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Large volume, intermediate-felsic magma reservoirs are the source of melt and mineralising fluids which generate porphyry copper deposits. Cooling and crystallisation of hydrous magmas drives the exsolution and expulsion of a magmatic volatile phase-a process which remains challenging to constrain in porphyry Cu systems where the record of magma volatile compositions is rarely preserved. Here, we use the halogen compositions of apatite inclusions shielded as inclusions within zircon to constrain volatile evolution in magma reservoirs which pre-date and are synchronous with porphyry Cu mineralisation at Quellaveco, Southern Peru. Geochemical and textural data confirm that the zircon-included apatites escaped re-equilibration with hydrothermal fluids, unlike apatites found in the groundmass of the same rocks. We, therefore, recommend that future studies attempting to reconcile magmatic volatile budgets using apatite in porphyry Cu systems should focus on apatite inclusions in zircon. By combining the apatite inclusion data with numerical modelling, we find evidence that the magma reservoir sourcing porphyry Cu mineralisation remained fluid-saturated for the entire period recorded by apatite crystallisation. By contrast, the pre-mineralisation batholith shows more variable, potentially fluid-undersaturated behaviour. Our modelling suggests that in order to attain the porphyry melt volatile compositions inferred from apatite, the magma reservoir must have exsolved a large proportion of its volatile budget, consistent with having been held at high crystallinity (40-60% crystals). This crystallisation interval coincides with peak chlorine and copper extraction from intermediate-felsic magmas, and would have permitted efficient fluid migration and accumulation at the roof of the system. We suggest that the storage of large-volume, long-lived, crystal-rich magma reservoirs in magmatic arcs may be a critical step in generating world-class porphyry copper deposits., Author(s): Chetan L. Nathwani [sup.1] [sup.2] [sup.4], Simon J. E. Large [sup.1], Emily R. Brugge [sup.1] [sup.2], Jamie J. Wilkinson [sup.1] [sup.2], Yannick Buret [sup.1] Author Affiliations: (1) grid.35937.3b, 0000 [...]

High-temperature conditions for ore deposition and anomalous abundances of base metals in orogenic gold deposits are frequently attributed to magmatic-hydrothermal fluids instead of metamorphic orogenic fluids. Zona Basal is a shear zone-related gold and base metals-rich mineral occurrence recently discovered in the northwestern portion of the Quadrilátero Ferrífero mining district in southeast Brazil. Mineralogical and geochemical characterization has revealed a two-stage mineralizing system. An Au-W-As stage is marked by a late- to post-tectonic arsenopyrite-pyrrhotite-pyrite assemblage (crystallized from ca. 491 to 404 °C), associated with native gold and scheelite, which was formed during the transition from the peak of greenschist facies metamorphism to retrograde metamorphism. A following, clearly post-tectonic stage comprises a pyrrhotite-pyrite-sphalerite-chalcopyrite-galena-ullmannite ± meneghinite ± fahlore ± miargyrite ± pyrargyrite assemblage crystallized from ca. 400 to < 170 °C, corresponding to the Ag and base metals stage. Fahlores from the Zona Basal mineral occurrence have been found to be sufficiently enriched in Ag, due to the Ag-Cu cation-exchange reaction 13PbS (galena) + 10AgSbS.sub.2 (in galena) = 3Ag.sub.3SbS.sub.3 (pyrargyrite) + Ag(Cu).sub.-1 (exchange in fahlore) + CuPb.sub.13Sb.sub.7S.sub.24 (meneghinite), to jump across the (Cu, Ag).sub.10(Fe,Zn).sub.2Sb.sub.4S.sub.13 miscibility gap. Two mechanisms may explain the abrupt chemical change between the two mineralization stages: (1) participation of magmatic-hydrothermal fluids of relatively low temperature (acting mostly as a chemical source for fluids) associated with a syn- to late tectonic granite intrusion; or (2) local re-concentration of base metals by unmixing of an early metamorphic orogenic fluid due to fluid immiscibility after the transition from ductile to brittle conditions, generating coexisting fluids with different salinities (i.e., extremely low and moderate salinity). The findings in this paper illustrate the importance of considering retrograde or cooling reactions and mineral re-equilibrium while addressing complex ore-formation., Author(s): Felipe Emerson André Alves [sup.1] [sup.2], Atlas Vasconcelos Corrêa Neto [sup.3], Mariana Brando Soares [sup.4], Reiner Neumann [sup.2], Gabriel Machado da Silva [sup.1], Gabriela Pereira Silva [sup.1], Ana Clara [...]

