Metasomatic Rocks Research Articles

Simple kinetic model for replacement reactions involving solid solutions: the significant role of geofluids

We develop a kinetic model that illustrates the relative role of intergranular diffusion and surface reactions in overall metamorphic net-transfer reactions involving solid solutions. The behavior and state of the system were observed to depend on the non-dimensional parameter(s), defined herein by gamma equiv (k cdot L cdot K^{n-1}) /D^{mathrm{eff}}, where k denotes the dissolution rate constant of the mineral, L denotes the distance between the two minerals participating in the reaction, K denotes the coefficient of the equilibrium abundance ratio of the precipitated component in the intergranular fluid to those in the mineral, n denotes the order of the dissolution reaction, and D^{mathrm{eff}} denotes the effective diffusion coefficient of the precipitated component in the intergranular fluid. When gamma is small, the system becomes homogeneous, which is controlled by the surface reaction. In contrast, a large gamma implies a heterogeneous system controlled by both surface reaction and diffusion. Geofluid is among the most important influencers of kinetic regimes and reaction textures in metamorphic and metasomatic rocks because the intergranular fluid distribution considerably affects the intergranular diffusivity (D^{mathrm{eff}}). Our conceptual model can potentially reveal the nature of the reaction kinetics and the fluid–rock interactions and capture the complex behaviors of natural systems in future extensions.

Rare-Earth Elements in Magmatic and Metasomatic Rocks of Murovdag Ore Area (Lesser Caucasus)

Abstract—The article discusses the distribution of rare-earth elements in the igneous and metasomatic rocks of the Murovdag ore district of the Lesser Caucasus. It has been established that the metasomatites and host rocks of the Murovdag ore district are characterized by the predominance of light rare-earth elements over heavy ones. The whole complex of the data obtained indicates that the components of the magmatic nature prevailed in the composition of the ore-forming fluids. However, mixing of magmatic fluids with meteoric waters is not excluded. It has been found that metasomatites and host magmatic rocks of the studied deposits have similar REE distribution patterns, which may indicate that metasomatic rocks were developed after igneous protolith of this composition.

Aportes a la metalogénesis del basamento proterozoico superior-cámbrico de las Sierras Pampeanas Orientales: mineralogía, termometría e isótopos de azufre del skarn Malagueño (Cu- Fe±Zn), Sierra Chica, Córdoba, Argentina

