Mineral Equilibria Modelling Research Articles

Petrology and phase equilibrium modeling of spinel-sapphirine-bearing mafic granulite from Akarui Point, L^|^uuml;tzow-Holm Complex, East Antarctica: Implications for the P-T path

We report new petrological and P-T data for spinel-sapphirine-bearing mafic granulite from Akarui Point in the upper amphibolite- to granulite-facies transitional zone of the Lützow-Holm Complex (LHC), East Antarctica, and provide unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during the Neoproterozoic. The reaction microstructures in the rock suggest the stability of the prograde pargasite + garnet + orthopyroxene + plagioclase + hematite assemblage followed by the formation of plagioclase + orthopyroxene + spinel + sapphirine symplectite after garnet. The application of mineral equilibrium modeling on the mafic granulite in the system NCFMASHTO yields a prograde condition of 11-12 kbar and ∼ 900 °C, followed by decompression to the peak stage (Pgs + Pl + Opx + Spl ± H2O) of 5-6 kbar and 900-920 °C, and subsequent cooling to less than 890 °C, possibly along a clockwise P-T path. The high-temperature and possibly ultrahigh-temperature condition observed at Akarui Point might reflect an effect of a local thermal event or the presence of an exotic crustal block within the LHC, which are essentially associated with the complex collisional evolution of this region during the assembly of Gondwana.

Petrology and phase equilibrium modeling of sapphirine + quartz assemblage from the Napier Complex, East Antarctica: Diagnostic evidence for Neoarchean ultrahigh-temperature metamorphism

A synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine + quartz assemblage from two localities (Tonagh Island (TI) and Priestley Peak (PP)) in the Napier Complex, East Antarctica, provides unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during Neoarchean. The reaction microstructures associated with sapphirine + quartz vary among the samples, probably suggesting different tectonic conditions during the metamorphic evolution. Sapphirine and quartz in TI sample were probably in equilibrium at the peak stage, but now separated by corona of Grt + Sil + Opx suggesting near isobaric cooling after the peak metamorphism, whereas the Spr + Qtz + Sil + Crd + Spl assemblage replaces garnet in PP sample suggesting post-peak decompression. The application of mineral equilibrium modeling in NCKFMASHTO system demonstrated that Spr + Qtz stability is lowered down to 930 °C due to small Fe3+ contents in the rocks (mole Fe2O3/(FeO + Fe2O3) = 0.02). The TI sample yields a peak p-T range of 950–1100 °C and 7.5–11 kbar, followed by cooling toward a retrograde stage of 800–950 °C and 8–10 kbar, possibly along a counterclockwise p-T path. In contrast, the peak condition of the PP sample shows 1000–1050 °C and >12 kbar, which was followed by the formation of Spr + Qtz corona around garnet at 930–970 °C and 6.7–7.7 kbar, suggesting decompression possibly along a clockwise p-T trajectory. Such contrasting p-T paths are consistent with a recent model on the structural framework of the Napier Complex that correlates the two areas to different crustal blocks. The different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within a complex Neoarchean subduction/collision belt.

Granulite formation in a Gondwana fragment: petrology and mineral equilibrium modeling of incipient charnockite from Mavadi, southern India

We report a new occurrence of incipient charnockite from Mavadi in the Trivandrum Granulite Block (TGB), southern India, and discuss the petrogenesis of granulite formation in an arrested stage on the basis of petrography, geothermobarometry, and mineral equilibrium modeling. In Mavadi, patches and lenses of charnockite (Kfs + Qtz + Pl + Bt + Grt + Opx + Ilm + Mag) of about 30 to 220 cm in length occur within Opx-free Grt-Bt gneiss (Kfs + Qtz + Pl + Bt + Grt + Ilm). The application of mineral equilibrium modeling on the charnockite assemblage in the NCKFMASHTO system to constrain the conditions of charnockitization defines a P–T range of 800 °C at 4.5 kbar to 850 °C at 8.5 kbar, which is broadly consistent with the results from the conventional geothermobarometry (810–880 °C at 7.7–8.0 kbar) on these rocks. The P–T conditions are lower than the peak metamorphic conditions reported for the ultrahigh-temperature granulites from this area (T > 900 °C). The heterogeneity in peak P–T conditions within the same crustal block might be related to local buffering of metamorphic temperatures by the Opx-Bt-Kfs-Qtz assemblage. The result of T versus mole H2O (M(H2O)) modeling demonstrated that the Opx-free assemblage in the Grt-Bt gneiss is stable at M(H2O) = 0.3 to 1.5 mol%, and orthopyroxene occurs as a stable mineral at M(H2O) <0.3 mol%, which is consistent with the petrogenetic model of incipient charnockite related to the lowering of the water activity and stabilization of orthopyroxene through the breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid from external sources. We also propose a possible alternative mechanism to form charnockite from Grt-Bt gneiss through slight variations in bulk-rock chemistry (particularly for the K- and Fe-rich portion of Grt-Bt gneiss) that can enhance the stability of orthopyroxene rather than that of biotite, with K-metasomatism playing a possible role.

