FIELD TRIP GUIDE TO THE SERIDÓ PEGMATITE PROVINCE

Abstract

The pegmatites of the Seridó Pegmatite Province show a granitic bulk mineralogical composition, composed of K-feldspar, albite, quartz, muscovite and occasionally biotite. Detailed mineralogical studies of the major minerals are scarce. There is, on the other hand, a relatively larger number of published contributions concerning pegmatite accessory minerals. This field trip guide focus on the mainly occurrence locations of these last minerals, which are group here by their most frequently recovered rare metals and gems: (1) Ta-Nb: Tantalite-columbite is the main mineral group, even though other rare varieties may occasionally be recovered as microlite, tapiolite, strueverite, simpsonite, ixiolite, among others. Their grain size varies from the centimeter to decimeter scale. The mineralization is disseminated (less frequently), in pockets, or as aggregates of crystals, usually associated with "replacement bodies" at the contact between Zone III and the quartz core. They also occur sporadically in Zone II and occasionally in homogeneous pegmatites, as crystals in subcentimeter to centimeter scale, frequently disseminated in the quartz-feldspar-muscovite groundmass. In this case, they display lamellar habit and normally lower Ta concentration. (2) Be: Beryl is the main Be-mineral present. Its grain size varies from centimeter to meter, it is normally prismatic and shows diverse colors (white, green, blue, pink, yellow), with green and white prevailing. It occurs in Zones II, III and IV (quartz core), as well as in association with the "replacement bodies". In homogeneous pegmatites, it occurs as centimetric crystals, disseminated in the groundmass. Some pseudoconical forms are also observed, and contain muscovite and quartz inclusions. The gem varieties (aquamarine, morganite, heliodor) are scarce. Beryl and the Ta-Nb minerals, are the main ores exploited in these pegmatites. (3) Li: Li-minerals recovered from the pegmatites are spodumene, lepidolite and amblygonite. The spodumene prevails over amblygonite and lepidolite, although the reserves of Li are still not completely known. Spodumene occurs in individual crystals or aggregates in decimeter to meter dimensions, rarely forming individualized zones. Lepidolite occurs mainly in albitized pegmatites associated with deep blue tourmalines (named Paraíba Tourmaline). Amblygonite occurs in pockets of decimeter to meter width and may be associated with other phosphates such as lithiophilite, triphylite, siclerite, tavoríte, among others. (4) Sn: Cassiterite is the main Sn-ore, although Sn is also present in the crystallographic structure of Ta-Nb minerals. It is found disseminated in centimeter grain size in the "replacement bodies" or in Zone I of heterogeneous pegmatites, and occurs occasionally in some homogeneous pegmatites. Cu (e.g. djurleite), U (e.g. uraninite) and REE (e.g. xenotime) minerals are considered important resources exploted or as potential for further exploration.