On-going mining operations in a marble quarry (Cantera Centro) from Malagueño, Sierra Chica de Córdoba, Argentina, have unearthed veins, veinlets and lenses of sulfides (pyrrhotite>pyrite≅
 chalcopyrite>>sphalerite). These veins and lenses are up to 0.3 m thick and 2-3 m long, although intermittently can extend about a hundred meters. They are associated with skarns. The metasedimentary host sequence, largely composed of gneisses, amphibolites and marbles, was intruded by amphibolic metagabbro and metadiorite dykes, metatonalite plutons and alkali-feldspar metagranites; the whole complex was metamorphosed into medium to high amphibolite facies and strongly deformed as a result of the a regional event M2-D2/D3 that affected the Neoproterozoic-Cambrian basement during the Pampean orogeny. Except for gneisses, all the other metamorphic lithologies register evidence of differential alteration into skarn, although the process was preferentially developed on marbles, amphibolites and metagranite, and to a lesser exten on mafic and mesosilicic dykes. The metasomatic rocks are characterized by a garnet>>pyroxene skarn (Grs27-48Adr22-34Alm15-27Sps9-21), formed after the replacement of the metagranite; a pyroxene-rich skarn (Hd42-63Di32-50Jo3-5) developed after para-amphibolite, and a garnet (Adr54-71Grs22-40Alm4-7Sps1-2) (±wollastonite) skarn that replaced a calcic marble. The dykes show poor metasomatic replacement and lack sulfides of metasomatichydrothermal origin. Retrograde mineral associations include hastingsite, ferroactinolite, epidote, clinozoisite, sericite, plagioclase (An18), chamosite and calcite. The sulfide mineralization is paragenetically associated with late-stage, infilling skarn-hydrothermal minerals that were sequentially deposited as: calcite→clinochlore→quartz→pyrite→pyrrhotite→chalcopyrite+sphalerite; these phases occur as veins and veinlets within the garnetpyroxene skarn, and as massive pyrrhotite lenses in the piroxene-rich skarn. Microthermometric data from fluid inclusions in sulfide related calcite, together with the geothermometric data from the Fe/Mg ratios in clinochlore and the phase equilibria data from intergrown high and low T ºC pyrrhotite phases, all constrain the infilling gangue phases and sulfide crystallization temperature within the ~360 °C to 250 °C range; the gradual termal decrease is in agreement with the temporal depositional sequence of the infilling phases. Fluid inclusion petrographic data and salinity estimations suggest that sulfide precipitation was triggered by boiling, from a fluid of moderate to high salinity (~14.5 to 33.5 wt% eq. NaCl). Evidence that the fluid evolved under dominantly reducing conditions are the high Fe+2/(Fe+2+Fe+3) ratios and molar proportions of subcalcic garnet (Alm+Sps) in garnet-pyroxene skarn, the presence of fluid inclusion sulfide daughter crystals in calcite, the high H2S/SO42- ratios in the sulfide-bearing fluid and the presence of pyrrhotite among the sulfide phases. Disseminated primary chalcopyrite and pyrite in metagabbro-diorite dykes (Cu ~300 ppm) and the high contents of Cu in amphibolite (~900 ppm) suggest that these protolithic lithologies were the probable sources of metals (Cu>>Zn±Ag). Values of δ34SΣfluid between ~-4 to +1.4‰ obtained from the fractionations factors of pyrite, pyrrhotite and chalcopyrite within the thermal range 350-150 ºC, indicate a magmatic source for sulfur, likely provided by leaching or desulfidation of primary sulfides of the metagabbro-diorite dykes. Metals and sulfur supplied by these dykes and amphibolite would have been redistributed in the skarn after the circulation of the metasomatic-hydrothermal fluids. Skarn bodies would have formed by infiltration of deep metasomatic fluids and fluid-rock interaction which affected lithologies of the Upper Proterozoic-Cambrian metamorphic basement. Fluids could have derived from hidden Cambrian intrusives, or from the surrounding regional migmatization, channeled along lithological contacts and faults/fractures. Field setting, textural and mineralogical evidence of sulfide mineralization in the skarn assemblage of Cantera Centro, suggest a Cambrian age associated to the post-deformational stage of the Pampean Orogeny.

Zinc-rich and copper-bearing amphiboles from sulfide-free ore occurrences of the Pelagonian Massif, Republic of North Macedonia

This paper describes specific features of isomorphism of unusual amphiboles containing up to 23 wt% ZnO and up to 1.3 wt% CuO from sulfide-free metasomatic rocks of the “Mixed Series” metamorphic complex, Pelagonian Massif, Republic of North Macedonia, in which chalcophile elements (Pb, Cu, Zn, Sb, As) occur in high concentrations in the form of oxides and oxysalts. The studied amphiboles belong to six mineral species: magnesio-riebeckite, ferri-winchite, F-dominant analogue of ferri-winchite (“ferri-fluoro-winchite”), ferri-barroisite, ferri-nyboite, and a zinc-rich amphibole related to clino-suenoite. The main substitution schemes are Zn → Mg and Na + Fe3+ → Ca + Mg. The amphibole related to clino-suenoite has been studied in the most detailed way. Its empirical formula is (Mg2.99Zn2.47Mn1.26Fe3+0.30Ca0.07)Σ7.09(Si7.89Fe3+0.06Al0.05O22)(OH)2, and it is the most Zn-rich amphibole among those that have ever been described in literary sources. This amphibole is characterized by single-crystal and powder X-ray diffraction, IR spectroscopic and optical data. The role of high barium activity in the formation of sulfide-free ores of chalcophile elements has been substantiated. A possible mechanism of sulfur immobilization can be described by the scheme S2− + 2O2 ↔ SO42−; SO42− + Ba2+ → BaSO4↓.

