Caracterização química e mineralógica de fosfatos de alumínio da falha de Samambaia, Rio Grande do Norte

Abstract

The Samambaia Fault in Rio Grande do Norte represents a seismogenic structure of relevance in the tectonic context of Brazil, characterized by intense neotectonic activity and hydrothermal alterations in fracture zones. In this scenario, aluminum phosphate mineralizations with potential role in sealing and diagenetic evolution of these zones were identified. This research aims to characterize texturally and chemically the phosphate phases present in veins and breccias of the Samambaia Fault, understand their formation mechanisms, and discuss their implications in the hydrothermal and supergene alteration history of the region. Integrated analyses of XRD, QEMSCAN, SEM/EDS, and EPMA were employed on samples collected from different fault zones. The data allowed identification of mineralogy, chemical composition, and textural relationships between phosphate phases and associated minerals. The main aluminophosphates identified were variscite, crandallite, and millisite/wardite, associated with kaolinite and quartz. Variscite is the predominant mineral, with wide compositional variation (Al³⁺⇌Fe³⁺ substitutions). Crandallite and millisite/wardite occur subordinately, suggesting evolution under different fluid pulses. Textural evidence indicates that phosphates precipitated in acidic and low-temperature environments, with predominance of meteoric fluids. The presence of aluminophosphates in the Samambaia Fault results from multiple chemical alteration events, with strong pH control and phosphorus activity. These minerals may play a relevant role in permeability reduction in fractured zones. Future studies should integrate isotopic, petrophysical, and experimental data to elucidate fluid origin, mineral stability, and impacts on fault dynamics.