Phase equilibrium modeling of incipient charnockite formation in NCKFMASHTO and MnNCKFMASHTO systems: A case study from Rajapalaiyam, Madurai Block, southern India

Incipient charnockites represent granulite formation on a mesoscopic scale and have received considerable attention in understanding fluid processes in the deep crust. Here we report new petrological data from an incipient charnockite locality at Rajapalaiyam in the Madurai Block, southern India, and discuss the petrogenesis based on mineral phase equilibrium modeling and pseudosection analysis. Rajapalaiyam is a key locality in southern India from where diagnostic mineral assemblages for ultrahigh-temperature (UHT) metamorphism have been reported. Proximal to the UHT rocks are patches and lenses of charnockite (Kfs + Qtz + Pl + Bt + Opx + Grt + Ilm) occurring within Opx-free Grt-Bt gneiss (Kfs + Pl + Qtz + Bt + Grt + Ilm + Mt) which we report in this study. The application of mineral equilibrium modeling on the charnockitic assemblage in NCKFMASHTO system yields a p-T range of ∼820 °C and ∼9 kbar. Modeling of the charnockite assemblage in the MnNCKFMASHTO system indicates a slight shift of the equilibrium condition toward lower p and T (∼760 °C and ∼7.5 kbar), which is consistent with the results obtained from geothermobarometry (710–760 °C, 6.7–7.5 kbar), but significantly lower than the peak temperatures (>1000 °C) recorded from the UHT rocks in this locality, suggesting that charnockitization is a post-peak event. The modeling of T versus molar H2O content in the rock (M(H2O)) demonstrates that the Opx-bearing assemblage in charnockite and Opx-free assemblage in Grt-Bt gneiss are both stable at M(H2O) = 0.3 mol%–0.6 mol%, and there is no significant difference in water activity between the two domains. Our finding is in contrast to the previous petrogenetic model of incipient charnockite formation which envisages lowering of water activity and stabilization of orthopyroxene through breakdown of biotite by dehydration caused by the infiltration of CO2-rich fluid. T-XFe3+ (=Fe2O3/(FeO + Fe2O3) in mole) pseudosections suggest that the oxidation condition of the rocks played a major role on the stability of orthopyroxene; Opx is stable at XFe3+ <0.03 in charnockite, while Opx-free assemblage in Grt-Bt gneiss is stabilized at XFe3+ >0.12. Such low oxygen fugacity conditions of XFe3+ <0.03 in the charnockite compared to Grt-Bt gneiss might be related to the infiltration of a reduced fluid (e.g., H2O + CH4) during the retrograde stage.

