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1.
Summary Electrum, hessite, petzite and sylvanite have been recorded from veins at Tyndrum, Scotland. Electron probe micro-analyses have also revealed two un-named Ag-Te-S phases. Fluid inclusion studies suggest that the mineralising fluids responsible for the precious metal mineralization contained 7.0 mol % CO2 and 7 wt % NaCl. TH (temperature of homogenisation) determinations were in the range 295°C to 325°C and a depth of vein formation 4 km is indicated. Mineral precipitation was probably caused by cooling and adsorption of gold onto pyrite. 34S values of + 1.8%o for galena from the Au + Ag + Te veins suggest a different (possibly igneous) sulphur source to that producing the Pb +Zn vein mineralization in the Tyndrum area. Although an age of 380 Ma was obtained using K-feldspar in the veins the data are not conclusive. It is argued that the Au + Ag mineralization at Tyndrum is due to hydrothermal activity related to Cu +Mo mineralization associated with the Late Caledonian granites.
Die Gold-Silber-Gang-Vererzung von Tyndrum, Schottland
Zusammenfassung In der Gang-Vererzung von Tyndrum, Schottland, kommen eine Gold-Silber-Legierung, Hessit, Petzit und Sylvanit vor. Außerdem ergaben Mikrosondenanalysen das Vorhandensein von zwei unbekannten Ag-Te-S Phasen. Die Untersuchungen von Flüssigkeitseinschlüssen zeigen, daß die für die frühe Mineralisation verantwortlichen Lösungen 7.0% Mol.% CO2 und 7 Gew.% NaCl enthalten haben. Die Homogenisie-rungstemperaturen liegen im Bereich von 295-325°C und weisen auf eine Tiefe der Gangentstehung von 4 km hin. Die Au-Ag-Te Mineralbildung kam vermutlich durch Abkühlung und Adsorption von Gold auf Pyrit zustande. 34S-Werte von + 1.8%0, gemessen an Bleiglanz aus Au-Ag-Te-Gängen deuten eine magmatische Schwefelherkunft an, während die Pb-Zn-Gangmineralisation eine andere Schwefelquelle vermuten läßt. Die Altersdatierung der Gänge von 380 Mio. Jahren läßt sich derzeit noch nicht sinnvoll interpretieren. Es wird angenommen, daß die Au-Ag-Vererzung von Tyndrum durch hydrothermale Aktivität im Zusammenhang mit einer an spätkaledonische Granite gebundenen Cu-Mo-Vererzung entstanden ist.

With 5 Figures  相似文献   

2.
Late Hercynian U-bearing carbonate veins within the metamorphic complex of La Lauzière are characterized by two parageneses. The first is dominated by dolomite or ankerite and the second by calcite and pitchblende. Fluids trapped in the dolomites and ankerites at 350–400° C are saline waters (20 to 15 wt % eq. NaCl) with D –34 to –49. In the calcite they are less saline (17 to 8 wt % eq. NaCl) and trapped at 300–350° C with D –50 to –65. All fluids contain trace N2, CO2 and probably CH4. The carbonates have 13C –8 to –14. and derived their carbon from organic matter. Evolution of the physico-chemical conditions from dolomite (ankerite) to calcite deposition was progressive.H and O-isotope studies indicate the involvement of two externally derived fluids during vein development. A D-rich ( –35) low fO2, saline fluid is interpreted to have come from underlying sediments and entered the hotter overlying metamorphic slab and mixed with more oxidizing and less saline U bearing meteoric waters during regional uplift. This evidence for a sedimentary formation water source for the deep fluid implies that the metamorphic complex overthrusted sedimentary formations during the Late-Hercynian.  相似文献   

3.
During low-grade regional metamorphism, pyrrhotite can form from gypsum by the reaction: CaSO4·2H2O+Fe sol. 2+ +2Corg.FeS+Ca sol. 2+ +2H2O+2CO2. This reaction takes place in the anchizone, below 350°C and might be initiated by the thermal dehydration of gypsum (200°C) and aided by the generation of gaseous hydrocarbons. Evidence for the reaction is the occurrence in dolomitic layers in the Ballachulish Slate, East Larroch quarry, Argyll of lath and swallow-tail shaped quartz+dolomite+pyrrhotite pseudomorphs after gypsum. Quartz+pyrrhotite bodies in slate represent replaced and deformed (mainly flattened) crystals, concretions and possible veinlets of gypsum. Pyrite porphyroblast growth, after the peak of metamorphism and under relatively high fS2 conditions, failed to destroy some early pyrrhotite because it is encapsulated in quartz. Pyrite-silicate reactions and hydrothermal exhalations have been suggested previously to account for pyrrhotite-enriched horizons in regionally metamorphosed rocks. Replacement of gypsum by pyrrhotite is an additional explanation for pyrrhotite-enriched horizons, especially in dolomitic and graphitic lithologies.  相似文献   

