite mineralization and a gangue assemblage which includes quartz, muscovite and K-feldspar. Most fluid inclusions in quartz veins homogenize in the range 100–350°C and have a salinity of 1–13 eq. wt.% NaCl. They display Th-salinity covariation consistent with a hypothesis of dilution of magmatic water by influx of meteoric water. CO2-bearing inclusions in an intensely mineralized vein at Mace Head provide an estimated minimum trapping temperature and pressure for the mineralizing fluid of 355°C and 1.2 kb and are interpreted to represent a H2O-CO2 fluid, weakly enriched in Mo, produced in a magma chamber by decompression-activated unmixing from a dense Mo-bearing NaCl-H2O-CO2 fluid. δ34S values of most sulphides range from c. 0‰ at Murvey to 3–4‰ at Mace Head and are consistent with a magmatic origin. Most quartz vein samples have δ18O of 9–10.3‰ and were precipitated from a hydrothermal fluid with δ18O of 4.6–6.7‰. Some have δ18O of 6–7‰ and reflect introduction of meteoric water along vein margins. Quartz-muscovite oxygen isotope geothermometry combined with fluid inclusion data indicate precipitation of mineralized veins in the temperature range 360–450°C and between 1 and 2 kb. Whole rock granite samples display a clear δ18O-δD trend towards the composition of Connemara meteoric waters. The mineralization is interpreted as having been produced by highlyfractionated granite magma; meteoric water interaction postdates the main mineralizing event. The differences between the Mace Head and Murvey mineralizations reflect trapping of migrating mineralizing fluid in structural traps at Mace Head and precipitation of mineralization in the granite itself at Murvey.

Notes: Abstract.  Emerald deposits in Swat, northwestern Pakistan, occurring in talc-magnesite and quartz-magnesite assemblages, have been investigated through stable isotope studies. Isotopic analyses were performed on a total of seven emeralds, associated quartz (seven samples), fuchsite (three samples) and tourmaline (two samples) from the Mingora emerald mines. The oxygen isotopic composition (δ18O SMOW) of emeralds shows a strong enrichment in 18O and is remarkably uniform at +15.6±0.4‰ (1σ, n=7). Each of the two components of water in emerald (channel and inclusion) has a different range of hydrogen isotopic composition: the channel waters being distinctly isotopically heavier (δD=−51 to −32‰ SMOW) than the other inclusion waters (δD=−96 to −70‰ SMOW). Similarly the oxygen isotopic compositions of tourmaline and fuchsite are relatively constant (δ18O=+13 to +14‰ SMOW) and show enrichment in 18O. The δ18O values of quartz, ranging from +15.1 to +19.1‰ SMOW, are also high (+16.9±1.4‰; 1σ, n=7). The mean δD of channel waters measured from emerald (−42±6.6‰ SMOW) and that of fluid calculated from hydrous minerals δDcalculated (−47±7.1‰ SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measured δD values of the hydroxyl hydrogen in fuchsite (−74 to −61‰ SMOW) and tourmaline (−84 and −69‰ SMOW) with pegmatitic muscovite and tourmaline suggests that the mineralization was probably caused by modified (18O-enriched) hydrothermal solutions derived from an S-type granitic magma. The variation in the carbon and oxygen isotopic composition of magnesite, locally associated with emerald mineralization, is also very restricted (δ13C∼−3.2±0.7‰ PDB; δ18O∼ +17.9±1.2‰ SMOW). On the basis of the isotopic composition of fluid (δ13C≈−1.8±0.7‰ PDB; δ18O≈+13.6±1.2‰ SMOW calculated for the 250–550 °C temperature), it is proposed that the Swat magnesites formed due to the carbonation of previously serpentinized ultramafic rocks by a CO2-bearing fluid of metamorphic origin.

Notes: Abstract The sulphide deposits of the Iberian Pyrite Belt (IPB) represent an ore province of global importance. Our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores. Measured 34S values range from −26 to +10‰ mostly for massive and stockwork ores, in agreement with data previously published. In situ laser 34S analyses reveals a close correlation of 34S with texture. Primary diagenetic textures are dominated by relatively low 34S (−8‰ to −2‰), whereas stockwork feeder textures are dominated by higher 34S (∼+3‰ to +5‰). Intermediate textures (mainly coarse textures in stratiform zones) have intermediate 34S, although they are mostly dominated by the high 34S component. Rare barite has a homogeneous 34S around +18‰, which is consistent with direct derivation from Lower Carboniferous seawater sulphate. A dual source of sulphide sulphur in the IPB deposits has been considered. A hydrothermal source, derived from reduction of coeval seawater sulphate in the convective systems, is represented by sulphide in the feeder zones. Here variations in 34S are caused by variations in the extent of the sulphate reduction, which governs the SO4:H2S ratio. The second end-member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface, as reflected in the primary textures. A distinct geographical variation in 34S and texture from SW (more bacteriogenic and primary textures) to NE (more hydrothermal textures and 34S) which reflects a variation in the relative input of each source was likely controlled by local geological environments. Given that the sulphur isotope characteristics of the IPB deposits are unlike most VMS and Kuroko deposits, and noting the dominance of a mixed reduced sedimentary and volcanic environment, we suggest that the IPB could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types.