Metasomatic Formation Research Articles

Structural factors controlling ore-bearing metasomatites of the Middle Dnieper megablock of the Ukrainian shield

Purpose. Determination of the patterns of structural control of metasomatic formations with a complex composition of mineralization to substantiate the most promising areas for the placement of ore formations of hydrothermal-metasomatic genesis. Methods. To substantiate the patterns of spatial distribution of ore-bearing metasomatite formations within the Middle Dnieper megablock of the Ukrainian Shield, a graphic comparison of metasomatism sites was carried out with maps of deep fault systems identified in accordance with the rotation hypothesis of K.F. Tyapkin. In accordance with the sequence of system-hierarchical subordination of structural control elements a comparison of metasomatism sites with fault systems was carried out on maps at scales 1:1000000, 1:500000, 1:200000 and 1:50000. To determine the material composition of metasomatic formations, petrographic, mineragraphic and mineralogical research methods were used. Findings. Structural and tectonic patterns of control of ore-bearing metasomatites by deep fault systems within the Middle Dnieper megablock have been established. The most productive systems of deep faults in terms of ore-mineralization control have been identified. The material composition of metasomatic formations that are promising for the mineralization of precious metals is characterized. The ore-controlling role of deep fault intersection in the formation of polychronous ore-bearing metasomatic formations is substantiated. The originality. The factors of structural-tectonic control of ore-bearing metasomatite formations by deep fault systems within the Middle Dnieper megablock are substantiated and the spatial connection of telescoped metasomatic formations with complex mineralization with deepfaultsintersection is determined. Practical implementation. The most promising areas of distribution of metasomatic ore-bearing formations of deep fault zones have been identified for metallogenic prediction.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 7. Genesis of lithium-bearing objects: rare-metal granites of the Ukrainian Shield

In the genetic sense, the following lithium-bearing objects have been characterized: vein Li-F granites and areal Liznykivsky granites of the Korosten pluton, rare-metal granites of the Kamianomohylsky complex, and ongonites of the Azov region. Most researchers consider Li-F granites as the final phase of magmatism of the Korostensky complex. Our conclusion differs from the mainstream view: vein Li-F granites of the pluton are products of crystallization of deep fluid flows that penetrated into the fissure cavities of the pluton rocks after the formation of magmatic massifs. In our opinion, the Liznykivsky granites are metasomatic formations that have developed on different-age rapakivi-like biotite-amphibole granites under the influence of deep fluid flows. The rare-metal granites of the Kamianomohylsky complex form three massifs in the Azov region: Kamiana Mohyla, Katerynivsky and Starodubivsky. Initially, all granite massifs were formed in the process of crystallization of magmatic melt, which later changed to metasomatic alteration of granites. Post-magmatic changes in granites that contributed to the development of rare metal mineralization include: 1) biotitization; 2) microclinization; 3) early albitization; 4) greisenization; 5) late albitization. According to the data of thermobarogeochemical studies, the stage-by-stage development of the post-magmatic alteration of the Kamiana Mohyla granitoids was substantiated and physicochemically characterized: microclinization (500-600 °С), albitization (430 °С), topazization (430 °С), silicification (430-440 °С), sulfidization (320 °С), late albitization (290-420 °С), crystallization of xenotime (200-210 °С), vein fluoritization and the growth of fluorite crystals (125-155 °С). The crystallization of ongonites took place at high temperature, which was 1140 °C at the final stage of quartz phenocrysts growth.

LITHIUM IN THE SUBSOIL OF UKRAINE Part 7. Genesis of lithium-bearing objects: rare-metal granites of the Ukrainian Shield

In the genetic sense, the following lithium-bearing objects have been characterized: vein Li-F granites and areal Liznykivsky granites of the Korosten pluton, rare-metal granites of the Kamianomohylsky complex, and ongonites of the Azov region. Most researchers consider Li-F granites as the final phase of magmatism of the Korostensky complex. Our conclusion differs from the mainstream view: vein Li-F granites of the pluton are products of crystallization of deep fluid flows that penetrated into the fissure cavities of the pluton rocks after the formation of magmatic massifs. In our opinion, the Liznykivsky granites are metasomatic formations that have developed on different-age rapakivi-like biotite-amphibole granites under the influence of deep fluid flows. The rare-metal granites of the Kamianomohylsky complex form three massifs in the Azov region: Kamiana Mohyla, Katerynivsky and Starodubivsky. Initially, all granite massifs were formed in the process of crystallization of magmatic melt, which later changed to metasomatic alteration of granites. Post-magmatic changes in granites that contributed to the development of rare metal mineralization include: 1) biotitization; 2) microclinization; 3) early albitization; 4) greisenization; 5) late albitization. According to the data of thermobarogeochemical studies, the stage-by-stage development of the post-magmatic alteration of the Kamiana Mohyla granitoids was substantiated and physicochemically characterized: microclinization (500-600 °С), albitization (430 °С), topazization (430 °С), silicification (430-440 °С), sulfidization (320 °С), late albitization (290-420 °С), crystallization of xenotime (200-210 °С), vein fluoritization and the growth of fluorite crystals (125-155 °С). The crystallization of ongonites took place at high temperature, which was 1140 °C at the final stage of quartz phenocrysts growth.