4.
The fluid inclusions occurring in quartz of cassiterite-bearing quartz veins from two localities of Southern Brittany have been studied (microthermometry and chemical analysis). In both localities, two sorts of fluids have been recognized: 1. Early fluids, related to the precipitation of cassiterite. Those fluids are closed to a chloride-bearing aqueous solution, with very little CO2 and hydrocarbons. The salinity is rather low (6 to 9 wt % eq. NaCl). The inclusions homogenize between 150 à 300 °C. The K/Na atomic ratio is about 0,1. From these data and the mineralogical associations (muscovite + kaolinite), the physical and chemical properties of the solution at the time of cassiterite crystallization have been calculated: temperature 350°C, pressure 800 bars; molalities of NaCl, KCl and HCl are, respectively about 1, 0.1 and 0.01 (pH at 25 °C, 1 bar 2, lower than 3 in any case). 2. Late fluids, related to an important kaolinization. They are generally colder, and have a either lower or higher salinity, than the early fluids.  相似文献   

5.
A basanitoid flow of Miocene age, exposed near the West Kettle River, 25 km southeast of Kelowna, British Columbia, contains abundant ultramafic and mafic nodules. The subangular nodules are 1–20 cm across and typically show granular textures. A study of 250 nodules indicates that spinel lherzolite (60%) is the dominant type with subordinate olivine websterite (10%), websterite (7%), clinopyroxenite (4%), wehrlite (4%), pyroxene gabbro (4%), dunite (2%), harzburgite (1%) and granitic rocks (8%). Ultramafic nodules are of two types. Most of the wehrlites and clinopyroxenites belong to the black pyroxene (aluminous clinopyroxene) series, whereas the other clinopyroxene-bearing nodules belong to the green pyroxene (chromian diopside) series. Some spinel lherzolite nodules have distinctive pyroxene- and olivine-rich bands. Microprobe analyses of the constituent minerals of more than thirty nodules from the green pyroxene series indicate that grain to grain variations within individual nodules are small even when banding is present. Olivine, orthopyroxene, clinopyroxene and spinel in spinel lherzolite have average compositions of Fo90, En90, Wo47Fs5En48, Cr/(Cr+ Al+Fe3)=0.1 and Mg/(Mg+Fe2+)=0.8. Equilibration temperatures, which were calculated using the two pyroxene geothermometer of Wells (1977), range between 920–980° C. Based on published phase stability experiments, pressures of equilibration are between 10–18 kbar. In summary, the upper mantle beneath southern British Columbia is dominated by spinel lherzolite but contains some banding on a scale of cm to meters. The temperature in the upper mantle is 950° C at a depth of 30–60 km.On leave from the Geological Institute, University of Tokyo, Japan  相似文献   

6.
The 40Ar/39Ar dating of alteration muscovite from the Rosebery Zn-Pb-Cu and Mount Lyell Cu deposits, Tasmania, Australia, has determined a succession of deformation events which occurred from 400-378 Ma, and comprises the Devonian Tabberabberan Orogeny. The dates from Rosebery range from 400-390 Ma, are a minimum age for mineralization, indicate the time of deformation, and provide a maximum age limit for granitoid emplacement in the vicinity of the deposit. The ages from the Mount Lyell field range from 400-378 Ma, are a minimum for mineralization, and date cleavage development. The North Lyell Cu mineralization, which was probably broadly coeval with deformation, may have formed at 400 Ma. All pre-Devonian alteration micas in the Rosebery and Mount Lyell areas have been recrystallized or reset. The Tabberabberan deformation in western Tasmania was broadly contemporaneous with widespread crustal shortening in southeastern Australia, as established from the dating of alteration minerals associated with deformation-related precious and base metal deposits.  相似文献   

7.
Origin and differentiation of picritic arc magmas,Ambae (Aoba), Vanuatu   总被引：3，自引：2，他引：1  
Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg 80 to 93.4) and clinopyroxene (mg 80 to 92), and include accessory Cr-rich spinel (cr 50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to 20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with 10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo2 FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has 15 wt% MgO and an mg Fe2 value of 81. It requires deep (3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T p 1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.  相似文献   

