MINERALOGICAL CHARACTERIZATION OF TOPAZ FROM MIAROLITIC PEGMATITES AND W-BEARING GREISEN IN THE A-TYPE EL PORTEZUELO GRANITE, PAPACHACRA (CATAMARCA PROVINCE), ARGENTINA

COLOMBO, F.; LIRA, R.; PANNUNZIO MINER, E.V.

Revista de la Asociación Geologica Argentina

Lugar: Buenos Aires; Año: 2008

0004-4822

The El Portezuelo granite (Catamarca Province, NW Argentina), probably of late Devonian or Early Carboniferous age, was emplaced at < 2 kbar and displays the chemical characteristics of an A-type granite. It hosts small miarolitic pegmatites and dikes that belong to the NYF petrogenetic family, miarolitic class, miarolitic-REE subclass, with features more similar to those reported for the gadolinite-fergusonite type. Topaz is widespread both in pegmatites and in a W-bearing greisen genetically related to the granite. In pegmatites it occurs as crystals up to over 15 cm long, showing the forms {120} and {041} (dominant), and also {100}, {010}, {001}, {110}, {130}, {230}, {410}, {150}, {340}, {201}, {203}, {011}, {021}, {031}, {111}, {112}, {221}, {223}, and {445}. Crystals are colorless to pale yellow, more rarely brownish or bluish. Electron-microprobe analyses of gemmy crystals from pegmatites and greisen show that topaz is F-rich (between 19.01 and 20.12 wt. % F, most crystals being slightly zoned) and has very low MgO and TiO2 (< 0.02 wt.%), low  FeO (< 0.03 wt.%), and up to 0.10 wt.% CaO. Refractive indices and unit-cell dimensions reflect the F-rich nature of the studied topaz. This occurrence produces the finest crystals yet known from western South America but probably has no economic potencial as gemstock producer.