Variação química mineral em contexto de mineralização W-Skarn: o caso do alvo Umbuzeiro no distrito mineral de Bodó, Província Borborema

Abstract

The skarns of the Bodó Mining District, located in the Borborema Province (northeastern Brazil), are important targets due to the tungsten mineralization associated with scheelite. This study investigated the Umbuzeiro target, aiming to characterize the hydrothermal and metasomatic processes that affected the metasedimentary rocks of the Jucurutu Formation, with an emphasis on the transition between preserved lithologies and altered zones. Petrographic analyses using optical microscopy, SEM-EDS images, and quantitative mineral chemistry of biotite, amphiboles, and feldspars from four lithologies were integrated: biotite feldspathic schist (original rock), biotite-amphibole schist, amphibole schist (transitional), and skarn (altered zone). The results show that skarnification occurred in two main stages: a prograde stage, characterized by the infiltration of hydrothermal fluids rich in Na⁺ and Ca²⁺, which promoted the replacement of the original biotite (XFe = 0.6–0.7) by calcic amphiboles (pargasite and Mg-hornblende; XMg = 0.5–0.6) and albite alteration of plagioclase (An₂₀–₃₀ → An₁₀). The dissolution of K-feldspar released K⁺, which redistributed into late-stage veins. The second stage was a retrograde stage, associated with the cooling of fluids (T < 400°C), resulting in the formation of actinolite (XMg = 0.7–0.8), epidote, calcite, and titanite. Scheelite, the tungsten ore mineral, precipitated in this stage, often associated with sulfides and titanite. Biotite in the altered rocks shows enrichment in Mg (XMg = 0.4–0.5) compared to the original rock (XMg = 0.3–0.4). Plagioclase evolved to albite compositions (An₁₀–₂₀). Texturally, altered zones are marked by poikiloblastic amphibole with quartz inclusions, replacement of biotite by epidote, and development of clinozoisite on grain boundaries. This work establishes petrogenetic criteria (e.g., Mg/(Mg+Fe) ratio in biotite, extreme albite alteration) for skarn mapping, in addition to reinforcing the potential of the Seridó Mineral Province for W-skarn exploration associated with hydrothermal fluids.