The textures and compositions of hydrothermal apatite grains have been reported in many iron oxide-apatite (IOA), porphyry, and rare-earth elements (REE) polymetallic deposits. However, such information for apatite in hydrothermal gold deposit is not available. In this study, we present integrated textural, elemental, and in situ oxygen isotopic data of hydrothermal apatite grains from the Hatu gold deposit (56t Au, average grade: 5 g/t) in the West Junggar, Northwest China, to investigate the detailed mineralization processes and nature of ore-forming fluids. Hydrothermal apatite is present within the quartz-sulfide ores at the Hatu gold deposit. Apatite grains of the Hatu gold ores contain abundant fluid inclusions and fine sulfide crystals, indicating that they are closely related to gold mineralization. Three types of hydrothermal fluorapatite (Ap1, Ap2, and Ap3) were revealed by Cathodoluminescence (CL) imaging. The Ap1 and Ap2 formed in the auriferous ore-forming stage, whereas the Ap3 formed in the post-ore stage. The three types of apatite have distinct total REE concentrations (216-751 ppm for Ap1, 14-120 ppm for Ap2, and 914-1422 ppm for Ap3) as well as chondrite-normalized REE distribution patterns. The textures and compositions of different apatites are inconsistent with dissolution-re-precipitation processes but instead point to three episodes of apatite formation in the Hatu hydrothermal system. The Ap3 has different REE and lower Eu/Eu* values (0.97-1.72) than Ap1 (1.34-4.50) and Ap2 (1.69-4.95), indicating a change of fluid compositions and a decrease of oxygen fugacity from the auriferous ore-forming stage to post-ore stage. All three types of apatite (Ap1, Ap2, and Ap3) show relatively high and consistent oxygen isotope compositions ranging from 14.2 ± 0.3â° to 15.2 ± 0.2â°, from 14.5 ± 0.2â° to 15.0 ± 0.3â°, and from 13.8 ± 0.3â° to 15.4 ± 0.2â°, respectively. Their calculated [delta].sup.18O.sub.fluid values are higher than magmatic-hydrothermal fluids but consistent with those from metamorphic fluids derived from meta-basalts. The near-zero [delta].sup.34S values of hydrothermal pyrite grains in the gold ores are much higher than those of pyrite framboid grains in the ore-hosted sedimentary rocks (- 41.1 to - 24.3â°), suggesting the sulfur may be derived from meta-basalts rather than sedimentary rocks in the mining area. Taken together, we suggest that Hatu is an orogenic gold deposit formed by fluids and metals derived from metamorphic de-volatilization of meta-basalts at depth. Our study highlights that a combination of CL imaging, in situ trace elements, and oxygen isotope compositions of hydrothermal apatite can be a novel and powerful tool to trace the nature of ore-forming fluids in the hydrothermal gold deposits., Author(s): Jiahao Zheng [sup.1] [sup.2] [sup.3], Ping Shen [sup.3], Wanyi Feng [sup.1] Author Affiliations: (1) grid.411604.6, 0000 0001 0130 6528, Zijin School of Geology and Mining, Fuzhou University, , 350108, [...]

