MINERALES DE ELEMENTOS DE TIERRAS RARAS EN LA CARBONATITA DE LA SIERRA DE MAZ (LA RIOJA, ARGENTINA)

AGUSTÍN KRISCAUTZKY; FERNANDO COLOMBO; EDGARDO G.A. BALDO

Tucuman

Congreso; XX Congreso Geológico Argentino; 2017

Asociación Geológica Argentina

Rare earth minerals in the carbonatite from Sierra de Maz (La Rioja, Argentina)REE-bearing minerals have been identified using quantitative electron microprobe analyses in the syenite-carbonatite complex cropping out at Sierra de Maz, La Rioja Province. In order of decreasing abundance they are: monacite, ideally Ce(PO4), ancylite, CeSr(CO3)2(OH)·H2O, hydroxylbastnäsite, Ce(CO3)(OH), calcioancylite, (Ce,Ca,Sr)(CO3)(OH,H2O) and parisite, CaCe2(CO3)3F2 (all Ce-dominant). With the exception of monazite, they are restricted to a dolomite-bearing facies located near the northern end of the complex.Monazite-(Ce) is very poor in Y (0.11%Y2O3), Th (0.22% ThO2) and U (0.08% UO2), unlike its counterparts from granites and metapelites. Ancylite-(Ce) shows extensive substitution of Sr by rare-earth elements (REE), with up to 36% of the Sr site occupied by REE. Hydroxyl-bastnäsite-(Ce) has a F/(F+OH) ratio of 0.28-0.42. Yttrium is enriched in fluorocarbonates (0.75% Y2O3 in parisite, 0.50% Y2O3 in hydroxylbastnäsite). REE exchange schemes aredifferent among the species. In ancylite and hydroxylbastnäsite the dominant substitution is La1Ce-1, whereas monazite shows (Pr+Nd)1La-1, with Ce approximately constant; this last substitution scheme possibly reflects preferential incorporation of REE in different growing faces of the crystal. Part of the monazite may be primary, but other crystals occurring as rims in fluorapatite grains are most probably of hydrothermal origin. Ancylite, calcioancylite and hydroxylbastnäsite are hydrothermal products (usually associated with dolomite). Parisite is found as an alteration of hydroxylbastnäsite due to a rise in the product aCa2+aF- of the final solutions. Apatite also represents an important REE host in this carbonatite, because despite its lower REE concentrations (0.7% Ce2O3, 0.20 % La2O3, 0.40% Nd2O3) it is very abundant.