Petrology and geochemistry of rare-metal of syenites in the Ukrainian Shield

Bezsmolova N.V. *

N.P. Semenenko Institute of‰ geochemistry, mineralogy and ore formation NAS Ukraine, Kyiv, Ukraine

Nepheline-free rare-metal syenites, which are characterized by the high content of Zr, TR,Y, Nb, are typical of the Ukrainian Shield. Two deposites and some occurances of the mentioned rare metals are connected with such type of rocks syenites. These rocks are located in three regions of the Ukrainian Shield: Azov area (its Eastern part), south margin of Korsun ó Novomirgorod anortosite-rapakivi granite pluton. ‰The common characteristic of nepheline-free rare metal syenites of the Ukrainian Shield is given.

‰‰‰‰‰‰‰‰‰‰‰ Nepheline-free rare-metal syenites, which are characterized by the high content of Zr, TR,Y, Nb, are typical of the Ukrainian Shield. Two deposites and some occurances of the mentioned rare metals are connected with such type of rocks syenites. Rare-metal syenites distinguish for high iron-contents of femic minerals (fayalite, hedenbergite, aegirine, riebeckite, aegirine-hedenbergite, ferrohastingsite), and high background level of zirconium (0.1-0.3%) and rare earth elements (0.05-0.2%). These syenites are genetically and spatially related to anorthosite-rapakivi granite plutons.

‰Nepheline-free rare-metal syenites are located in three regions of the Ukrainian Shield: Azov area (its Eastern part), south margin of Korsun ó Novomirgorod anortosite-rapakivi granite pluton and Korosten pluton. A wide distribution of such syenites is observed in Azov area, where they compose half of the South-Kalchyk massif.

Azov deposit is located in the same massif. Different gabbroids ó ferrogabbro, ultramafics, gabbro and gabbro-norite, were also exposed by mapping drilling in the South-Eastern part of the South-Kalchyk massif. Isotopic age of the rocks, which compose South-Kalchyk massif, is 1.8 Ma.

‰‰‰‰‰‰‰‰‰‰‰ Syenites of South-Kalchyk massif belongs to subalkaline series and two-feldspat verieties. Dark-coloured minerals are characterized by the high iron-contents (fayalite, hedenbergite, aegirine). The contents of dark-colour minerals in syenites varies from 10 to 30-40%. Besides, FeO dominates over Fe₂O_3. Accessory minerals in syenites are represented by apatite, ilmenite, orthite, and chevkinite. Leikocratic as well as melanocratic varieties of syenites occur. Accoding to their structural features, all varieties - from fine-grained to coarse-grained and pegmatoide are observed.

‰‰‰‰‰‰‰‰‰‰‰ Syenites of Azov deposit are characterized by increasing of alkalinity of rocks and minerals (absence of oligoclase, high contents of alkalines in amphiboles and clinopyroxenes). Moreover, in these syenites britholite, orthite and fluorite appears (in ore amount), by high content of zircon. Syenites of Azov deposit principally don?t different from the syenites of South-Kalchyk massif by their structure signs and ratio of salic and mafic minerals.

‰‰‰‰‰‰‰‰‰‰‰ Syenites of Velyka Vyska massif which spatially related to the South margine of Korsun-Novomirgorod pluton is located among granites rapakivi, and some of them ó among Novoukrainska granites. In this massif fayalite-hedenbergite with ferrohastingsite sienites dominate, which contain about 10-15% of dark-coloured minerals. The content of plagioclase in them usually is 5-10%. Sometimes melanocratic, enriched in zircon and ilmenite varieties of syenites are observed. These syenites,on their chemical and mineralogical features, are similar to syenites of the Azov deposit. These features include: mainly feldspar composition, increased content of acmite component (up to 9%) in clinopyroxene, high content of zirconium (1,5% and more) and also often observed presence of accessory orthite and chevkinite, sometimes fluorite. In syenites of Velyka Vygka massif are chacteristic higher contents of lanthanoids (up to 0.2%) and zirconium (up to 1%), compare to two-feldspar syenites of South-Kalchyk massif.