New Findings of Rare Minerals in Alkaline Rocks of Ukrainian Shield

Over recent years, new rare minerals have been discovered in the alkaline rocks of the Ukrainian Shield. Agpaitic varieties of alkaline magmatic and metasomatic rocks turned out to be especially abundant in rare minerals. Numerous findings are related to alkaline metasomatites which are considered to be fenites and apofenite albitites of the Dmytrivka quarry. It is well known primarily by the presence of various accessory (Nb, REE, and Zr) minerals, as well as silicate and oxide minerals that are rare for Ukraine. The most common albite microcline fenites of this quarry are characterized by rare-earth mineralization, whereas the concentration of REE decreases in apofenite albitites and Zr and Nb increase. New rare minerals were also found in the essentially albite rock with astrophyllite, alkaline pyroxene and amphibole of the Malatersa massif and agpaitic phonolites of the Oktyabrsky massif. In the rocks of the mentioned massifs and occurrences of alkaline rocks the most interesting are the findings of the perraultite — jinshajiangite series. They were found in three points of the Azov area and include 1) perraultite and jinshajiangite in the alkaline metasomatites of the Dmytrivka quarry; 2) only perraultite in agpaitic phonolites of the Oktyabrsky massif (Kam’yana gully); 3) jinshajiangite in a veined albite rock among the gabbro of the Malatersa massif. Baotite and minerals of the hejtmanite — bafertisite series were also found in the metasomatites of the Dmytrivka quarry. The latter belong to intermediate varieties in terms of MnO (10-17 wt.%) and FeO (10-17 wt.%) which distinguishes them from Fe-rich bafertisite from other regions. A silicate mineral with high content of Na, Zr, Mn and elevated Ti and Nb is rarely observed as small inclusions in the kupletskite grains from alkaline metasomatite of the Dmytrivka quarry. According to the chemical composition it was previously diagnosed as janhaugite. Tainiolite was found in some occurrences of alkaline metasomatites in the Azov region. In addition small aggregates of the REE-enriched epidote were found in fenites of the Kaplany village, which is probably the first finding in Ukraine. Two new Zr minerals have been found in the aegirine syenites of the Korsun-Novomyrhorod pluton: elpidite and mineral with a high content of Y2O3 (13-14 wt.%) (Y-hagatalite ?).

Tracing subduction zone fluids with distinct Mg isotope compositions: Insights from high-pressure metasomatic rocks (leucophyllites) from the Eastern Alps

Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of −0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from −0.64‰ to −1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from −0.29‰ to −0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about −0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg (<−1.3‰) fluid produced by dissolution of mainly Mg-calcite at HP conditions; (2) a high δ26Mg (>0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite mélange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as −1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries.

Forms of Arsenic Concentration in Sulfide-Free Endogenic Pb–Zn–Sb Ores of the Pelagonian Massif, Republic of North Macedonia

The chemical composition, substitutions, and zoning of minerals that concentrate arsenic in specific sulfide-free ore-bearing metasomatic rocks at the contact of the Early Paleozoic metarhyolites and aporhyolite schists with dolomite marbles and baryte schists in the Pelagonian massif, near the village of Nežilivo, Republic of North Macedonia, have been studied. These metasomatic rocks are enriched in chalcophile elements (Zn, As, Sb, Pb, Cu), which occur in oxides and oxysalts. Partitioning mechanisms of various constituents (Mg, Zn, Cu, Ca, Pb, Y, Ln, F, OH) between associated arsenates, As-bearing phosphates and mottramite, are discussed.

