G. S. Ripp

Chrome-bearing mineral phases in the carbonatites of northern Transbaikalia

This paper presents the results of a study of the Vesely carbonatite occurrence in a new carbonatite-bearing area of northern Transbaikalia. The REE patterns, oxygen and carbon isotope compositions of carbonates and magnetites, and strontium isotope composition of dolomite allow us to classify the rocks as carbonatites. This study focused on minerals from xenoliths that were identified as mantle-derived on the basis of structure and mineral composition. The compositions of Cr-bearing chlorite, phengite, magnetite, rutile, ilmenite, and titanite are reported. Aggregates of closely intergrown magnetite and rutile occurring in the xenoliths resemble exsolution structures. A possible origin of these aggregates is discussed.

U-Pb (SHRIMP-II) isotope dating of zircons from alkali rocks of Vitim province, West Transbaikalia

A U-Pb (SHRIMP-II) geochronological study of rocks from the Sayzhenski, Snezhny and Nizhne-Burulzayski massifs, incorporated into the Vitim alkali province (Sayzhenski Complex) has been made. The acquired data indicate that the rocks formed in the interval of 520-486 Ma. The proximity of their age to accretionary-collision processes in the Central Asian fold belt, accompanied by intràplate magmatism, has been noted. Two independent stages in the evolution of Vitim Province magmatism are suggested: Early Paleozoic (520-485 Ma) and Late Paleozoic (350-290 Ma).

Geochronology of the Gulkhen Massif, Vitim alkali province, Western Transbaikalia

The Saizha Complex (first identified in [1])includes more than 20 massifs composed by pyroxenites, gabbropyroxenites, ijoliteurtites, nepheline andalkali syenites. The petrotypical plutons of this complex are the Saizha and Gulkhen plutons. The geochronological dating on Gulkhen massif rocks yieldedthe values ranging from 462 to 279 Ma, so it is impossible to infer the unambiguous time of its formation.The present work considers the data of more preciseU–Pb and Ar–Ar geochronological analyses of themain rock species of the massif. The obtained resultsindicate two stages of rock formation: Early and LatePaleozoic.For dating, the samples of gabbroids (Gulkh376),pyroxenites (Gulkh371), and ijolites (Gulkh379)were collected. U–Pb (SHRIMPII) dating wasimplemented in the Center for Isotope Studies,Karpinskii AllRussia Research Geological Institute(St. Petersburg). The U–Pb ratios were measured bythe technique from [2]. The obtained data were treatedusing SQUID software [3]. The plots were constructedusing ISOPLOT/EX software [4].

Isotopic systematics of the rocks of the Khalyuta carbonatite complex of western Transbaikalia

This paper reports the results of isotopic investigations of the rocks of the Khalyuta carbonatite complex (carbonatites, comagmatic silicate rocks, fenites, and hydrothermal rocks) and host limestones and granites. Pyroxene, amphibole, magnetite, potassium feldspar, apatite, phlogopite, calcite, dolomite, strontianite, celestite, and barite were investigated. The isotopic compositions of C, O, Sr, and S were analyzed. The character of the distribution of oxygen isotopic composition in minerals (carbonates, silicates, phosphates, and oxides) suggests their equilibrium formation. It was supposed that the evolutionary trend of C and O isotopic compositions is mainly related to the processes of differentiation in the melt-fluid system and indicates the absence of significant contamination by carbon and oxygen from a crustal source during rock formation from the magmatic stage to the hydrothermal stage. The isotopic compositions of S and Sr did not change.

Mantle(?) xenoliths in the carbonatites of northern Transbaikalia

The composition and nature of high-Cr minerals in lithic clasts from the carbonatites of the Veseloe occurrence, northern Transbaikalia, were considered. In order to determine their source, the Cr-bearing phases were compared with chromite, magnetite, and rutile from ultrabasic rocks, mantle xenoliths, and eclogites. It was suggested that the xenoclasts studied were formed at great depths, whereas the carbonatites were directly derived from the mantle rather than formed by the crustal differentiation of a silicate-carbonate melt.

Chronology of the formation of the gabbro-syenite-granite series of the Oshurkovo pluton, western Transbaikalia

This paper reports the results of Rb-Sr, 40Ar-39Ar, and U-Pb geochronological investigations for igneous and metamorphic rocks from the regions of the Oshurkovo basic massif. It was shown that the gabbro-syenite-granite complex that was formed there is similar to the bimodal basalt-rhyolite series of volcanic associations. Three major stages of magmatic activity were recognized: syenite-granite (132–127 Ma), basic (126–117 Ma), and granite (121–112 Ma). The silicic igneous rocks were formed owing to anatexis under the influence of heat released from the parent chamber of alkaline gabbroids.

Mineralogy of metamorphosed carbonatite of the Vesely occurrence, Northern Transbaikal region, Russia

Metamorphism of carbonatite is exemplified in the Vesely occurrence. According to available data, the age of the carbonatite is 596 ± 3.5Ma, whereas metamorphism is dated at 550 ± 14 Ma. The rocks at the Vesely occurrence were metamorphosed under conditions of greenschist facies (epidote-muscovite-chlorite subfacies) under elevated pressure. Microthermometry of fluid inclusions in minerals indicates that the temperature of metamorphism is 377−450°C and the pressure estimated from phengite geobarometer is 6−8 kbar. The low-grade metamorphism led to the partial recrystallization of carbonates and apatite with removal of trace elements. This process resulted in a change of the oxygen isotopic composition of the studied minerals. Metamorphism was accompanied by formation of talc, phengite, chlorite, quartz, tremolite-actinolite, and anthophyllite, which are not typical of carbonatite. The data obtained show that the metamorphism exerted an effect on the mineralogical, isotopic, geochemical, and technological properties of the carbonatite. The effect of metamorphism should be taken into account in determination of the nature of ore mineralization and estimation of ore quality and perspective of the occurrence.

Phosphate and sulfate-phosphate mineralization in sillimanite-bearing rocks at the Kyakhta deposit, western Transbaikal region

The phosphate and sulfate-phosphate minerals in the sillimanite-bearing rocks of the Kyakhta deposit are considered. The mineral assemblages of the high-Al rocks were formed during prograde and retrograde stages of metamorphism. The first stage is characterized by the formation of sillimanite, corundum, muscovite, quartz, rutile, titanohematite, magnetite, feldspar, biotite, lazulite, and wagnerite. The muscovite composition showed that sillimanite paragenesis was formed at temperatures above 510–600°C. According to oxygen isotope thermometry, the minimum metamorphic temperature for quartz and titanohematite is 690°C. Andalusite, diaspore, quartz, pyrophyllite, muscovite, and a wide range of phosphates and sulfate-phosphates crystallized during the retrograde stage. The decrease in temperature and increase in the water content led to the following sequence of mineral formation: Mg-Fe-Al-Ca-REE-rich phosphates (lazulite, scorzalite, augelite, apatite, and monazite) → Ca-Sr sulfate-phosphates (woodhouseite and svanbergite) → sulfate (barite) → Sr-Ca-Ba aluminophosphates (goyazite, crandallite, and gorceixite). The chemical compositions of phosphates and sulfate-phosphates minerals and their formation conditions are discussed.

Micas from the Khaluta carbonatite deposit, western Transbaikal region

The Khaluta carbonatite deposit located in the western Transbaikal region was formed during the Late Mesozoic rifting in the southern framework of the Siberian Craton. Carbonatite is associated with shonkinite and syenite and is accompanied by fenitization. The composition of mica in more than 160 samples of country rocks, carbonatites, silicate rocks, and fenites was studied. The Fe3+ and Fe2+ contents, as well as oxygen isotopic composition, were determined. The Mg and Fe contents increase, whereas the Ti and Al contents decrease in micas when passing from silicate rocks and fenites to carbonatites. Micas from carbonatites are depleted in Al, enriched in Fe3+, and distinguished by high Si and F contents. According to our calculations, in some cases Al replaces Si in the tetrahedral site instead of replacement of Fe3+ as is characteristic of tetraferriphlogopite. Formally, the mica from carbonatites falls within the tetraferriphlogopite field, but typical inverse pleochroism is not always observable. The δ18O values of micas from carbonatite, shonkinite, syenite, and fenite are similar to those of mantle-derived silicate minerals. The δ18O values in the minerals coexisting with phlogopite testify to their isotopic equilibrium and make it possible to calculate the crystallization temperature of carbonatite.

Genetic nature of apatite–magnetite ore in North Gurvunur deposit, western Transbaikal region

The paper gives a mineralogical and geochemical characterization of the North Gurvunur deposit, which was discovered in the Eravna ore district. The ore is composed of apatite–magnetite paragenesis. Apatite is distinguished by elevated LREE concentrations; some of them are contained in emulsion-type impregnation of monazite. Hematitization, carbonate, quartz, and pyrite veinlets formed at the postore stage, and gypsum–anhydrite mineralization is widespread in the supraore sequence. Two groups of endogenic minerals are distinguished by oxygen isotopic composition. One of them comprises magnetite and apatite, which are characterized by a homogeneous composition throughout the section of the ore lode and are close to the mantle source. The oxygen–isotope temperature calculated for the apatite–magnetite couple (620–800°C) provides evidence for magmatic origin of ore. The δ18O of fluid in equilibrium with hematite is 8.0–8.5‰ and shows a certain enrichment in crustal component; carbonates of postore veinlets reveal participation of meteoric water. The study has made it possible to refer the North Gurvunur deposit to the Kiruna type.