Melt-enhanced strain localization and phase mixing in a large-scale mantle shear zone (Ronda peridotite, Spain)

Abstract. Strain localization in upper-mantle shear zones by grain size reduction and the activation of grain-size-sensitive deformation mechanisms is closely linked to phase mixing. With its mylonitic grain size (50–100 µm) and well-mixed phase assemblage, the kilometer-scale shear zone at the northwestern boundary of the Ronda peridotite is, in this respect, no exception. In transects across the high-strain mylonitic into the low-strain tectonitic part of this shear zone, the following four dominant microstructural domains were identified: (1) olivine-rich matrix, (2) mixed matrix, (3) neoblast tails of clinopyroxene porphyroclasts, and (4) neoblast tails of orthopyroxene porphyroclasts. In these domains, phase mixing and its impact on strain localization were investigated by a combination of microstructural (optical microscopy), textural (EBSD), and geochemical (EPMA) analysis. The dominant microstructural domain of all samples is the mixed matrix composed of olivine, orthopyroxene, and clinopyroxene. Its homogenous distribution of interstitial pyroxenes contradicts mechanical mixing. Instead, extensive phase mixing under near-steady-state conditions is documented by the constant grain size and by phase boundary percentages > 60 % for the entire mylonitic unit and all the microstructural domains. Lobate phase boundaries, homogenous phase mixing, and secondary-phase distribution, as well as continuous geochemical trends that are independent of the microstructural domain, point to a reaction-driven, metasomatic formation of the mixed matrix and pyroxene porphyroclast tails in the entire shear zone. An OH-bearing metasomatism by small fractions of evolved melts is indicated by amphibole abundance in pyroxene neoblast tails, olivine B-type-crystallographic-preferred orientations (CPOs), and the microstructural consistency of the garnet–spinel (grt–spl) mylonites from both major peridotite massifs of the Gibraltar arc, Ronda, and Beni Bousera (Morocco). The established syn-deformational temperature of 800–900 ∘C at 1.95–2.00 GPa suggests that the metasomatism did not reset the equilibrium temperatures. Consistent geochemistry and phase assemblage in mylonites and tectonites but a change from equiaxial (tectonites) to wedge-shaped pyroxenes aligned parallel to the foliation (mylonites) point to a pre- to syn-deformational metasomatism, with the potential annealing of the tectonites. For the mylonitic mixed matrix, wedge-shaped pyroxenes, and neoblast tail formation in pyroxene porphyroclast stress shadows point to the activity of incongruent dissolution–precipitation creep. Apart from the dissolution–precipitation creep, strong CPOs of all major phases (ol, opx, and cpx) suggest dislocation creep as being the major deformation mechanism in the entire shear zone.

Petrographic composition and ore potential of low-temperature metasomatites of the Middle-Dniprean mega-block of the Ukrainian Shield

Purpose. To substantiate the formation conditions, distribution patterns and metallogenic specialization of low-temperature metasomatites of the Middle Dnipro megablock of the Ukrainian Shield based on the research results obtained. Methodology. The authors used a set of field and laboratory methods. Petrographic, mineragraphic, petrochemical studies were carried out. Interpretation of the results of chemical, laser microspectral, X-ray diffraction, scintillation spectral, thermal analyses is carried out. Findings. A generalized petrological description of low-temperature metasomatites of the Middle Dnipro megablock of the Ukrainian Shield is carried out, taking into account the composition of the original rocks, characteristics of newly formed mineral parageneses, metallogenic specialization and patterns of distribution of metasomatic formations. The study on occurrences of vertical and horizontal metasomatic zoning within the greenstone structures of the Middle-Dniprean region was carried out. The composition of telescoped metasomatites spatially related to the intersection of deep faults is characterized. The metallogenic specialization of low-temperature metasomatic formations is determined and the process of formation of ore concentrations of complex composition in the nodes of intersection of deep faults has been characterized. Based on the results of studies on the composition and ore content of metasomatites, an insignificant level of erosional truncation of vertical ore-metasomatic columns within greenstone structures is substantiated. Originality. The aspects of petrology and ore content of the group of low-temperature metasomatic formations of the Middle Dnipro megablock of the Ukrainian Shield have been revealed. The spatial relationship of the metasomatic areas with the systems of deep faults has been substantiated, which justifies using the identified metasomatic formations as geological indicators of deep fault zones. The features of ore content of low-temperature metasomatic formations of the Middle Dnipro megablock are characterized. It is shown that formations of telescoped metasomatites with a complex composition of mineralization are spatially associated with the intersection of deep faults. Practical value. The metallogenic specialization of low-temperature metasomatic formations has been determined, which can be used as a search criterion for various types of deposits of metallic and non-metallic minerals within the Ukrainian shield.

