Ronaldo Mello Pereira

The Glória quartz-monzodiorite: isotopic and chemical evidence of arc-related magmatism in the central part of the Paleoproterozoic Mineiro belt, Minas Gerais State, Brazil

The Glória quartz-monzodiorite, one of the mafic plutons of the Paleoproterozoic Mineiro belt, is intrusive into banded gneisses, amphibolites, schists and phyllites of the Rio das Mortes greenstone belt, in the southern portion of the São Francisco Craton, State of Minas Gerais, Brazil. The Glória quartz-monzodiorite yields a SHRIMP U-Pb zircon age of 2188 +/- 29 Ma, suggesting a tectonic relationship with the pre-collisional phase of the Mineiro belt. According to the Nd isotopic evidence (epsilonNd(T) = -3.4; T DM = 2.68 Ga) the original magmas was formed by a mixture among Archean crustal material and Paleoproterozoic juvenile magma. The Glória quartz-monzodiorite shows metaluminous and calc-alkaline tendency with intermediate K content, comparable to that of volcanic-arc rocks. The primary mineralogical assemblage was partly modified by metamorphism, dated between 2131-2121 Ma in nearby coeval plutons. Such metamorphism is significantly older than the reported metamorphic episodes of the Mineiro belt in the Quadrilátero Ferrífero region (2059-2041 Ma) in the eastern portion of the study area. This evidence, together with chemical and isotopic data from other mafic and felsic plutons coeval with the Glória quartz-monzodiorite, indicate a tectonic and magmatic migration within the Mineiro belt from west to east.

The graphite mineralization in the Aracoiába-Baturité District (CE): geotectonic and metallogenetic implications

Na regiao de Aracoiaba-Baturite, a subunidade Baturite (Grupo Ceara) foi dividida em dominios Baturite I e II. O Dominio Baturite I compreende uma sequencia supracrustal paraderivada, formada em ambiente correspondente a zona de talude continental, sobreposta a rochas ortoderivadas paleoproterozoicas do Complexo Gnaissico-Migmatitico. Os gnaisses aluminosos portadores de grafita associados ao Dominio Baturite I foram comparados aos tipos encontrados nos cinturoes khondaliticos de diversas partes do mundo. O Dominio II apresenta ortoanfibolitos e metaturbiditos, tendo estas rochas sido formadas em zona de sope de talude-planicie abissal em facies de mar profundo. As ocorrencias de grafita dispoem-se desde Aracoiaba ate a regiao de Caninde, acompanhando a Subunidade Baturite, que representa um importante metalotecto regional. Os depositos de grafita sao do tipo grafita disseminada e macica, as quais estao, respectivamente, associadas a gnaisse grafitoso (tipo disseminado) e a veios (tipo macico) e apresentam diferentes origens geneticas (singenetica e epigenetica). Valores de δ 13C entre -30 e -20% indicam que a grafita e derivada de materia orgânica. As mineralizacoes epigeneticas sao encontradas no entorno do pluton de Pedra Aguda (Complexo Gabro-Dioritico Anelar de Pedra Aguda) a uma distância maxima do corpo de 2,5 km. A faixa grafitosa do Distrito de Aracoiaba-Baturite foi considerada como um remanescente de uma zona de subduccao/zona de sutura derivada do processo de fechamento de um antigo oceano com a consequente colagem continental de dois microcratons (bloco Baturite a norte e bloco Caio Prado a sul) durante a orogenia Brasiliana.

Hafnian zircon as a tracer for rare metal exploration

The major difficulty for the application of tracer minerals as mineralogical tool in geological ex- ploration is to differentiate, in geochemical samples (like soil or stream sediment), the ore-related resistant indicator minerals from the barren-related grains. Hafnian zircon or high hafnium zircon can be such a tool to prospect rare metal granite and pegmatite bodies. Here we present some habits of the Hf-rich zircon associated to granites and pegmatites from the southeastern Brazil. Some particular characteristics of the mineral, like its luster, colour and especially its crystal forms, would allow the separation, with a simple visual analysis under a stereomicroscope, of the possible hafnian zircon from the common zircon grains. Microprobe or SEM/EDS analysis are still necessary to confirm the hafnium content in zircon crystals.