8.
Summary Amphibole and mica lamprophyres and related dykes of Tertiary age from the Kreuzeck Mountains, Central Alps, Austria, have been investigated petrographically and geochemically. They intrude a sequence of early Palaeozoic metapelites, greenstones and amphibolites to the north of the Cretaceous Periadriatic Lineament, a major suture zone of 700 km E-W extent. The dykes are spatially associated with Sb, W, Hg, and Cu-Ag-Au deposits.Most lamprophyres are characterized by primitive chemistry (mg-numbers > 60 and Cr > 200 ppm) and have high contents of LIL elements (K, Rb, Sr and Ba). Geochemically, five different subgroups of calcalkaline/shoshonitic to alkaline affinity can be distinguished. These are: Group 1, amphibole-bearing shoshonitic lamprophyres (0.5–1.0 wt% Ti02, Zr < 150 ppm, Nb < 13 ppm, Ba/Rb < 10); Group 2, mica-bearing shoshonitic lamprophyres (1–1.5 wt% TiO2, Zr 180 ppm, Nb < 17 ppm, Ba/Rb > 20); Group 3, alkaline lamprophyres (1.5–2.1 wt% TiO2, Zr > 250 ppm, Nb > 30 ppm, Ba/Rb 10–25); Group 4, low-MgO alkaline lamprophyres ( 2.5 wt% TiO2, mg-number < 57, Nb 20 ppm, Ba/Rb 20); Group 5, calc-alkaline basaltic dykes ( 2.2 wt% TiO2, mg-number <55, Nb < 10 ppm, Ba/Rb < 10). Group 2,3 and 4 dykes have NE-SW orientations and are of Oligocene age (K-Ar age 27–32 Ma); Group 1 and 5 dykes are of Lower Oligocene age (K-Ar age 36 Ma) but have mostly E-W orientations.The Kreuzeck lamprophyres were generated in post-collisional magmatic events, which were probably linked to extensional tectonics following oblique continent-continent collision between the African and Eurasian Plates during the Eocene. Group 1, 2 and 5 dyke rocks have typical calc-alkaline geochemical signatures indicating that they represent partial melting products of subduction-modified lithosphere. Group 3 and 4 alkaline lamprophyres have geochemical features transitional between calc-alkaline and within-plate alkaline igneous rocks (e.g. Ba/Nb 30–70) indicating that their mantle source-region includes both subduction-modified lithospheric and OIB-type asthenospheric components.There is no apparent relationship between mineralization in the Kreuzeck region, thought to be of Ordovician-Devonian age, and much later lamprophyre intrusion. Alteration of the dykes by late-magmatic fluids has resulted in the formation of secondary minerals, and has occasionally led to increased Au and PGE values in the 10–35 ppb range particularly in close proximity to Cu-Ag-Au deposits.
Shoshonitische und alkalische Lamprophyre mit erhöhten Au- und PGE-Gehalten aus der Kreuzeckgruppe, Ostalpen, Österreich
Zusammenfassung Die vorliegende Arbeit untersucht die Petrographie und Geochemie tertiärer Lamprophyre und genetisch verwandter Ganggesteine aus der zentralalpinen Kreuzeckgruppe, nördlich des Periadriatischen Lineamentes, in Kärnten, Österreich. Die Ganggesteine durchschlagen die altpaläozoischen Metapelite, Grünsteine und Amphibolite des Altkristallins diskordant und stehen in räumlichem Zusammenhang mit Sb, W, Hg und Cu-Ag-Au Lagerstätten, die bereits seit dem Mittelalter abgebaut wurden.Die meisten Lamprophyre zeigen primitiven Charakter (Mg-Zahlen >60 und Cr > 200 ppm) und besitzen hohe Gehalte an LILE (K, Rb, Sr und Ba). Geochemisch lassen sich fünf verschiedeneGruppen mit kalkalkalisch/shoshonitischem bis alkalischem Charakter unterscheiden: Gruppe 1, Amphibol-führende shoshonitische Lamprophyre (0.5–1.0 Gew% TiO2, Zr < 150 ppm, Nb < 13 ppm, Ba/Rb < 10);Gruppe 2, Glimmer-führende shoshonitische Lamprophyre (1–1.5 Gew% TiO2, Zr 180 ppm, Nb < 17 ppm, Ba/Rb > 20); Gruppe 3, alkalische Lamprophyre (1.5–2.1 Gew% TiO2, Zr > 250 ppm, Nb > 30ppm, Ba/Rb 10–25); Gruppe 4, alkalische Lamprophyre mit geringen MgO-Anteil ( 2.5 Gew% TiO2, Mg-Zahl < 57, Nb 20 ppm, Ba/Rb 20); Gruppe 5, kalkalkalisch basaltische Ganggesteine ( 2.2 Gew% TiO2, Mg-Zahl < 55, Nb < 10 ppm, Ba/Rb < 10). Die Lamprophyre der Gruppen 2, 3 und 4 zeigen nordöstliches Streichen und oligozänes Intrusionsalter (K-Ar Alter 27–32 Ma), während die Ganggesteine der Gruppen 1 und 5 überwiegend östliches Streichen und UnterOligozänes Intrusionsalter (K-Ar Alter 36 Ma) aufweisen.Die Intrusionen erfolgten während einer tektonischen Dilatationsphaseim Oligozän nach der Kontinent-Kontinent Kollision zwischen derAfrikanischen und der Eurasischen Platte im unteren Eozän. Ganggesteine der Gruppen 1, 2 und 5 besitzen typisch kalkalkalischen Charakter und stellen vermutlich Produktevon aufgeschmolzener, subduzierter Lithosphäre dar. Die Geochemie der alkalischen Lamprophyre derGruppen 3 und 4 (e.g. Ba/Nb 30–70) deutet auf ihre genetische Zwischenstellung zwischen subduction-related und within-plate regime.Zwischen den tertiären Gangintrusionen und den vermutlich paläozoischen Vererzungen der Kreuzeckgruppe besteht kein genetischer Zusammenhang. Die Alteration der Ganggesteine durch postmagmatische Lösungen hat jedoch zur Bildung von sekundären Mineralen und teilweise zu überdurchschnittlich erhöhten Au und PGE Gehalten von bis zu 35 ppb geführt.

