Mineralogy of a highly fractionated replacement unit from ‘Ángel’ Pegmatite, Comechingones Pegmatitic Field, Córdoba, Argentina

DEMARTIS, MANUEL; MELGAREJO DRAPER, JOAN CARLES; ALFONSO, PURA; CONIGLIO, JORGE; PINOTTI, LUCIO; D'ERAMO, FERNANDO

Asociación Geológica Argentina, Serie D, Publicación Especial Nº14

Asociación Geológica Argentina

Lugar: Mendoza; Año: 2011 p. 69 - 70

0328-2767

INTRODUCTION The Comechingones Pegmatitic field (CPF), southeastern Pampean Pegmatitic Province (Galliski 1994a and b; Hub 1994 and 1995; Galliski 1999), locates in the northwestern Sierra de Comechingones, Córdoba province, Argentina. It is composed of a great number of Be–Nb–Ta–P–U-rich pegmatites of the rare-element class (some of them grading to the muscovite class), beryl–columbite–phosphate type (Galliski 1994a). These pegmatites were emplaced synkinematically during the compressive deformation of the crustal scale Guacha Corral shear zone (Demartis 2010). The pegmatites are generally well zoned. Border and wall zones are composed of fine- to medium-grained quartz + muscovite ± microcline ± albite. Coarse- to very coarse-grained intermediate zones are composed of graphic microcline + quartz + muscovite ± albite ± garnet; phosphates and uranium minerals occur in these zones. Core is composed of milky quartz with subordinated microcline and muscovite, with accessory crystals of beryl and minerals of the columbite series. Replacement units are rare and have been found only in a few cases, as in the “Ángel” pegmatite, and they generally affect graphic microcline + quartz intermediate zones. “Ángel” pegmatite, cropping out in the southern part of the CPF (~ 32º 18' S and 64º 54' W), is particularly known because of its varied primary and secondary uranium mineralogy (Angelelli, 1950). The aim of this contribution is to describe the extremely fractionated minerals from the replacement units. THE REPLACEMENT UNIT from “Ángel” pegmatite This unit consists of an extremely foliated and ductily deformed rock mostly composed of fine- to medium-grained albite + micas + quartz. It was generated by an almost complete metasomatic replacement of the graphic microcline. Radial platy variety of albite (cleavelandite) is commonly present in crystals of a few centimetres long, but tabular euhedral polysynthetic albite grains predominate. Accessory minerals such as zircon, Ta-rich oxides, and phosphates were also recognized. Silicates Li-rich micas (lepidolite or polylithionite-trilithionite series; Tischendorf et al. 2004) generally show a pink to pale pink color in hand samples. A zoned pattern can be usually recognized using both optical and BSE-scanning electron microscopy, reflecting changes in the Rb content of the micas. Zircon usually crystallizes in ~100 µm euhedral to subhedral grains. They are zoned and strongly corroded, either in the crystal centers and the grain borders. The rims tend to be euhedral and are strongly enriched in Hf. Černý et al. (1985) and Corbella & Melgarejo (1993) also found extreme enrichment in Hf in crystals produced during late events in evolved pegmatites, and emphasized in the progressive enrichment in Hf from core to rim in individual zircon grains. Albite uses to be present in tabular crystals, some hundreds of microns in dimension. Oxides Manganotantalite and members of the microlite subgroup are found as anhedral crystals scattered among the albite grains. These minerals are very common, although fine-grained (less than 100 microns in size). Manganotantalite is not zoned and is partially replaced by metamictic microlite. On its turn, several generations of microlite are distinguished, owing to changes in the contents of Na, Bi and U in the A position. Native elements Native bismuth is widespread in these units, and it is found only as fine-grained crystals of less than 5 microns in diameter. These grains are found filling small veinlets in microlite and in the cleavages of the surrounding micas. Hence, bismuth may correspond to a product of alteration of bismutomicrolite. Phosphates Primary phosphates in the albite units comprise F-rich members of the montebrasite-amblygonite series, and they are widespread as anhedral grains of ~1 mm in diameter. These phosphates are intergrown with albite and lithium micas. In addition, secondary phosphates are found as vein and porosity infillings, and also as some pseudomorphs of the primary phosphates. Crandallite occurs in fine-grained crystals, and it can be associated with an undefined interstitial Hf- and Zr- bearing phosphate which may correspond to a new mineral species. This phosphate is found as fine-grained interstitial intergrowns between albite and mica crystals. DISCUSION AND CONCLUSIONS According to primary K-feldspar (Demartis 2010) and columbite-group mineral (Galliski & Černý 2006) geochemistry, pegmatites from the southern CPF could represent poorly evolved melts with at least intermediate fractionation degree. However, the mineral assemblage found in this replacement unit suggests a metasomatic event produced by highly fractionated fluids during the late stages of pegmatite crystallization. Extreme fractionation is indicated by relatively high Ta and Mn contents in members of the columbite group and pyrochlore supergroup, and by relatively high Hf content in zircon, as was determined by EDS spectra. Positive correlation between Hf and Ta has been also described in some cuppolas of extremely evolved granites (Ru Cheng Wang et al. 1996). The occurrence of Hf-bearing phosphates need more study.