Mineralogy and crystal chemistry of micas from the A-type El Portezuelo Granite and related pegmatites, Catamarca (NW Argentina)

Abstract

The A-type El Portezuelo Pluton (Catamarca, NW Argentina) is parental to an intragranitic suite of pegmatites of NYF-type affiliation (miarolitic class, miarolitic-rare earth element subclass, with features more similar to those reported for the gadolinite-fergusonite type). This study was performed on samples from the host granite and several zones of pegmatites, including crystals growing in miarolitic cavities and fine-grained overgrowths. Micas from the granite and massive pegmatites are rather homogeneous, but crystals coming from miarolitic cavities are usually sharply zoned with monocrystalline trioctahedral inner zones overgrown by polycrystalline dioctahedral rims. Dioctahedral micas are always paragenetically later. Micas from the granite are intermediate members of the annite-siderophyllite series. From the outer pegmatite zones inwards the substitution (SiLi)([4]AlFe)-1 in trioctahedral micas leads to compositions intermediate between siderophyllite and polylithionite, up to the composition KLiFe2+Al(AlSi3)O10(F,OH)2 (formerly called zinnwaldite). Dioctahedral micas also show a trend from near end-member muscovite to (Fe, Mg, Li)-rich muscovite (phengite), according to the substitution (R2+Si)([4]Al[6]Al)-1, where R2+ = Fe, Mg, Mn; there is a compositional gap between dioctahedral and trioctahedral micas. Zoned individual crystals have trioctahedral cores enriched in Fe, Mn and F; Na, Li and Ti are usually also enriched in the cores, whereas Mg is usually depleted compared with the dioctahedral rims. Micas in El Portezuelo are the most important F-bearing species (for their elevated F contents and their modal abundance) and they also have a major role in the distribution of Li and Rb. The overall evolutionary trend is very similar to that found at the Pikes Peak Batholith (Colorado USA) and is characteristic of NYF-type granite-miarolitic pegmatite systems.