Apatite-rich rocks from marine phosphorites, alkaline igneous complexes, and iron-oxide copper gold (IOCG) deposits commonly contain elevated but sub-economic contents of rare earth elements (REE). In this study, we selected fluorapatite from the Paleoproterozoic Yinachang IOCG deposit in Southwest China to evaluate the mechanisms of REE remobilization, transportation and precipitation during fluorapatite alteration. Four texturally and compositionally distinct types of fluorapatite were identified. Ap1, related to mantle-derived magmatic-hydrothermal fluid from dolerite intrusions, contains the highest (REE + Y) content (> 1 wt%). Ap2 was directly deposited from a magmatic-hydrothermal fluid related to Cu mineralization, rich in Cl and S but poor in REE + Y. Pervasive REE-rich mineral inclusions and micro-pores in the altered domains from Ap1 and Ap3-1 indicate that extensive metasomatic alteration occurred via a coupled dissolution-reprecipitation process, which was related to a late oxidizing hydrothermal fluid. After the end of the dissolution-reprecipitation process, Ap3-2 formed through regrowth of Ap3-1. Ap4 in late monazite-sulfide veins has extremely high positive Eu anomalies (24-32) and the highest Eu contents, which are related to a late reduced hydrothermal fluid. This study demonstrates that the geochemical and textural features of altered fluorapatite from the Yinachang IOCG deposit were controlled by both a sorption processes and the oxidation states of the hydrothermal fluids. The most salient feature in the Yinachang deposit is that REE remobilization associated with the alteration of fluorapatite by Neoproterozoic hydrothermal fluids is not strictly localized in extent, and REE in fluorapatite can also be transported in distance and precipitate as REE-rich minerals in independent mineralized veins., Author(s): Bing Xiao [sup.1] [sup.2], Yuanming Pan [sup.3], Hao Song [sup.4], Wenlei Song [sup.5], Yu Zhang [sup.6], Huayong Chen [sup.1] [sup.2] Author Affiliations: (1) grid.9227.e, 0000000119573309, Key Laboratory of Mineralogy [...]

Peralkaline granites and pegmatites are a prime repository of REE and HFSE, critical raw materials. Although it is accepted that magmatic processes are fundamental in concentrating these metals, the role of hydrothermal fluids in concentrating and fractionating these elements remains unclear. This paper investigates the global reproducibility of the magmatic-hydrothermal evolution of alkaline silica-saturated systems using alkali pyroxene and amphiboles from six alkaline complexes. These minerals contain significant amounts of REE and other HFSE, and pyroxene is stable throughout the magmatic and hydrothermal stages. Amphibole consists of mostly unzoned arfvedsonite, leakeite, and katophorite, while pyroxene is always aegirine. Two types of aegirine were defined. In all complexes, type-I aegirine is zoned; its core is enriched in Ca, REE, Zr, Hf, Sc and Sn, and the rims in Na, Fe.sup.3+ and contains secondary rare-metal bearing minerals and fluid inclusions. Type-II aegirine replaces amphibole and is oscillatory zoned. We interpret the amphiboles and REE-rich cores of type-I aegirine to have grown during the magmatic stage, whereas the rims of REE-poorer type-I and II aegirine are formed during the hydrothermal stage. During magmatic crystallization, REE intake into amphiboles and pyroxene as well as LREE-HREE fractionation were favored by their crystallographic properties and by competition among them and other minerals. During subsequent hydrothermal stages, REE and other HFSE were remobilized, locally reconcentrated and fractionated in mineral pseudomorphs and secondary pyroxene. These observations point out the importance of studying rock-forming minerals such as pyroxenes and amphiboles to unravel geological events controlled by common processes globally., Author(s): Cyrielle Bernard [sup.1], Guillaume Estrade [sup.1], Stefano Salvi [sup.1], Didier Béziat [sup.1], Martin Smith [sup.2] Author Affiliations: (1) grid.462928.3, 0000 0000 9033 1612, GET, CNRS, UPS, Université de Toulouse [...]

The 1370 Ma Gifford Creek Carbonatite Complex (GCCC) comprises a diverse suite of alkaline dyke and sill complexes that cover an area of ~ 250 km.sup.2 in the Gascoyne Province, Western Australia. Most carbonatite types are interpreted to be related products of fractional crystallisation, with calcite carbonatites representing cumulate rocks and dolomite carbonatites representing crystallised products of the derivative liquids. Genetic relationships between these carbonatites and other alkaline igneous units are less clear. The ankerite-siderite carbonatites and magnetite-biotite dykes are likely of related magmatic origin as both have distinctly high LREE and low HFSE contents. The ankerite-siderite carbonatites have mantle-like [delta].sup.13C isotope values of - 6.1 to - 7.1â° and similar geochemistry to other known magmatic ferrocarbonatites. Silica-rich alkaline veins found near the centre of the complex have trace element signatures that are antithetic to the magnetite-biotite dykes, so these veins are interpreted to represent products of alkali- and F-rich magmatic-hydrothermal fluids exsolved from the magnetite-biotite dykes during their emplacement. Carbon, O, Sr, and Nd isotope data are consistent with an enriched mantle source for the origin of the GCCC, with mantle enrichment likely caused by plate convergence processes associated with the c. 2.0 Ga Glenburgh Orogeny. There is no evidence to link mantle plume activity with formation of the GCCC; rather, alkaline magmatism is interpreted to result from low degree melting of the metasomatised mantle during reactivation of the crustal suture zone at 1370 Ma. The carbonatitic magmas utilised the Lyons River Fault to traverse the crust to be emplaced as the GCCC. Post magmatic alteration has variably modified the O and Sr isotope compositions of carbonates from these rocks. We therefore appeal for careful evaluation of isotopic data from ancient carbonatites, as isotopic resetting may be more common than currently recognised., Author(s): Paul Slezak [sup.1] [sup.2], Carl Spandler [sup.1] Author Affiliations: (1) 0000 0004 0474 1797, grid.1011.1, Department of Geoscience, College of Science and Engineering, James Cook University, , 4811, Townsville, [...]

