Isotopic study (UPb, SmNd, and PbPb) of rocks and auriferous mineralizations from the Volta Grande do Xingu deposits, Bacajá Domain, Amazonian craton

Abstract

The Três Palmeiras volcano-sedimentary sequence in the northern Bacajá Domain, southeastern of the Amazonian craton, hosts gold occurrences and deposits, classified as orogenic type. In the Volta Grande do Xingu area, samples of outcrops and drill holes allowed us to identify three main lithological units: (1) a sequence of mafic metavolcanic rocks, represented by metabasalt, mafic schist and restricted metagabbro bodies; (2) a sequence of intermediate metavolcanic rocks, represented by porphyritic meta-andesite and metavolcanoclastic rocks; both sequences are cut by (3) quartz-diorite and granodiorite bodies, correlated with the Oca Granodiorite, previously dated at 2160 ± 3 Ma. Gold mineralization occurs as microinclusions in pyrite, chalcopyrite and/or quartz. Dating results indicate that meta-andesite and metavolcanoclastic rocks are, respectively, 2417 ± 4 Ma and 2410 ± 7 Ma by Pb-evaporation in zircon method, whereas the metavolcanoclastic rock was dated at 2415 ± 50 Ma by the UPb (LA-ICP-MS) zircon method, age corroborated by a SmNd whole-rock isochron of metabasalt and meta-andesite samples at 2465 ± 17 Ma. These ages denote the presence of Siderian volcanic rocks, predating those previously recorded in the Bacajá Domain (2359 ± 2 Ma). Pyrite (leachates) hosted in meta-andesite and quartz-diorite was dated at 2353 ± 43 Ma and 2146 ± 19 Ma, respectively, according to Pb-isotopic data, whereas gold particles yielded ages between 2.40 and 2.18 Ga by Pb-evaporation. The ages for pyrite and gold particles contained in dioritic suite samples imply at least two generations of pyrite, suggesting the mineralization likely relates to the Rhyacian granitogenesis recorded in the Bacajá Domain. Pb whole-rock isotopic analyses of meta-andesite and metabasalt samples indicate an age of 2403 ± 92 Ma, with high MSWD and error, but consistent with ages obtained by other methods for this sequence. The Nd-T_DM of 2.49 to 2.39 Ga and εNd_(t) (+2.03 to +3.63) indicate essentially a juvenile mantle origin for the metavolcanic rocks. The integration of field, mineralogical, isotopic, and geochronological data suggest that the mineralization event was most likely connected to metamorphic-deformational processes and granitoid intrusion associated with the Transamazonian cycle, complemented by inherited components from the Três Palmeiras sequence.