Poster #68 A NEW PERSPECTIVE ON ANHEDONIA IN SCHIZOPHRENIA

A synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine + quartz assemblage from two localities (Tonagh Island (TI) and Priestley Peak (PP)) in the Napier Complex, East Antarctica, provides unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during Neoarchean. The reaction microstructures associated with sapphirine + quartz vary among the samples, probably suggesting different tectonic conditions during the metamorphic evolution. Sapphirine and quartz in TI sample were probably in equilibrium at the peak stage, but now separated by corona of Grt + Sil + Opx suggesting near isobaric cooling after the peak metamorphism, whereas the Spr + Qtz + Sil + Crd + Spl assemblage replaces garnet in PP sample suggesting post-peak decompression. The application of mineral equilibrium modeling in NCKFMASHTO system demonstrated that Spr + Qtz stability is lowered down to 930 °C due to small Fe3+ contents in the rocks (mole Fe2O3/(FeO + Fe2O3) = 0.02). The TI sample yields a peak p-T range of 950–1100 °C and 7.5–11 kbar, followed by cooling toward a retrograde stage of 800–950 °C and 8–10 kbar, possibly along a counterclockwise p-T path. In contrast, the peak condition of the PP sample shows 1000–1050 °C and >12 kbar, which was followed by the formation of Spr + Qtz corona around garnet at 930–970 °C and 6.7–7.7 kbar, suggesting decompression possibly along a clockwise p-T trajectory. Such contrasting p-T paths are consistent with a recent model on the structural framework of the Napier Complex that correlates the two areas to different crustal blocks. The different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within a complex Neoarchean subduction/collision belt.

The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome, Bohemian Massif

AbstractEclogite, felsic orthogneiss and garnet–staurolite metapelite occur in a 5 km long profile in the area of Międzygórze in the Orlica–Śnieżnik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high‐pressure and medium‐pressure rocks. The structural succession in all lithologies shows an early shallow‐dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallow‐dipping axial‐plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet‐rich (grs = 22–29 mol.%) and omphacite‐rich (jd = 33–36 mol.%) layers with oriented muscovite (Si = 3.26–3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of ∼19–22 kbar and ∼700–750 °C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from ∼9 kbar and ∼730 °C to 5–6 kbar and 600–650 °C. The S3 fabric contains in addition domains with albite, chlorite, K‐feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from ∼5 kbar and ∼520 °C to ∼7 kbar and ∼610 °C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to ∼7.5 and ∼630 °C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550 °C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P–T paths during the development of the subvertical fabric reflects their exhumation together.

Hydrogeological investigations of thermal waters in the Sfax Basin (Tunisia)

The Sfax Basin in eastern Tunisia is bounded to the east by the Mediterranean Sea. Thermal waters of the Sfax area have measured temperatures of 23–36°C, and electrical conductivities of 3,200 and 14,980 μS/cm. Most of the thermal waters are characterized as Na–Cl type although there are a few Na–SO4–Cl waters. They issue from Miocene units which are made up sands and sandstones interbedded with clay. The Quaternary sediments cap the system. The heat source is high geothermal gradient which are determined downhole temperature measurements caused by graben tectonics of the area. The results of mineral equilibrium modeling indicate that the thermal waters of the Sfax Basin are undersaturated with respect to gypsum, anhydrite and fluorite, oversaturated with respect to kaolinite, dolomite, calcite, microcline, quartz, chalcedony, and muscovite. Assessments from various chemical geothermometers, Na–K–Mg ternary and mineral equilibrium diagrams suggest that the reservoir temperature of the Sfax area can reach up to 120°C. According to δ18O and δ2H values, all thermal and cold groundwater is of meteoric origin.

Metamorphic P– T evolution of garnet–kyanite–staurolite schist and garnet amphibolite from Bodonch area, western Mongolian Altai: Geothermobarometry and mineral equilibrium modeling

We report the first detailed petrological data from the pelitic schists and amphibolites of Bodonch area, southwestern Mongolia, which occupies a significant part of the Paleozoic history of the Altai Orogen in the southwestern margin of the Central Asian Orogenic Belt (or Altaids), and discuss pressure–temperature ( P–T) evolution of the area. The dominant mineral assemblages of pelitic schist in Bodonch area are garnet + kyanite + staurolite + biotite + paragonite, garnet + biotite + staurolite + cordierite, and garnet + biotite + sillimanite with quartz, plagioclase, and ilmenite, while amphibolite contains calcic amphibole + quartz + plagioclase + garnet + ilmenite assemblage. Application of geothermobarometers and mineral equilibrium modeling on kyanite-bearing pelitic schists yielded peak P– T conditions of 640–690 °C/6.3–10.7 kbar. Slightly lower conditions were obtained from sillimanite-bearing schist (580–600 °C/3.6–4.5 kbar) and amphibolite (560–570 °C/3.0–3.5 kbar). The peak high-pressure amphibolite-facies condition and clockwise P– T evolution of Bodonch area estimated for the first time in this study is consistent with available reports of P– T conditions from other localities in the Altai Orogeny outside Mongolia. For instance, P– T conditions of 630 °C/8.7 kbar estimated for kyanite-bearing pelitic schists (garnet + biotite + staurolite + kyanite + quartz) from Xinjiang in Chinese Altai, which is about 340 km WNW from our locality along the regional structure, are comparable with our results. We suggest both Bodonch and Xinjiang areas in the Altai Orogeny underwent a similar metamorphism possibly related to regional accretionary and collisional evolution of the Central Asian Orogenic Belt.