Patterns of Forming the Urban Surface Deposited Sediments

The contemporary sedimentation processes participate in the formation of fluxes of the solid substances in an urban area, forming together a source of pollution, transit and depositing media. The patterns of forming the urban surface deposited sediments were determined on the example of Russian metropolis Nizhny Novgorod. Two schemes of the research were implemented in residential districts of the city in the summer field season 2018. The first research design scheme represented a detailed survey of residential quarters of the city. It included a collection of 25 samples of environmental compartments from five residential quarters with multi-storey buildings. The sampling was conducted in various functional landscape parts at the courtyard and facade areas. Each sample was separated into particle size fractions. Chemical and mineral analysis was performed in each granulometric subsample of the collected samples. The second scheme represented the urban geochemical study based on sampling the sediments from local surface depressed zones of microrelief in the city. The 40 samples were collected at the territory of the city on an irregular grid. The sampling was carried out in residential areas in blocks with multi-storey buildings. One sample represented a combined sample of sediments from local surface zones of microrelief by 3-5 localizations from the area of the block. Chemical analysis of the collected samples was conducted. The maximum content of dust (0.002-0.01 mm) 39% was found in soil, while the lower mean dust content was found in the samples of sediment from local surface depressed zones of microrelief (30 %) and in the road deposited sediments (30 %). The mineral composition of soil and sediment in Nizhny Novgorod is represented by quartz sand (approx. 55 %). In the samples of sediments, the minerals’ content is basically characteristic for acid and metasomatic rocks. Building materials’ content in the sediments reaches up to 20%. The maximum organic matter content was found in fine dust fraction 2-10 microns 21% in soils and 5% on the roads. Differentiation of the mineral content in the samples is observed for the various particle size fractions and different functional parts of the quarters. The highest content of metals of anthropogenic association (such as Pb, Zn and Cu) was found in the dust fraction. The Pb and Zn concentrations in samples of sediment of depressed areas of microrelief and road dust were significantly lower than in the soil.

Uranium deposits of Mongolia, their exploration and mine development

Most of the known uranium deposits in Mongolia are related to volcano-tectonic structures of the Late Mesozoic and permeable sandstone aquifers of terrigenous sedimentary rocks in Late Mesozoic-Cenozoic basins. A young uranium deposit has been recently discovered at the surface in Quaternary alluvial sediments. Uranium deposits are classified here according to the International Atomic Energy Agency (IAEA) classification scheme. Beside of supergene and hydrothermal uranium discoveries, uranium mineralization can also be found in the Lower Cretaceous lignite seams, in Mesozoic alkaline intrusive rock related REE, Th, U enriched mineral systems and in metasomatic rocks, as well as in migmatites and pegmatites localized in Precambrian metamorphic rocks. Currently, no uranium can be produced in Mongolian facilities except pilot testing being done first. A pilot test was carried out at the Khairhan and Kharaat deposits in Mongolia and these experiments have demonstrated the ore to be amenable to acid leach (sulphuric acid) with the addition of an oxidizing agent. These tests confirmed that hydraulic control can be maintained and that the uranium solubilization and mobilization can be controlled. The results of the test were encouraging, with the well production rate, uranium concentration in produced solutions, chemical usage, and estimated uranium recovery all within ranges expected for normal commercial operations. There are a number of Mongolian uranium deposits are in the stage of mine development. The Government of Mongolia has approved the agreement of mine development of the Zuuvch ovoo and Dulaan uul deposits. Pilot testing on these deposits is underway and uranium will be extracted by in-situ leaching soon.

The First Data on the Structure and Mineral Composition of Ores at the Tarakon Potential Gold Field, Russian Far East

The original materials obtained, along with the literature data (hydrochemical contrasting halos), make it possible to predict a new (Tarakon) gold ore field within the eastern part of the Argun superterrane. Commercial gold concentrations are noted in quartz metasomatic rocks after granite and limestone. The amount of ore minerals usually does not exceed 5–10%. They commonly occur in the rocks as micron and submicron particles mainly of pyrite, less often, of galena, sphalerite, chalcopyrite, ilmenite, hematite, magnetite, fahlore, and native high-grade gold. It is assumed that formation of the Tarakon gold ore field, as well as of most ore bodies in the region, is related to Late Mesozoic magmatism.

Clinopyroxene episyenites in a Proterozoic rapakivi granite, SE Finland \u2014 recrystallization textures, mass transfer and implications for the petrology of A-type granite complexes

