Mineralogia e geoquímica dos skarns de W-Mo da mina Brejuí, Faixa de Dobramentos Seridó, Rio Grande do Norte

Abstract

The Brejuí Mine is geologically situated within the Seridó Fold Belt, located in the Borborema Province - one of the main geotectonic terrains of northeastern Brazil that formed during the Brasiliano/Pan-African Orogeny (Neoproterozoic), where Ediacaran to Cambrian-aged granitic intrusions favored the development of various mineralization types, particularly the skarn-type deposits of the Seridó Scheelite Province, such as the W-Mo deposits addressed here. The objective of this work is to analyze the mineral parageneses of the skarns at Brejuí Mine and their associated hydrothermal alteration processes. These skarns, locally called "tactites", are calc-silicate rocks hosted at the contact between lithotypes of the Seridó Group, specifically between marbles and paragneisses of the Jucurutu Formation, where hydrothermal fluids rich in volatiles and metals promoted the formation of these calcic skarns and subsequent scheelite deposition. The W-Mo mineralizations are associated with multiple stages of metasomatic/hydrothermal alteration, as evidenced by studies conducted in this work involving the integration of optical microscopy, electron microscopy (SEM/EDS) and Xray fluorescence (XRF) data, where three main stages of hydrothermal alteration were identified, compatible with the regional metallogenic evolution: (1) an intermediate stage between the metasomatic and hydrothermal phases, marked by the replacement of grossular and diopside by epidote and vesuvianite, indicating Ca-W-Si-rich fluids; (2) a secondary hydrothermal stage with predominance of epidote, calcite and vesuvianite; and (3) an advanced hydrothermal stage characterized by the deposition of scheelite, sulfides (chalcopyrite, molybdenite) and magnetite, confirming S- and metal-enriched fluids. Furthermore, late-stage parageneses (zeolites, sulfides) suggest variable redox conditions during hydrothermal evolution, which remain poorly explored in the literature. This work reinforces the importance of integrating analytical techniques for understanding mineralized systems in complex environments like Seridó, and highlights the need for future studies in isotopic geochemistry and fluid microthermometry to completely elucidate the deposit's genesis.