Efeitos hidrotermais em rochas carbonáticas-siliciclásticas da Formação Itaituba, Pensilvaniano da Bacia do Amazonas, região de Uruará (PA)

Abstract

Carbonate and siliciclastic rocks from the Neocarboniferous period are widely recorded in the western and central-western portions in the Uruará region, State of Pará, on the southern edge of the Amazon Basin. These deposits are represented by the Itaituba Formation, from which a 35 m drill was described and five carbonate microfacies (calcimudstone, dolomudstone, bioclastic wackestone, bioclastic packstone and bioclastic grainstone rich in terrigenous) and eigth siliciclastic facies (massive claystone, black shale, siltstone with truncated crosslamination, siltstone with low-angle cross-lamination, siltstone with parallel laminate, sandstone with truncated cross-laminate, sandstone with parallel laminate and massive sandstone) were describe. From petrographic analysis, cathodoluminescence, X-ray diffraction, scanning electron microscopy and dispersive energy spectroscopy, it was possible to identify the main hydrothermal and diagenetic processes that affected these rocks. In the carbonate rocks, micritization, dissolution, calcitization, mechanical and chemical compaction and cementation were observed. In sandstones, quartz overgrowth, carbonate cementation and chemical compaction occur. During the Triassic-Jurassic period, great volcanism event occurred in the central portion of the West Gondwana supercontinent, known as the Central Atlantic Magmatic Province (CAMP), with the placement of dikes and basalt sills represented in the Amazon Basin by the Penatecaua Magmatism. The percolation of hydrothermal fluids in the late phase of this magmatism, promoted several changes in the mineralogy and textures of these rocks, among them hydraulic fracturing, formation of vugular porosity, precipitation of mineral assemblages typical of hydrothermalism such as - saddle dolomite, calcite, apatite, pyrite, chalcopyrite, galena, sphalerite, iron and titanium oxides, chlorites, talc, garnet, fluorcarbonate enriched in REE, saponite and corrensite - in the venules, and silicification of the matrix and carbonate grains. Organic geochemistry analysis in the samples of carbonate and shale, exhibit extremely low total organic carbon values, with low maturity, generating type IV kerogen, probably originated from oxidized organic matter. Thus, the present work intends to expand the study of rocks that occur in subsurface in the Uruará region, enabling the understanding of depositional and diagenetic processes active in their formation and its alteration due to the hydrothermal effect.