Na-metasomatic augite and aegirine-augite episyenites are hosted by subalkaline amphibole granites in the 1.644 Ga Suomenniemi rapakivi granite complex, southeastern Finland. In an examined drill core, episyenites are bordered by up to 50-cm-wide zones of Na-enriched augite-bearing granite in which alkali feldspar forms rims on plagioclase, and aggregates of augite and magnetite have formed by fluid-induced dehydration of hastingsite and biotite. In the episyenites, quartz has been partially removed, alkali feldspar (An<1Ab50Or50) and plagioclase have been partially recrystallized to granoblastic polygonal aggregates, and clinopyroxene (augite or aegirine-augite) has coarsened and been enriched in the aegirine component. Mass balance modeling implies addition of mainly Na, depletion of Si, F, Rb, Ba, Sr and 10–20% volume loss (by quartz dissolution) in the episyenitized zones in the drill core. Quartz-depletion and recrystallization may have been caused by several superimposed stages of alteration, probably related to voluminous dike- and A-type granite magmatism in the Suomenniemi area 10–15 m.y. after the emplacement of the Suomenniemi complex. Because of the coarse recrystallization textures and near-solidus recrystallization temperatures, distinguishing these fenite-like metasomatic rocks from igneous syenites is not trivial. In epizonal A-type granite complexes, high-temperature episyenites may be more common than currently thought.

Serpentinite-derived low δ7Li fluids in continental subduction zones: Constraints from the fluid metasomatic rocks (whiteschist) from the Dora-Maira Massif, Western Alps

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior.

Mineral associations and Mo-W ore types of the Slepaya Zalezh' orebody at Tyrnyauz deposit

Slepaya Zalezh is the largest ore body of the Tyrnyauz Mo-W deposit. It is the priority object on the renewal of its operation. A dissemination of molibdoscheelite, scheelite and molibdenite, controling ore concentrations, is confined to metasomatic rocks of the productive stage, laid on skarns, near-skarn rocks, hornfelses and marbles. The types of the ores are divided accordingly of ratio in them of different facies of ore-bearing metasomatic and initial rocks, which is a working out mass of ore. 7 types of the ores are divided and maped on the ore body vertical projection. Aposkarn ores predominate clearly in the I and II types, accordingly laid over endo- (substituting hornfelses) or exo- (apomarble) skarns. In the III type there are ores laid over endo-skarns and near-skarn rocks in equal parts. Relative to it the type IV contain less of of near-skarn rocks, but more of hornfelses. Ores replacing exo-skarns and to a lesser degree (up to 25%) – marbles dominate in the V type. The type VI is an intermediate one between the types III and IV. In the type VII all initial rocks are presented. Every of the types has a stable mineral and chemical composition inherited from the initial rocks. The ores of the specific type have convenient simple distinguishing features for a hand sorting of specimens, samples, and a preliminary ore separation. The types of ores have narrow change limits in size and form of ore mineral grains and their intergrowthes, percentage of minerals harmful for the flotation. Minor differences in contents of the useful components of each ore type offer possibilities of their selective working off and burdening. Mapping results contain data of the distribution of accompany sorts of mineral resources: garnet and wollastonite in the skarns and productive metasomatic rocks, pyroxene-plagioclase near-skarn rocks and hydrothermal alterated rocks with Cu, Au and other mineralization.

Geochemistry of sillimanite-magnetite-kaolinite of metasomatic rocks of the island of Great Tyuters (gulf of Finland, Russia)

Among the secondary quartzites of the island of the Great Tyuters there are extended linear zones with a capacity of up to 3 meters, clearly visible in detailed satellite images. They are composed of sillimanite-magnetite-kaolinite metasomatites discovered for the first time. The formation of metasomatites is associated with fragmentation of quartzites and the subsequent introduction of fluids of a substance saturated with various components. Metasomatites are enriched with Al2O3 (16–23 wt.%), in contrast to 2–5 wt. per cent of this oxide is present in their substrate – quartzite. In metasomatic rocks dominated by iron oxide, quartzite – ferrous. The rocks do not contain any alkaline and alkaline-earth elements. They are enriched with Zr (146–199 g/t) and a number of other elements, including impurities and rare earth, among which prevail Ce (34 g/t) and La (17 g/t). REE of both breeds is characterized by a small and close degree of fractionation ((La/Yb)n=6.55 and 6.17). Metasomatites on a set of minerals do not differ from quartzites, but differ in their quantitative ratios and composition. Of particular interest is the magnetite of metasomatites. It contains many quartz inclusions with kaolinite edges. Quartz inclusions, in turn, contain inclusions of titanium magnetite crystals. In the formation of metasomatites, there were several stages of the metasomatic process with a changing composition of fluids and different acidity-alkalinity of the mineral formation medium. The development of metasomatites in linear zones, the disintegration of their substrate, the high content of kaolinite – all this gives them a similarity with weathering cows. However, the same set of minerals in metasomatite and its substrate, the complete absence of sulfides and sulfur, the presence of magnetite with double mineral inclusions – such features distinguish these rocks from the classical weathering crust.