Features of structural tectonics and geochemistry of Tomulakh metamorphism zone

The methods of geological and geochemical modeling for prospecting of mineralization in areas associated with the deposits of the Talnakh ore-forming system are considered. The geological and geochemical model is based on the geological and structural analysis of the spatial relationships of geochemical anomalies, which allows us to quantify the geochemical types of halos caused by both the multi-stage formation of deposits and the presence of different types of mineralization, which contributes to solving the forecasting problems. The subject of the study is the anomalous Tomulakh metamorphism zone (TMZ), the tectonic conditions of its formation, the material composition of metamorphic and metasomatic formations and facies, and their geochemical expression. The TMZ, located in the north of the ore-bearing Kharaelakh intrusive of the Talnakh ore-magmatic system, is traced along the Axial-Dyangin fault. The high density of faulting and the composition of the tectonic breccias in the upper exocontact zone ensure propagation of metamorphosing fluids over a considerable distance in the vertical and lateral directions. Within the TMZ, which is the flank part of the fluid-magmatic system, fluids evolve in space and time from alkaline to acidic. Alkaline and acidic near-fracture metasomatites developing along tectonic breccias are marked by the geochemical associations Y-Mo-Zr and Be-Mo-Zr. Ore mineralization in the form of rare scattered inclusions of pyrite, chalcopyrite, pyrrhotite, and sphalerite is mapped by the Zn-Cu-Ni-Co specialization. The development of ore mineralization in the contact-metamorphosed rocks of the upper Devonian TMZ is considered as a continuation of the contact halo of the Ni-Cu-Co specialization of the Kharaelakh intrusive. Calciteanhydrite metasomatites (americophile) and anhydrite marble in the top exocontact TMZ occur in the zone of dispersed mineralization and Sr anomalies traceable along the Axial-Dyingin fault and being the extension of the contact envelope of the Kharaelakh intrusion. It is assumed that the TMZ is formed under the influence of the fluid-magmatic Talnakh ore-forming system.

Types of Tellurium Mineralization of Gold Deposits of the Aldan Shield (Southern Yakutia, Russia)

The published and original data on the tellurium mineralization of gold ore deposits of the Aldan Shield are systematized and generalized. The gold content is related to hydrothermal-metasomatic processes caused by Mesozoic igneous activity of the region. The formation of tellurides occurred at the very late stages of the generation of gold mineralization of all existing types of metasomatic formations. 29 tellurium minerals, including 16 tellurides, 5 sulfotellurides and 8 tellurates have been identified. Tellurium minerals of two systems predominate: Au-Bi-Te and Au-Ag-Te. Gold is not only in an invisible state in sulfides and in the form of native gold of different fineness, but also is part of a variety of compounds: montbrayite, calaverite, sylvanite, krennerite and petzite. In the gold deposits of the Aldan Shield, three mineral types are distinguished: Au-Ag-Te, Au-Bi-Te, and also a mixed one, which combines the mineralization of both systems. The decrease in the fineness of native gold is consistent with the sequence and temperatures of the formation of Te minerals and associated mineral paragenesis from the epithermal–mesothermal Au-Bi-Te to epithermal Au-Ag-Te. The conducted studies allowed us to determine a wide variety of mineral species and significantly expand the area of distribution of Au-Te mineralization that indicates its large-scale regional occurrence in the Aldan Shield.