With 6 Figures  相似文献   

9.
Mineral parageneses of the polymetallic, Sbrich deposit at Dúbrava has been formed during five separated stages. A fluid inclusion study demonstrates that the earliest stages with scheelite, molybdenite and arsenopyrite have been associated with immiscible CO2 (± CH4)-rich, low-saline fluids at temperatures between 300 and 400 °C and pressures as much as 2 kbar. Deposition of the main, superimposed ores, stibnite and zinckenite, has been intimately connected with circulation of aqeuous, moderately saline fluids (15.5–23.5 wt% NaCl equiv.) upon epithermal conditions. Salinity of the aqueous fluids associated with tetrahedrite is clustered around 10 wt% NaCl equiv. Quartz from the latest, barite stage has precipitated from aqueous fluids enriched in divalent cations. These fluids are believed to be genetically linked with Triassic evaporite formations preserved in the region. Temperature-salinity diagrams constructed from microthermometry data indicate influx of diluted meteorite water in the stibnite, tetrahedrite and barite stages. Isotopic data are in accordance with model. The 18O values between –9.3 and +1.5 have been derived for water in equilibrium with quartz, coexisting with sphalerite, tetrahedrite and barite, thus confirming the participation of isotopically lighter meteoric waters in the mineral-forming solutions. The ( 18O) values between +3.3 and +8.5 estimated for the water associated with the scheelite and arsenopyrite stages, are suggestive for the majority of metamorphic and/or magmatic water in the mineral-forming, CO2-rich solutions.  相似文献   

10.
Mesothermal gold mineralization at the Samdong mine (5.5–13.5 g/ton Au), Youngdong mining district, is situated in massive quartz veins up to 1.2 m wide which fill fault fractures within upper amphibolite to epidote-amphibolite facies, Precambrian-banded biotite gneiss. The veins are mineralogically simple, consisting of iron- and base-metal sulfides and electrum, and are associated with weak hydrothermal alteration zones (<0.5 m wide) characterized by silicification and sericitization. Fluid inclusion data and equilibrium thermodynamic interpretation of mineral assemblages indicate that the quartz veins were formed at temperatures between 425 and 190°C from relatively dilute aqueous fluids (4.5–13.8 wt. % equiv NaCl) containing variable amounts of CO2 and CH4. Evidence of fluid unmixing (CO2 effervescence) during the early vein formation indicates approximate pressures of 1.3–1.9 kbars, corresponding to minimum depths of 5–7 km under a purely lithostatic pressure regime. Gold deposition occurred mainly at temperatures between 345 and 240 °C, likely due to decreases in sulfur activity accompanying fluid unmixing. The 34S values of sulfide minerals (-3.0 to 5.3 ), and the measured and calculated O-H isotope compositions of ore fluids (18O = 5.7 to 7.6; = –74 to –80) indicate that mesothermal gold mineralization at the Samdong mine may have formed from dominantly magmatic hydrothermal fluids, possibly related to intrusion of the nearby ilmenite-series, Kimcheon Granite of Late Jurassic age.  相似文献   