Thermal gradient and timing of high‐T–low‐P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia

AbstractThe Wongwibinda Metamorphic Complex (WMC) is a high‐temperature, low‐pressure (HTLP) belt in the southern New England Orogen. It is characterized by a high metamorphic field gradient exposed in variably metamorphosed siliceous turbidites. The Abroi Granodiorite and the Rockvale and Tobermory adamellites, S‐type granitoids of the Hillgrove Plutonic Suite, intrude the metaturbidites. Six samples of metaturbidite were studied from an ∼3 km long field traverse. Integrated petrography, mineral chemistry, and mineral equilibria modelling indicate a peak metamorphic temperature of 350–450 °C in the lowest grade rocks and ∼660 °C in the highest‐grade rocks. Maximum pressure does not exceed 3.5 kbar anywhere, implying a maximum depth of 12 km and indicating an average vertical gradient of at least 55 °C km−1, though our calculations suggest this is not linear. Metamorphic isograds show no apparent relationship with distance to the exposed margins of the Hillgrove Suite granitoids. Electron microprobe U–Th–Pb monazite data indicate a date of 296.8 ± 1.5 Ma for the thermal peak of the HTLP metamorphism. Laser ablation inductively coupled plasma mass spectrometry indicates a zircon U–Pb crystallization age of 290.5 ± 1.6 Ma for the Abroi Granodiorite, confirming that the pluton post‐dates the peak HTLP metamorphism. Consequently, magmatic advective heat transfer from depth via emplacement of a large volume of granitoid is unlikely to be the key local driver of the high‐grade metamorphism. It is concluded that published evidence of an extensional geodynamic setting around the Carboniferous–Permian boundary supports conductive heat transfer as the key driver of HTLP metamorphism for the WMC. It is not possible to exclude magmatic advective heat transfer via emplacement of mantle derived basaltic magmas in the deeper crust.

Protolith heterogeneity as a factor controlling the feedback between deformation, metamorphism and melting in a granulite-hosted gold deposit

Abstract: Structures and alteration associated with mineralizing, pre-anatectic hydrothermal alteration at the Challenger gold mine in South Australia strongly influenced and preferentially focused subsequent metamorphism, partial melting and deformation. This event resulted in efficient deformation-enhanced melt extraction from the host rocks immediately adjacent to mineralization, leaving restitic garnet, cordierite, K-feldspar rich mineral assemblages. Progressive reintegration of silicate melt into the highly restitic proximal gneiss bulk composition, using mineral equilibrium modelling in the system NCKFMASHTO, indicates that hydrothermal alteration stabilized a higher proportion of hydrous minerals leading to increased fertility and thus melting in the proximal gneiss. Further modelling at lower temperatures provides an insight into mineral assemblages that may have been associated with mineralization. Temperature–composition modelling provides evidence that composite gold–sulphide melts were armoured by quartz and as such were immobile during peak metamorphism. At Challenger, melt permeability of the mineralized zones was enhanced via the formation of a structural fabric, folding, boudinage, fracturing and the formation of shear zones. Giving consideration to the relationships and orientations of these structures, a model for preferential deformation and melt migration within the mineralized shear zone under transpression is developed, where melt is essentially pumped out of the system parallel to the maximum principal elongation direction.