СУЛЬФОАРСЕНИДНОЕ ОРУДЕНЕНИЕ С ЗОЛОТОМ В ЗОНЕ СМЯТИЯ НА ВОСТОЧНО-ЗАБАЙКАЛЬСКОЙ ВЕТВИ МОНГОЛО-ОХОТСКОЙ СУТУРЫ (МЕСТОРОЖДЕНИЕ ПОГРОМНОЕ, ВОСТОЧНОЕ ЗАБАЙКАЛЬЕ, РОССИЯ)

Pogromnoe deposit of gold-sulfide-quartz formation of stringer-disseminated ore type is formed into shear zone between overthrusts within the block of volcanic-sediment basin along the part of the Mongol-Okhotsk suture. Paragenesises of zonal quartz-albite-mica metasomatic formation correspond to profile of the moderate acid leaching. Near ore metasomatic replacements are dated as 139.5± 1.2 Ma. The absolute age of gold-bearing stockworks formation is 131±1.2 Ma. Ore minerals are arsenopyrite, pyrite, additional sphalerite, chalcopyrite, pyrrohotite, sulfosalts, gersdorffite, rare molybdenite and magnetite. Diffusion coefficient of As was calculated into zonal metacrystals of arsenopyrite and pyrite in ores. The established temperature interval of ore gold sedimentation is based on the physico-chemical modeling with SEJEKTOR software as equal 350–150°C. The ore gold formation was generated by sulfoarsenide hydrothermal solution containing gold as AuAsS2 and H2AuAsS3 complexes for concentration of dissolved S ≥ 0.01 ÷ 0.001 m and As ≥ 0.005 ÷ 0.0005 m respectively.

Magmatic haggertyite in olivine lamproites of the West Kimberley region, Western Australia

Abstract We report the first occurrence of magmatic haggertyite (BaFe6Ti5MgO19) from the Miocene lamproites of the West Kimberley region of Western Australia. This contrasts with the metasomatic formation reported in an olivine lamproite host at the type locality, Prairie Creek, Arkansas. Haggertyite occurs in the groundmass of a diamondiferous olivine lamproite pipe in the Ellendale field, and within the large zoned Walgidee Hills lamproite where it forms part of an extensive suite of Ba- and K-bearing titanate and Ti-rich silicate minerals. The haggertyite co-exists with chromian spinel, perovskite, and ilmenite in the Ellendale lamproite, and with priderite and perovskite and, in one locality, with priderite, jeppeite, ilmenite, and perovskite, in the Walgidee Hills lamproite. Unlike priderite and perovskite, which are common groundmass phases in the Ellendale olivine lamproites and present throughout the Walgidee Hills lamproite, haggertyite appears restricted in its occurrence and crystallization interval, with sparse ilmenite apparently mostly crystallizing as an alternative phase. In the Walgidee Hills lamproite the haggertyite-bearing assemblage is succeeded by the Ba-titanate assemblage priderite plus jeppeite in the evolved central part of the body. The haggertyite in the main zone of the Walgidee Hills lamproite has an average composition of (Ba0.7K0.3)1.0(Ti5.0Fe2.13+Cr0.1Fe3.82+Mn0.2Mg0.6Na0.1)12O19 and is thus very similar to the original haggertyite described from xenoliths in the Prairie Creek lamproite apart from being poorer in Cr and Ni. Haggertyite in the groundmass of the Ellendale olivine lamproite and the central zone of the Walgidee Hills lamproite, in addition to variations in Mg and Cr, show significant variation in Ti and Fe contents and in calculated Fe3+ and Fe2+. A linear inverse relationship between Ti and Fe, and Ti and Fe3+, indicates that Fe3+ is accommodated by the coupled substitution Ti4+ + Fe2+ ⇆ 2 Fe3+. A marked trend to higher Fe3+ in the haggertyite in Ellendale 9 olivine lamproite is ascribed to increasing oxidation during crystallization, with fO2 estimated from the olivine-spinel thermometer and oxygen barometer at Dlog FMQ = –1 to +3 at temperatures of 790–660 °C. The haggertyite in the central zone of the Walgidee Hills lamproite, in contrast, shows a marked trend to Fe2+ enrichment, which is associated with decreasing Fe in perovskite. This is inferred to indicate formation under more reducing conditions, but sufficiently oxidized to permit Fe3+ in co-existing priderite and jeppeite. Trace-element analysis by LA-ICP-MS shows the Walgidee Hills haggertyite contains minor amounts of Na, Si, Ca, V, Co, Zn, Sr, Zr, Nb, and Pb, and only traces of Al, P, Sc, Rb, REE, Hf, and Ta. Moreover, the haggertyite is preferentially enriched in certain lithophile (Ba, Sr), siderophile (Mn, Fe, Co, Ni), and chalcophile (Zn, Pb) elements relative to co-existing priderite. Haggertyite crystallization appears to be a consequence not only of the very high Ba, Ti, and K contents of the lamproite, but of relatively high-Fe concentrations and low temperatures in evolved olivine lamproite magma with the Fe3+/Fe2+ ratio determined by the prevailing fO2. The new data suggest that haggertyite might also be present but previously unrecognized in the evolved groundmass of other olivine lamproites. Haggertyite is one of an increasing number of new minerals in upper mantle rocks and volcanics derived from the upper mantle hosting large-ion-lithophile and high field strength cations.

