Late Magmatic Fluids Research Articles

The Penikat Intrusion, Finland: Geochemistry, Geochronology, and Origin of Platinum–Palladium Reefs

The Palaeoproterozoic Penikat layered ultramafic–mafic intrusion in northern Finland is one of the most richly mineralized layered intrusions on Earth, containing at least six platinum-group element (PGE) enriched horizons exposed along >20 km of strike, amongst them the SJ reef, which at ∼3–7 ppm Pt + Pd over a width of ∼1–2 m is surpassed by few other PGE reefs globally in terms of its endowment in PGE. Important PGE enrichments also occur in the PV reef (average 2·6 ppm Pd, 4 ppm Pt over 1·1 m) and AP1 reef (average 6·2 ppm Pd, 1·7 ppm Pt over 0·7 m). Here we present new major and high-precision trace element and Nd isotope data from a traverse across the intrusion, and a new U–Pb age of 2444 ± 8 Ma for the intrusion. We show that the PGE reefs formed by predominantly orthomagmatic processes as, for example, reflected by well-defined positive correlations between Pt + Pd and Os + Ir + Ru contents. Late-magmatic fluids played no significant role in concentrating PGE. There are at least six cyclic units in the intrusion, displaying a progressive upward decrease in differentiation indices Mg# and Cr/V. Subdued stratigraphic variations in incompatible trace element ratios (Ce/Sm mostly 5–10) and Nd isotope compositions (eNd –3 to –1) indicate that mixing of magmas of distinct lineage, or in situ contamination with country rocks, was not required to form the PGE reefs. There is also no evidence for addition of external sulphur to the magma, based on S/Se ratios at, or below, primitive mantle levels. Instead, we propose that sulphide melt saturation at Penikat was reached in response to fractionation of a siliceous, high-magnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes.

Fe-Ti oxide micro-inclusions in clinopyroxene of oceanic gabbro: Phase content, orientation relations and petrogenetic implication

Fe-Ti-oxide micro-inclusions in clinopyroxene of oceanic gabbro from the mid Atlantic ridge have been studied using electron backscatter diffraction and electron probe microanalyses. A first generation of Fe-Ti-oxide inclusions occurs as needles or elongated plates lying in the (010) plane of the clinopyroxene host. The inclusions show distinct elongation directions following “irrational” planes either nearly parallel to the “c” or nearly parallel to the “a” axis of the clinopyroxene host. The habit planes correspond to inclusion-host interfaces, where densely packed oxygen layers of the inclusion and host phases are coherent across the interface. Both inclusion types have distinct crystallographic orientation relations to the host, which are determined by the nearly parallel alignment of densely packed oxygen layers in the inclusions and the clinopyroxene host. Based on the angle between the two elongation directions primary formation of the inclusion at 800° to 900°C was inferred. Initially, homogeneous Ti-bearing magnetite was precipitated together with titanian pargasite lamellae due to reaction of early magmatic clinopyroxene with late magmatic fluid or melt. After cooling below 600°C the Ti-magnetite decomposed into an oriented magnetite+ulvospinel intergrowth. Late stage hydrothermal alteration leads to the corrosion of the magnetite-ulvospinel inclusions and partial replacement by ilmenite as well as newly precipitated homogeneous ilmenite plates that are closely associated with actinolite lamellae lying in the (100) plane of the clinopyroxene host.

Britholite-group minerals as sensitive indicators of changing fluid composition during pegmatite formation: evidence from the Keivy alkaline province, Kola peninsula, NW Russia

The Keivy alkaline province, Kola Peninsula, NW Russia, consists of vast alkali granite massifs and several dike-like nepheline syenite bodies. It contains numerous rare-metal occurrences, formed by a complex sequence of magmatic, late-magmatic and post-magmatic (including pegmatitic) processes. The Sakharjok nepheline syenite pegmatite contains a remarkably diverse number of britholite group minerals, pointing to different physico-chemical conditions in the fluid. REE and actinides distribution in the host rock indicates that the late-magmatic (and pegmatitic) fluids were alkaline, with significant amounts of F and CO2. From REE and F variations of the britholite group minerals possible fluid compositions at different stages are suggested. The earliest fluorbritholite-(Ce) formed locally from a late magmatic, high temperature F-rich fluid. Fluorbritholite-(Y) presumably crystallized from a F-bearing and CO2-rich fluid; marked F saturation resulted in precipitation of abundant fluorite due to a temperature drop. Variations in REE and F contents in the most abundant fluorcalciobritholite indicate a successive decrease of F in the fluid during its evolution. The relationship between intergrown fluorapatite and fluorcalciobritholite and the presence of zones with a REE-rich fluorapatite between them indicate a continuous to sudden crystallization in this mineral sequence. The сrystallization of the latest “calciobritholite” is related to the input into the fluid of CO2 and/or H2O.

