Estudo petrográfico e isotópico (Pb-Pb, U-Pb e Sm-Nd) de rochas metavulcânicas, mineralizações auríferas e rochas granitoides relacionadas ao Greenstone Belt Três Palmeiras, Volta Grande do Xingu, Domínio Bacajá, Pará

Abstract

The present study was based on studies of petrography, isotopic geochemistry and geochronology, aiming to discuss the possible genetic and temporal relationship between the formation of auriferous mineralization, its host and the metavulcanic sequences that form the Três Palmeiras Greenstone Belt. Descriptions of outcrop and drilling holes samples allowed the definition of the following lithotypes in the study area: (1) a sequence of metabasalts, mafic schists and restricted metagabbro bodies (Anphibolite Itatá); (2) a sequence of porphyritic metandesites and metavulcanoclastic rocks (Micashist Bacaja). These rocks cropout nearby of the Xingu River, as elongated bodies, with WNW-ESE direction and record the effects of the predominant deformation in the area. These sequences are sectioned by bodies of diorites and granodiorites, with 2160 ± 3 Ma in age, comprising Oca Granodiorite. The gold deposits are composed of mineralized quartz veins in pyrite, chalcopyrite and gold, housted in the bodies of quartz-diorites and granodiorites, in the form of microinclusions in these sulphides. Pb-Pb isotopic data, obtained on pyrite from quartz-diorite and metandesite samples by Pb-leaching technique, provided ages of 2146 ± 19 Ma and 2353 ± 43 Ma, respectively, whereas the Pb-evaporation method on zircon provided ages of 2417 ± 4 Ma (MSDW = 0.64) for a meta-andesite sample and 2410 ± 4.3 Ma (MSWD = 4.3) for a sample of metavulcanoclastic rock occurring in the same area. These ages are similar to the age furnished by a whole-rock Sm-Nd isochron (2465 ± 17 Ma). Finally, the Pb-evaporation method on gold particles provides the age of 2189 ± 11 Ma (MSWD = 31). These ages indicate that mineralization has formed during the Riacian period. Based on the ages already determined for the granitic rocks present in the area, it is probable that the mineralization has a genetic and temporal relationship with these rocks. Isotopic data discard, so far, any possible genetic relationship as greenstone metavulcanic sequences. NdTDM model ages of 2.49 to 2.39 Ga obtained in this work for metandesite and metadacite samples from TPGB and its εNd(T) (+2.03 and +3.33) suggest that these rocks originated from the mantle in the Paleoproterozoic, followed by a rapid crustal residence.