Mineralogy and geochemistry of chloritholites from the Nepryakhino gold field, South Urals

The chlorite rocks (chloritolites) exposed in a bedrock of the Mokhovoe boloto (Moss swamp) gold placer (East Uralian Megazone, South Urals), which occurs on ultramafc rocks, are studied. The trace element composition of chloritolites is characterized by elevated contents of Mn, Ti, V (hundreds of ppm), Cu, zn, Ni, Co, Cr, zr, Li, Sc (tens of ppm), w, zr, Y and REE. Chloritolites contain up to 3 vol. % of disseminated magnetite, ilmenite and accessory minerals (rutile, xenotime, monazite, zircon, apatite, scheelite, U-bearing thorite) from a mineral assemblage, which cocrystallize with the main volume of chlorite. The mineralogical and geochemical features of the Mokhovoe boloto chloritolites and gold-bearing chloritolites of the Karabash massif in the Main Uralian Fault zone are slightly similar. The elevated Ti and P contents of the studied chloritolites, the level of REE contents corresponding to mafc rocks, and the lack of relict chromite indicate their possible metasomatic formation after dolerite dikes known within Chebarkul-Kazbai ultramafc complex. Figures 8. Tables 7. References 14.

Geochronological constraints on uranium mineralization within the Hüttenberg siderite deposit (Eastern Alps, Austria)

A small-scale hydrothermal uranium mineralization hosted within the siderite deposit at Hüttenberg (Eastern Alps, Austria) was re-investigated using modern scanning electron microscope (SEM) and microprobe (EMPA) methods. The uranium mineralization comprises brannerite, coffinite and uraninite, and is spatially associated with Fe-, Ni- and Co-arsenides (loellingite, rammelsbergite, safflorite), bismuth and bismuthinite, as well as rare accessory silver-bearing minerals and gold locally. The U-bearing fluids also carried P, Y and REEs, which precipitated as xenotime and P–Y coffinite. The uranium mineralization paragenetically postdates the metasomatic formation of the host siderite ore.Uraninite allows for precise single spot EMPA ages to be calculated (±2 Ma) due to its high radiogenic lead content. There is an excellent internal consistency in the uraninite data with calculated dates ranging between 77 and 84 Ma. We interpret that these record the crystallization age of the uraninite, and that any influences of lead loss or common Pb are minimal. Brannerite was also analyzed but U-total Pb microprobe data scatter (c.30–80 Ma) with evidence of lead loss, particularly in altered domains. It does, however, provide a minimum constraint on the timing of mineralization that is consistent with the crystallization age of the uraninite. Coffinite proved completely unsuitable for U-total Pb geochronology due to extreme lead loss. However, hydrothermal xenotime yields a U-total Pb age of 78 ± 5 Ma, consistent with the uraninite age. Therefore, the U-total Pb geochronological data support a late Cretaceous age for the U-mineralization of c. 80 Ma. Considering this timing constraint together with other geological and indirect thermochronological aspects, we conclude that the siderite deposit of Hüttenberg is older and formed at c. 90−80 Ma.

On the question of creation of secondary porousity in large-depth sediments of the south-east of the Dneprovsk-Donetsk depression