Metastable persistence of pelitic metamorphic assemblages at the root of a Cretaceous magmatic arc – Fiordland, New Zealand

AbstractFour aluminosilicate‐bearing, amphibolite facies pelitic schists sampled from the root of the long‐lived eastern Gondwana continental magmatic arc now exposed in southwest Fiordland, New Zealand, record remarkably different P–T–t histories. The four samples were collected from within 20 km of each other within the Fanny Bay Group and Deep Cove Gneiss near Dusky Sound. Integrated petrography, mineral chemistry, mineral equilibria modelling and in situ electron microprobe chemical dating of monazite shows that the sample of the Fanny Bay Group south of the Dusky Fault records a Carboniferous history with peak conditions of 4–4.5 kbar at 570–590 °C, while one sample of the Deep Cove Gneiss from Long Island records a Cretaceous history with apparent peak conditions of 7.5 kbar at 650 °C. Two other samples of the Deep Cove Gneiss from Resolution Island record mixed Carboniferous and Cretaceous histories with apparent peak conditions of 7 kbar at 650 °C and 3–7 kbar at 640–720 °C. The metapelitic schists on Resolution Island were intruded by arc magmas including the voluminous high‐P Western Fiordland Orthogneiss, yet they lack mineralogical evidence of the Cretaceous high‐P (&gt;12 kbar) event. Analysis of water isopleths in a model system shows that the amount of water accommodated in the rock mineral assemblage increases with pressure. With the exhaustion of all free water, and without the addition of external water, these rocks persisted metastably within the deep arc during the high‐P event. The emplacement of large volumes of diorite (i.e. the Western Fiordland Orthogneiss) into the root of the Early Cretaceous continental magmatic arc did not lead to regional granulite facies metamorphism of the country rock schists, as large volumes of amphibolite facies rock metamorphosed under medium‐P conditions persisted metastably in the deep arc crust.

Hydrogeochemical factors effecting the scaling problem in Balçova geothermal field, İzmir, Turkey

Reservoir fluid compositions have been assessed from analytical data on water samples collected from thermal and cold waters in Balcova geothermal field. The results of mineral equilibrium modelling indicate that the waters, with some exceptions, are systematically supersaturated with respect to calcite, aragonite, dolomite, chalcedony and quartz, but undersaturated with respect to amorphous silica, celestite, anhydrite and gypsum and undersaturated or supersaturated with respect to barite, low-albite, K-feldspar, gibbsite and Fe(OH)3(a). Calculation of mineral saturation states and geochemical analyses of scale and field observations show that carbonate minerals (calcite, aragonite and dolomite) are most likely to be precipitated as a scale type. Besides carbonates, scale formation risk of amorphous silica, Fe(OH)3(a), anhydrite, barite and celestite minerals should be taken into account in some wells and surface equipment. Most of the waters, with some exceptions, have carbonate scaling risk at all temperatures, whereas the other scaling risks only exist over a limited temperature range. While silica, Fe(OH)3(a) and barite show a scaling tendency at low temperatures, anhydrite and celestite scaling occurs at higher temperatures.

On the roles of deformation and fluid during rejuvenation of a polymetamorphic terrane: inferences on the geodynamic evolution of the Ruker Province, East Antarctica

AbstractEvaluating pressure–temperature (P–T) conditions through mineral equilibria modelling within an amphibolite facies polymetamorphic terrane requires knowledge of the fluid content of the rocks. The Archean‐Palaeoproterozoic basement rocks of the Ruker Province, East Antarctica, preserve evidence of three metamorphic events (M1–M3). Of particular interest is the M3 event, which is constrained to the early Palaeozoic (c. 550–480 Ma). Evaluation of the tectonic setting during this time is important because the Ruker Province is located within a critical region with respect to models of Gondwana assembly. Structural evidence of the early Palaeozoic event is preserved as large (up to ∼500 m wide) high strain zones that cut the orthogneiss‐metasedimentary basement (Tingey Complex) of the Ruker Province. Rocks within these zones have been thoroughly recrystallized and preserve a dominant shear fabric and M3 mineral assemblages that formed at P–T conditions of 4.0–5.2 kbar and 565–640 °C. Distal to these zones, rocks preserve more complex petrographic relationships with S1 and S2 foliations, being incompletely overgrown by M3 retrograde assemblages. We show that the mineral assemblages preserved during the M3 event are highly dependent on the availability of fluid H2O, which is strongly influenced by the structural setting (i.e. proximity to the high‐strain zones). P–T structural and fluid flow constraints support a model of basin inversion during early Palaeozoic crustal rejuvenation in the Ruker Province.

