Petrologia e metalogenia do Depósito de Cobre Bom Jardim de Goiás (GO)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

The Bom Jardim de Goiás Deposit is located at western Goiás State, on the western border of the Arenópolis magmatic arc. A reserve of 4,575,660 t, containing 0,92% Cu, was estimated by Serviço Geológico do Brasil (CPRM). The country rocks comprise vulcanoclastic rocks assigned to the Córrego da Aldeia formation from the Bom Jardim de Goiás Group, with age interpreted as close to 900 Ma. The rocks as characterized by the presence of crystals, lithic fragments, ash and pumices. They were classified as crystalline and cineritic tuffs and are commonly cut by veins and veinlets without a preferential orientation, containing the following hydrothermal minerals: biotite, quartz, chlorite, calcite, pyrite, chalcopyrite, titanite, actinolite, epidote, plagioclase and magnetite. The tuffs are calc-alkaline and have composition predominately riodacitic to dacitic and geochemical characteristics of magmas from volcanic arcs. The magmatic biotite from the tuffs has composition intermediary between annite and phogopite ((Mg/(Mg+Fe) ~ 0.5; Al ~ 2.5) and chemical characteristics of biotites from orogenic calc-alkaline suites. εNd(t) values lie between +3.5 and +7.4 and the model ages vary from 0.8 to 1.1 Ga. These values are coherent to juvenile magmas generated in volcanic arcs settings. Two types of hornblende-biotite monzogranite outcrop east of the deposit, pink and white. Both are interpreted as Serra da Negra granites. The monzogranites are equigranular and medium grained. West of the deposit a biotite sienogranite was identified. The granite, interpreted as Macacos Granite, is red, isotropic and coarse grained. The granites ages are probably about 580-600 Ma. The Serra Negra granite contains mafic enclaves, which probably resulted from magma mixing. The granites have geochemical characteristics of I-type calc-alkaline granites, formed in postcollisional setting. The biotite from the Macacos Granite is completely transformed to chlorite, while the biotite from the Serra da Negra Granite has composition intermediary between annite and phogopite ((Mg/(Mg+Fe) ~ 0.4; Al ~ 2.2) and chemical characteristics of biotites from intra-plate suites. The TDM values lie between +1.1 and +1.9 Ga and may represent either the formation of the continental crust (εNd(t) = -5 to -2) or mixing of sources of different ages. The isotopic data are coherent with the geochemical data. Afanitic basic rocks occur either intercalated with the vulcanoclastic rocks from the deposit or as dikes cutting the granites. The basalt from the deposit has composition similar to those of tholleitic basalts from volcanic arc settings, while the dikes have composition of intra-plate tholleitic basalts. The εNd(t) and TDM values are respectively +3 to +4 and 1.1-1.2 Ga for the analyzed basic rocks. The Bom Jardim de Goiás Deposit does not contain alteration halos. Only a zone of intense silicification associated with the mineralized veinlets and an external zone of choritization with restricted epidote, are recognized. The copper mineralization occurs disseminated and confined to the veins and veinlets in the tuffs, with no relationship with the granites that outcrop in the deposit region. The ore is mainly pyrite and chalcopyrite and is represented by the following ore paragenesis: pyrite + chalcopyrite pirrotite gold electrum esfalerite magnetite ilmenite hematite. Although there is no expressive zone of massive sulfide in the deposit, its characteristics allow suggest that the mineralization occurs in a stockwork zone rich in veinlets filled by pyrite and chalcopyrite, comprising a deposit similar to volcanic massive sulfide deposits.

ASSUNTO(S)

cobre goiás geologia arco magnético

ACESSO AO ARTIGO

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