Genesis and alteration mechanisms of britholite group minerals from ore bodies – related to a Keivy peralkaline granitenepheline syenite complex, Kola Peninsula, NW Russia Dmitry ZOZULYA1, Lyudmila LYALINA1, Raymond MACDONALD2, Boguslaw BAGIŃSKI2, Yevgeny SAVCHENKO1, Piotr DZIERŻANOWSKI2 1Geological Institute, Kola Science Centre, Russian Academy of Sciences, Apatity, Russia 2Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Warsaw, Poland Alkaline rocks of the Kola Peninsula, NE Fennoscandian Shield The Keivy peralkaline granite-syenite complex consists of 2670-2660 Ma aegirine-arfvedsonite granites (six sheet-like massifs of a few hundred meters thickness and a total exposure area of ca. 2500 km2) that intrude the TTG basement of the Central Kola terrane. Small dike-like bodies of 2610 Ma nepheline syenite cut the West Keivy peralkaline granite massif. West Keivy & White Tundra massifs Numerous Zr-Y-REE-Nb ore occurrences and deposits are associated with different lithologies and formed by different genetic (late- and post-magmatic) processes: mineralized apical and veined granites, mineralized nepheline syenites, quartzolites (silexites), various metasomatic rocks, pegmatites. Geochemical constraints on the origin of Keivy peralkaline granite-syenite complex Keivy peralkaline granites have the geochemical features of within-plate or post-collisional Atype granitoids. Associated nepheline syenite is of OIB-like affinity. Sakharjok nepheline syenite (n=30) The rocks of the Keivy complex are extremely enriched in Zr (300-5000 ppm), Y (40-500 ppm), Nb (20-600 ppm), REE (100-1000 times chondrites), which is explained by enriched mantle source of primary melts and extreme fractionation processes. The REE patterns for some samples are of V-shape form, which is a characteristic of hydrothermal alteration. Geology and WR geochemistry of ore bodies Quartzolite (silexite): Mineralized nepheline syenite: - small irregular-shaped bodies (1-2 m) and veins in the endocontact (type 1) and exocontact (type 2) zones of granite massifs; - ore bodies are confined to the nepheline syenite and represent the linear zones of 200-1350 m length and of 3-30 m thickness; - REE (9600-67800 ppm), Y (4570-19600 ppm), Nb (240-14300 ppm), Zr (11200-79700 ppm); - REE2O3 (0.2-0.3 wt. %), Y (500-900 ppm), Nb (150-1200 ppm), Zr (10000-16000 ppm); - RM minerals associations: zircon-britholiteyttrialite (type 1), zircon-fergusonite (type 1), zircon-chevkinite (type 2). - ore minerals: zircon (0.5-1.2 vol. %, rarely up to 2.5 vol. %), britholite (0.2-1.0 vol. %) and pyrochlore. 100000 Quartzolite-1 10000 1000 Quartzolite-2 Nepheline-feldspar pegmatite: - pegmatite body is of zonal structure, outcrops in the area of 30 m2 and consists mainly of nepheline, albite, analcime, pyroxene, biotite; - RM minerals association: britholite- and apatite-group minerals, zircon, meliphanite, leucophanite, gadolinite, hainite, behoite. 100 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu General description of britholite group minerals (BGM, with exception of tritomite species) • • • • • • Britholite-(Ce) (Ce,Ca)5(SiO4,PO4)3OH Britholite-(Y) (Y,Ca)5(SiO4,PO4)3OH Fluorbritholite-(Ce) (Ce,Ca)5(SiO4,PO4)3F Fluorbritholite-(Y) (Y,Ca)5(SiO4,PO4)3F Fluorcalciobritholite (Ca,REE)5(SiO4,PO4)3F «Calciobritholite» (Ca,REE)5(SiO4,PO4)3OH • • • • • (Slovakia, UK, granite) • «Britholite-(La)» (La,Ca)5(SiO4,PO4)3(OH,F) (Angola, carbonatite) • Common accessory REE mineral of alkaline, acid and carbonatitic rocks; Highly variable chemical composition with different REE/Ca, P/Si, F/OH, depending mainly on chemistry of crystallization media; Ore quantities in several REE deposits; Ce species are mostly confined to Si-undersaturated rocks, Y species – to Si-oversaturated rocks; Mostly of late- and post-magmatic origin; Several species may crystallize in a single rock due to different genetic processes. 