Tin-tantalum-niobium mineralization in the Penouta deposit (NW Spain): Textural features and mineral chemistry to unravel the genesis and evolution of cassiterite and columbite group minerals in a peraluminous system

Abstract

The Sn-Ta-Nb Penouta granite is a highly evolved rare-metal post-kinematic leucogranite located in north-western Spain, which was intermittently mined from Roman times until 1985, when falling metal prices led to the closure of most mines in the country.Mineralization consists mainly of cassiterite (cassiterite I) and columbite group minerals (CGM) disseminated in the leucogranite, that increase in abundance and crystal size towards the upper part of the granitic cupola, especially in banded pegmatite-like dikes. Upwards quartz veins and greisen zones associated with the wall rock (mainly augen gneisses) were developed, containing coarse-grained cassiterite (cassiterite II). In contrast to nearly homogenous cassiterite chemistry, CGM may show complex compositional zonation patterns in relation with compositional variations of the leucogranite. Taking into account the mineral chemistry and textural features of CGM, two main groups were distinguished: (i) the first one (Group I) follows a general main trend from columbite-Fe to columbite-Mn, and finally to tantalite-Mn, which is in accordance with the evolution of the magma observed from the less to the more evolved zones of the leucogranite and from bottom to top of the body, and responds to common fractionation patterns of decreasing Nb/Ta ratios in the melt; and (ii) a second group (Group II), with intermediate values of Mn/(Mn+Fe) and Ta/(Ta+Nb) due to local supersaturation effects of the melt during crystallization, resulting in complex oscillatory zoned CGM crystals.Textural and chemical features of cassiterite and CGM support the magmatic origin for both minerals, although cassiterite crystallized later in the sequence in both generations. Mineral chemistry evolution of CGM is in good agreement with a greater solubility of tantalite in the melt at the upper levels, where, besides, the fluorine contents of apatite and muscovite were increased and the onset of fluorite crystallization occurred. So, the fluorine enrichment in the upper levels seems likely to be an important component to explain the mineral chemistry of the Penouta granite, although the same mineral chemistry scheme could also be explained in terms of white mica fractionation upwards. Furthermore, the absence of typical textures originated by fluids in the CGM as well as the lack of these minerals in the quartz veins point to a low influence of the hydrothermal fluids in dissolution and recrystallization processes of the primary oxides.