Li-Na-metasomatism related to I-type granite magmatism: A case study of the highly fractionated La Pedriza pluton (Iberian Variscan belt)

This study describes rare albitites associated with highly fractionated I-type granitic melts, showing a mineral paragenesis that resembles those of high-temperature metasomatized rocks (fenites) associated with alkaline and carbonatitic intrusions. The studied albitites occur as lenticular bodies on the meter scale, hosted in peraluminous granites, and are scattered in a narrow sector next to the peraluminous and highly fractionated I-type La Pedriza leucogranite (LPL) pluton in the Spanish Central System. Albitites show sharp contacts and preserve textural features of the host granites (i.e., isovolumetric metasomatism). These rocks consist of a variably albitized magmatic feldspar framework (70–85 modal %) and interstitial domains where singular Na-Li-Fe3+-rich amphiboles and pyroxenes crystallized with albite or quartz. Titanite, andradite-grossular garnet, Mg-Li-rich mica, magnetite, fluorite, and apatite are metasomatic accessory minerals. Mass balance indicates enrichments of Na2O, FeO, MgO, MnO, Li, F, Be, and some HFSE (high-field-strength elements, such as Zn, Sc, and Sn) and LILE (large-ion lithophile elements) depletion. U-Pb data from metasomatic titanite yield 307 ± 10 Ma, which is coeval with the LPL and within the range of the regional granite emplacement (about 309–298 Ma). The LPL also shows strong compositional affinities (F, Li, Na, Sn, and Zn enrichment) with albitites. Similar Sr-Nd-δD isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant during metasomatism. However, the heterogeneity of δ18O, which shifts toward lower values in the more distant outcrops, suggests contribution of meteoric in origin low-salinity waters during prograde metasomatism. P-T-fO2 conditions are estimated at 0.2–0.3 GPa, 645–700 °C, and relatively oxidized conditions. We propose that Na-HFSE-flux-rich residual melts of the LPL expelled two immiscible hot liquids: an alkaline low-density liquid and a high-density hydrosilicate liquid (HSL). The low-density fluid dissolved magmatic quartz and provided albitization of the magmatic feldspar framework by a diffusive mechanism. Quartz dissolution created microchannels for infiltration of the relatively low viscosity HSL, forming tubular channels. This fluid was enriched in network modifiers (Li, Na, F, Be) and cations (Ca, Al) released after albitization, and it precipitated in these microchannels to form the new metasomatic paragenesis (quartz, albite, uncommon Li-rich mafic silicates, and accessory minerals).

Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics

AbstractBoninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 &gt; 52 and Ti8 &lt; 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si boninites (Si8 &lt; 57) and high-Si boninites (Si8 &gt; 57). Satisfying one but not both of the boninite criteria are rocks with Si8 &gt; 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 &lt; 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main boninite sources. Nonetheless, arc-basin boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic boninites, they provide evidence for some form of subduction during the Archean.

Study of diffraction data sets using factor analysis: a new technique for comparing mineralogical and geochemical data and rapid diagnostics of the mineral composition of large collections of rock samples

This paper presents an example of comparing geochemical and mineralogical data by means of the statistical analysis of the X-ray diffraction patterns and the chemical compositions of bulk samples. The proposed methodology was tested on samples of metasomatic rocks from two geologically different objects. Its application allows us to mathematically identify all the main, secondary and some accessory minerals, to qualitatively estimate the contents of these minerals, as well as to assess their effect on the distribution of all petrogenic and investigated trace elements in a short period of time at the earliest stages of the research. We found that the interpretation of the results is significantly influenced by the number of samples studied and the quality of diffractograms.