Magmatic–hydrothermal evolution of highly fractionated granites: Evidence from the Kuiqi miarolite in Fujian province, SE China

The Kuiqi miarolite, a highly evolved alkaline granite pluton, is situated in Fujian province, southeastern China. Quartz crystals from the Kuiqi miarolite contain abundant inclusions, which have preserved a complete record of the magmatic–hydrothermal evolution. Five inclusion type were identified, i.e., the melt (M-type), melt–fluid (ML-type), daughter mineral-bearing (S-type), vapor-rich (V-type), and aqueous (L-type) inclusions. Integrated petrographic, microthermometric and laser Raman spectroscopy studies demonstrate that the whole magmatic–hydrothermal evolution at Kuiqi comprises three stages, i.e., the magmatic, magmatic–hydrothermal and hydrothermal stages. The M-type inclusions (minimal trapping temperatures: ca. 710–800°C) represent the melts entrapped at the magmatic stage of crystallization, while the occurrence of euhedral prismatic arfvedsonites in these melt inclusions indicates that the granitic magma was probably volatile-rich. At the magmatic–hydrothermal stage, both the ML-type inclusions (Th=ca. 500–710°C) and coexisting M- and V-type inclusions reflect the presence of late magmatic fluid circulation and melt-volatile immiscibility, exhibiting a transition toward the hydrothermal stage. At the hydrothermal stage, the pressure and temperature (700–485°C) drop may have led to oversaturation of the primary saline fluids, resulting in the phase separation a highly saline and a volatile-rich fluid occurred, accompanied by the widespread vapor–brine immiscibility. The L-type inclusions in the miarolitic quartz display bimodal salinity distribution: The Th and salinity of the inclusions at the bottom (L1) miarolitic quartz are ca. 303–427°C and ca. 6.6–17.9wt.% NaCl equiv, respectively, while that of the inclusions at the top (L2) are ca. 165–232°C and ca. 0.2–5.7wt.% NaCl equiv, respectively. This indicates a drop of fluid temperature and salinity during the miarolitic cavity formation. Moreover, the low homogenization temperatures and salinities of the L-type inclusions suggest an influx of meteoric water during the late hydrothermal evolution stage.

Zirconosilicates in the kakortokites of the Ilímaussaq complex, South Greenland: Implications for fluid evolution and high-field-strength and rare-earth element mineralization in agpaitic systems

AbstractThe layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.

The origin of the mineralizing fluids in different type mineralizations associated with the Upper Cretaceous Elazig Magmatic Complex, Turkey; an isotopic approach

This study examined the origin and properties of mineralized fluids by using C, O and S isotopes in different type mineralizations associated with the Upper Cretaceous Elazig Magmatic Complex. The isotopic compositions of vein type mineralizations show that the thrust zone affects the formation of the Karakas iron mineralization by meteoric and magmatic hydrothermal solution mixtures due to the average δ18OH2O value 6.40‰. The calculated δ18OH2O composition values is 5.20‰ in biotite from the Kızıldag vein type Cu–Pb–Zn mineralizations, which is consistent with a magmatic origin of the fluids. The calculated δ18OH2O composition is 4.30‰ that indicates a medium and low temperature magmatic hydrothermal fluid effect. The skarn type mineralizations isotopic compositions indicate that the calculated δ13CCO2 values are between −12.70‰ and −36.39‰ that could be late magmatic fluids that were modified by interaction with the host meta-sedimentary rocks and with meteoric water at the Birvan and Asvan iron mineralizations. Also the δ18OH2Ovalues in quartz of the Meseli iron mineralization are between 0.70‰ and 1.30‰. The lower δ18OH2O oxygen isotope composition compared to magmatic origins must be hydrothermal solutions mixing with meteoric waters. In the massive sulfide type Kavallı and Derince pyrite samples, δ34SH2S values are between 17.73‰ and 20.63‰. These values clearly indicate the volcano-sedimentary effect on hydrothermal solutions, which form the mineralization. The first findings of this study present information that all of the measured isotopic composition was modified by mixing metamorphic, magmatic and meteoric waters in the final stages of the hydrothermal solutions circulation.

Age, petrochemistry, and origin of a REE-rich mineralization in the Longs Peak-St. Vrain batholith, near Jamestown, Colorado (U.S.A.)

