ALBERT

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: Abstract We determined the δ18O and δ13C composition of the same fixed growth increment in severalPorites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and δ18O are inversely related. We explain this in terms of McConnaughey's kinetic isotopic disequilibria model. Annual trends in δ18O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophylla content and δ13C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal δ13C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, δ13C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in °13C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater δ13C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in13C and18O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in δ18O and δ13C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.

Notizen: Abstract.  We determined the δ18O and δ13C composition of the same fixed growth increment in several Porites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and δ18O are inversely related. We explain this in terms of McConnaughey’s kinetic isotopic disequilibria model. Annual trends in δ18O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophyll a content and δ13C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal δ13C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, δ13C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in δ13C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater δ13C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in 13C and 18O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in δ18O and δ13C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.

Notizen: Abstract Dalradian metamorphic rocks, Lower Ordovician meta-igneous rocks (MGS) and Caledonian granites of the Connemara complex in SW Connemara all show intense retrograde alteration. Alteration primarily involves sericitization and saussuritization of plagioclase, the alteration of biotite and hornblende to chlorite and the formation of secondary epidote. The alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs, and was the result of a phase of fluid influx into these rocks. In hand specimen K-feldspar becomes progressively reddened with increasing alteration. Mineralogical alteration in the MGS and Caledonian granites took place at temperatures ∼275±15°C and in the MGS Pfluid is estimated to be ≤1.5 kbar during alteration. The °D values of alteration phases are:-18 to-29‰ (fluid inclusions),-47 to-61‰ (chlorites) and-11 to-31‰ (epidotes). Chlorite δ18O values are +0.2 to +4.3‰, while δ18O values for quartz-K-feldspar pairs show both positively sloped (MGS) and highly unusual negatively sloped (Caledonian granites) arrays, diverging from the normal magmatic field on a δ-δ plot. The stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200–140°C. The retrograde fluid had δD ∼-20 to-30‰ in all lithologies, but the fluid δ18O varied both spatially and temporally within the range-4 to +7‰. The fO2 of the fluid that deposited the epidotes in the MGS varied with its δ18O value, with the most 18O-depleted fluid being the most oxidizing. The δD values, together with low (〈0‰) δ18O values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin. This meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration. The variation in retrograde fluid δ18O values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid-rock interaction. Infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the Galway granite soon after intrusion at ∼400 Ma. However convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later.

Notizen: Abstract Fluid inclusions in granite quartz and three generations of veins indicate that three fluids have affected the Caledonian Galway Granite. These fluids were examined by petrography, microthermometry, chlorite thermometry, fluid chemistry and stable isotope studies. The earliest fluid was a H2O-CO2-NaCl fluid of moderate salinity (4–10 wt% NaCl eq.) that deposited late-magmatic molybdenite mineralised quartz veins (V1) and formed the earliest secondary inclusions in granite quartz. This fluid is more abundant in the west of the batholith, corresponding to a decrease in emplacement depth. Within veins, and to the east, this fluid was trapped homogeneously, but in granite quartz in the west it unmixed at 305–390 °C and 0.7–1.8 kbar. Homogeneous quartz δ18O across the batholith (9.5 ± 0.4‰n = 12) suggests V1 precipitation at high temperatures (perhaps 600 °C) and pressures (1–3 kbar) from magmatic fluids. Microthermometric data for V1 indicate lower temperatures, suggesting inclusion volumes re-equilibrated during cooling. The second fluid was a H2O-NaCl-KCl, low-moderate salinity (0–10 wt% NaCl eq.), moderate temperature (270–340 °C), high δD (−18 ± 2‰), low δ18O (0.5–2.0‰) fluid of meteoric origin. This fluid penetrated the batholith via quartz veins (V2) which infill faults active during post-consolidation uplift of the batholith. It forms the most common inclusion type in granite quartz throughout the batholith and is responsible for widespread retrograde alteration involving chloritization of biotite and hornblende, sericitization and saussuritization of plagioclase, and reddening of K-feldspar. The salinity was generated by fluid-rock interactions within the granite. Within granite quartz this fluid was trapped at 0.5–2.3 kbar, having become overpressured. This fluid probably infiltrated the Granite in a meteoric-convection system during cooling after intrusion, but a later age cannot be ruled out. The final fluid to enter the Granite and its host rocks was a H2O-NaCl-CaCl2-KCl fluid with variable salinity (8–28 wt% NaCl eq.), temperature (125–205 °C), δD (−17 to −45‰), δ18O (−3 to + 1.2‰), δ13CCO2 (−19 to 0‰) and δ34Ssulphate (13–23‰) that deposited veins containing quartz, fluorite, calcite, barite, galena, chalcopyrite sphalerite and pyrite (V3). Correlations of salinity, temperature, δD and δ18O are interpreted as the result of mixing of two fluid end-members, one a high-δD (−17 to −8‰), moderate-δ18O (1.2–2.5‰), high-δ13CCO2 (〉 −4‰), low-δ34Ssulphate (13‰), high-temperature (205–230 °C), moderate-salinity (8–12 wt% NaCl eq.) fluid, the other a low-δD (−61 to −45‰), low-δ18O (−5.4 to −3‰), low-δ13C (〈−10‰), high-δ34Ssulphate (20–23‰) low-temperature (80–125 °C), high-salinity (21–28 wt% NaCl eq.) fluid. Geochronological evidence suggests V3 veins are late Triassic; the high-δD end-member is interpreted as a contemporaneous surface fluid, probably mixed meteoric water and evaporated seawater and/or dissolved evaporites, whereas the low-δD end-member is interpreted as a basinal brine derived from the adjacent Carboniferous sequence. This study demonstrates that the Galway Granite was a locus for repeated fluid events for a variety of reasons; from expulsion of magmatic fluids during the final stages of crystallisation, through a meteoric convection system, probably driven by waning magmatic heat, to much later mineralisation, concentrated in its vicinity due to thermal, tectonic and compositional properties of granite batholiths which encourage mineralisation long after magmatic heat has abated.

Notizen: Abstract Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ 18O(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault.