Syomka V. Petrology of molybdenum- and tungsten-bearing metasomatites of the Ukrainian Shield

The thesis is aimed at solving actual problems of petrology of molybdenum- and tungsten-bearing metasomatites of the Ukrainian Shield. For the first time in the Ukrainian Shield the mineral facies of sheelite-bearing metasomatites has been distinguished, economic prospection on tungsten ore of the specific skarn types has been proven. Ore-bearing skarns are related to apomagnesian calsic and calsic infiltration varieties. According to investigation results sheelite skarn mineralization is formed during processes of quartz-feldspar metasomatic alterations and acid leaching (actinalitization, fluoritization, silification) of primary barren skarns. The widespread distribution of greisens with wolframite-cassiterite mineralization has been found. Such type of mineralization was found only in the Volyn’ megablock and for first time it has been identified in the Kryvyi Righ-Kremenchuk suture zone. This fact essentially rises the prospection of such type of mineralization for the Precamrian of the Ukrainian Shield. Proterozoic greisens mineralogical facies and ore mineral associations are found to be similar to that ones found in Phanerozoic tin-tungsten deposits that include main resources of these metals of the world. It has been discovered that molibdenum greisen type is widely distributed in Precambrian rock associations of the Ukrainian Shield and it is prospective on tungsten. The presence of several underevaluated deposits in the Volyn’ and Middle Dnieper megablock can testify for this fact. Mineralogy and petrographic features of molybdenum-bearing greseins are found to be dependent on composition of host rocks, after which they are formed. For the first time, metasomatites with rare earth-molybdenum-uranium-thorium genetic type of mineralization , early known only in the Ingul megablock, has been found widely distributed in the Ukrainian Shield. First findings of this type of ore mineralization in the Volyn’ (Bezimennyy ore occurrence) and Near Azovian (Dibrova deposit) megablocks testify for this. It has been proven that common feature of these deposits and ore occurrences is their spatial relation with aplite-pegmatitic granites formed during Paleoproterozoic and Mesozoic tectonic and magmatic activization. This type of mineralization is characterized by stockwork nature of rich ores represented by minerals of U, Th, Zr, Mo, Bi. Typomorphic features of minerals indicative for molybdenum and tungsten ore mineralization, such as sheelite, wolframite and molybdenite, has been identified. For the first time in rocks surrounding skarns the tungsten-containing rutile and sphene have been found and studied. It is shown that composition of trace elements is dependent on composition of the rocks hosting ore-bearing skarns. Investigation on molybdenite polytypism has revealed that 3R-polytype is characteristic for slightly eroded molybdenum deposits. It has been established that high rhenium grades are typical for molybdenytes from greenstone belt rocks which are formed during metamorphic alteration of deep basic rocks (mostly volcanites). It has been proven that molybdenum-tungsten-bearing metasomatites are formed within wide temperature range, from high temperature (skarns and fenites) though middle temperature (greisens, alkaline and silica-potassium metasomatites) and low temperature (secondary quartzite) stages. Deposition of ores occurred within temperature range of 200-400 єC, at low pressure and conditions of infiltration through active faults. Genetic classification of molybdenum- and tungsten-bearing metasomatites has been developed. It has been established that the multiple stage deposition and regeneration nature are characteristic features of Mo-W deposition in Precambrian rock associations of the Ukrainian Shield.