An unusual rare earth element (REE) mineralization occurs at a locality known as the “Rusty Gold” within the anorogenic 1.4 Ga Longs Peak-St. Vrain monzo- to syenogranite Silver Plume-type intrusion near Jamestown, Colorado (U.S.A.). Irregular-shaped centimeter- to decimeter-sized mineralized pods and veins consist of zoned mineral assemblages dominated by fluorbritholite-(Ce) in a gray-colored core up to 10 cm thick, with monazite-(Ce), fluorite, and minor quartz, uraninite, and sulfides. The core zone is surrounded by a black, typically millimeter-thick allanite-(Ce) rim, with minor monazite-(Ce) in the inner part of that rim. Bastnasite-(Ce), tornebohmite-(Ce), and cerite-(Ce) appear in a thin intermediate zone between core and rim, often just a few hundreds of micrometers wide. Electron microprobe analyses show that the overall REE content increases from rim to core with a disproportionate increase of heavy REE (∑ HREE increases 10-fold from 0.2 to 2.1%) compared to light REE (∑ LREE increases twofold from 21.3 to 44.3%). The fluorbritholite-(Ce) contains minor U, Th, Fe, Mn, and Sr (total 0.10 apfu), with Al, Mg, Na, K, Ti, Pb, S, and Cl below instrument detection limits. Cerite-(Ce) is a minor constituent of the thin zone between the inner rim and the core. The cerite-(Ce) is Fe-rich with low Ca, and minor Al, Mg, and Mn, whereas tornebohmite-(Ce) is Al-rich and Ca-poor. Monazite-(Ce) and uraninite U-Th-Pb microprobe ages yield 1.420(25) and 1.442(8) Ga, respectively, confirming a co-genetic relationship with the host ca. 1.42(3) Ga Longs Peak-St. Vrain granite. We suggest the origin of the REE mineralization is a F-rich and lanthanide-rich, either late-magmatic hydrothermal fluid or residual melt, derived from the granite. This late-stage liquid, when becoming progressively enriched in REE as it crystallized, could explain the observed concentric mineralogical and geochemical zoning.

China: Tosaf/Colloids – masterbatch

Ion adsorption deposits, in which the rare earth elements (REE) occur adsorbed onto clay mineral surfaces, currently provide the world’s dominant supply of heavy REE (Gd-Lu). Concentration of REE within ion adsorption deposits has been proposed to be a dominantly supergene process, where easily degradable REE-minerals (e.g. REE-fluorcarbonates) break down and release REE that are then adsorbed onto clay mineral surfaces in the weathered material. Here we present data from the Cenozoic Ambohimirahavavy alkaline complex in Madagascar, with the aim of further constraining controls on the formation and HREE enrichment processes in ion adsorption deposits.The laterite weathering profiles described here are developed on alkaline igneous rocks, including both SiO2-undersaturated and oversaturated lithologies. The latter group includes REE mineralised peralkaline granitic pegmatites and granitic dykelets. The weathering mineralogy includes gibbsite, clay minerals and minor Fe and Mn oxyhydroxides. X-ray diffraction and infrared spectroscopy show that the clay fraction in all sites is dominated by kaolinite and halloysite (7 Å and 10 Å). Extraction with ammonium sulfate (0.5 M (NH4)2SO4, pH = 4; removes weakly adsorbed metals retained on the solid surface and dissolves carbonates) indicates that, in the different profiles and along the same profile, the leachable REE content is heterogeneous, ranging from 5 to 2300 mg/kg total REE, with 1 to 32% heavy REE. Leaching with magnesium chloride (0.5 M MgCl2, pH = 6; removes only weakly adsorbed REE) releases similar total REE concentrations, suggesting that most of the leachable REE are adsorbed onto mineral surfaces. In most of the laterite profiles, the amount of leachable REE continuously increases with depth up to the saprock. Recovery rates depend on the REE atomic number, generally decreasing from La to Lu except for Ce which is invariably low.In the Ambohimirahavavy complex, the nature of the protolith is the main factor controlling the amount of easily leachable REE in the laterite weathering profile. Hydrology and topography are secondary factors. The most favourable protoliths include SiO2-undersaturated volcanic lithologies and altered mudstone with granitic dykelets. The main primary REE minerals include agpaitic minerals (eudialyte) and allanite-(Ce). Locally in granitic pegmatite dykes, autometasomatism by late magmatic fluids inhibits formation of ion adsorption ore by transforming easily weathered agpaitic minerals into unweatherable zircon.

