The Complex Nb-Ta-Ti-Sn Oxide Mineral Intergrowths in the La Calandria Pegmatite, Cañada del Puerto, Córdoba, Argentina

GALLISKI M. A.; MÁRQUEZ-ZAVALÍA MARÍA FLORENCIA; CERNÝ P.; LIRA R.; FERREYRA K.

Bartlett, New Hampshire

Simposio; PEG 2013-6th International Symposium on Granitic Pegmatites; 2013

IntroductionThe origin of accessory minerals in granitic pegmatites, especially those that concentrate highfield-strength elements (W, Zr, Nb, Ta, Ti, Sn, Hf, Th, U), is occasionally multi-episodic under variablephysico-chemical conditions. In this paper we describe an assemblage of ?ixiolite?, tantalian rutile,wodginite-group and columbite-group minerals, cassiterite and pyrochlore-supergroup minerals that occurs in a rare-element granitic pegmatite of the beryl-columbite-phosphate subtype in the Eastern Pampean Ranges, Argentina. Geological Setting of the Parent Pegmatites.The studied paragenesis is found in the La Calandria pegmatites, located on the western slopeof the Sierra Grande de Córdoba at Cañada del Puerto, central Argentina. The pegmatite dikes are mostly emplaced in biotite-muscovite mylonitic gneisses (?augen gneiss?), and subordinatemetaquartzites and calc-silicate layers. Locally, these dikes also crosscut lens-shaped bodies ofgabbro. These dikes outcrop intermittently over a distance of  250 m. Thicknesses vary in eachpegmatite and along strike from 0.2 to 1.5 m. Pegmatites are well zoned and symmetrical. Thecontact with the countryrock is sharp and easily traced, followed by a border zone 1-2 cm thick,mostly composed of albite with subordinate quartz grains (1 cm). The border zone grades into another coarser-grained zone (2-2.5 cm) made of quartz, Kfeldspar, albite and some muscovite which extends about 3 cm inwards. Zonation continues with coarser grain sizes, including 1 to 3cm-sized topaz crystals (frequently andpartially replaced by 2M1 yellowish green muscovite) grading into a zone rich in K-feldspar and quartz where cm-sized nodules of triplite, some rounded grains of pyrochloresupergroup minerals, and 1-2 cm aggregates of dark Nb-Ta oxides are found. In some sectors, a quartzcore is developed.The largest mining pit (5 m deep, up to 4m wide) was excavated at the southernmost outcrop on a 1 m thick pegmatite dike.This part of the pegmatite is intruded into a dark bluish-gray, medium-sized gabbro body. Thepegmatite zoning starts with a border zone 1 cm thick composed of Qtz-Pl-Ms, followed inwards by Qtz-Pl-Kfs. Other mineral species found in this pit are rare biotite, topaz, triplite, Nb-Ta-Ti oxide minerals (up to 1cm sized crystals, partially altered), and pyrochlore-supergroup minerals.Mineralogy of the Oxide Mineral Intergrowths The oxide mineral intergrowths are composed of a very intricate and complex association of morethan one generation of ?ixiolite?, wodginite-group minerals, tantalian rutile, columbite-group minerals, cassiterite, and pyrochlore-supergroup minerals.Ixiolite-wodginite-group minerals: These minerals occur in three principal forms: (i) as mmsizedirregular grains with abundant exsolutions of tantalian rutile and secondary fluorcalciomicrolite,(ii) as fine exsolutions regularly dispersed in grains of tantalian rutile, and (iii) as very thin rims thatform the interphase between major grains of tantalian rutile and ixiolite-wodginite of anothergeneration.The chemical composition of these phases is variable, but in most of the grains there is a slightpredominance of Nb over Ta and a clearly defined predominance of Fe over Mn; the composition in terms of Mn#-Ta# is outside the ixiolite field of Černý & Ercit (1989) and in a gap in thetitanianixiolite ? titanian columbite from selected worldwide occurrences (Černý et al. 1998).Titanium is abundant with maximum and [average] contents of 13.67 [6.02] TiO2 wt.%, tungsten values are 3.66 [1.84] wt.% WO3, whereas SnO2 shows 7.82 [2.79] wt%, and ZrO2 1.59 [0.46] wt.