Mesoarchean subalcalic granitoids and connected TR-U-Th ores in the Azov Domain of the Ukrainian Shield
Syomka V.O., Ponomarenko O.M., Bondrenko S.M., Donskoy N.A., Syomka L.V.
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation, Kiev, Ukraine
Distinct spatial and genetic interrelation of thorium-rare-metallic deposits known by recent time to alkalic and subalkalic rock associations has been found (Kichurchak, 1991). Genetic types of the deposits are different. They have been formed during all stages of postmagmatic process: starting with early pneumatolytic-hydrothermal and finishing late medium-, low-temperature. Particular genetic type of thorium-rare-metallic deposit depends on combination of four factors: type of origin alkalic rock association related; position of ore mineralization during the stage of postmagmatic process; host rocks; combination of different useful minerals. Three alkalic rock associations related to different types of postmagmatic thorium-rare-metallic deposits have been distinguished. There are such rock associations: ultrabasic alkalic related genetically and spatially to carbonatite deposits; alkalic and nepheline syenites; finally, subalkalic and alkalic granites, granosyenites and syenites.
The postmagmatic thorium-rare-metallic deposits are associated with rigid areas of the Earth crust. They have been occurred in edge of shields, ancient platforms, middle massifs, margin zones of early consolidation of fold belts. Characteristic feature of rare-metallic deposits is strong spatial interrelation to parental intrusives. The deposits are often located in endocontact zone of intrusive, and seldom next to exocontact one.
TR-U-Th deposits are characteristic formations of south-western part of Ingul domain of the Ukrainian Shield (Belevtsev, 1995; Syomka, 2008). Although for the first time in recent years the TR-U-Th occurrence has been found in Volyn domain too (Syomka, 2006). General feature of the deposits is spatial interrelation to subalkalic aplite-pegmatoid granites of Paleoproterozoic tectonic-magmatic activaton, stockwork ores (U, Th, Zr, Mo, Bi minerals). Specific ore minerals are broggerite, zircon, monazite, thorite, molybdenite, bismuthite and native bismuth. Average age of uranium ores is 2000 Ma.
Within the junction zone of north-western part of the Western Azov block with the Dnieper-Donetsk depression the Dibrov deposit of TR-U-Th ores has been discovered over recent years (Syomka, 2009; Kichurchak, 1991). The Dibrov brachysyncline is laterally divided in two granite domes: northern and southern ones. Northern dome is oval, 300x400 m. It is extended in submeridional direction. The southern dome’s size is greatly larger. It is extended in sublatitudinal direction. Contacts of granite massifs with host rocks are distinct and active. The southern dome is more significantly eroded, broken up with faults of E-W direction. Host rocks are metamorphic rocks of Mesoarchean kayinkulak strata. Exactly in northern exocontact of this dome the TR-U-Th deposit is located. On chemical composition the Dibrov aplite-pegmatoid granites are like the Mesoarchean mokromoskovsk granites and Paleoproterozoic yanvarsk and krasnokutsk ones. General difference is increased content of alkali (9.4%), higher agpaitic coefficient (0.7). These features allow belonging them to subalkalic type which is specific for middle- and low-temperature TR-U-Th deposits (Nevsky, 1972).
In order to find out age of secondary quartzites and Dibrov aplite-pegmatoid granite the study of zircons using «Element-2» mass-spectrometer has been carried out in Bristol University (UK). Average Mesoarchean ages of aplite-pegmatoid granite (3296±22 Ma), aplite (3171±53 Ma), ore quartzite (2906±170 Ma) have been clear defined as result of the study.