Calcite veins are a common product of hydrothermal fluid circulation. Clumped-isotope palaeothermometry is a promising technique for fingerprinting the temperature of hydrothermal fluids, but clumped-isotope systematics can be reset at temperatures of > ca. 100 °C. To model whether the reconstructed temperatures represent calcite precipitation or closed-system resetting, the precipitation age must be known. LA-ICP-MS U-Pb dating of calcite is a recently developed approach to direct dating of calcite and can provide precipitation ages for modelling clumped-isotope systematics in calcite veins. In this study, clumped-isotope and LA-ICP-MS U-Pb calcite analyses were combined in basalt-hosted calcite veins from three settings in Scotland. Samples from all three localities yielded precipitation temperatures of ca. 75-115 °C from clumped-isotope analysis, but veins from only two of the sites were dateable, yielding precipitation ages of 224 ± 8 Ma and 291 ± 33 Ma (2[sigma]). Modelling from the dated samples enabled confident interpretation that no closed-system resetting had occurred in these samples. However, the lack of a precipitation age from the third location meant that a range of possible thermal histories had to be modelled meaning that confidence that resetting had not occurred was lower. This highlights the importance of coupling clumped-isotope thermometry and LA-ICP-MS U-Pb calcite dating in determining the temperature of hydrothermal fluids recorded in calcite veins. This paired approach is shown to be robust in constraining the timing and precipitation temperature of calcite formation, and thus for tracking hydrothermal processes., Author(s): J. M. MacDonald [sup.1], J. W. Faithfull [sup.2], N. M. W. Roberts [sup.3], A. J. Davies [sup.4], C. M. Holdsworth [sup.1], M. Newton [sup.1], S. Williamson [sup.1], A. Boyce [...]

We report on the presence of the serpentine-type antigorite in abyssal-serpentinized peridotite. At mid-ocean spreading ridges, antigorite crystallizes under retrograde metamorphic conditions during tectonic exhumation of the newly formed oceanic lithosphere. Using optical microscopy and micro-Raman spectroscopy, we identified antigorite in 49 samples drilled at the Hess Deep (East Pacific Rise) and the Atlantis Massif (Mid-Atlantic Ridge, 30°N), and dredged along the Southwest Indian Ridge (62°-65°E). Overall, antigorite is common, but occurs in limited modal amounts. SEM and TEM investigations reveal its frequent crystallization after lizardite and chrysotile via dissolution-recrystallization processes and a local association with olivine or talc. We explain antigorite crystallization by the interaction with seawater-derived hydrothermal fluids moderately enriched in silica (metasomatism). The origin of silica is attributed to alteration of mafic intrusions or pyroxenes. Antigorite can, therefore, be considered a marker of preferential fluid pathways under rock-dominated conditions during exhumation of a portion of the oceanic lithosphere. We also measured the in-situ major and trace-element composition of antigorite and the predating and postdating phases. Most of the elements are immobile during the mineralogical transitions. Other elements (Ni, Ca, Al, and Ti) evolve within the serpentine textures, including antigorite, as a result of chemical exchanges accompanying the development of the sequence of serpentine textures. A further category includes elements that are specifically enriched (Mn, Sn) or depleted (Fluid-Mobile Elements: B, Sr, As, U, Sb, and Cl) in antigorite compared to lizardite and chrysotile. These enrichments and depletions possibly reflect a change of the fluid physicochemical characteristics allowing a change in element mobility during the dissolution-recrystallization accommodating the lizardite/chrysotile-to-antigorite transition. Such depletion in FME is comparable to depletions described in studies of serpentinization and antigorite formation in subduction zone setting, which suggests that the origin of antigorite in some subducted samples could be reevaluated., Author(s): Stéphane Rouméjon [sup.1] [sup.2] , Muriel Andreani [sup.3] , Gretchen L. Früh-Green [sup.1] Author Affiliations: (Aff1) 0000 0001 2156 2780, grid.5801.c, Institute of Geochemistry and Petrology, ETH Zürich, , [...]