11.
Monomineralic domains of chlorite, corundum and Cr muscovite coexist over a kilometer scale within ultramafic schists of the Harare greenstone belt (2.73 Ga). This exotic lithological association includes the conjunction of some of the most aluminous (Al2O388 wt%) and potassic (K2O10 wt%) rocks known. The paragenetic sequence developed from chloritecorundumcorundum+ diaspore: Cr muscovite variably overprinted both the corundum and chloritite domains. Terminal stages were marked by sporadic production of andalusite+quartz, and finally margarite.Chlorite (Cr2O3=0.31–2.65 wt%), corundum (0.79–2.66 wt%), and diaspore are all Cr-rich varieties. The chromian (Cr2O33.86 wt%) paragonitic muscovite incorporates up to 17% of the paragonite molecule, and significant Mg and Fe substitutions.The suite of rocks are characterized by chondritic Ti/Zr ratios (–x=107), systematically enhanced Cr (up to 14000 ppm) and Ni (up to 1200 ppm) abundances, low levels of the alteration-insensitive incompatible elements Th, Ta, Nb. Chlorite, corundum and Cr muscovite represent progressive stages in the incremental metasomatic alteration of a komatiite precursor. Mass balance calculations, constrained by the isochemical behaviour of Ti, Zr and Hf reveal that the komatiite chloritite transformation involved volumetric contractions of 60% by hydrothermal leaching of Si, Fe, Mn, Ca and Na. Reaction of chloritite to corundum involved further volumetric reductions of 50% due to essentially quantitative loss of Si, Fe, Mn, Mg, K and Ca. Conversion of corundum to muscovite required additions of Si, K, Fe, Mn, Mg, Rb and Ba at 50–200% dilation. K, Rb, Ba, Li and Cs are enriched by up to 2×103 over background abundances in ultramafic rocks, and the suite is also enriched in B, Se, Te, Bi, As, Sb and Au. REE were extensively leached during chloritite-corundum stages, whereas LREE additions accompany development of muscovite. Ti, Zr, Hf and Al were all concentrated by selective leaching of mobile components, but absolute additions of Al accompanied development of the corundum domains due to Al precipitation in response to depressurization.Corundum ( 18O=3.5–4.8), muscovite ( 18O=6.7–7.5) and chlorite (4.5–5.6) are isotopically uniform and formed at 380–520° C from a fluid where 18O=5.6–6.9. The corundum is 18O depleted relative to either igneous or anatectic counterparts (Ocor=7.6–8.2), or to gibbsitic laterites ( 18O=12–17).Previous genetic schemes involving metamorphism of exhalites or bauxite, or Si-undersaturation of magmas, can all be ruled out from the data. The chloritite, corundum, Cr-muscovite association represents sequential alteration products of ultramafic rocks by high temperature, low pH hydrothermal solutions carrying LIL-elements, and in which excursions of pH and/or degree of quartz undersaturation account for the mineralogical transitions. A deep level acid epithermal system, or fluid advection across steep inverted thermal gradients in a thrust regime could account for required hydrothermal conditions.  相似文献   

12.
A pink-colored (HUE 5R-7/4), predominantly calcium saturated, clay mineral from Tirebolu, NE-Turkey, is shown to be a dioctahedral, high-charge manganiferous smectite. It is probably an alteration product of volcanic tuffs of rhyodactitic composition. Ionic formula: (Si7.71Al0.29)IV(Al3.04Fe 0.12 3+ Fe 0.02 2+ Mg0.92Mn 0.07 2+ )VI O20 (OH)4 0.79 M+ Cation exchange capacity: 92 me/100 g (air dry sample). Unit cell parameters (ca. 20 ° C; ca. 40% RH): a09.02 Å b05.21 Å Mg++... Mn-smectite: c014.65 Å Ca++... Mn-smectite: c014.83 Å K+... Mn-smectite: c012.10 Å
Zusammenfassung Ein rosa (HUE 5R-7/4), weitgehend Calcium-gesättigtes Tonmineral von Tirebolu (NO-Türkei) wurde als ein dioktaedrischer, hochgeladener manganhaltiger Smektit identifiziert. Er ist wahrscheinlich ein Umwandlungsprodukt eines rhyodazitischen Tuffs. Strukturformel: (Si7.71Al0.29)IV(Al3.04Fe 0.12 3+ Fe 0.02 2+ Mg0.92Mn 0.07 2+ )VI O20 (OH)4 0.79 M+ Kationaustauschkapazität: 92 me/100 g (luftgetrocknet). Gitterkonstanten (ca. 20° C; ca. 40% RF): a09.02 Å b05.21 Å Mg++... Mn-Smektit: c014.65 Å Ca++... Mn-Smektit: c014.83 Å K+... Mn-Smektit: c012.10 Å
  相似文献   

