Metamictic griphite from the Difunta Correa pegmatite, sierra de Los Llanos, La Rioja (Argentina)

BALDO, EDGARDO; KRISCAUTZKY, AGUSTÍN; COLOMBO, FERNANDO; PANNUNZIO MINER, ELISA VICTORIA

Melbourne

Congreso; XXII Meeting of the International Mineralogical Association; 2018

International Mineralogical Association

Griphite, an uncommon phosphate, has been found at the Difunta Correa granitic pegmatite (30º 20´ 45´´ S ? 66º 32´ 2.3´´ W), in La Rioja Province, NW Argentina.This pegmatite, a tabular body that reaches 2 m thick, is hosted in diorite, but bears no genetic link with it. In addition to quartz, microcline, plagioclase and muscovite, it also hosts zwieselite-triplite [with Fe/(Fe+Mn) = 0.35-0.53] with minor amounts of wyllieite, alluaudite, wolfeite, lazulite and fluorapatite, as elongated (due to deformation) nodules in the intermediate zone. Griphite occurs as blackish brown veinlets, up to 4 mm thick, crosscutting potassic feldspar, with intense greasy luster and conchoidal fracture. It is not associated to other phosphates. Griphite is amorphous, with total absence of diffraction peaks in a powder X-ray diagram. Its chemical composition, measured with electron microprobe (wt.%, n = 4), is P2O5 43.10, SiO2 0.28, Al2O3 9.91, FeOtotal 14.75 (Fe2O3 calc 0.76, FeOcalc 14.75), MnO 19.47, ZnO 0.13, MgO 0.07, CaO 6.05, Li2O calc 2.34, Na2O 2.12, F 2.64, H2Ocalc 0.59, total 101.23, with Fe3+/(Fe2++Fe3+), Li and H calculated from stoichiometry. The empirical formula, based on 6 (P+Si) atoms, is Ca (Mn2.69Fe2+1.01Li1.53Na0.67Ca0.06Mg0.02Zn0.02)6Fe2+(Al1.91Fe3+0.09)2 (P5.95Si0.05O4)6. Trace elements and U were analyzed using mass spectrometry, after dissolving the sample. The total content of rare earth elements (REE) is 273.46 ppm, with a very pronounced negative Eu anomaly (Eu/Eu*= 0.006). The condrite-normalized REE pattern shows the curvatures known as the tetrad effect (Fig. 1), which along with the Eu/Eu* value suggest the involvement of pegmatitic fluids in the origin of the mineral (Irber 1999). The U content is 4410 ppm (0.50 wt.% UO2), whereas Th reaches only 43.8 ppm. Therefore, U is the main cause of metamictization, as found by Peacor and Simmons (1972) for griphite from the Sitting Bull and Riverton Lode pegmatites in South Dakota (USA). The Pb content (16 ppm) is very low (even assuming all Pb is radiogenic). The application of the equation of Montel et al. (1996) gives an age of 26 Ma. According to the current knowledge (e.g. Verdecchia et al. 2014), the major igneous and metamorphic events in Sierra de los Llanos are Ordovician. Thus, the low Pb content indicates Pb loss from griphite.This is the first report of phosphates from Sierra de Los Llanos, and the first recorded occurrence of a Li-bearing mineral in this mountain range. ReferencesIrber, W. (1999) The lanthanide tetrad effect and its correlation with K/Rb, Eu/Eu*, Sr/Eu, Y/Ho, and Zr/Hf of evolving peraluminous granitic suites. Geochimica et Cosmochimica Acta, 63: 489-508.Montel, J. M., Foret, S., Veschambre, M., Nicollet, C. and Provost, A. (1996) Electron microprobe dating of monazite. Chemical Geology, 13: 37-53.Peacor, D.R. and Simmons, W.B. Jr. (1972) New data on griphite. American Mineralogist, 57: 269-272.Verdecchia, S.O., Murra, J.A., Baldo, E.G., Casquet, C., Pascua, I. and Saavedra, J. (2014) Geoquímica de las rocas metasedimentarias del Cámbrico medio al Ordovícico temprano de la Sierra de Los Llanos (Sierras Pampeanas, Argentina): Fuente de sedimentos, correlación y ambiente geotectónico. Andean Geology, 41: 380-400.