Carolina Dallagassa, MSc
C. Dallagassa1, F. Cuadros1, N. Botelho1
1Institute of Geosciences, University of Brasília, Brasília, Distrito Federal, Brazil
The global expansion of low-carbon technologies has intensified the search for lithium in new frontiers. The Brasília Orogen (Tocantins Province) represents a major Neoproterozoic belt formed during the assembly of Gondwana (640–500 Ma). Its external zone hosts several peraluminous granitic-pegmatitic systems with exploration potential for critical minerals. This study focuses on three suites: Mata Azul (MA), Aurumina (AU), and São Domingos (SD). Methods included mapping 15 outcrops and sampling 6 drill cores, supported by petrographic analysis (n=47), whole-rock geochemistry (ICP-OES/MS), and mineral chemistry via electron probe micro-analysis (EPMA-WDS) on feldspars, micas, tourmalines, garnets, spodumene, Li-Al phosphates, and Nb-Ta oxides. Whole-rock geochemistry confirms that all suites are strongly peraluminous (A/CNK 1.07–1.68 and A/NK 1.11–2.25), consistent with evolved magmatism. In the Mata Azul Suite (560–520 Ma), five pegmatitic facies were identified, ranging from quartz-muscovite and graphic textures to specialized lepidolite-albite facies with whole-rock Li up to 12,729 ppm and K/Rb ratios as low as 29. Zonation is well-developed, with core-related pods containing lepidolite and rubellite. Lepidolite in these facies yields high Li₂O (~4.7–6.1 wt.%), F (up to 10.3 wt.%), and Rb (up to 1.1 wt. % Rb₂O). Garnets are spessartine-rich (Sps₄₄₋₆₈), and tourmalines evolve from schorl to elbaite (up to ~1.69 wt.% Li₂O). The Aurumina Suite (2.11–2.16 Ga) comprises syeno/monzogranites, albitites, and pegmatites with lepidolite and total Li contents up to 8,555 ppm. Mylonitic granites in this suite host spodumene as porphyroclasts. Nb-Ta oxides evolve from columbite-(Mn) to ferrotitanowodginite, while Li-Al phosphates correspond to the amblygonite-montebrasite series [F/(F+OH) 0.15–0.64]. This suite exhibits the highest degree of fractionation and trace-element enrichment, with Be up to 258 ppm, Cs up to 1,351 ppm, Ta up to 176 ppm, and K/Rb ratios reaching highly evolved values of 20. The São Domingos Suite (2.1–2.0 Ga) consists of albite-rich pegmatites hosting coarse cassiterite and montebrasite. It is geochemically distinct, being P-rich (up to 1.29 wt.% P₂O₅) and exhibiting low K/Rb ratios of 23, with Li contents reaching 993 ppm. The presence of lithium minerals (lepidolite, spodumene, and montebrasite) and highly fractionated geochemical signatures (low K/Rb and high Be-Cs-Ta-Li) demonstrates extreme magmatic differentiation processes required for the development of LCT-type mineralization. These results characterize these suites as fertile environments for critical mineral exploration within the Brasília Orogen.