13.
The Lewis Ponds Zn–Pb–Cu–Ag–Au deposit, located in the eastern Lachlan Fold Belt, central western New South Wales, exhibits the characteristics of both volcanic-hosted massive sulphide and carbonate-hosted replacement deposits. Two stratabound massive to disseminated sulphide zones, Main and Toms, occur in a tightly folded Upper Silurian sequence of marine felsic volcanic and sedimentary rocks. They have a combined indicated resource of 5.7 Mt grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au. Main Zone is hosted by a thick unit of poorly sorted mixed provenance breccia, limestone-clast breccia and quartz crystal-rich sandstone, whereas Toms Zone occurs in the overlying siltstone. Pretectonic carbonate–chalcopyrite–pyrite and quartz–pyrite stringer veins occur in the footwall porphyritic dacite, south of Toms Zone. Strongly sheared dolomite–chalcopyrite–pyrrhotite veins directly underlie the Toms massive sulphide lens. The mineralized zones consist predominantly of pyrite, sphalerite and galena. Paragenetically early framboidal, dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate occur throughout the breccia and sandstone beds that host Main Zone, but are rarely preserved in the annealed massive sulphide in Toms Zone. Main and Toms zones are associated with a semi-conformable hydrothermal alteration envelope, characterized by texturally destructive chlorite-, dolomite- and quartz-rich assemblages. Dolomite, chlorite, quartz, calcite and sulphides have selectively replaced breccia and sandstone beds in the Main Zone host sequence, whereas the underlying porphyritic dacite is weakly sericite altered. Vuggy and botryoidal textures resulted from partial dissolution of the dolomite-altered sedimentary rocks and unimpeded growth of base metal sulphides, carbonate and quartz into open cavities. The intense chlorite-rich alteration assemblage, underlying Toms Zone, grades outward into a weak pervasive sericite–quartz assemblage with distance from the massive sulphide lens. Limestone clasts and hydrothermal dolomite at Lewis Ponds are enriched in light carbon and oxygen isotopes. The dolomite yielded 13CVPDB values of –11 to +1 and 18OVSMOW values of 6 to 16. Liquid–vapour fluid inclusions in the dolomite have low salinities (1.4–7.7 equiv. wt% NaCl) and homogenization temperatures (166–232°C for 1,000 m water depth). Dolomitization probably involved fluid mixing or fluid–rock interactions between evolved heated seawater and the limestone-bearing facies, prior to and during mineralization. 34SVCDT values range from 2.0 to 5.0 in the massive sulphide and 3.9 to 7.4 in the footwall carbonate–chalcopyrite–pyrite stringer veins, indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater. The sulphide mineral assemblages at Lewis Ponds are consistent with moderate to strongly reduced conditions during diagenesis and mineralization. Low temperature dolomitization of limestone-bearing facies in the Main Zone host sequence created secondary porosity and provided a reactive host for fluid-rock interactions. Main Zone formed by lateral fluid flow and sub-seafloor replacement of the poorly sorted breccia and sandstone beds. Base metal sulphide deposition probably resulted from dissolution of dolomite, fluid mixing and increased fluid pH. Pyrite, sphalerite and galena precipitated from a relatively low temperature, 150–250°C hydrothermal fluid. In contrast, Toms Zone was emplaced into fine-grained sediment at or near the seafloor, above a zone of focused up-flowing hydrothermal fluids. Copper-rich assemblages were deposited in the Toms Zone footwall and massive sulphide lenses in Main and Toms zones as the hydrothermal system intensified. During the D1 deformation, fracture-controlled fluids within the Lewis Ponds fault zone and adjacent footwall volcanic succession remobilized sulphides into syntectonic quartz veins. Lewis Ponds is a rare example of a synvolcanic sub-seafloor hydrothermal system developed within fossiliferous limestone-bearing facies. The close spatial association between limestone, hydrothermal dolomite, massive sulphide and dacite provides a basis for new exploration targets elsewhere in New South Wales.Editorial handling: D. Lentz  相似文献   

14.
We report the result of H2O-undersaturated melting experiments on charges consisting of a layer of powdered sillimanite-bearing metapelite (HQ36) and a layer of powdered tonalitic gneiss (AGC150). Experiments were conducted at 10 kbar at 900°, 925° and 950°C. When run alone, the pelite yielded 40 vol% strongly peraluminous granitic melt at 900°C while the tonalite produced only 5 vol% weakly peraluminous granitic melt. At 950°C, the pelite and the tonalite yielded 50 vol% and 7 vol% granitic melt, respectively. When run side by side, the abundance of melt in the tonalite was 10 times higher at all temperatures than when it was run alone. In the pelite, the melt abundance increased by 25 vol%. When run alone, biotite dehydration-melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt. When run side by side only garnet was produced in addition to granitic melt. Experiments of relatively short duration, however, also contained Al-rich orthopyroxene. We suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of Al2O3 in the melt, which lowers the temperature of the biotite dehydration-melting reaction. In the pelite, the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite-layer to the pelite-layer. This has the effect of changing the bulk composition of this layer in the direction of minimum-temperature granitic liquids. Our results show that rocks which are poor melt-producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase(s) and facilitate dehydration-melting. Because of this effect, the continental crust may have an even greater potential for granitoid melt production than previously thought. Our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks, which will be reflected in their isotopic and geochemical compositions.  相似文献   

15.
Ilmenite macrocrysts in olivine melilitites from Namaqualand-Bushmanland, South Africa, have decomposed by subsolidus reduction to form oriented Mg-titanomagnetite along {0001} ilmenite planes. Residual ilmenite contains 10–11 wt% MgO, 1 wt% MnO, and 0.1 wt% Cr2O3. This macrocryst assemblage is mantled by an annulus of Mg-titanomagnetite, followed by an overgrowth of radiating magnetite + perovskite. Terminal compositions of these magnetites are similar to groundmass spinels, and to the outermost margins of magnetite macrocrysts that have very high Fe3+ core contents. The assemblages are remarkably similar to oxide intergrowths in kimberlites and an upper mantle derivation is proposed for ilmenite macrocrysts in these melilitites. Oxidation states in the source regions are also very similar, whether on-or off-craton, being slightly above FMQ (NNO), but reduced to FMQWM with the onset of decompression, volatile loss, and carbonate immiscibility. In the case of the melilitites, late stage, low pressure crystallization above NNO precipated abundant magnetite + perovskite. The oxide fO2 data are consistent with, and refine the fO2 estimates obtained previously for the behavior of Fe/Mg and Ni contents in olivine from the same suite of samples.  相似文献   