Origin of peralkaline granites of the Jurassic Bokan Mountain complex (southeastern Alaska) hosting rare metal mineralization

The Jurassic Bokan Mountain complex (BMC), composed of arfvedsonite and/or aegirine-bearing peralkaline A-type granitic rocks, is a circular body about 3 km in diameter located in southeastern Alaska. Like many other highly fractionated granitic bodies, the BMC granites were affected by late magmatic or post-magmatic processes, which, however, did not modify the contents of major elements. The granitic rocks are distinctly enriched in high-field-strength elements (HFSEs), rare earth elements (REEs), Y, Th, and U but depleted in Ba, Sr, and Eu and have high positive ɛNd(T) values. Unlike the variations in the major elements, Sr and Ba, which can be accounted by fractional crystallization, the abundance of REE, Y, HFSE, U, and Th (the elements which are hosted in accessory phases) were modified by F-rich hydrothermal fluids. The BMC hosts significant rare metal mineralization related to the late-stage crystallization history of the complex involving late magmatic and/or post-magmatic fluids. The mineralization includes two types: (1) a U–Th deposit which was exploited at the former Ross-Adams mine and (2) REE and Y mineralization mostly hosted in felsic dikes. Thermodynamic modelling of granites and spatially associated mafic rocks using the programme Rhyolite-MELTS implies that the granites can be derived from the mafic rocks by fractional crystallization. It is suggested that such a process (i.e. derivation of peralkaline granitic magma from the alkali or transitional basaltic magmas derived by partial melting from a lithospheric source metasomatically enriched in rare metals) can be invoked for other peralkaline granitic rocks hosting rare metal deposits.

New evidences of hydrothermal fluids circulation at the Devonian Kess Kess mounds, Hamar Laghdad (eastern Anti‐Atlas, Morocco)

New evidences based on a combination of field and laboratory investigations reinforce the hypotheses that the circulation of warm fluids has remarkably contributed to the origin and development of the Devonian Kess Kess mounds of the Hamar Laghdad Ridge (eastern Anti‐Atlas, Morocco). The limestones of the Hamar Laghdad Ridge were deposited above a structural high generated by calc‐alkaline volcanic activity that has probably fuelled the circulation of warm fluids throughout the overlying carbonate units. The geological and palaeontological attributes described throughout the succession of the Hamar Laghdad Ridge (from the Lochkovian to Frasnian intervals) are interpreted as the result of hydrothermal processes related to a volcanic system. In particular, these attributes seem consistent with a chemo‐physical environment fuelled by the circulation of warm and late magmatic fluids. These attributes include a very low oxygen stable isotope signature (δ18O ~ −10‰) for carbonates. Evidences for a late magmatic fluid circulation consist of volcanic glass and pyroclasts replacement with hydrothermal minerals such as quartz, anatase and clinochlore. Fluids circulating through veins and pores into sediments, and venting to the seafloor, probably induced the formation of cavities where monospecific trilobite communities were detected. The partially silicified trilobite remains are associated with traces of goethite. This iron‐bearing oxide mineral is also present in the upper part of the Hamar Laghdad Ridge. All these attributes are here interpreted as possible evidences for a low‐temperature hydrothermal venting system active during the Lochkovian–Frasnian time span. This study combines an updated revision with new petrographic, geological and geochemical results aimed at providing an overall framework on the origin and early diagenesis of the Devonian succession of Hamar Laghdad. Copyright © 2014 John Wiley & Sons, Ltd.

Oxygen isotope ratio of quartz veins from the auriferous Ramagiri–Penakacherla schist belt and surrounding granitoids in the Eastern Dharwar craton: A case for a possible link between gold mineralization and granite magmatism

In the Eastern Dharwar craton, among the many shear zone-hosted lode gold deposits, those at Ramagiri and Penakacherla are located near the western margin of the craton. Mineralized quartz (±sulfide±carbonate) veins are hosted by the schistose (metavolcanic and carbonaceous metasedimentary) rocks, in close spatial association with granitoids having quartz and quartzofeldspathic veins representing hydrothermal activities associated with them. Mineralized quartz veins from the ore zones (in Ramagiri and Penakacherla regions) and quartz (or pegmatitic) veins from the surrounding granitic terrane were chosen for δ18O analysis. Samples from the schistose and granitic domains show δ18Oquartz values in the range of 10.4–14.9 and 9.3–10.9‰ respectively. The ore-zone fluids from the Ramagiri and Penakacherla regions give δ18O values of 7.9±1.5 and 5.1±0.8‰, calculated at pressure-corrected temperatures obtained from fluid inclusion microthermometry. The late-magmatic fluid is relatively 18O-poor with δ18O values estimated at 4.5±0.7‰ and the value is closer to what is obtained for the ore zones. Based on the δ18O values reported and a possible magmatic contribution to ore fluid deciphered from fluid inclusion characteristics, a genetic relationship between granitic magmatism and gold mineralization is surmised. The observed increase in the 18O/16O ratio from the magmatic fluid to ore fluid in the shear zone is attributed to interaction of the magmatic fluid with host metasediments, that agrees well with the variation in the CO2/CH4 ratio of carbonic component in such fluids.