Crustal melting and melt extraction, ascent and emplacement in orogens: mechanisms and consequences

Significant volume of wet melting requires an influx of H 2 O-rich volatile phase. In hydrate-breakdown melting, initial melt accumulation is diffusion-controlled and melt accumulates around peritectic phases in low-pressure sites. As the melt-bearing rock weakens, it becomes porous at a few volume per cent melt, initiating an advective flow regime; as melt volume reaches the melt connectivity transition, melt may be lost from the system in the first of several melt-build-up–melt-loss events. Using mineral equilibria modelling, major and accessory phase controls on melt chemistry are evaluated. In residual migmatites and granulites, microstructures indicate the former presence of melt whereas leucosome networks record melt extraction pathways. Sites of initial melting nucleate shear instabilities; strain and anisotropy of permeability control the form of millimetre- to centimetre-scale inferred melt channels and strong anisotropy promotes high fluid focusing. Focused melt flow occurs by dilatant shear failure of low melt volume rocks, which leads to the formation of melt flow networks allowing accumulation and storage of melt, and forming the link for melt flow from grain boundaries to ascent conduits. Melt ascent is via ductile fractures, which may propagate from dilation or shear bands. Fractures are characterized by blunt tips; also they may exhibit zigzag geometry close to the tips and petrographic continuity between leucosome in the host and granite in the dyke. Horizontal tabular and wedge-shaped intrusions commonly are associated with the brittle–ductile transition zone. Vertical lozenge-shaped intrusions represent congelation of magma in ascent conduits and blobby plutons record lateral expansion localized by instability; these intrusion types characterize emplacement at deeper levels in the crust.

Hydrogeochemical and hydrogeological investigations of thermal waters in the Usak Area (Turkey)

Thermal waters of the Usak area have temperatures ranging from 33 to 63°C and different chemical compositions. These waters hosted by the Menderes Metamorphic rocks emerge along fault lineaments from two geothermal reservoirs in the area. The first reservoir consists of gneiss, schists, and marbles of the Menderes Metamorphic rocks. The recorded reservoir is Pliocene lacustrine limestone. Hydrogeochemical studies indicate that thermal waters were mixed with surface waters before and/or after heating at depth. The results of mineral equilibrium modeling indicate that all the thermal waters are undersaturated at discharge temperatures for gypsum, anhydrite, and magnesite minerals. Calcite, dolomite, aragonite, quartz, and chalcedony minerals are oversaturated in all of the thermal waters. Water from the reservoir temperatures of the Usak area can reach upto120°C. According to δ18O and δ2H values, all thermal and cold groundwater are of meteoric origin.

Timing of Gold Mineralization Relative to the Peak of Metamorphism at Bronzewing, Western Australia

The Bronzewing gold deposit is hosted within the middle to upper greenschist facies, tholeiitic metabasalt of the Yandal greenstone belt within the Archean Yilgarn craton. Gold mineralization at Bronzewing is surrounded by an alteration halo hundreds of meters in thickness, composed of distal chlorite, intermediate chlorite-calcite, and proximal carbonate-K-mica zones. Chlorite (in the distal zone) and muscovite and biotite (in the proximal zone) define a strong foliation that is locally crenulated, indicating that alteration minerals and K2O addition were produced prior to the crenulation event. Within the proximal alteration zone, biotite, ankerite, and calcite cut the crenulated foliation-forming potassic minerals, implying changes in metamorphic conditions subsequent to mineralization. Mineral equilibria modelling indicates that the stable mineral assemblage of biotite-muscovite-calcite-ankerite-albite-quartz can only be produced at a temperature of about 440°C (at 2.5 kbars) with a fluid composition of about XCO2 = 0.5–0.6. However, the textural relationships cannot be explained, and the observed alteration zones surrounding mineralization cannot be reproduced by infiltration of fluid (XCO2 = 0.5–0.6) into a slightly carbonated actinolite-bearing mineral assemblage at this temperature. A fluid of XCO2 = 0.15–0.3, between 330° and 375°C (at 2.5 kbars), can reproduce the observed pre-peak metamorphic alteration assemblages at Bronzewing. Calculated internal buffering paths, which model the mineral assemblage and evolution of fluid composition upon further heating, show that the postmineralization assemblage can be explained by further heating to the peak of metamorphism at about 440°C with fluid composition evolving to XCO2 = 0.5–0.6. The implication of the proposed timing relationship is that synmineralization alteration at Bronzewing occurred at temperatures significantly lower than that of peak metamorphism (60°–120°C lower).

