Mapeamento geológico de detalhe e caracterização mineralógica em corpos pegmatíticos de Condado, município de Quixeramobim (CE)

Abstract

The southeastern portion of Quixeramobim (CE), the study area of this work, is situated within the geotectonic context of the Ceará Central Domain (DCC) of the Borborema Province (BP). The objective of this study is to present the results of detailed geological mapping (1:10,000), carried out over an area of approximately 3.3 km², which allowed the recognition of three lithostratigraphic units (Acopiara Unit, Banabuiú Granite, and Pegmatites), as well as the petrographic, lithogeochemical, and mineral-chemical characterization of the rocks in the area. The first unit corresponds to the Paleoproterozoic gneissic basement (Acopiara Unit), represented by partially migmatized gneisses interlayered with amphibolites and schists. These lithotypes exhibit an Sn+1 foliation with preferred NE–SW and N–S orientations and moderate to steep dips towards the west. Mineral assemblages of this unit, including amphibole (Am) + quartz (Qtz) + plagioclase (Pl) ± K-feldspar (Kfs), suggest metamorphism in the upper amphibolite facies. The second unit is represented by Neoproterozoic granites (Banabuiú Granite), composed of S-type, two-mica granites, classified mainly as monzogranites, predominantly falling within the leucocratic association field, with a peraluminous character and showing Ms > Bt. The third unit corresponds to Paleozoic pegmatites, occurring as two distinct bodies in the area, distinguished by their mineralogy and textural features: the first, occupying most of the study area, displays characteristic pegmatitic texture and a significant modal proportion of K-feldspar, which imparts a marked pink coloration; in some bodies, remnants of metasomatic “pockets” were observed. The second, concentrated in the northwestern portion of the area, contains higher concentrations of albite and biotite, in dioritic-composition zones where biotite displays a “fishbone” texture. The mapped pegmatite bodies host gemological minerals such as beryl (aquamarine) and varieties of tourmaline (schorl and elbaite-rubellite), as well as lithium-bearing minerals such as lepidolite. Geochemical data obtained through flame photometry and a portable lithium analyzer indicate significant lithium oxide concentrations in the minerals analyzed, including 42,725 ppm Li in lepidolite, 23,890 ppm in elbaite-rubellite, 2,507 ppm in muscovite, 1,533 ppm in beryl, and 371 ppm Li in black tourmaline.