The age of gold-copper-lead mineralization in the Katuma Block of the Ubendian Belt remains controversial because of the lack of radiometric ages that correlate with the age of tectonothermal events of this poly-orogenic belt. Previous studies reported whole rock and mineral Pb-Pb ages ranging between 1,660 and 720 Ma. In this study, we report U-Th-total Pb ages of monazite from hydrothermally altered metapelites that host the Au-Cu-Pb-bearing veins. Three types of chemically and texturally distinct types of monazite grains or zones of grains were identified: monazite cores, which yielded a metamorphic age of 1,938 ± 11 Ma (n = 40), corresponding to known ages of a regional metamorphic event, deformation and granitic plutonism in the belt; metamorphic overgrowths that date a subsequent metamorphic event at 1,827 ± 10 Ma (n = 44) that postdates known eclogite metamorphism (at ca. 1,880 Ma) in the belt; hydrothermally altered poikilitic monazite, formed by dissolution-precipitation processes, representing the third type of monazite, constrain the age of a hydrothermal alteration event at 1,171 ± 17 Ma (n = 19). This Mesoproterozoic age of the hydrothermal alteration coincides with the first amphibolite grade metamorphism of metasediments in the Wakole Block, which adjoins with a tectonic contact the vein-bearing Katuma Block to the southwest. The obtained distinct monazite ages not only constrain the ages of metamorphic events in the Ubendian Belt, but also provide a link between the metamorphism of the Wakole metasediments and the generation of the hydrothermal fluids responsible for the formation of the gold-copper-lead veins in the Katuma Block. Electronic supplementary material The online version of this article (doi:10.1007/s00410-014-1088-1) contains supplementary material, which is available to authorized users., Author(s): Emmanuel Owden Kazimoto[sup.1] [sup.2] , Volker Schenk[sup.1] , Peter Appel[sup.1] Author Affiliations: (1) Institut für Geowissenschaften, Christian-Albrechts-Universität Kiel, Ludewig-Meyn-Str. 10, 24118, Kiel, Germany (2) Department of Geology, University of [...]

The Iceland Deep Drilling Project Well 1 was designed as a 4- to 5-km-deep exploration well with the goal of intercepting supercritical hydrothermal fluids in the Krafla geothermal field, Iceland. The well unexpectedly drilled into a high-silica (76.5% Si[O.sub.2]) rhyolite melt at approximately 2.1 km. Some of the melt vesiculated while extruding into the drill hole, but most of the recovered cuttings are quenched sparsely phyric, vesicle-poor glass. The phenocryst assemblage is comprised of titanomagnetite, plagioclase, augite, and pigeonite. Compositional zoning in plagioclase and exsolution lamellae in augite and pigeonite record changing crystallization conditions as the melt migrated to its present depth of emplacement. The in situ temperature of the melt is estimated to be between 850 and 920 °C based on two-pyroxene geothermometry and modeling of the crystallization sequence. Volatile content of the glass indicated partial degassing at an in situ pressure that is above hydrostatic (~16 MPa) and below lithostatic (~55 MPa). The major element and minor element composition of the melt are consistent with an origin by partial melting of hydrothermally altered basaltic crust at depth, similar to rhyolite erupted within the Krafla Caldera. Chondrite-normalized REE concentrations show strong light REE enrichment and relative flat patterns with negative Eu anomaly. Strontium isotope values (0.70328) are consistent with mantle-derived melt, but oxygen and hydrogen isotope values are depleted (3.1 and -118 ‰, respectively) relative to mantle values. The hydrogen isotope values overlap those of hydrothermal epidote from rocks altered by the meteoric-water-recharged Krafla geothermal system. The rhyolite melt was emplaced into and has reacted with a felsic intrusive suite that has nearly identical composition. The felsite is composed of quartz, alkali feldspar, plagioclase, titanomagnetite, and augite. Emplacement of the rhyolite magma has resulted in partial melting of the felsite, accompanied locally by partial assimilation. The interstitial melt in the felsite has similar normalized Si[O.sub.2] content as the rhyolite melt but is distinguished by higher [K.sub.2]O and lower CaO and plots near the minimum melt composition in the granite system. Augite in the partially melted felsite has re-equilibrated to more calcic metamorphic compositions. Rare quenched glass fragments containing glomeroporphyritic crystals derived from the felsite show textural evidence for resorption of alkali feldspar and quartz. The glass in these fragments is enriched in Si[O.sub.2] relative to the rhyolite melt or the interstitial felsite melt, consistent with the textural evidence for quartz dissolution. The quenching of these melts by drilling fluids at in situ conditions preserves details of the meltwall rock interaction that would not be readily observed in rocks that had completely crystallized. However, these processes may be recognizable by a combination of textural analysis and in situ analytical techniques that document compositional heterogeneity due to partial melting and local assimilation. Keywords Krafla * Iceland * Geothermal * Rhyolite * Basalt partial melting * Stable isotope * Strontium isotope, Introduction The Iceland Deep Drilling Project (IDDP) is an industry-government consortium with the goal of exploring for supercritical hydrothermal fluids that could be utilized for energy production (Frioleifsson and Elders [...]

