2011

Òåçèñû ìåæäóíàðîäíîé êîíôåðåíöèè

Ðóäíûé ïîòåíöèàë ùåëî÷íîãî, êèìáåðëèòîâîãî

 è êàðáîíàòèòîâîãî ìàãìàòèçìà

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Granitoids of Subpolar Urals (Kulemshor massif) and related

rare-metal/rare-earth mineralization

Udoratina O.V.^*, Varlamov D.A.^**, Kapitanova V.A.^*

^* Institute of Geology of Komi SC UB RAS, Syktyvkar (Russia), ^** Institute of Experimental Mineralogy, Chernogolovka (Russia)

udoratina@geo.komisc.ru

The rare-metal mineralization in the north of Urals Mountains is most often related to occurrences of granitoid magmatism. The Kulemshor Late Cambrian granite massif, which majority of rocks contains abundant ore mineralization (Zr, Nb, U+Th, REE, Ti), is the typical representative of similar type of magmatism. The mineralization is represented by ilmenite-magnetite-titanite association with zircon, allanite, apatite and also different late minerals of Y, REE, Nb, Th, U.

Granitoids of the Kulemshor massif are developed in the upper courses of River Torgovaya in Subpolar Urals. The massif is the most southern outcrop of a large Torgovsky massif exposed within the southern part of the Lyapin megaanticlinorium of the Central Ural uplift.

The majority of researchers are carried by rocks of massif to salnersky-mankhambovsky granodiorite-granite complex. The granitoids penetrate green shale metamorphites of Sablegorskaya formation (R-G₁sb), in the western part accurate intrusive contacts are observed; in the eastern part the contact is tectonic. Rocks of the massif are overlapped by Lower Ordovician sediments of Telposskaya formation (O₁tl), which contain abundant products of granitoid rocks destruction. Granite bodies conform to enclosing sediments.

The massif is multi-phase, its basic part is composed of coarse-grained, frequently gneiss-like and porphyry-like, biotite and bimica granites. The presence of granodiorites and tonalities is observed, which were related earlier to hybrid rocks of endocontact facies. The vein rocks are represented by dykes of metadolerites and aplites. According to many methods the temperature of rock formation estimates in range 535-600^oÑ. By the set of signs the rocks were formed in hypabyssal conditions and they are close to derivatives of dry granitoid magmas in general.

The authors have analyzed granitoids of the southern termination of the massif (in riverhead of River Mort-Kulem-Shor, river basin of Torgovaya), and so-called metagranites (altered albitized granites), enriched by uranium and thorium. The rocks were sampled across the strike of ore zones (the distance between profiles was 250 m) through 100 m between samples; only granite rocks were chosen for studies, metamorphites were not included in the collection.

Granitoids (granodiorites and granites) have light-grey color, taxitic gneiss-like texture and medium-coarse-grained structure. Textural heterogeneities are caused by the linear dislocation of dark minerals.

Granodiorites have medium-coarse-grained blastogranitic and porphyry-like structure, and “relic’s” areas with conserved granitic, subhedral-grained structure. The rocks are composed of (hereinafter – in vol.%) plagioclase (An_1-45), (andesine, oligoclase, albite) – 30, quartz – 20-25, pertite potassium feldspar – less 5, chloritized amphibole – to 20, biotite – to 10. The secondary minerals are represented by albite, chlorite, epidote and zoisite, and their content can reach 20 vol.%.

Granites are characterized by a medium-coarse-grained blastogranitic structure with areas with conserved granitic and graphic structures. Mineral composition is represented by plagioclase (oligoclase, albite – An_3-16) – 15-40, quartz – 20-40, microcline-pertite – 20-40, biotite – < 10, muscovite – 1. The secondary minerals are represented by stilpnomelane, sericite, chlorite and epidote.

Albitized varieties of granites with inclusions of ore minerals are characterized by a medium-coarse-grained granitic, porphyry-like, blastogranitic structure, with areas with conserved granitic, graphic and newly formed granoblastic structures. Mineral composition is represented by plagioclase (oligoclase, albite – A_3-16) – to 20, quartz – 30, microclin-pertite – 40, biotite – 1, muscovite – 1, ore minerals (accessory) – up to 5. Neogenic aegirine and albite are observed.

The basic accessory minerals are represented by allanite, titanite, zircon, (fluorine)apatite, garnet, epidote; the most of them has distinct zonation. Ore minerals – magnetite (seldom Ti-magnetite) and Mn-ilmenite are widely developed; the secondary minerals are represented by stilpnomelane, sericite, chlorite and epidote. Common occurrence of magnetite and ilmenite is characteristic.

The rocks, which form ore zones, distinctly display dinamometamorphism expressed in cataclase, and often in milonitization of rocks. In our opinion it is defining for localization of mineralization. Microstructure of rocks changes from graphic, granitic  structures to structures with developed initial cataclase and further to heavy cataclastic rocks and even to structures of initial milonitization.