Mineralogy and Geochemistry of Cu-Rich Ores from the McCreedy East Ni-Cu-PGE Deposit (Sudbury, Canada): Implications for the Behavior of Platinum Group and Chalcophile Elements at the End of Crystallization of a Sulfide Liquid

Copper-rich massive sulfides are an important source of Pt and Pd in magmatic Ni-Cu-platinum group elements (PGE) ore deposits. At the McCreedy East deposit, Sudbury, they constitute a classic magmatic assemblage of chalcopyrite, cubanite ± pentlandite, located in sharp-walled footwall veins. These Cu-rich ores represent the subsolidus (<600°C) exsolution products of intermediate solid solution (ISS) that crystallized (950°–800°C) from a highly fractionated sulfide liquid rich in Pt, Pd, Ag, As, Bi, Cd, Pb, Se, Sn, Te, and Zn. Laser ablation-inductively coupled plasma-mass spectrometry and scanning electron microscope analyses have revealed the distribution of PGE and trace elements in these ores, which is important for a better understanding of the petrogenesis of Cu-rich sulfide deposits and to improve PGE extraction. Chalcopyrite and cubanite are the dominant hosts of Se and Sn, with Co in pentlandite. Lead is hosted by galena and Zn and Cd by sphalerite, with only a small proportion of these elements present at trace level in ISS (now equally distributed between chalcopyrite and cubanite). Platinum, Pd, Au, As, Bi, and Te, however, are not concentrated in the base metal sulfides and are accounted for almost entirely by the platinum group minerals (PGM) assemblage, which is dominated by Pt-Pd-Bi-Te phases, such as michenerite [(Pt,Pd)BiTe] and froodite [PdBi2], with minor sperrylite [PtAs2] and Sn-bearing PGM, such as niggliite [PtSn], paolovite [Pd2Sn], and an unnamed Pt-Sn-Te phase. The PGM form complex, composite grains hosted at the grain boundaries of the base metal sulfides. They typically comprise a core of Sn- or As-bearing PGM (stable at higher temperatures) hosted in Bi-Te-PGM (lower temperatures), which are commonly surrounded by accessory tellurides (Ag, Bi, Pb bearing) and sulfides (galena, sphalerite, and stannite [Cu2FeSnS4]). Primary chloride minerals such as cottunite [PbCl2] and ferropyrosmalite [(Fe,Mn)8Si6O15(OH,Cl)10] also form composite grains with hessite [Ag2Te] and galena. In contrast to much of the previous work at Sudbury, which has invoked the role of late-magmatic and/or hydrothermal fluids in the collection of precious metals, we show that, in this case, PGM in Cu-rich ore have a magmatic origin. Due to the incompatibility of Pt, Pd, Bi, and Te during the crystallization of ISS, these elements became concentrated in a small volume of late-stage S-bearing melt trapped between intermediate solid solutions. A sequence of PGM, followed by accessory tellurides and sulfides, crystallized from this late-stage melt and formed composite grains. Toward the end of crystallization, the small amount of Cl that was soluble in the sulfide liquid crystallized as primary chloride minerals at low temperatures (<500°C), either from the late-stage melt or from an exsolved Cl-rich late-magmatic fluid. Some of the primary PGM have been partially altered by Cl-rich fluids (late magmatic and/or hydrothermal) that leached Bi in preference to Pd to form an unnamed Pd-Bi-O-Cl phase. Many PGM also show scalloped edges and truncated grain boundaries, indicating partial corrosion and dissolution, most likely by late-magmatic/hydrothermal fluids that probably remobilized and deposited PGE in the footwall surrounding the veins.