Introduction. Currently, most researchers associate the main prospects of oil and gas potential with great depths. The most important component of the problem of oil and gas potential at great depths is the problem of reservoir availability. This problem still has no clear solution. From the point of view of the classical theory of stadial catagenesis, with depth, the primary porosity naturally and irreversibly decreases, and we can only talk about how quickly this happens. In this case, large depths appear to be a zone of fading oil and gas potential without any special prospects. The theory of superimposed catagenesis comes to the rescue, which shows that there are processes that naturally lead to the formation of secondary porosity, a secondary reservoir, and then large depths represent a special deep zone of gas accumulation. According to the views of many researchers, including the authors of this work, quartz sandstones with regenerative quartz cement – quartzite sandstones - are the most promising in terms of reservoir properties at great depths. The aim of the work is to establish the regularities of compaction of quartzite sandstones with depth, to identify anomalies in the compaction process and to try to interpret them using mineral indicators of hypogenic-allogeneic porosity formation – one of the varieties of superimposed catagenesis of great depths. Previous study. The theory of metasomatic reservoir formation as a result of hypogenic-allogeneic porosity formation is one of the most developed. It is based on the penetration of a certain favorable composition of deep aggressive fluids into the reservoir during the period of tectonic-magmatic activation with leaching of some components and deposition of others. It is assumed that this process is accompanied by the crystallization of certain indicator minerals. Result of work. The graphs constructed in the course of this work reflect both a natural change in the porosity of this type of Sandstone with depth, and show an anomaly associated with some superimposed process that allows for different explanations. It is natural to use indicator minerals to solve the problem, but the analysis shows that specific indicator minerals of hypogenic-allogeneic decompression have not yet been found, and all the minerals involved can be interpreted as minerals of one of the stages of stadial catagenesis. This does not disprove the possibility of the formation of a secondary metasomatic reservoir by the above mechanism, but it is not possible to prove its implementation by the indicator minerals noted in the studied samples. Relevance. The question of the formation of secondary porosity is of great practical importance for assessing the prospects of gas content: either large depths are represented by the area of naturally decreasing gas accumulation, or the area of naturally occurring secondary reservoirs with good oil and gas prospects, which can be distinguished as a separate deep zone with its own specific power sources, reservoirs and cap rocks.

Cretaceous molybdenite in metasomatic epidosite associated with the Pounamu ophiolite, New Zealand

ABSTRACTEpidosites occur in association with pelagic and quartzofeldspathic metasediments overlying the Pounamu suprasubduction zone ophiolite. They are dominated by epidote with minor diopside, tremolite, titanite, zircon and quartz. Detrital zircon age spectra are similar to those of associated quartzofeldspathic schist, with minimum ages of Cretaceous (c. 106 Ma). Epidosite was derived by metasomatic replacement of this metasedimentary protolith by Ca-rich fluids, likely derived from the underlying ophiolite. One epidosite contains laminae and veinlets of molybdenite, which yields a Re/Os age of c. 89.7 ± 0.6 Ma. Metasomatic formation of epidosite and molybdenite mineralisation occurred during the interval between c. 106 and c. 70 Ma when the Pounamu terrane component of the Alpine Schist underwent accretion, subduction, hydrothermal alteration, mineralisation and metamorphic recrystallization. Molybdenite occurrence is unique in the context of Haast Schist metallogenesis, with Mo most likely derived by leaching and redistribution from the sedimentary cover of the Pounamu ophiolite.

ORE CONTROL OF KHIZOVAARA STRUCTURE DEPOSITS

Abstract
 Ore-controlling factors determine the patterns of formation and localization of mineralization within ore regions and deposits. The need for this study arises from the importance of integrated assessment of mineral resources and improvement of metasomatic formation techniques. This is especially important for geological materials which are mined for their direct commercial value (industrial materials). This article is devoted to the study of the ore control of complex industrial minerals. The Khizovaara structure belongs to the Tikshozero greenstone belt. Within the structure, a multistage metamorphism and metasomatism processes are manifested. The totality of lithological, structural and petrologic ore control factors determines the existence within the structure of several deposits. These are deposits of industrial minerals, such as garnet, quartz, muscovite, kyanite, staurolite. In almost all cases, the ores are complex. The following objects were studied: Southern Lens (kyanite + quartz) deposit, Northern lens (kyanite + quartz) deposit, East Khizovaara (muscovite + quartz) deposit, Vysota-181 (garnet + staurolite + kyanite + muscovite + quartz) deposit, ore occurence Fuxit (decorative rocks). For the ores of each site, the processes of regional metamorphism of the amphibolite facies of kyanite-biotite and muscovite-chlorite-kyanite subfacies are important. Metamorphism, tectonic regime and geological connection with rocks has been studied as a ore control factor, based on this, data on the quantitative distribution of industrial minerals of metamorphic genesis have been obtained. Acidic and alkaline metasomatites of each site are considered. On the basis of these data, metasomatic processes that lead to the formation of complex ores are revealed. The process of superposition of metasomatosis products of the late stage on the products of early stage metasomatosis was studied. This process leads to the formation of complex ores of three or four minerals. The result of the work is a general scheme of metamorphic and metasomatic ores control

Petrologic nature of the active subarc crust-mantle boundary: Mixed magmatic-metasomatic processes recorded in xenoliths from Sabtang island, Luzon arc

Some arc magmas reside in the uppermost mantle and the lower crust. Their deep-seated behavior determines the composition of magmas that erupt at the surface. Mafic-ultramafic xenoliths newly found in Sabtang island, Batanes group of islands of the Luzon arc record subarc processes. The xenolith suite is comprised mainly of dunites, orthopyroxenites, clinopyroxenites, hornblendites, and gabbros, all hosted in basaltic to andesitic lavas. Petrographic characteristics suggest the metasomatic formation of orthopyroxenites and hornblendites from dunites and clinopyroxenites, respectively. The apparently primary minerals are homogeneous in composition. Olivine is relatively magnesian (Fo82–90) and chromian spinel is rich in Cr# (=Cr/[Cr + Al], around 0.7) in dunites. Clinopyroxene is relatively magnesian (Mg# = Mg/[Mg + Fe2+] = 0.73–0.93) in clinopyroxenites and gabbros, and plagioclase is highly anorthitic (An89–98) in the gabbros. The primary mineral assemblage reflects crystallization of olivine and spinel followed by clinopyroxene all occurring in the uppermost mantle and lower crust of the Luzon arc. The orthopyroxenes and amphiboles were metasomatically produced at the expense of olivine and clinopyroxene, respectively. Clinopyroxene in the xenoliths is in equilibrium with the magmas that formed the Sabtang volcanics. They have relatively elevated contents of large-ion lithophile elements and light-rare-earth elements, which suggest derivation from an enriched mantle. The Sabtang xenoliths evidence the very active modification of the subarc mantle-crust boundary zone by mantle-derived magmas and slab-derived melts/fluids so that the mineral assemblage of the resultant rocks is similar to that of the predominant recent magma.

Decoding of Mantle Processes in the Mersin Ophiolite, Turkey, of End-Member Arc Type: Location of the Boninite Magma Generation

The Mersin ophiolite, Turkey, is of typical arc type based on geochemistry of crustal rocks without any signs of mid-ocean ridge (MOR) affinity. We examined its ultramafic rocks to reveal sub-arc mantle processes. Mantle peridotites, poor in clinopyroxene (<1.0 vol.%), show high Fo content of olivine (90–92) and Cr# [=Cr/(Cr + Al) atomic ratio] (=0.62–0.77) of chromian spinel. NiO content of olivine is occasionally high (up to 0.5 wt.%) in the harzburgite. Moho-transition zone (MTZ) dunite is also highly depleted, i.e., spinel is high Cr# (0.78–0.89), clinopyroxene is poor in HREE, and olivine is high Fo (up to 92), but relatively low in NiO (0.1–0.4 wt.%). The harzburgite is residue after high-degree mantle melting, possibly assisted by slab-derived fluid. The high-Ni character of olivine suggests secondary metasomatic formation of olivine-replacing orthopyroxene although replacement textures are unclear. The MTZ dunite is of replacive origin, resulted from interaction between Mg-rich melt released from harzburgite diapir and another harzburgite at the diapir roof. The MTZ dunite is the very place that produced the boninitic and replacive dunite. The MTZ is thicker (>1 km) in Mersin than in MOR-related ophiolite (mostly < 500 m), and this is one of the features of arc-type ophiolite.

Diamond ages from Victor (Superior Craton): Intra-mantle cycling of volatiles (C, N, S) during supercontinent reorganisation

The central Superior Craton hosts both the diamondiferous 1.1 Ga Kyle Lake and Jurassic Attawapiskat kimberlites. A major thermal event related to the Midcontinent Rift at ca. 1.1 Ga induced an elevated geothermal gradient that largely destroyed an older generation of diamonds, raising the question of when, and how, the diamond inventory beneath Attawapiskat was formed. We determined Re–Os isotope systematics of sulphides included in diamonds from Victor by isotope dilution negative thermal ionisation mass spectrometry in order to obtain insights into the age and nature of the diamond source in the context of regional tectonothermal evolution. Regression of the peridotitic inclusion data (n=14 of 16) yields a 718 ± 49 Ma age, with an initial 187Os/188Os ratio of 0.1177 ± 0.0016, i.e. depleted at the time of formation (γOs −3.7 ± 1.3). Consequently, Re depletion model ages calculated for these samples are systematically overestimated. Given that reported 187Os/188Os in olivine from Attawapiskat xenoliths varies strongly (0.1012–0.1821), the low and nearly identical initial Os of sulphide inclusions combined with their high 187Re/188Os (median 0.34) suggest metasomatic formation from a mixed source. This was likely facilitated by percolation of amounts of melt sufficient to homogenise Os, (re)crystallise sulphide and (co)precipitate diamond; that is, the sulphide inclusions and their diamond host are synchronous if not syngenetic.The ∼720 Ma age corresponds to rifting beyond the northern craton margin during Rodinia break-up. This suggests mobilisation of volatiles (C, N, S) and Os due to attendant mantle stretching and metasomatism by initially oxidising and S-undersaturated melts, which ultimately produced lherzolitic diamonds with high N contents compared to older Kyle Lake diamonds. Thus, some rift-influenced settings are prospective with respect to diamond formation. They are also important sites of hidden, intra-lithospheric volatile redistribution that can be revealed by diamond studies. Later emplacement of the Attawapiskat kimberlites, linking the carbon cycle to the surface, was associated with renewed disturbance during passage of the Great Meteor Hotspot. Lherzolitic diamond formation from oxidising small-volume melts may be the expression of an early and deep stage of the lithospheric conditioning required for the successful eruption of kimberlites, which complements the late and shallow emplacement of volatile-rich metasomes after upward displacement of a redox freezing front.

COMMENTS ON «THE OPEN LETTER TO THE MEMBERS OF THE BUREAU OF THE NATIONAL STRATIGRAPHIC COMMITTEE OF UKRAINE AND FELLOW PRECAMBRIAN GEOLOGISTS» V.P. KYRYLYUK

V.P. Kyrylyuk proposals as to stratigraphic subdivision of early Precambrian formations of Ukrainian Shield as a whole and rock association of Dniester-Bug megablock in particular are analysed. On an example of supercrustal formations of Dniester-Bug megablock it is shown, that among rocks of granulitic association, in addition to that stratogenic formations (primarily sedimentary, volcanogenic, chemogenic, and others) there are more late (intrusive-magmatic, metasomatic) rocks which formation is caused by the superimposed endogenic activity. The morphology of these units, their relations with other supercrustal rocks and occurrence modes are formed as a the result of the same superimposed endogenic activity. Simple assumption of these formations to be the treated as suites will lead only to increase in volumes of these stratigraphic units and the interpreation of contacts of their vein (intrusive magmatic and metasomatic) varieties as primary stratification can cause wrong interpretation of tectonic structure of the territory. Plutonic (metamorphosed dikes of diabases) and metasomatic (biotite, a garnet-biotite gneisses, granulites and eulysite) formations are also found in rhythmical-layered alumina-basic formations. If these rocks be excluded from geological section, remained petroparagenesis will include hyperstene, bi-pyroxene plagiogneisses and crystalloshists, that is typical of Tyvrivka strata (enderbite-gneiss formation). Leucocratic biotite and garnet-biotite gneisses that comprize about 80 % of high-alumina-quartzitic formation volume (Kosharo-Oleksandrivka suite) are also metasomatic in origin.

Geochemical typification of kimberlite and related rocks of the North Anabar region, Yakutia

The results of geochemical typification of kimberlites and related rocks (alneites and carbonatites) of the North Anabar region are presented with consideration of the geochemical specification of their source and estimation of their potential for diamonds. The content of representative trace elements indicates the predominant contribution of an asthenospheric component (kimberlites and carbonatites) in their source, with a subordinate contribution of vein metasomatic formations containing Cr-diopside and ilmenite. A significant contribution of water-bearing potassium metasomatic parageneses is not recognized. According to the complex of geochemical data, the studied rocks are not industrially diamondiferous.

Porosity characteristics of the Eocene mixed-sediment reservoirs in the saline lacustrine basin, Qaidam, China

In recent years, significant progress has been achieved in the field of deep exploration in the Yingxi area of the Qaidam Basin. The Eocene was characterized by mixed sediments in the lacustrine basin. The area consists of lacustrine carbonate, detrital material, and mud. All rocks contain numerous salt minerals. Via core observation, physical property analysis, capillary pressure tests, image logging, and field emission scanning electron microscopy, results reveal that the rocks contain a double porosity system. The first are fractures and salt mineral dissolution pores possessing a large pore radius, and the second are cracks between the salts and dolomite intercrystalline pores with a small pore radius. These two types of pores caused the double-platform feature with a lower narrow portion and upper wide portion on capillary pressure curves. Overall, the physical properties of the rocks are deficient, with low porosity-ultra-low permeability. Moreover, pores and throats are generally acceptable. Mineralogical composition analysis based on electronic probe shows that the rocks possess a high content of both silicon and aluminum, but low content of iron and manganese. The average FeO content in the dolomicrite is 1.322%, and that of MnO is 0.060%, both of which are lower than average Fe or Mn content in hydrothermal dolomite. It is believed that the dolomicrite possesses a penecontemporaneous metasomatic formation mechanism. The dolomitization process produces a large number of shrunken intercrystalline pores. These kinds of matrix pores, functioning as storage space, constitute an important support factor for the sustained and stable production of oil and gas in the study area.