‰‰‰‰‰‰‰‰‰‰‰ Yastrybetsky syenite massif is located 30 km to the North-West from Korosteny pluton within Sushchany-Perga zone and lies among granites of Perga complex. Massif is represented by differentiated layered intrusion. Yastrybetsky massif consists of endocontact syenites, syenites of the main layered series, quarts syenites (central core), syenites of the upper layered group. The thickness of syenites of the main layered series is more than 1000 m and the layer (cumulative deposits) of the rich zirconium ores is related to them. Namely this kind of rocks is identical to syenites of the Azov deposit.

‰‰‰‰‰‰‰‰‰‰‰ Syenites of Yastrybetsky massif are represented by alkaline feldspar varieties. Oligoclase can be observed in small quantity only in endcontact syenites. Dark-coloured minerals of syenites are strongly enriched in iron and are represented by hedenbergite, aegirine-hedenbergite, aegirine, ferrohastingsite, ferroendenite, riebeckite. Fayalite had not been found. Characteristic accessory minerals are zircon, fluorite, orthite, britholite, ilmenite, bastensite, parisite. Apatite rarely occurs only in endcontact syenites. Structurally syenites of the Yastrybetsky massif are represented by from medium ó to coarse grained up to pegmatoid varieties.

‰‰‰‰‰‰‰‰‰‰‰ Yastrybetskiy massif represents a layered intrusion. Rhythmic layering of the massif is displayed by interlaying of leucocratic and mesocratic syenites. The cryptic layring is expressed by compositional trand of the rock-forming minerals ( to the central core of the massif, from down to up in ‰the main layered series, and from up to down in the upper layered group) of the chemical composition of the rock forming minerals. Also the alkalinity of pyroxenes and amphiboles increase in this direction, decrease the aluminaty of biotite and increase the content of orthoclase minal in the alkaline feldspar. The age on zircon of syenites of the Yastrybetskiy massif, ‰according to the last dating is 1,76 Ma.

‰‰‰‰‰‰‰‰‰‰‰ The syenites of the Davydky massif also belongs to the rare-metal type and this massif is located on the North-East margine of the Korosten pluton. Massif is composed by the layered series of gabbroids (gabbro, troctolites, gabbrodiabases, andesine anorthosites). The central part is represented by syenites. In the Eastern cryptic part of the massif some andesite anortozites are developed, but not in big quantity. Syenites of the Davydky massif are less differentiated than syenites of Veilyka Vygkiy and Yastrybetsky massifs. Syenites of the Davydky massif have increased content of titanium in pyroxene titanium hedenbergite or ferroaugite are, characterized by less zirconium content (up to 0.1%) and lanthanoids. Besides zirconium minerals are represented by zircon as well as baddeleite.

‰‰‰‰‰‰‰‰‰‰‰ ‰As to other deposites of rare-metal syenites of the Ukrainian Shild, Khochyn and Karpylovka consist of biotite and amfibolite syenites with the high content of zircon, orthite and apatite. In general they are similar to the biotite-amfibole syenites of the Yastrybetsky massif.

‰‰‰‰‰‰‰‰‰‰‰ So, nepheline-free rare-metal syenites, enriched in zirconium and lantanoids have quite a wide distribution within the Ukrainian Shield. Deposites of the rich ‰Zr, TR, Y ores are related to these syenites.

References:

Kryvdik S.G. Rare-metal of syenites in the Ukrainian Shield. Kiev: Geokhimia. 2002. ²7. P. 707-717 (in Russian).

Kryvdik S.G., Tkachuk V.I. Petrology of rare-metal of rocks in the Ukrainian Shield. Kiev: Nauk. dumka, 1990. 408p. (in Ukraine).