THE EXPEDITO MASSIVE SULFIDE DEPOSIT, MATO GROSSO

RENATO DANTAS NEDER, BERNARDINO RIBEIRO FIGUEIREDO, CHARLES BEAUDRY, CHRIS COLLINS, JAYME ALFREDO DEXHEIMER LEITE

Resumo


The Expedito massive sulfide Pb-Zn deposit is located 14 Km north of the Aripuanã town, in the northwestern portion of the State of Mato Grosso, western Brazil. The deposit occurs within a thick pile of acid to intermediate volcanic rocks and co-genetic intrusions of the Uatumã Group. These rocks are believed to be related to Mesoproterozoic intracontinental rift, supported by U-Pb zircon age of dacitic volcanics (1,762 ± 6 M.y.) and of a granite (1,755 ± 5 M.y.). The deposit is hosted by a horizon of dacitic lapilli and crystal tuff interlayered with massive dacitic porphyritic flows and carbonate and chert layers. The deposit consists of several discordant and discontinuous lenses of massive to semi-massive pyrrhotite, pyrite, sphalerite, galena, chalcopyrite, and arsenopyrite. Sulfides also occur disseminated or in veins. There are two types of ores: sphalerite-rich, more prominent, and chalcopyrite-rich with high Cu-Au grades. These deposits are enveloped by a hydrothermal alteration halo consisting of chlorite, biotite, and a peculiar assemblage of calc-silicate minerals associated with carbonate. Magnetite-rich zones overprint the massive sulfides and suggest post-depositional sulfide replacement. The ore lenses are intimately associated with the volcanic rocks, suggesting an exhalative origin. However, the absence of typical exhalative textures, intense calc-silicate alteration, and syn-deformational character of the deposits do not fit the classical VMS model. Therefore, an intrusion-related, epizonal hydrothermal replacement model is proposed.

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