Estudo geocronológico e de geoquímica isotópica da área Pojuca (Província Mineral de Carajás - PA)

Abstract

The Pojuca area, located in the northern portion of the Mineral Province of Carajás (southeast of the State of Pará – Brazil), presents Archean units such as: metavolcano-sedimentary sequences of the Igarapé Pojuca Group, with important deposits of Cu (Au, Ag, Mo); deformed granites (Itacaiúnas Deformed Granite); sedimentary rocks of the Águas Claras Formation; and metagabbroic dykes. The Pojuca Granite is an intrusive Proterozoic body in Archean rocks. Geochronological and isotopic geochemical studies were carried out on these lithologies using the methods Pb-Pb by evaporation in zircon, Pb-Pb in whole rock and phosdspar and Rb-Sr in whole rock and minerals (all results calculated with deviations of 2σ) . In the metavolcanic rocks of the Igarapé Pojuca Group two distinct types of zircon (metamictic and non-metamictic) were observed. Metamitic zircons show high contamination of foreign Pb, possibly caused by the action of hydrothermal fluids during thermal events (Archaean regional metamorphism, intrusions of the Itacaiúnas Deformed Granite and the Pojuca Granite). The non-metamictic zircons did not show Pb amounts detectable by the mass spectrometer. The analyzes carried out by the Pb-Pb method in whole rock and feldspars did not provide good results due to the dispersion of the analytical points in the graph 206Pb/204Pb vs. 207Pb/204Pb. However, the values of the Th/U ratios (2.5) indicated a mantle source for the metavolcanic rocks. The Pb-Pb method by zircon evaporation was successfully applied to the Itacaiúnas Deformed Granite, obtaining an age of 2560 ± 37 Ma, interpreted as the minimum crystallization age of these rocks. Confirming for this body an Archean age close to that previously obtained by the Rb-Sr method in whole rock (2480 ± 40 Ma), being the same correlated to the age of the Ancient Salobo Granite (2573 ± 2 Ma, U-Pb in zircon). Analyzes by the Pb-Pb method on whole rock and feldspars did not provide good results on the Itacaiúnas Deformed Granite. The ages obtained (1597 ± 13 Ma and 2215 ± 86 Ma) indicate a disturbance of the Pb isotopic system, possibly caused by the intrusion of the Pojuca Granite. This same event affected and rehomogenized, on a mineral scale, the Rb-Sr system of this body, as indicated by the Rb-Sr age in whole rock and minerals (1810 ± 57 Ma). In the Pojuca Granite, only one zircon made it possible to obtain an age of 1873 ± 59 Ma, similar to the previously obtained U-Pb age in zircon (1874 ± 2 Ma). Using the Pb-Pb method in whole rock and minerals, the age of 1856 ± 23 Ma was obtained for the Pojuca Granite, also interpreted as the crystallization age of the body. The value of µ = 10.5 ± 0.32, as well as the high ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, indicate a crustal source for the granitic magma. The zircons of metagabbroic rocks (dikes) are petrographically similar to the metamict zircons of volcanic rocks. It is not possible to obtain satisfactory geochronological results. In the rocks analyzed by the Pb-Pb method in whole rock and feldspars from the Pojuca area, very high 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios were found. In the Itacaiúnas Deformed Granite and in the Pojuca Granite these ratios are interpreted as a product of the enrichment of the source magmas of these granites in the U and Th elements. In metavolcanic rocks it is believed that the high ratios reflect an isotopic contamination due to hydrothermal fluids associated with the intrusions of the Itacaiúnas Deformed Granite and the Pojuca Granite. Plotting the Pb isotopic ratios on the 206Pb/204Pb vs. 208Pb/204Pb, 204Pb/206Pb vs. 207Pb/206Pb and 204Pb/206Pb vs. 208Pb/206Pb, it is noted that each studied unit occupies a different domain. However, there is a slight overlap between the fields defined by the isotopic ratios of the metavolcanic rocks and the Pojuca Granite, which may indicate contamination of the metavolcanic rocks by granite.