Carbonatite in a post-collisional tectonic setting: Geochronology and emplacement conditions at Naantali, SW Finland

New geochronological data from monazite and zircon in the Naantali carbonatite, southwest Finland is presented. Inclusion free, light pink, gemmy zircon grains were analysed with the ion microprobe. The zircon data indicate that carbonatite emplacement occurred at 1796±9Ma (2σ). The calcite–prehnite–epidote–actinolite assemblage in the Naantali fenites indicates emplacement was in the upper crust at pressures of <2kbar. This is the earliest age reported for upper-crustal magmatism related to tectonic extension in southwest Finland. Fluorapatite phenocrysts displaying irregular zoning patterns and inclusion-rich alteration zones were analysed by electron microprobe. Unaltered domains of the fluorapatite are enriched in Th–Si–LREEs and have positive Eu anomalies. Altered domains have lower concentrations of Th, Si and the LREEs and contain inclusions of monazite-(Ce), quartz, allanite-(Ce)±bastnäsite-(Ce). The textures of the altered domains are characteristic of dissolution–reprecipitation reactions that indicate disequilibrium between early fluorapatite and late carbonatitic fluid. Electron probe microanalysis total U–Th–Pb ages for monazite inclusions in the fluorapatite are 1797±34Ma (2σ). Correlation of the zircon age data with field relationships and textural observations indicates that fluorapatite alteration occurred via reaction with late magmatic fluid during dyke emplacement and crystallisation.

Mineralogy, geochemistry and petrogenesis of igneous inclusions within three inactive diapirs, Zagros belt, Shahre-kord, Iran

AbstractThe Kaj-Rostam Abad, Dashtak and Doab diapirs are part of the Precambrian–Cambrian Hormuz series that are rich in igneous inclusions concentrated by dissolution of diapiric salt. They are situated in the Iran–Pakistan salt range and commonly associated with inclusions of basalt, trachyte, andesite, micro-gabbro, gypsum and anhydrite, with lesser amounts of carbonate rocks. The mineral assemblage in these inclusions developed in three stages: (I) magmatic stage (diopside, Ti-augite, kaersutite, plagioclase, apatite, biotite and opaque minerals), (II) late magmatic stage (biotite, quartz, chlorite, albite, calcite, titanite, epidote, actinolite and opaque minerals) and (III) vein mineralization (quartz, chlorite, albite, calcite, garnet, epidote, opaque minerals and actinolite). Clinopyroxene is diopside to Ti-augite. Actinolite, kaersutite, albite and pycnochlorite are constituents of the metasomatic rocks of the area. Chlorite geothermometry yielded a temperature of 330–500 °C for chlorite formation. Clinopyroxene thermobarometry ranges from 960 ≤T≤ 1440 °C and 1 ≤P≤ 10 kbar. The presence of halite-bearing fluid inclusions in hydrothermal quartz veins with homogenization temperatures between 320 and 350 °C points to strong evidence of hydrothermal events. The salinity of these fluids is 39.8–42.7 wt% NaCl. δ18O data on hydrothermal quartz veins range from 14.89 to 22.09 ‰ (SMOW), indicating that the studied samples were affected by fluids originated from sedimentary-evaporitic rocks. Meteoric water that penetrated the evaporitic rocks likely mixed with late magmatic fluids while subjected to magmatic heat, when buried to depths of several kilometres by the Phanerozoic cover sequence. Whole-rock geochemistry data for the studied rocks emphasize their alkaline to sub-alkaline affinities, in a transitional magmatic series.

Formation of baotite - a Cl-rich silicate - together with fluorapatite and F-rich hydrous silicates in the Kvaløya lamproite dyke, North Norway

Baotite occurs as a late phase in the Kvaloya lamproite dyke and in the fenitized granite adjacent to the dyke, suggesting that baotite formed during reactions between rock and fluids derived from a volatile-rich lamproitic magma. Most of the analyzed grains of baotite from the Kvaloya lamproite show compositions close to the ideal Nb-free end-member Ba4Ti8Si4O28Cl. Compilation of all published baotite analyses suggests that the major compositional variations of baotite occur between the Nb-free end member Ba4Ti8Si4O28Cl, and a Nb-rich end member Ba4Ti2Fe²+ 2Nb4Si4O28Cl. However, a Pb-bearing baotite, showing significant concentrations of Ca, Sr, Pb and K, and approximately 3 Ba p.f.u., was also identified from the Kvaloya lamproite. Euhedral fluorapatite formed as an early phase during crystallization of the lamproite magma, while anhedral REE-rich fluorapatite overgrowths on the euhedral grains formed during reactions with the late magmatic fluid. Fluorapatite contains up to 1.2 F p.f.u., but only traces of Cl. Other F-rich, but Cl-poor minerals of the lamproite include fluoro-potassic-magnesio-arfvedsonite, fluoro-phlogopite, and yangzhumingite. The presence of baotite together with a range of high-F, but low-Cl mineral phases suggests that the minerals formed in equilibrium with a high-F, Cl-bearing hydrous fluid. The high Cl-content of baotite demonstrates that Cl is strongly partitioned into this mineral in the presence of a Cl-bearing F-rich hydrous fluid. We suggest that a combination of high aSi, aTi, aBa, and fO2, but low aCa of the fluid enabled baotite formation.