The lawsonite paradox: Mineral equilibria modelling in the system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O

The lawsonite paradox: Mineral equilibria modelling in the system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O

Mineral saturation and scaling tendencies of waters discharged from wells (>150 şC) in geothermal areas of Turkey

Aqueous species distribution was calculated from the chemical composition of water discharges from 27 selected production wells, with reservoir temperatures >150 şC, in seven geothermal areas including Kızıldere, Salavatlı, Germencik, Kavaklıdere-Sazdere, Salihli-Caferbeyli, Simav, and Tuzla. Twenty-five of the water compositions are relatively dilute with electroconductivity values of 1826 to 7200 μS/cm and are dominated by Na (410 to 2027 mg/kg), Cl (45 to 1882 mg/kg), and alkalinity-CO 2 (491 to 2312 mg/kg). Two water samples from Tuzla are highly saline connate waters with Cl of 35 273 to 44 140 mg/kg and Na of 18 200 to 22 250 mg/kg. Mineral equilibrium modeling indicates that the aquifer waters in these selected geothermal wells, with some exceptions, are oversaturated with respect to calcite, aragonite, and celestite, but undersaturated with respect to gypsum, anhydrite, fluorite, Ca-montmorillonite, anorthite, albite-low, gibbsite, illite, kaolinite, and K-feldspar. The waters are at near saturation with respect to chalcedony, quartz, amorphous silica, dolomite, and strontianite. Calculation of mineral saturation states, geochemical studies, and field observations show that carbonate minerals (calcite, aragonite, and dolomite), amorphous silica, and sulfate minerals (celestite and anhydrite) are most likely to be precipitated as scales in geothermal wells. Assessment of calcite and amorphous silica scaling tendencies for selected well waters indicates that hot injection is favorable for Tuzla well T-2 (∼50–170 şC) and for Kızıldere wells R-1 and KD-6 (around 100 şC). For the other wells, cold injection (<50 şC) is favored if calcite and amorphous silica accumulation is to be avoided in injection wells.

Water geochemistry of the Seferihisar geothermal area, İzmir, Turkey

The Na–Cl thermal waters of Seferihisar have measured temperatures of 30–153°C, and conductivities of 7400–55200 μS/cm. They issue from Upper Cretaceous to Paleocene Bornova melange rocks, which are made up of sandstone–shale intercalations, conglomerate, mafic submarine volcanics, limestone lenses, serpentinite and limestone bodies, and their complexes, through the intersection of faults. The clay-rich zones of the overlying Neogene terrestrial sediments cap the system. The heat source is the high geothermal gradient caused by graben tectonics of the area. Na and Cl ions mainly dominate the chemistry of thermal waters, thus thermal waters of the Seferihisar area appear to be mixtures of groundwaters and seawater. The seawater contribution in thermal waters varies from 10% to 80%. Assessments from empirical chemical geothermometers, mineral equilibria and silica-mixing model applied on the thermal waters of the Seferihisar geothermal area suggest that the temperatures of the reservoir systems vary between 60 and 180°C. The Seferihisar geothermal area has been physically divided into five main groups and estimated reservoir temperatures provided by the entire methods give the increase as: Doğanbey Burnu, Karakoç spa, Doğanbey, Tuzla and Cumali spa areas in ascending order of reservoir temperature. Hydrogeochemical assessments indicate that thermal waters were mixed with cold waters before and/or after heating at depth in different proportions. The results of mineral equilibrium modeling indicate that the thermal waters of Seferihisar are undersaturated with respect to gypsum and amorphous silica, oversaturated with respect to calcite, dolomite and aragonite, and undersaturated or oversaturated with respect to quartz, chalcedony and anhydrite.