16.
DC and AC electrical conductivities were measured on samples of two different crystals of the mineral aegirine (NaFeSi2O6) parallel () and perpendicular () to the [001] direction of the clinopyroxene structure between 200 and 600 K. Impedance spectroscopy was applied (20 Hz–1 MHz) and the bulk DC conductivity DC was determined by extrapolating AC data to zero frequency. In both directions, the log DC – 1/T curves bend slightly. In the high- and low-temperature limits, differential activation energies were derived for measurements [001] of EA 0.45 and 0.35 eV, respectively, and the numbers [001] are very similar. The value of DC [001] with DC(300 K) 2.0 × 10–6 –1cm–1 is by a factor of 2–10 above that measured [001], depending on temperature, which means anisotropic charge transport. Below 350 K, the AC conductivity () (/2=frequency) is enhanced relative to DC for both directions with an increasing difference for rising frequencies on lowering the temperature. An approximate power law for () is noted at higher frequencies and low temperatures with () s, which is frequently observed on amorphous and disordered semiconductors. Scaling of () data is possible with reference to DC, which results in a quasi-universal curve for different temperatures. An attempt was made to discuss DC and AC results in the light of theoretical models of hopping charge transport and of a possible Fe2+ Fe3+ electron hopping mechanism. The thermopower (Seebeck effect) in the temperature range 360 K < T <770 K is negative in both directions. There is a linear – 1/T relationship above 400 K with activation energy E 0.030 eV [001] and 0.070 eV [001]. 57Fe Mössbauer spectroscopy was applied to detect Fe2+ in addition to the dominating concentration of Fe3+.  相似文献   

17.
The solubility of water in melts in the NaAlSi3O8–H2O system at high P and T was deduced from the appearance of quenched products and from water concentrations in the quenched glasses measured by ion probe, calibrated by hydrogen manometry. Starting materials were gels with sufficient water added to ensure saturation of the melts under the run conditions. Experiments were carried out for 10–30 h in an internally heated argon pressure vessel (eight at 1400° C and 0.2–0.73 GPa and three at 0.5 GPa and 900–1200° C) and for 1 h in a piston-cylinder apparatus (three at 1200° C, 1–1.3 GPa). No bubbles were observed in the glasses quenched at P<0.5 GPa or from T<1300° C at 0.5 GPa. Bubble concentration in glasses quenched from 1400° C was low at 0.5, moderate at 0.55 GPa and very high at 0.73 GPa and still higher in glasses quenched in the piston cylinder. Water concentration was measured in all glasses, except for the one at 0.55 GPa, for which it was only estimated, and for those at 0.73 GPa because bubble concentration was too high. Inferred water solubilities in the melt increase strongly with increasing P at 1400° C (from 6.0 wt% at 0.2 GPa to 15 at 0.55 GPa) and also with increasing T at 0.5 GPa (from 9.0 wt% at 900° C to 12.9 at 1400° C). The T variation of water solubility is fundamental for understanding the behaviour of melts on quenching. If the solubility decreases with T at constant P (retrograde solubility), bubbles cannot form by exsolution on isobaric quenching, whereas if the solubility is prograde they may do so if the cooling rate is not too fast. It is inferred from observed bubble concentrations and from our and previous solubility data that water solubility is retrograde at low P and prograde at and above 0.45 GPa; it probably changes with T from retrograde below to prograde above 900° C at 0.5 GPa. Moreover, the solubility is very large at higher pressures (possibly>30 wt% at 1.3 GPa and 1200° C) and critical behaviour is approached at 1.3 GPa and 1200° C. The critical curve rises to slightly higher P at lower T and intersects the three-phase or melting curve at a critical end point near 670° C and 1.5 GPa, above which albite coexists only with a supercritical fluid.  相似文献   

