Ferronigerite with dominant substitution Ti Sn_(-1) in muscovite+chlorite aggregate from massive quartz nodule associated with a petalite-rich aplite-pegmatite of the Barroso-AIvao pegmatite field, Northern Portugal

摘要

Zoned to homogeneous, euhedral to subhedral tabular crystals (up to ~ 100 um in size) of feiTonigerite-6N6S 'ferro- nigerite-2N1S with Ti-enriched core occur in fine-grained muscovite+chlorite aggregate from a quartz nodule enclosed in a mica schist host. The quartz nodule is located close to the petalite-rich aplite-pegmatite dike Lousas from the Barroso-Alvao pegmatite field. Ferronigerite-6N6S exhibits dominant substitution TiSn ,, Sn/(Sn+Ti) = 0.99-0.61 and Fe > Zn > Mg > Mn. Normalization to 66 cations yielded SUM Sn+Ti from 5.94 apfu in Ti-rich to 6.64 apfu in Sn-rich compositions and high SUM negative charges 190.43 and 191.95; normalization to 186 negative charges gave SUM Sn+Ti 5.80-6.43 and low SUM cations = 63.96-64.46, respectively. Calculation of Li by normalization to 66 cations and 192 negative charges (OH = 6) yielded SUM Sn+Ti = 5.73-6.21 apfu and SUM oxides = 99.20-100.86. These data are closest to the ideal formula of ferronigerite-6N6S although calculated Li contents of 0.88 to 1.12 wt. percent Li_2O (2.35-3.12 apfu) seem to be too high. The quartz nodule consists of an aggregate of zoned tourmaline, medium-grained aggregate of muscovite and fine-grained aggregate of muscovite+chlorite (likely a pseudomorph after cordierite-sekaninaite). This finegrained aggregate contains accessory ferronigerite, fluorapatite, paragonite, beryl, staurolite, and rutile with rare small grains (< 1 mu m) of Fe, Mn, Y, Nb, Ta, and Ti-oxides. The studied assemblage represents a new paragenetic type of nigerite showing obvious peraluminous signature, high amount of Fe and Mg along with close relationship to a highly evolved granitic rock. The geochemi-cal signature of this assemblage indicates influx of highly evolved fluids enriched in F, P, B, Sn, Zn, Be, Nb, Ta and potentially Li into the quartz nodule.