Fluid inclusion and trace-element analysis of the rare-element pegmatite bodies Altim and Tamanduá from the Borborema Province, Brazil

Fluid inclusion studies on pegmatitic bodies are performed to gain insight on the conditions during the formation of the pegmatites as FIs can be seen to directly mirror the fluids that were present during the formation of, most commonly, the quartz in the pegmatite. Continued progress in the methodology of measurements specifically of HFSE from FIs with relatively new methods Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry enables us to measure elements that previously have not been able to be quantified. The study reports the pressure-temperature conditions and trace-element compositions of the two pegmatite bodies Altim and Tamanduá formations using microthermometry and LA-ICP-MS on fluid inclusions and shows the major and trace-element composition of the muscovite and feldspars with Electron Probe Microanalyzer (EPMA) and LA-ICP-MS respectively. The rare-element pegmatite bodies Altim and Tamanduá are located within the Borborema Pegmatite Province (BPP) in North-eastern Brazil. The BPP is emplaced within the Northern Domain (Northern Tectonic Sub-Province) of the Borborema Province as a 75 by 150 km large area trending NNE-SSW direction, along the eastern part of the Seridó belt. The rare-element pegmatites are considered to have been formed from the highly undercooled residual melts of the granitic intrusions. The G4 leucocratic pegmatoid granite and the GR3A and GR3B pegmatitic granites are considered to be the source of the pegmatites in the BPP as their ages are the closest match to the 509–525 Ma age measured from the pegmatites. The fluid inclusion analyses were performed on the quartz from the different zones (I–IV) of the pegmatite bodies where zone I is the outermost zone and zone IV the quartz core and from the euhedral quartz found from miarolitic cavities in the pegmatite. The Altim samples are mostly fluid rich, L 80% + V (+ S) (L:Liquid; V:Vapor; S:Solid) inclusions and some of the larger (primary) inclusions within the euhedral quartz samples show multiple S. The Tamanduá samples show larger diversity of Fluid Inclusion Assemblages (FIA); aqueous-carbonic, monophase vapour, vapour-rich two-phase and FIA with immiscible liquids (possible phase separation). The immiscible liquids have possibly been formed through boiling of the fluids within the pegmatite providing a possible deposition mechanism for the columbite-tantalite ore minerals found in the Tamanduá locality. The two pegmatite bodies have clearly different major and trace-element compositions with specifically elevated Zn content in the Altim pegmatite body which clearly defines the two pegmatite bodies as two different formations. Furthermore, the salinities for the two pegmatite bodies have a clear division with low–moderate aqueous-carbonic FI in the Tamanduá and moderate–high salinities in the Altim pegmatite body. The two pegmatite bodies appear to have no connection to each other through a connected evolutionary history and the Tamanduá pegmatite appears to be the more highly fractionated of the pegmatites. LA analyses on fluid inclusions provided trace-element data for the previously poorly reported HFSEs Nb and Ta showing concentrations of 0.1–3.2 ppm (excluding extreme outliers) with Nb/Ta average abundance ratios between 1.5 and 3.5. The highest concentrations of Nb and Ta directly correlating with the highest concentrations of Al.