%. The contents of Ti and Sn in the most enriched samples are in excess of those from other worldwide occurrences of titanianixiolite studied by Černý et al. (1998), but in the same range that some of the compositions given by Beurlen et al. (2007) for ixiolites from the Borborema pegmatite province.The sum of these elements, when the data are plotted in the Mn + Fet ? Nb + Ta ? Ti + Sn + Zrdiagram, shows a field that encompasses the fields of columbite-, wodginite-group minerals and, to a degree, of ixiolite, mostly along the line 2:1, but outside from the domains of ixiolite from theBorborema pegmatite province (Beurlen et al.2007).Tantalian rutile: It is present as mm-sized grains irregularly associated with ixiolite-wodginite-groupminerals or as anhedral inclusions in these minerals. The largest grains locally contain irregularexsolutions of ixiolite or infrequent exsolutions of cassiterite or wodginite. This last phase usuallyshows frequent patchy zoning in the largest grains and also in the exsolutions, which occasionallyshow very thin rims of titanowodginite or ixiolite. Less frequently, especially when some crystals grow on the border of the ore mineral aggregates in contact with quartz, oscillatory compositionalzoning is present. Titanium contents are high with a maximum of 61.67 wt.% TiO2. Ta2O5 is also high at 44.97 [36.45] wt.%, whereas Nb2O5 is lower at 22.17 [10.25] wt.%. Iron, dominantly as Fe2O3, is also a major oxide at 11.61 [7.09] wt.% Fe2O3 and 7.55 [4.65] wt.% FeO. Tin is also a main element, peaking at 8.84 [2.16%] wt% SnO2. Tungsten contents are invariably low, up to 0.56 wt.% WO3[0.26%].Columbite-group minerals: Columbite-(Mn) is present in a single grain sampled in the inner part ofthe pegmatite, with chemical compositions very depleted in Fe and with low contents of W, Ti andSn. However, in the periphery of the grain, these elements gradually increase and plot in the domainof ixiolite or titanowodginite.Cassiterite:This mineral was only detected as ≤ 100 μm irregular exsolutions in tantalian rutile,close to the contact with a grain of a possible wodginite-group mineral.The cassiterite exsolutionshave variable compositions with SnO2 up to >90 wt.% and 5 wt.% Fe2O3. Other exsolutions, with compositions of titanowodginite and ferrotitanowodginite, coexist with the cassiteriteexsolutions in tantalian rutile.ConclusionsThe minerals present in the examined assemblage are not in equilibrium and comprisemagmatic and subsolidus phases distinguished texturally and chemically. The primary, magmaticstage of mineralization possibly crystallized ixiolite I + tantalian rutile I in the outer zones of thepegmatite and, less frequently, local columbite-Mn in the inner part. Subsolidus unmixing of ixiolite Iproduced wodginite-group minerals I + tantalian rutile II. Contemporaneously, tantalian rutile Ilocally exsolved wodginite-group mineral II + cassiterite. Localized Ca- F-rich hydrothermaloverprint transformed tantalian rutile I to tantalian rutile III + ixiolite II + fluorcalciomicrolite. Besides, the hydrothermal overprint produced peripheral transformation of columbite-(Mn) to ixiolite III and widespread distribution of fluorcalciomicrolite throughout the polygranular assemblage.Acknowledgements: The authors are grateful to A. Falster for the comments.ReferencesBeurlen, H., Barreto, S. B., Silva, D., Wirth R. & OlivierP. (2007): Titanianixiolite - niobian rutileintergrowths from the Borborema Pegmatite Province,Northeastern Brazil. Can. Mineral. 45, 1367?1387.Černý, P., &Ercit, T.S. (1989): Mineralogy of niobiumand tantalum: crystal chemical relationships,paragenetic aspects and their economic implications.In Lanthanides, Tantalum and Niobium (P. Möller, P.Černý, & F. Saupé,eds.). Springer-Verlag, Berlin, 27-79.Černý, P., Ercit, T. 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