Geochemistry of Sillimanite–Magnetite–Kaolinite Metasomatic Rocks on Great Tyuters Island, Gulf of Finland, Russia

Secondary quartzites on Tyuters Island in the Gulf of Finland host long linear zones, up to 3 m thick, that are easily discernible in detailed satellite imagery. The zones consist of sillimanite–magnetite–kaolinite metasomatites, rocks that were first found in these zones. The metasomatites were produced at shattering of the quartzites and subsequent fluid introduction of material enriched in various components. The metasomatites are enriched in Al2O3 (16–23 wt %, as compared to 2–5 wt % in the quartzites). In contrast to the quartzites, iron in the metasomatic is contained mostly in the form of ferric oxide (whereas it occurs dominantly as ferrous oxide in the quartzites). The metasomatites contain practically no alkalis and alkali-earth elements and are enriched in Zn (146–199 ppm) and some other trace elements, including REE, first of all Ce (34 ppm) and La (17 ppm). Both the metasomatites and the quartzites show similar and insignificant REE fractionation: (La/Yb)n = 6.55 and 6.17. The metasomatites consist of the same minerals as the quartzites, but the proportions of these minerals and their composition in these rock types are different. It is worth mentioning magnetite of the metasomatites: it hosts abundant quartz inclusions with kaolinite rims. The quartz inclusions host, in turn, inclusions of magnetite crystallites. The metasomatites were produced in a number of stages of a metasomatic process, with changes in the fluid composition and its pH. The development of the metasomatites in the linear zones, the disintegration of their protolithic quartzites, and the high kaolinite contents in the rocks make these metasomatites similar, in a sense, to weathering crusts. However, the facts that the metasomatites and their protolithic quartzites consist of the same minerals, that the metasomatites contain absolutely no sulfides and sulfur but contain magnetite with complicated mineral inclusions make these rocks different from classic weathering crusts.

Perovskites of the Tazheran Massif (Baikal, Russia)

The paper provides details of local geology and mineralogy of the Tazheran Massif, which was the sampling site of perovskite used as an external standard in perovskite U-Pb dating by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively-coupled plasma (LA–ICP–MS) mass spectrometry. The Tazheran Massif is a complex of igneous (mafic dikes, syenite, nepheline syenite), metamorphic (marble), and metasomatic (skarn, calc–silicate veins) rocks. Metasomatites are thin and restricted to the complex interior being absent from the margins. Perovskite has been studied at four sites of metasomatic rocks of three different types: forsterite–spinel calc–silicate veins in brucite marble (1); skarn at contacts between nepheline syenite and brucite marble (2), and skarn-related forsterite–spinel (Fo-Spl) calc–silicate veins (3). Pervoskite from Fo-Spl calc–silicate veins (type 1) is almost free from impurities (&lt;1 wt.% in total: 0.06%–0.4% REE2O3, 0.10%–0.22% Nb2O5, ≤0.1% ThO2). The U contents are from 0.1 to 1.9 wt.% UO2 and are relatively uniform in perovskites from the same vein but differ from vein to vein of this type. Perovskite from the contact skarn (type 2) contains 1.5 to 4.5 wt.% REE2O3 but is poor in other impurities. Perovskite grains from skarn-related Fo-Spl calc–silicate rocks (type 3) belong to two generations that differ in REE, Nb, Th, Fe, and Na concentrations. Early-generation perovskites occurs as compositionally homogeneous octahedral or cubic-octahedral crystals with contents of impurities higher than in other varieties (3.6 wt.% REE2O3, 1.6 wt.% Fe2O3, 1.3 wt.% Nb2O5, 0.7 wt.% ThO2, 0.6 wt.% UO2, and 0.6 wt.% Na2O) but the lowest is at the respective site. Late-generation varieties show highly variable impurity concentrations of 1.5 to 22.7 wt.% REE2O3, 0.4 to 8.4 wt.% Nb2O5, and 0.8 to 4.5% ThO2, while the perovskite component may be as low as 65%. In addition to the lueshite and loparite, components, they contain REEFeO3 and Th0.5TiO3 endmembers which have no natural analogs.

Rare-earth mineralization in Karelia’s alkaline and moderately alkaline complexes, associated metasomatic rocks and ores

Rare-earth mineralization in Karelia’s alkaline and moderately alkaline complexes, associated metasomatic rocks and ores