SÍNTESE DOS RECURSOS MINERAIS DO CRÁTON DO SÃO FRANCISCO E FAIXAS MARGINAIS EM MINAS GERAIS

Abstract

The organization of the main mineral resources of Minas Gerais State, Brazil, in the São Francisco Craton and its marginal belts permits the establishment of mineral provinces, districts and fields. This has been performed, taking into account the types of resources, their distribution in time and space, and the similar geological contexts of their host rocks. The representation of these units at a scale of 1:2 500 000 is overlain on a base map of regional geological environments, and is the result of an extensive bibliographic survey focusing on this past decade.

The Archaean granite-greenstone terrains in the cratonic domain are host to auriferous mineralizations, the subject of controversy regarding their syn- or epigenetic origins in the rocks of the Nova Lima Group, Quadrilátero Ferrífero. Gold- and sulphide-bearing quartz veins occur in rocks correlative to this Group, in the Pitangui-Pará de Minas, Mateus Leme and Conselheiro Lafaiete regions, respectively at the northwest and south of the Quadrilátero Ferrífero. The two former regions comprise agalmatolitic (massive pyrophyllitic) districts, and the latter is manganiferous. Mylonitic metaconglomerates of the Maquiné Group are mineralized in gold near Capanema. Chromitie boudinage lenses are associated with peridotites at Piumhi.

The main mineralizations hosted in metasedimentary cover sequences of the Transamazonian cratonic basement are those of the Quadrilátero Ferrífero Itabirite District. It comprises extensive deposits of high-grade hematite, friable itabirites and weathered hematite, in the Minas Supergroup. Gold-bearing quartz and hematite boundins occur in shear zones affecting the itabirites. At Mariana, gold is associated with quartz veins along the contact between the Cauê Itabirite and underlying schists.

Gold and uranium are present in paleoplacer metaconglomerates in the Gandarela and Ouro Fino Synclines. Quartz veins contain topaz near Ouro Preto. Graphitic Iodes occur at Itapecirica. Transamazonian tin-tantalum-lithium-niobium-bearing pegmatites crop out near São João del Rei.

The Neoproterozoic cratonic cover hosts lead-zinc mineralizations in the Bambuí Group rocks in Januária region. Gas wells are present in places in the sediments of the Bambuí Group. In the Alto Paranaíba region, alluvial diamonds are, in part, considered to be derived from Precambriam metasediments.

In the Araçuai Belt, chrome occurrences of the Serro meta-ultramafic bodies, with subordinate fold, are significant. Manganese is present in gondites of the Dom Silvério Group. Gold-related hydrothermal mineralization in shear zones is hosted in metavolcanosediments in the granite-gneissic Porteirinha Complex. Oxide-facies banded iron formations constitute iron ore at Morro do Pilar. Graphitic  Iodes occur in metamorphic rocks at Pedra Azul. Nickel is associated with meta-ultramafic rocks at Ipanema, in the Atlantic Belt.

Diamond is associated with fluviatile metaconglomerate units of the Sopa-Brumadinho Formation of the Espinhaço Supergroup, and is recovered from their alluvial deposits. Gold and quartz occur in hydrothermal veins near Diamantina.

Iron is present in glacial-marine hematitic diamictites of the Macaúbas Group in the Rio Vacaria Valley. In southern exposures of this Group, surficial chemical weathering produces manganiferous laterites. Tungsten occurs in contact aureoles around intrusive granites between Coronel Murta and Itinga.

Pegmatites related to the Brasiliano-age magmatism are divided into six districts, in the Araçuaí, Governador Valadares, Santa Maria de Itabira, Caparaó and Juiz de Fora regions. They are exploited for alexandrite, aquamarine, beryl, emerald, tourmaline, lithium, niobium, tantalum, tin and kaolin.

In the Alto Rio Grande Belt, gold and limestone are present in rocks of the São João del Rei Group. Lode gold is related to shear zones truncanting metamorphic rocks of the Andrelândia Group at São Gonçalo do Sapucaí. Nickel, south of Liberdade, and kyanite also occur in the Andrelândia Group.

In the Brasília Belt, the Morro do Ferro Greenstone Belt hosts nickel-cooper-cobalt and platinum group elements in gossans associated with komatiitic peridotites near Fortaleza de Minas. At Nova Rezende, it hosts nickel-cooper-zinc sulphide mineralizations which occur in fault zones in komatiitic ultrabasics and metasediments, and along the contacts between them. Lead-zinc and phosphate respectively comprise deposits in the Vazante and Lagamar regions, in the Vazante Formation. Gold-bearing boudins constitute ore in the Paracatu Formation.

During the last decade, the volume of scientific contributions to the field of study is significant. Discussions on hydrothermal, structurally-controlled shear zone-hosted, and granitoid-related mineralizations are highlighted as the most notable. In particular, tha syn-and/or epigenetic relations remain the subject of academic debate.