Alkaline magmatism of the Vitim province, West Transbaikalia, Russia: Age, mineralogical, geochemical and isotope (О, C, D, Sr and Nd) data

In this paper, we study the geochronology, mineral chemistry, and whole-rock elemental, stable (O, C, D) and Sr-Nd isotopic data for alkaline ultrabasic–basic massifs of the Vitim alkaline province (Sayzhenski complex) in the Central Asian Orogenic Belt, near the boundary with the Siberian craton, to evaluate their petrogenesis and geodynamic significance. U-Pb zircon dating results in Early Paleozoic (520–486Ma) and Late Paleozoic (306–294Ma) stages of alkaline rock formation. The mineralogy and geochemistry exhibit a wide range of SiO2 (38–73wt.%), enrichment in Sr, Ba, LREE and Ta and, most significantly, in Na and Al. The rocks crystallized from a parental CO2- and H2O-rich silica-undersaturated melt. Isotopically, the rocks are highly variable, with (87Sr/86Sr)i – 0.705595–0.707729 and (143Nd/144Nd)i – 0.512237–0.512643. The geochemical and isotope data suggest that the rocks were derived from a source composed of three distinct components: PREMA, EM II and marine carbonate. Additionally, stable (O, C, D) isotope data display the shifting influence of assimilated organic sediments in the source of melts and a partial secondary isotope exchange between the late-magmatic fluids and minerals.

Fluid inclusions in Irish granite quartz: monitors of fluids trapped in the onshore Irish Massif

ABSTRACTFluid inclusion studies of granite quartz provide an opportunity to study fluid flow associated with igneous activity and post-emplacement fluid processes. This study presents new fluid inclusion data from the late Caledonian Donegal granites and Newry granodiorite, and the Tertiary Mourne Mountains granite in Ireland, which identify three distinct fluids. Aqueous-carbonic fluids (Type 1) have been recorded in late Caledonian granites with a significant mantle component (Newry granodiorite and the Ardara and Thorr granites in Donegal). These fluids represent late-magmatic fluids trapped at high temperatures (up to 575°C), and the ultimate source of these carbonic fluids is linked to sub-lithospheric processes during the Caledonian orogeny. The dominant fluid type (Type 2) in late Caledonian granites is a H2O+NaCl±KCl fluid which may be related to thermal convection cells around granite bodies and/or to regional scale influx of surface derived fluids at the end of the Caledonian orogeny. High salinity NaCl–CaCl2 fluids (Type 3) overprint quartz in the Ardara granite in Donegal, and in the Newry granodiorite, and are interpreted to represent basinal brines, sourced in overlying sedimentary basins, which circulated through the crystalline basement during a period of crustal extension (possibly during the Carboniferous or the Triassic). Fluid inclusion studies of the Tertiary Mourne Mountains granites have identified only Type 2 fluids related to thermal convection cells, consistent with stable isotope evidence, which indicates that this younger granite is unaffected by regional-scale fluid influxes.

Fluid-Rock Interaction in the Miocene, Post-Caldera, Tejeda Intrusive Complex, Gran Canaria (Canary Islands): Insights from Mineralogy, and O- and H-Isotope Geochemistry