Minimum contents of U and Th in rocks of the Dibrov location have been found in Mesoarchean metamorphized volcanogenic-sedimental formations of kayinkulak strata framed with slightly eroded granite Dibrov dome. Average content of uranium in these rocks varies 2-5 ppm, 2-12 ppm of thorium. Average content of uranium in aplite-pegmatoid granite is 12 ppm, thorium is 16 ppm; that is it exceeds in 2-4 times. Accordingly contents of uranium and thorium are increased in postmagmatic series of granites enriched with potassium and fluids, in aplites (U 16 ppm, Th 31 ppm) and pegmatites (U 27 ppm, Th 87 ppm). Most increased contents (U 418 ppm, Th 591 ppm) have been found in secondary quartzite formed during acid-leach process of aplite and pegmatite. Mineral-concentrators of uranium and thorium are monazite, pitchblende and brannerite.
Rare-earth elements in ore secondary quartzite are represented with cerium group ones. On mass-spectrometric data the sum of lanthanide elements’ content varies from 0.45 до 2.25 %, average content is 1.06 %. Cerium group rare-earth elements content is 98 % of the sum, yttrium one is 2 % only.
On data of spectrometric research accompanying indicator elements of TR-U-Th ores are Nb, Pb, Mo, Bi, Cu, Zn and Sn. By comparison with of Azov domain rocks these elements contents in ore zones are higher in ten times.
In regard to petrochemical features of aplite-pegmatoid granites it should to note the following. On chemical composition they are like Mesoarchean mokromoskovsk granites, Paleoproterozoic yanvarsk and krasnokutsk granites. General difference is increases content of alkali, agpaitic coefficient. It allows belonging them to sublakalic type, which is characteristic for hydrothermal middle- and low-temperature TR-U-Th deposits (Nevsky, 1972).
Bi-feldspar composition and little prevalence of sodium over potassium (K2O/ Na2O=0.9) is a characteristic feature of early stages of Dibrov aplite-pegmatoid granites. In the rocks of late stages (pegmatite and aplite) the total alkalinity has been increased (10.4 %) during microclinization (K2O/Na2O=3.5). During acid-leach process of aplite and pegmatite alkali, magnesium and iron have been got away from the rocks and formed thin biotitic veins which contents increased lithium and fluorine content. Titanium in rutile with pseudomorphic brannerite has been indicated in secondary quartzites while biotite destroying. Correlation between U and Th, silica coefficient (SiO2/Al2O3), железистости (Fe2O3/ FeO), TiO2 content has been established in ore secondary quartzites.
Mineralogic-petrographical and isotopic-geochemistry data given in the paper demonstrate that new genetic type of complex TR-U-Th ores in Mesoarchaean subalkalic granites of Western part of Azov domain which related to secondary quartzites has been found.
Belevtsev Ya.N., Koval V.B. Genetic types and location principles of uranium deposits in Ukraine. Kiev: Naukova Dumka, 1995. 396 p (in Russian).
Kichurchak V.M., Boiko A.Z., Berzenin B.Z. About section of sachkinsk suite of centralno-priazov'e series in outside area north of Dibrov structure (Western Priazovie) // Geologic Journal. 1991. No 4. P. 101-107 (in Russian).
Nevsky V.A. Geology of postmagmatic thorium-rare-metallic deposits. Moscow: Atomizdat, 1972. 408 p (in Russian).
Syomka V.O., Bondarenko S.M. et al. New ore occurrence of potassium-uranium rock series in Kocherivka tectonic structure (North-Western region of the Ukrainian Shield) // Mineralogical Journal. 2006. Vol. 28. No 4. P. 59-75 (in Ukrainian).
Syomka V.O., Ivanov B.N. et al. Petrochemical peculiarities of rare-earth-thorium-uranium deposits in central part of the Ukrainian Shield. Problems of their origin // Mineralogical Journal. 2008. Vol. 30. No 1. P. 94-103 (in Ukrainian).
Syomka V.A., Ponomarenko A.N., Bondarenko S.N. New genetic type of rare-earth-thorium-uranium ores in the Ukrainian Shield / Abstr. vol. of Intern. Conference "Development peculiarities of raw materials base of uranium: exploration and enrichment" (Uranium-2009), September 15-17, 2009, Kiev. 2009. P. 76-78 (in Russian).