Pegmatites typically exhibit wallrock dispersion halos formed as a consequence of fluid exsolution from the melt. However, the precise nature of trace element partitioning between wallrock minerals and advecting fluids remains unresolved. This study presents trace element partition coefficients among biotite, amphibole, and fluid obtained via a metric distance profile at the Areias pegmatite sill wallrock gneiss, Seridó Pegmatite Province, northeastern Brazil. The gneiss predominantly retained its metamorphic minerals and texture, with minor local tourmalinization. LA-ICP-MS trace elements analyses of biotite near the pegmatite indicate enrichments in Li, F, Mg, Cu, Zn, and Ba, and depletions in Ti, V, Cs, and W, whereas amphibole is enriched in Li, F, Mn, Cu, Rb, Ba, and Pb, and depleted in Mg, Ca, Sc, and Ti. The partition coefficients indicate that biotite preferentially incorporates Cs (214), Rb (67), Ba (27), Li (21), Ta (2.6), Ti (2.1), W (1.9), F (1.7), Ni (1.5), Zn (1.4), Co-Nb (1.3), Ga (1.2), Cr (1.1), and V (1.0), whereas Be-Sr-Th (0.1), Sc (0.2), B-Pb (0.3), Cu (0.4), U (0.5), and Mn (0.7) are more likely partitioned into amphibole. The compositions of pegmatite fluid and melt are determined by combining a previous model of trace element ratios of the advected fluid at the Areias sill, along with published mineral/melt and mineral/fluid partition coefficients of Li for biotite and quartz. The calculated Li concentration of the pegmatite fluid applied to the pegmatite fluid trace element ratios yielded the following concentrations (ppm) of all modeled elements, including F=6496, Li=1096, Rb=818, Zn=114, U=54, Cs=34, and Nb=27. The obtained fluid/melt partition coefficients are Li=19.9, F=7.95, Zn=4.6, Rb =0.97, Cs=0.61, and Nb=0.35. Utilizing the average trace element compositions of wallrock biotite and amphibole, the calculated mineral/fluid partition coefficients for biotite are Zn=4.63, Cs=1.23, Rb=0.98, F=0.95, Li=0.47, and Nb=0.19, and for amphibole are Zn=3.28, F=0.55, Nb=0.16, Li-Rb=0.02, Cs=0.01, and U=0.003. When compared to published trace element compositions of fluid inclusions and geothermal waters, the Areias pegmatite fluid displays distinct magmatic unfractionated compositions with high Rb/Cs and Zn/Nb ratios. Furthermore, low Li/Cs and Li/Rb ratios indicate significant fractionation of Li into the exsolved fluid. Biotite and amphibole show Mg and Ti chemical gradients, validating Ti-geothermometry temperatures of 674 ± 23 °C and 671 ± 31 °C during pegmatite fluid advection. These elevated temperatures, combined with unfractionated Rb/Cs and Zn/Nb magmatic ratios, suggest early fluid exsolution in pegmatite crystallization. Thermochronometric constraints on fluid advection duration suggest 20 years under isothermal conditions and 100 years with a cooling rate of 1 ºC/year. These timescales are 2 to 4 orders of magnitude slower than thermal models for pegmatite crystallization, indicating that fluid flow commences early and persists during post-pegmatite crystallization.