The intra-caldera volcaniclastic deposits of the Miocene Tejeda caldera on Gran Canaria host an similar to 12 km diameter intrusive complex comprising a peralkaline, trachytic to phonolitic cone sheet swarm surrounding a central core of hypabyssal syenite stocks. Both intrusive rock types display textural and mineralogical features indicative of secondary fluid-rock interaction, including (1) deuteric mineral phases (e.g. aegirine, alkali-amphibole, analcime), (2) turbid alkali feldspars, and (3) hydrothermal mineral phases (phyllosilicates, Fe-Ti oxides, Mn-oxides, and quartz). Altered cone sheets have whole-rock delta O-18 values ranging from 0 center dot 1 to 10 center dot 0 parts per thousand (n = 22), and whole-rock delta D values between -62 and -149 parts per thousand (n = 28). Three altered syenite samples have whole-rock delta O-18 values of 2 center dot 5, 1 center dot 5, and 0 center dot 9 parts per thousand, and corresponding delta D values of -91, -99, and -121 parts per thousand. The H2O concentrations of the altered cone sheets range from 0 center dot 4 to 0 center dot 8 wt % (n = 28), and the altered syenites have H2O concentrations of 0 center dot 5, 0 center dot 5, and 0 center dot 6 wt %, respectively. The majority of altered samples are depleted in O-18 relative to the typical delta O-18 range for unaltered trachytes and syenites (delta O-18 = 6-8 parts per thousand), indicative of interaction with local meteoric water (delta O-18 c. -8 parts per thousand) at temperatures epsilon 150 degrees C. Only one cone sheet sample appears petrographically unaltered and has a typical 'igneous' isotopic composition (delta O-18 = 7 center dot 1 parts per thousand, delta D = -48 parts per thousand) and a relatively high H2O concentration (2 center dot 2 wt %). A weak correlation (r = 0 center dot 55) between delta D and H2O is observed in the cone sheets, reflecting the combined effects of magmatic H2O exsolution, and subsequent deuteric and hydrothermal alteration. No systematic variation in delta O-18 or delta D was detected across the cone sheet swarm, most likely reflecting overprinting of isotopic compositions during successive intrusive events. However, the highest delta O-18 values (8 center dot 2-10 center dot 0 parts per thousand) occur in clay-bearing cone sheets from the central part of the intrusive complex, suggesting enhanced infiltration of relatively cool meteoric water in this area. Overall, at least three phases of fluid-rock interaction can be distinguished: (1) deuteric alteration (c. 300-500 degrees C) by late magmatic fluids expelled from a solidifying crystal mush; (2) hydrothermal alteration (epsilon 150-300 degrees C) by meteoric water during the final stages of crystallization and/or immediately following solidification of the intrusive complex; (3) retrograde alteration related to the influx of relatively cool (150 degrees C) meteoric waters.

Compositional variation of the tennantite–tetrahedrite solid-solution series in the Schwarzwald ore district (SW Germany): The role of mineralization processes and fluid source

AbstractThe study presents analysis from members of the tennantite–tetrahedrite solid-solution series (‘fahlore’) from 78 locations in the Schwarzwald ore district of SW Germany. Electron microprobe analysis is used to correlate the compositional variations of the fahlores with mineral association, host rock, tectonic history and precipitation mechanisms. Results indicate that most fahlores from gneiss-hosted veins do not have distinctive geochemical characteristics and range from tetrahedrite to tennantite end-member composition with variable trace-element content. However, diagenetically formed fahlore has a near-end-member tennantite composition with very small trace-element content. Red-bed-hosted fahlore formed by fluid mixing is tennantite enriched in Hg that probably has its source in the red-bed sediments. Fahlore formed from granite-related late-magmatic fluids, or from mixing of fluids of which one has equilibrated with granitic basement rocks, is typically As- and Bi-rich (up to 22.2 wt.% Bi). Gneiss-hosted fahlore formed by fluid cooling is Ag-rich near-end-member tetrahedrite. Some fahlores reflect their paragenetic association, e.g. a large Ag content in association with Ag-bearing minerals or a large Co and Ni in association with Co- and Ni-arsenides.Although they have similar compositions, gneiss-hosted fahlores show systematic variations in Ag contents and Fe/Zn ratios between the Central and the Southern Schwarzwald with Fe-rich fahlore in higher stratigraphic levels (North) and Zn- and Ag-rich fahlore in lower stratigraphic levels (South). We show that fahlore composition varies with precipitation mechanism (cooling vs. mixing vs. diagenesis), depth of formation, paragenetic association and host rock. Comparison with fahlores from other European occurrences indicates that these conclusions are consistent with fahlore systematics found elsewhere, and could be used to infer details of ore-forming processes.

Sensitive high-resolution ion microprobe analysis of zircon reequilibrated by late magmatic fluids in a hybridized pluton

Zircon from a microgranular enclave in the ca. 315 Ma postcollisional Karkonosze pluton (Western Sudetes, northeastern Bohemian Massif) is characterized by unusual morphologies and reequilibration textures. Blocky, clustered, and skeletal Th-U–rich zircon grains are internally corroded along discrete boundary zones, and subsequently replaced by porous microgranular aggregates of zircon and various other minerals, including thorite. The boundary zones have textures and compositions characteristic of diffusion-controlled chemical reaction fronts, including enrichment in Ca, Ba, and light rare earth elements, whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation. Initial zircon crystallization occurred with the mingling of mafic magma into a cooler granitic melt, whereas zircon modification is attributed to the reaction of late magmatic fluids from the host granite with the enclave. Precise dating of reequilibrated zircon as 304 ± 2 Ma indicates that fluid activity, which is also responsible for scheelite mineralization, postdates the emplacement of the main part of the pluton by several millions of years.