100 µm fluorbritholite-(Y) Morphology and structure of BGM from quartzolite б 380 285 190 Ca 95 0 200 µm а 730 547.5 365 Y 182.5 0 100 75 50 25 Ce в 0 40 µm Altered rim with monazite-(Ce) alteration rim is composed of X-ray amorphous substance with lower Ca and REE contents (somewhere monazite and bastnaesite crystallized) Morphology and structure of BGM from mineralized nepheline syenite fluorbritholite-(Ce), rarely fluorbritholite-(Y) in amphibole syenite; - individual (up to 1 mm) subhedral and anhedral crystals (aegirine syenite); - large (up to 20 mm) agglomerations of grains (amphibole syenite); rim - includes the “porous” zircon, fluorite and albite; - post-crystallization inner-grain alteration: bud-type (1) and banded (2) zones; - two types of alteration rims: dense (epidote composition with Ap) and tracery (REE carbonates and silicates); - possible reaction for rim formation: britholite + fluid → apatite + epidote + REE carbonate Morphology and structure of BGM from nepheline-feldspar pegmatite Fluorcalciobritholite Fluorbrith(Y,La) Ap Fluorbrith-(Y) Brith 40 µm а 200 µm б 20 µm д Ap F-CaBrith Ap Fluorbrith-(Ce) Brith-(Ce) 1 mm в 100 µm - two morphological types: (1) euhedral prismatic crystals of 1.5-2 mm length and (2) anhedral grains up to 8-10 mm size; - intergrowths with apatite and fluorite; - alteration rims contain numerous apatite grains. г 400 µm fluorbritholite-(Ce), fluorbritholite-(Y), fluorcalciobritholite, britholite-(Се), «calciobritholite», “fluorbritholite-(La)” е Substitution schemes and the composition gap in apatite-supergroup minerals 8 7 6 Px NephSyen TNS Amph NephSyen PNS 5 REE+Y+Si (apfu) Neph-Fs Pegm PegEss Px NephSyen PegmNS 4 Quartzolite-2 Sill 2 Quartzolite-1 Sill 1 3 Q-Fs Pegmatite PegmGr Metasomatite metasom 2 Ap Essexite apatESS 1 Ap Quartzolite-2 apatSILL2 0 0 1 2 3 4 5 6 7 8 Ca+P (apfu) Inverse correlation; Shift from 1:1 line due to different substitution scheme; Gap is shortened due to calciobritholite and unusually REE-rich apatite. Substitution schemes and the composition gap in apatite-supergroup minerals 4,5 4 3,5 Px NephSyen TNS 3 REE+Y+Na (apfu) Amph NephSyen PNS Neph-Fs Pegm PegEss 2,5 Px NephSyen PegmNS Quartzolite-2 Sill 2 2 Quartzolite-1 Sill 1 1,5 Q-Fs Pegmatite PegmGr Metasomatite метасоматит 1 Ap Essexite apatiteESS Ap Quartzolite-2 apatiteSILL-2 0,5 0 0 2 4 6 2Ca (apfu) 8 10 12 Distribution of Y- and Ce- species of BGM in different rock types 3 Ce 2,5 Px NephSyen TNS 2 Amph NephSyen PNS Neph-Fs Pegm PegEss 1,5 Px NephSyen PegmNS SilQuartzolite-2 2 Quartzolite-1 Sill 1 1 Q-Fs Pegmatite PegmGr Metasomatite metasom apobas 0,5 Y 0 0 0,5 1 1,5 2 2,5 3 Chondrite-normalised REE patterns for BGM BGM from quartzolite-1, N=14 BGM from mineralized nepheline syenite, N=12 BGM from quartzolite-2, N=11 BGM from nephelinefeldspar pegmatite, N=7 La/Ndn as indicator of CO2 and H2O activities in fluid 18 16 14 CO2 Silexite-1 12 pNS 10 La/Ndn Pegm AlkGR WT 8 Silexite-2 6 tNS Pegm Essex 4 Apobas metas H2O 2 0 0 0,2 0,4 0,6 (La+Ce)/(REE+Y) , ppm 0,8 Conclusions and remarks on role of fluids in genesis and alteration of BGM from Keivy complex 1. 2. 3. 4. 5. BGM from Keivy complex formed during late-magmatic (in nephelinefeldspar pegmatite and nepheline syenite) and post-magmatic (in nepheline syenite and quartzolite) stages. Numerous genetic processes and different composition of crystallization media result in high diversity and variable composition of BGM. Alkaline and fluorine-rich fluids originated from nepheline syenite and peralkaline granite magmas, enriched in REE. At various crystallization and alteration stages, the CO2, H2O activities in the fluid were high: CO2- rich fluid is suggested for pegmatitic and aegirine-nepheline syenitic BGM, H2O-rich fluid – for amphobolenepheline syenitic and quartzolitic BGM. Post-crystallization alteration of Ce-dominant britholites results in formation of rims with lower F, La, Ce and crystallization of epidote, apatite, REE-carbonates; alteration of Y-dominant britholites leads to release of Ca, Y, P and crystallization of monazite and bastnaesite.
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