Zircon occurs in voids and cracks in phosphatic coprolites enclosed in siderite concretions in Mississippian shales near Edinburgh, Scotland. The zircon formed during hydrothermal alteration of early-diagenetic concretions and occurs as spherical aggregates of prismatic crystals, sometimes radiating. Vitrinite reflectance measurements indicate temperatures of ~270°C for the zircon-bearing concretions and the host shales. Molecular parameter values based on dibenzothiophene and phenanthrene distribution and occurrence of di- and tetra-hydro-products of polycyclic aromatic compounds suggest that the rocks experienced relatively high-temperature aqueous conditions related to hydrothermal fluids, perhaps associated with neighboring mafic intrusions. The zircon was dissolved from the concretions, transported in fluids, and reprecipitated in voids. This is the first record of the precipitation of authigenic zircon in sedimentary rock as a new phase, not as outgrowths. Keywords Zircon * Concretions * Diagenesis * Hydrothermal alteration * Vitrinite reflectance * PAHs, Introduction Zircon can crystallize under pressure (P)-temperature (T) conditions ranging from those in the mantle to the low P and T pertaining to diagenesis. Low-temperature zircon growth, including examples from [...]

Byline: Bin Fu (1,2), John W. Valley (1), Noriko T. Kita (1), Michael J. Spicuzza (1), Chad Paton (2), Tatsuki Tsujimori (3), Michael Brocker (4), George E. Harlow (5) Keywords: Zircon; Jadeite; Hydrothermal; Oxygen isotopes; SIMS; Greece; Japan; Guatemala; California Abstract: Jadeitites form from hydrothermal fluids during high pressure metamorphism in subduction environments however, the origin of zircons in jadeitite is uncertain. We report ion microprobe analyses of [delt[a].sup.18]O and Ti in zircons, and bulk [delt[a].sup.18]O data for the jadeitite whole-rock from four terranes: Osayama serpentinite melange, Japan Syros melange, Greece the Motagua Fault zone, Guatemala and the Franciscan Complex, California. In the Osayama jadeitite, two texturally contrasting groups of zircons are identified by cathodoluminescence and are distinct in [delt[a].sup.18]O: featureless or weakly zoned zircons with [delt[a].sup.18]O = 3.8 +- 0.6[per thousand] (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher [delt[a].sup.18]O = 5.0 +- 0.4[per thousand]. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar [delt[a].sup.18]O values to the latter from Osayama: Guatemala zircons are 4.8 +- 0.7[per thousand], and the Syros zircons are 5.2 +- 0.5[per thousand] in jadeitite and 5.2 +- 0.4[per thousand] in associated omphacitite, glaucophanite and chlorite-actinolite rinds. The [delt[a].sup.18]O values for most zircons above fall within the range measured by ion microprobe in igneous zircons from oxide gabbros and plagiogranites in modern ocean crust (5.3 +- 0.8[per thousand]) and measured in bulk by laser fluorination of zircons in equilibrium with primitive magma compositions or the mantle (5.3 +- 0.6[per thousand]). Titanium concentrations in these zircons vary between 1 and 19 ppm, within the range for igneous zircons worldwide. Values of [delt[a].sup.18]O (whole-rock) a [delt[a].sup.18]O (jadeite) and vary from 6.3 to 10.1[per thousand] in jadeitites in all four areas. These values of [delt[a].sup.18]O and Ti are higher than predicted for hydrothermal zircons, and the [delt[a].sup.18]O values of most zircons are not equilibrated with the coexisting jadeite at reasonable metamorphic temperatures. We conclude that while some zircons may be hydrothermal in origin, a majority of the zircons studied are best explained as relic igneous crystals inherited from precursor rocks they were not precipitated directly from hot aqueous fluids as previously assumed. Therefore, U--Pb ages from these zircons may date magmatic crystallization and do not establish the timing of high pressure metamorphism or hydrothermal activity. Author Affiliation: (1) WiscSIMS, Department of Geology and Geophysics, University of Wisconsin, Madison, WI, 53706-1692, USA (2) School of Earth Sciences, The University of Melbourne, Parkville, VIC, 3010, Australia (3) Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori, 682-0193, Japan (4) Institut fur Mineralogie, Universitat Munster, 48149, Munster, Germany (5) Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY, 10024-5192, USA Article History: Registration Date: 05/10/2009 Received Date: 17/05/2009 Accepted Date: 05/10/2009 Online Date: 24/10/2009 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0453-y) contains supplementary material, which is available to authorized users.

