CONICET | Buscador de Institutos y Recursos Humanos

Columbite-group minerals (CGM) from 30 rare-element-class granitic pegmatites of the Pampean Ranges were analyzed. Most of the pegmatites (87%) belong to a synorogenic LCT pegmatite belt of Lower Paleozoic age, emplaced dominantly in medium-grade metamorphic rocks. The pegmatites belong to rare-earth (2), beryl (6), albite (3), and complex types, this last one represented by spodumene (10), lepidolite (2) and petalite (1 or 2) subtype pegmatites. In contrast, 4 pegmatites (13%) are postorogenic, intragranitic bodies of non-typical beryl-columbite-phosphate subtype. Rare-earth-type pegmatites have CGM with [Ta] = Ta/(Ta+Nb) of 0.10 to 0.20 and [Mn] = Mn/(Mn+Fe) of 0.50 to 0.70, relatively high Mg, Y, Sn, W, Ti and U contents, and no defined fractionation trends. In beryl-type pegmatites, [Ta] of CGM varies from 0.10 to 0.60 but can attains 0.80, eventhough the range in a single body normally is not greater than 0.40. [Mn] varies between 0.30 and 0.90 but the range is commonly narrow in a single pegmatite. Albite-type pegmatites show platy columbites with variable [Ta] and [Mn] of 0.15 to 0.75 and 0.25 to 0.75, respectively, low Pb and U contents, intermediate Mg and high Zn. Spodumene-subtype pegmatites show extended [Ta] variation from 0.15 to 0.90, and [Mn] usually ranges from 0.50 to 1.00; however, in some pegmatites the early CGM can have [Mn] as low as 0.20. The content of trace elements like W, Ti, Sn, Mg, As, U or Sc is variable among different pegmatites. In spodumene-subtype pegmatites, some dikes shows positive correlation of [Ta] and [Mn] evolution, others show a range of [Ta] at almost constant and relatively higher [Mn]. The petalite- subtype pegmatites have CGM with [Ta] range from 0.15 to 0.80 and [Mn] from 0.50 to 0.70; the contents of W, Sb and Mg are moderate, those of As and Zn high. Lepidolite-subtype pegmatites show [Ta] in CGM from 0.15 to 0.70 and [Mn] from 0.50 to 0.90. The geochemistry of disordered CGM in non typical beryl-columbite-phosphate pegmatites shows a significant contrast to those of the LCT pegmatites. The CGM show relatively low values of [Ta] with variation from 0.05 to 0.30, and Fe- rich compositions with [Mn] range from 0.20 to 0.60. These characteristics and the high contents of W, Ti, U, Zr and Y combined are prominent, and suggestive of a petrochemical provenance of the parent pegmatites, different from that of the typical LCT populations.

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