Textural and mineral-chemistry constraints on columbite group minerals in the Penouta deposit: evidence from magmatic and fluid-related processes.

Abstract

The Penouta Sn-Ta deposit, in the northwest of Spain, is a greisenized granitic cupola where Ta minerals occur mainly as disseminations in a leucogranite body intruded in Precambrian-Lower Cambrian gneises and mica-schists. This leucogranite is a medium- to fine-grained inequigranular rock consisting mainly of quartz, albite, K-feldspar and muscovite. Accessory minerals are mainly of spessartine, zircon, cassiterite, Nb-Ta oxides, monazite, xenotime, native bismuth and pyrite. The alteration processes were mainly albitization, muscovitization and kaolinitization. This leucogranite is peraluminous and P-poor, with 0.03-0.07 wt.% P2O5, 900-1500 ppm Rb, 30-65 ppm Cs, 120-533 ppm Li, 80-140 ppm Ta, 51-81 ppm Nb and up to 569 ppm of Sn. Mineralogical characterization of Nb-Ta oxide minerals was determined by X-ray diffraction, scanning electron microscopy, electron microprobe analysis and mineral liberation analysis. Mn-rich members of the columbite-group minerals (CGM) are the most common Ta-bearing phases, but microlite, wodginite, tapiolite and Ta-rich cassiterite occur also. CGM crystals are commonly zoned concentrically, with a Nb-rich core surrounded by a Ta-rich rim, with a sharp boundary between them. Convoluted zoning occurs also. Dissolution textures resulting from the corrosion of columbite and tantalite rims, in particular, are common. TheMn/(Mn + Fe) ratio varies between 0.33 and 0.97 and the Ta/(Ta + Nb) ratio between 0.07 and 0.93. Wodginite and microlite formed as late replacements of CGM and occur associated with tantalite and cassiterite. Subhedral to anhedral cassiterite crystals, usually up to 200 μmacross, occur in two generations: the earlier one is Nb,Ta-poor whereas in the later generation, the Ta content can reach >9 wt.%of Ta2O5 and 1.7 wt.%of Nb. The presence of a fluid phase in the apical zone of the granite, probably related to the separation of a fluid/vapour of the melt, could explain the sponge-like textures, the Ta enrichment associated with these textures, the occurrence of Taenriched mineral phases (microlite and wodginite) and their common interstitial character.