Hydrothermal circulation at slow-spreading ridges is important for cooling the newly formed lithosphere, but the depth to which it occurs is uncertain. Magmas which stagnate and partially crystallize during their rise from the mantle provide a means to constrain the depth of circulation because assimilation of hydrothermal fluids or hydrothermally altered country rock will raise their chlorine (Cl) contents. Here we present Cl concentrations in combination with chemical thermobarometry data on glassy basaltic rocks and melt inclusions from the Southern Mid-Atlantic Ridge (SMAR; ~ 3 cm year.sup.-1 full spreading rate) and the Gakkel Ridge (max. 1.5 cm year.sup.-1 full spreading rate) in order to define the depth and extent of chlorine contamination. Basaltic glasses show Cl-contents ranging from ca. 50-430 ppm and ca. 40-700 ppm for the SMAR and Gakkel Ridge, respectively, whereas SMAR melt inclusions contain between 20 and 460 ppm Cl. Compared to elements of similar mantle incompatibility (e.g. K, Nb), Cl-excess (Cl/Nb or Cl/K higher than normal mantle values) of up to 250 ppm in glasses and melt inclusions are found in 75% of the samples from both ridges. Cl-excess is interpreted to indicate assimilation of hydrothermal brines (as opposed to bulk altered rock or seawater) based on the large range of Cl/K ratios in samples showing a limited spread in H.sub.2O contents. Resorption and disequilibrium textures of olivine, plagioclase and clinopyroxene phenocrysts and an abundance of xenocrysts and gabbroic fragments in the SMAR lavas suggest multiple generations of crystallization and assimilation of hydrothermally altered rocks that contain these brines. Calculated pressures of last equilibration based on the major element compositions of melts cannot provide reliable estimates of the depths at which this crystallization/assimilation occurred as the assimilation negates the assumption of crystallization under equilibrium conditions implicit in such calculations. Clinopyroxene-melt thermobarometry on rare clinopyroxene phenocrysts present in the SMAR magmas yield lower crustal crystallization/assimilation depths (10-13 km in the segment containing clinopyroxene). The Cl-excesses in SMAR melt inclusions indicate that assimilation occurred before crystallization, while also homogeneous Cl in melts from Gakkel Ridge indicate Cl addition during magma chamber processes. Combined, these observations imply that hydrothermal circulation reaches the lower crust at slow-spreading ridges, and thereby promotes cooling of the lower crust. The generally lower Cl-excess at slow-spreading ridges (compared to fast-spreading ridges) is probably related to them having few if any permanent magma chambers. Magmas therefore do not fractionate as extensively in the crust, providing less heat for assimilation (on average, slow-spreading ridge magmas have higher Mg#), and hydrothermal systems are ephemeral, leading to lower total degrees of crustal alteration and more variation in the amount of Cl contamination. Hydrothermal plumes and vent fields have samples in close vicinity that display Cl-excess, mostly of > 25 ppm, which thus can aid as a guide for the exploration of (active or extinct) hydrothermal vent fields on the axis., Author(s): Froukje M. van der Zwan [sup.1] [sup.6] , Colin W. Devey [sup.1] , Thor H. Hansteen [sup.1] , Renat R. Almeev [sup.2] , Nico Augustin [sup.1] , Matthias Frische [...]

