GARNET AND GAHNITE FROM THE BORBOREMA PEGMATITE PROVINCE, NORTHEASTERN BRAZIL, AS INDICATORS OF PEGMATITE EVOLUTION AND POTENTIAL FOR RARE-METAL MINERALIZATION

Although garnet and gahnite (ZnAl₂O₄) are common accessory phases in granitic pegmatites, little is known about the relationship between variations in their chemical composition during crystallization, pegmatite evolution, and the presence of rare metal (Li, Ta) mineralization. We present compositional data of garnet and gahnite from the Capoeira 2, Boqueirão, Alto Mirador, and Carrascao Li-Be-Ta-Nb-bearing pegmatites from the Borborema Pegmatite Province, northeastern Brazil.

Garnet from all four pegmatites represents a spessartine-almandine solid solution with minor amounts of pyrope and grossular (sps_71-96alm_2.5-27pyr_1-1.4grs_0.4-0.5). In a plot of MnO+CaO vs. FeO+MgO (wt%) garnet from Alto Mirador pegmatite plot in the field of high-Mn garnet from Core and Replacement Zones of Li pegmatites. In contrast, garnet from Carrascao, Boqueirão, and Capoeira 2 pegmatites plot in the field of Mn-Fe garnets in Core and Replacement Zones of Li-poor pegmatites. Based on these relationships, the tendency of the pegmatites to contain Li mineralization should increase in the order Capoeira 2, Boqueirão, Carrascao, and Alto Mirador. However, the Boqueirão pegmatite, which falls in the field of Li-deficient pegmatites, has been mined for Li. In addition, studies of tourmaline, mica, feldspar, and Nb-tantalum group minerals indicates that Capoeira 2 is more evolved than Boqueirão.

The composition of gahnite in pegmatites from the BPP (gah_90-97hc_0.6-9spl_0-5) falls within the compositional field of spinels in pegmatites and in the field of gahnite from rare-element granitic pegmatites (LCT-type) from Ontario. Gahnite from the Alto Mirador pegmatite has two different populations of compositions, one of them containing the highest Mg and lowest Zn content of gahnite from all pegmatites analyzed, indicating that gahnite recorded a change from a less evolved to a more evolved system during crystallization. This suggests that our understanding of the compositional variations of garnet and gahnite in pegmatites and the relationship with the formation of Li mineralization may still be limited. The major element composition of these minerals may provide complementing, but sometimes contrasting information, and trace element compositions may be required to better understand the evolution of granitic pegmatites.