Paleoproterozoic juvenile magmatism within the northeastern sector of the São Francisco paleocontinent: insights from the shoshonitic high Ba-Sr Montezuma granitoids

New, integrated petrographic, mineral chemistry, whole rock geochemical, zircon and titanite U?Pb geochronology, and zircon Hf isotopic data from the Montezuma granitoids, as well as new geochemical results for its host rocks represented by the Corrego Tingui Complex, provides new insights into the late- to post-collisional evolution of the northeastern S~ao Francisco paleocontinent. U?Pb zircon dates from the Montezuma granitoids spread along the Concordia between ca. 2.2 Ga to 1.8 Ga and comprise distinct groups. Group I have crystallization ages between ca. 2.15 Ga and 2.05 Ga and are interpreted as inherited grains. Group II zircon dates vary from 2.04 Ga to 1.9 Ga and corresponds to the crystallization of the Montezuma granitoids, which were constrained at ca. 2.03 Ga by the titanite U?Pb age. Inverse age zoning is common within the ca. 1.8 Ga Group III zircon ages, being related to fluid isotopic re-setting during the Espinhaco rifiting event. Zircon ?Hf(t) analysis show dominantly positive values for both Group I ( 4 to ?9) and II ( 3 to ?8) zircons and TDM2 model ages of 2.7?2.1 Ga and 2.5?1.95 Ga, respectively. Geochemically, the Montezuma granitoids are weakly peraluminous to metaluminous magnesian granitoids, enriched in LILES and LREE, with high to moderate Mg# and depleted in some of the HFSE. Their lithochemical signature, added to the juvenile signature of both inherited and crystallized zircons, allowed its classification as a shoshonitic high Ba?Sr granitoid related to a late- to post-collisional lithosphere delamination followed by asthenospheric upwelling. In this scenario, the partial melting of the lithospheric mantle interacted with the roots of an accreted juvenile intra-oceanic arc, being these hybrid magma interpreted as the source of the Montezuma granitoids. The Corrego Tingu? Complex host rocks are akin to a syn- to late-collisional volcanic arc granitoids originated from the partial melting of ancient crustal rocks. The results presented in this study have revealed the occurrence of juvenile rocks, probably related to an island arc environment, that are exotic in relation to the Paleo- to Neoarchean crust from the S~ao Francisco paleocontinent?s core

MINERALIZAÇÕES DE ZINCO E CHUMBO DO DEPÓSITO SALOBRO, PORTEIRINHA (MG)

A regional prospect program carried out by CVRD since the 70´s to find out gold and base metals deposits discovered the Salobro Zn-Pb deposit among others. It is located in the Porteirinha district in the northern area of Minas Gerais state. Three units were defined in the Riacho dos Machados Group (A, B, C from bottom to top), an Arquean or Paleoproterozoic metassedimentary sequence. The geological evaluation defined 8 mt@7% (Zn+Pb) measured for a cut off of 5% and a minimum width of 2,00 m. The Riacho dos Machados Group hosts the mineralization in a metachert layer in the unit B. The mineralization is essentially stratabound. Sphalerite and galena are the ore minerals, both with coarse granulometry imposed by metamorphic processes. The iron content of sphalerite ranges from 2,4% to 7,9%. The mineralogy of the ore-gangue is composed mainly by amphiboles, quartz and magnetite. Pyrite and pyrrhotite are present disseminated in the ore and in several massive and thick bodies. In spite of being a small deposit, Salobro has favorable features for exploitation.O depósito de zinco-chumbo Salobro, localizado no município de Porteirinha (MG), foi descoberto dentro do programa regional de prospecção de ouro e metais base executado pela CVRD por mais de três décadas (desde os anos 70) na região de Riacho dos Machados, no norte do estado de Minas Gerais. O Grupo Riacho dos Machados foi individualizado na região em três unidades litoestratigráficas denominadas A, B e C da base para o topo. A avaliação geológica definiu recursos de 8 mt@7% (Zn+Pb) para um teor de corte de 5% e uma espessura mínima de 2 m. As mineralizações ocorrem na seqüência metassedimentar do Grupo Riacho dos Machados e estão hospedadas em uma camada de metacherte da unidade B. A mineralização é essencialmente do tipo stratabound. Esfalerita e galena são os minerais econômicos, ambos apresentando granulometria grossa devido ao metamorfismo regional. O teor de ferro da esfalerita varia entre 2,4% e 7,9%. A ganga do minério é composta principalmente por quartzo, anfibólio e magnetita. Pirita e pirrotita estão presentes disseminadas no minério e em vários corpos maciços e espessos. Apesar de tratar-se de um depósito de dimensões reduzidas, o Salobro apresenta características competitivas e favoráveis à sua explotação

Petrography, geochemistry and geochronology of the potassic granitoids of the Rio Itacambiruçu Supersuite: implications for the Meso- to Neoarchean evolution of the Itacambira-Monte Azul block

ABSTRACT: In the Itacambira-Monte Azul block (BIMA) the Archean rocks are represented by tonalite-trondhjemite-granodiorite (TTGs) of the Porteirinha Complex and by potassic granitoids of the Rio Itacambiruçu Suite that crop out at the Pedra do Urubu (PUp), Rio Gorutuba (RGp), Barrocão (BAp) and Lagoa Nova (LNp) plutons. This work presents new data for the PUp, RGp and BAp. The petrographic and lithochemical analyses allowed the subdivision of the Rio Itacambiruçu Suite in two groups of granitoids: a medium potassium group, represented by the PUp and RGp, and a high potassium group, in which the BAp rocks are inserted. The geochronological data showed different ages for the medium and high potassium groups, with crystallization ages at ca. 2.92 Ga and 2.65 Ga, respectively. Despite the differences observed between these two groups, the chemical signatures of these rocks are compatible with the signature of biotite-granites generated by the reworking of ancient continental crust. Thereby, the data obtained in this work, added to some previously published results, allowed to the identification of two Archean crustal stabilization events recorded in BIMA, one of Mesoarchean age and the other of Neoarchean age. In addition, we propose the lithodemic reclassification of the Rio Itacambiruçu Suite to Rio Itacambiruçu Supersuite

Petrography, geochemistry and geochronology of the potassic granitoids of the Rio Itacambiruçu Supersuite: implications for the Meso- to Neoarchean evolution of the Itacambira-Monte Azul block

<div><p>ABSTRACT: In the Itacambira-Monte Azul block (BIMA) the Archean rocks are represented by tonalite-trondhjemite-granodiorite (TTGs) of the Porteirinha Complex and by potassic granitoids of the Rio Itacambiruçu Suite that crop out at the Pedra do Urubu (PUp), Rio Gorutuba (RGp), Barrocão (BAp) and Lagoa Nova (LNp) plutons. This work presents new data for the PUp, RGp and BAp. The petrographic and lithochemical analyses allowed the subdivision of the Rio Itacambiruçu Suite in two groups of granitoids: a medium potassium group, represented by the PUp and RGp, and a high potassium group, in which the BAp rocks are inserted. The geochronological data showed different ages for the medium and high potassium groups, with crystallization ages at ca. 2.92 Ga and 2.65 Ga, respectively. Despite the differences observed between these two groups, the chemical signatures of these rocks are compatible with the signature of biotite-granites generated by the reworking of ancient continental crust. Thereby, the data obtained in this work, added to some previously published results, allowed to the identification of two Archean crustal stabilization events recorded in BIMA, one of Mesoarchean age and the other of Neoarchean age. In addition, we propose the lithodemic reclassification of the Rio Itacambiruçu Suite to Rio Itacambiruçu Supersuite.</p></div