Metastable intermediate Na-K mica represents a product of hydrothermal alteration in volcanic rocks from the alteration halo of the Waterloo massive sulfide deposit, Australia. The XRD pattern of this solid solution between paragonite and muscovite is characterized by a rational series of basal reflections with d values intermediate between the end members. Transmission electron microscopy revealed that the intermediate N-K mica forms thick stacks that belong to a two-layer polytype. Na-rich intermediate N-K mica typically occurs together with paragonite whereas K-rich intermediate Na-K mica is intergrown with muscovite. The intermediate Na K mica is interpreted to have formed as a result of the incomplete transformation of K-rich mica to Na-rich mica through dissolution and recrystallization processes driven by compositional changes of the hydrothermal fluids interacting with the volcanic rocks. Alteration must have proceeded under non-equilibrium conditions because the composition of the solid solution falls into the miscibility gap separating paragonite and muscovite.

The age of gold-copper-lead mineralization in the Katuma Block of the Ubendian Belt remains controversial because of the lack of radiometric ages that correlate with the age of tectonothermal events of this polyorogenic belt. Previous studies reported whole rock and mineral Pb-Pb ages ranging between 1,660 and 720 Ma. In this study, we report U-Th-total Pb ages of monazite from hydrothermally altered metapelites that host the Au-Cu-Pb-bearing veins. Three types of chemically and texturally distinct types of monazite grains or zones of grains were identified: monazite cores, which yielded a metamorphic age of 1,938 ± 11 Ma (n = 40), corresponding to known ages of a regional metamorphic event, deformation and granitic plutonism in the belt; metamorphic overgrowths that date a subsequent metamorphic event at 1,827 ± 10 Ma (n = 44) that postdates known eclogite metamorphism (at ca. 1,880 Ma) in the belt; hydrothermally altered poikilitic monazite, formed by dissolution-precipitation processes, representing the third type of monazite, constrain the age of a hydrothermal alteration event at 1,171 ± 17 Ma (n = 19). This Mesoproterozoic age of the hydrothermal alteration coincides with the first amphibolite grade metamorphism of metasediments in the Wakole Block, which adjoins with a tectonic contact the vein-bearing Katuma Block to the southwest. The obtained distinct monazite ages not only constrain the ages of metamorphic events in the Ubendian Belt, but also provide a link between the metamorphism of the Wakole metasediments and the generation of the hydrothermal fluids responsible for the formation of the goldcopper-lead veins in the Katuma Block. Keywords: Monazite dating * Kibaran orogeny * Crustal thickening * Orogenic gold mineralization * Hydrothermally altered monazite, Introduction Despite many attempts and its importance in exploration, dating of hydrothermal gold mineralization in orogenic belts is still a challenge due to the scarcity of minerals formed during mineralization [...]

The Komsomolskaya kimberlite is one of numerous (> 1,000) kimberlite pipes that host eclogite xenoliths on the Siberian craton. Eclogite xenoliths from the adjacent Udachnaya kimberlite pipe have previously been geochemically well characterized; however, data from surrounding diamond-bearing kimberlite pipes from the center of the craton are relatively sparse.Here, wereportmajor-and trace-element data, as well as oxygen isotope systematics, for mineral separates of diamondiferous eclogite xenoliths from the Komsomolskaya kimberlite, suggesting two distinct subgroups of a metamorphosed, subducted oceanic crustal protolith. Using almandine contents, this suite can be divided into two subgroups: group B1, with a high almandine component (> 20 mol%) and group B2, with a low almandine component ( Keywords SCLM, Eclogites, Siberian craton, Mantle xenolith, TTG complex, Introduction Eclogites make up a minor (< 2 %), but important, component in mantle xenolith suites and are transported to the surface by kimberlite magmas; as such, these eclogites represent [...]