18.
The Sila batholith is the largest granitic massif in the Calabria-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica±cordierite±Al silicate granodiorite to leucomonzogranite. Mineral 40Ar/39Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293–289 Ma). SiO2 content in the granitic rocks ranges from 57 to 77 wt%; cumulate gabbro enclaves have SiO2 as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite±amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica±Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite±hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to 300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (0.40–4.5), Th/Sm (0.1–3.6), Th/Ta (0–70), Ba/Nb (1–150), and Ba/Ta (50–2100), as well as low values of Nb/U (2–28) and La/Th (1–10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock 18O ranges from +8.2 to +11.7; the mafic cumulate enclaves have the lowest 18O values and the two-mica granites have the highest values. 18O values for biotite±honblende tonalitic and granodioritic rocks (9.1 to 10.8) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites (10.7 to 11.7). The initial Pb isotopic range of the granitic rocks (206Pb/204Pb 18.17–18.45, 207Pb/204Pb 15.58–15.77, 208Pb/204Pb 38.20–38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tetonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex intractions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.  相似文献   

19.
The South Bay copper-zinc-silver orebody, 52 miles east of Red Lake in northwestern Ontario, lies in refolded siliceous volcanic rocks near the close of an Archaean volcanic cycle. The rock mass mechanics events at South Bay mainly happened during the endogenesis and uplift of the earth's crust at the Birch-Uchi greenstone belt. After the uplift the geological history of this region was of little significance for the structure of the ore-body. The paleographic properties of the mineral deposits have been deformed by two phases of geological events; namely, first, Rotational Stress — confined pressure (geometry of torsion structures) and, second, Spherical Stress — compressive pressure (geometry of ellipsoidic structures). And, finally the structure formation in the rock mass induced natural stresses which have components determined by the geometry of the deformations.
Zusammenfassung Das South-Bay Kupfer-Zink Lager, 80 km östlich des Red Lake im Nordwesten von Ontario gelegen, ist in gefalteten archaischen Vulkaniten von hohem Silikatgehalt eingebettet. Die gesteinsverformenden Bewegungen der South-Bay Region erfolgten hauptsächlich während der Endogenetik und der Anhebung der Erdkruste der Birch-Uchi Greenstone Zone. Nach dieser Anhebung haben die geologischen Ereignisse dieser Region die Struktur des Erzlagers kaum mehr beeinflußt. Die Petrogefüge des Minerallagers zeigen Deformationen während zweier tektonischer Phasen: zuerst freie Verdrehungsspannungen, die Torsionsstrukturen verursachten, gefolgt von allseitigem Druck, der dem Erzlager ellipsoidähnliche Strukturen aufprägte. In den Gesteinen existieren Spannungen, deren Komponenten durch die Geometry der Verformungen bestimmt sind.
Résumé La mine de Cuivre-Zinc-Argent de South-Bay, située à 80 km à l'est de Red-Lake dans le nort-ouest de l'Ontario, se trouve dans des volcanites, à haute teneur en silicates, plissées au cours de l'Archéen. Les mouvements déformatifs dans la région de South-Bay eurent lieu principalement durant l'endogénese et le soulèvement de la cróute terrestre, le long de la ceinture de roches vertes de Birch-Uchi. Aprèsce soulèvement, les manifestations géologiques dans cette région n'eurent guère d'influence sur la structure du gisement. Les textures pétrographiques du gisement montrent des déformations se rapportant à deux phases tectoniques: d'abord, une rotation engendrant des structures de torsion, et ensuite une compression qui imprima une structure ellipsoÏdale. Il a existé dans la masse rocheuse des tensions dont les composantes ont été déterminées par la géométrie des déformations.
/ , 80- -. , . , , Birch-Uchi. . : , : , . .
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20.
Summary The provenance of the Mid-Devonian clastic sediments in the Teplá-Barrandian Unit (TBU) of the Bohemian Massif was investigated by laser ablation ICP-MS U–Pb zircon dating, bulk sediment geochemistry and mineralogical study of the heavy mineral fraction. In contrast to the island arc provenance of the TBU Neoproterozoic sediments, the Early Palaeozoic sediments contain significant amounts of differentiated crustal material. The detrital zircon populations in the Barrandian Mid-Devonian siltstones and sandstones show ages ranging from Archaean (3.0Ga) to Early Palaeozoic (0.39Ga). Major age maxima are at 2.6Ga, 2.0–2.25Ga, 0.62 and 0.51Ga. The youngest identified zircons so far correspond to Lower and Mid-Devonian ages. The extensive mechanical abrasion of zircons having Archaean (3.0, 2.8 and 2.6Ga) to Paleoproterozoic ages (2.25–2.0Ga) suggest their provenance from recycled old sedimentary sequences. The relatively large number of zircons with ages between 2.0 and 3.0Ga may indicate the presence of relicts of the Archaean/Paleoproterozoic crust in the source areas of the studied Mid-Devonian sediments. The absence of detrital zircon ages between 0.9 and 1.2Ga and the presence of zircon ages of 2.0–2.25 and 0.5–0.8Ga correspond to the zircon age pattern from the Gondwana-related North African, rather than Gondwana-related South American and Baltic terranes. The material was entering the basin predominantly from the west and consisted primarily of detrital material of Cambrian granitoids and recycled material of Neoproterozoic meta-sedimentary sequences.  相似文献   

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