Metasomatically altered varieties (cataclastic granites) are albitized. The development of aegirine, biotite, zoned allanite, Mn-ilmenite (up to pyrophanite), often substituted by titanite, neogenic zircon is observed in intercataclase matrix together with quartz-albite aggregate. According to microprobe analysis number of minerals of rare-metal, rare-earth and radioactive elements are determined: thorite, aeshynite (Y,Ca,Fe,Th)(Ti,Nb)₂(O,OH)₆, fergusonite (YNbO₄, including samples with heavy enrichments by Yb and Dy), yttrialite (Y,Th)₂Si₂O₇, xenotime, monazite, bastnesite (Y,TR)(CO₃)F, synchysite Ca(Y,TR)(CO₃)₂F, calcioancylite (Ca,Sr)(Ce,TR)₃(CO₃)₄(OH)₃·H₂O, brannerite (U,Y,TR)(Ti,Fe)O₆, polycrase (Y,Ca,Ce,U,Th)(Ti,Nb)₂O₆, columbite, Nb-bearing rutile, baddeleyite. All yttrium-bearing minerals usually contain considerable impurities of heavy rare-earth elements of yttrium group (Dy, Yb etc. – up to 8-12 wt.% TR₂O₃). In addition, the following minerals are preliminarily diagnosed (they demand additional investigations): gerenite-(Y) Ca₂Y₃Si₆O₁₈·2H₂O, caysichite-(Y) Ca₃(Yb,Er)Y₄Si₈O₂₀(CO₃)₆(OH)·7H₂O, thorianite, various phosphate-silicates of thorium, vanadium-bearing (to 3 wt.% V₂O₅) epidote etc. A part of niobic minerals (usually in the form of inclusions in ilmenite – fergusonite, columbite, Nb-rutile), some part of thorite (inclusions in primary zircon), monazite and xenotime can be related to primary minerals. The major part of these minerals has obviously imposed character: they are visible in the form of coronas, borders and margins, fillings of fractures in primary accessory minerals and in intergranular cavities.

Probably, they were formed as a result of transformation of primary accessories – allanite, titanite, apatite, zircon under the influence of the latest sodium-carbon dioxide metasomatosis. The majority of primary accessory minerals (especially their central zones) contain significant admixtures of REE (apatite, allanite, even titanite – up to 3-4 wt.% Y₂O₃), thorium (allanite, zircon), strontium and fluorine (apatite) and their destruction or transformation liberates rare-earth elements, niobium, thorium, uranium, and also fluorine that conduct to formation of late paragenesis of rare-earth and thorium-uranium minerals. A part of rocks is characterized by yttric (+ heavy REE of yttrium group) specialization, and yttrium and HREE are concentrated in primary titanite, apatite and to lesser degree – in allanite. Their metasomatic transformation forms a complex of yttric minerals (caysichite, yttrialite, polycrase, gerenite, secondary fergusonite and xenotime, etc.). Allanite (central parts) and apatite are the major concentrators of light REE at a magmatic stage and their destruction/transformation results in the formation of rare-earth (fluorine)carbonates like calcioancylite, synchysite and bastnesite, secondary monazite. Thorium phases (sometimes occurring as borders on zircon) are most likely formed during destruction of primary thorium containing zircon. Uranium phases (unlike thorium) as a whole are not characteristic for the massif and occur only in some types of granites and it probably is related to local concentration of uranium at finishing stages of granitoid formation.

Petrochemically the rocks are characterized by a large change of quantity of silica from 67 to 77 weight % and non-continuous alumina content (Al₂O₃ – from 11 to 16 weight %). Granites are divided on two groups tending by the alumina content (wt.%) either to 11 or to 16; the agpaitic index depends on this parameter. At the graph Al₂O₃-SiO₂ points of compositions do not form isolated fields and they form a uniform trend, the alumina content decreases with increasing of silica acidity.

Granitoids are related to rocks of subalkaline series (Na₂O+K₂O) – about 8 wt.%, and they are characterized by potassic-sodic specificity. The rocks of the Kulemshor massif are related by their substrate characteristics to A-type granites. On the diagrams used for geodynamic reconstruction, points of the compositions are located in a figurative field of intraplate granites.

The following elements in the studied granites have contents above a Clarke: HREE, Th, Hf, Se, As, Sb, U, Zr, Y, Ni; below Clarke – Rb, Sr, Cr, Ba, Br, Ta, F.

The rocks are characterized by enrichment with large ionic lithophylous elements. The samples with high content of ore components are different among others on the lines of REE spectra normalized by the values of upper crust. The authors explain changes of REE spectra of samples by the high content of rare-earths containing minerals in rocks. As a whole the studied rocks are characterized by low REE content and moderately differentiated distribution of light REE in comparison to heavy ones, which is reflected by low value of ratio (La/Yb)_N equal on the average to 6. The observed insignificant negative europium anomaly (Eu/Eu* – 0.37) was proved by petrographic and mineralogical data about the presence calcium containing minerals in the rocks.

The obtained new geochemical data on rare-metal containing granitoids of the Kulemshor massif allows to assume the nature of their possible substrate, and also to determine their geodynamic position. The studied granitoids by petrogeochemical characteristics are related to calc-alkaline magmatic series and correspond to group of A-granites formed in intraplate conditions. The age of Kulemshor granite formation is Late Cambrian, corresponding in the history of Subpolar Urals to the beginning of Paleozoic rifting. The granites contain rare metal, rare-earths and radioactive accessory minerals, therefore the contents of Nb, Y, HREE and others rare elements in the rocks are increased and probably due to that the granites are related to A-type. The rare-metal mineralization located in granitoids is undoubtedly late in comparison to formation time of granitoids, since role of post granite’s cataclase is distinctly displayed for ore placement. How late?  This question remains open.

Probably, later there was only the redistribution of ore matter. In our opinion paragenetically the ore mineralization is related to granites, genetically – to processes of imposed sodic (+carbon dioxide) metasomatosis developing on cataclase areas.

The researches are conducted under financial support of integration project 09-C-5-1017, grants of Russian Fund of Basic Researches 09-05-00991, 11-05-01087.