Estudos geoquimicos e isotopicos das mineralizações auriferas e rochas associadas da Região de Pontes e Lacerda (MT)

AUTOR(ES)
DATA DE PUBLICAÇÃO

1996

RESUMO

Geochemistry and isotope studies were carried out on the gold deposits of Pontes e Lacerda region (Mato Grosso state, Brazil) where rocks of the Aguapei and Rondoniense mobile belts (SW of Amazon Craton) occur. The stratigraphy of this part of Amazon Craton consists of the Pontes e Lacerda Metavulcano-sedimentar Sequence (PLMVSS), Santa Helena Granite-Gneiss (SHGG), Maraboa Granite (MG), Aguapei Group (AG) and Nene Tonalite (NT). The PLMVSS rocks resemble ocean floor basalts in composition, and their isotopic signature indicates a Sr depleted and Nd enriched source in the the mande. SmlNd dating yielded an isochron age of 1910:t 387 Ma and a model age of 1200 Ma. These ages are not conclusive due to the limitations of the dating method. The foliated SHGG is sub-alkaline in composition.Its age at 1318 :t 24 Ma. (Rb/Sr isochron) may represent isotopic homogenization of a former granitic protolith. The 86Sr/87 Sr initiaI ratio of 0,711 indicates crustaI source for the magma. MG was separated tTom SHGG due to its isotropic texture and its Rb/Sr isochron age of 1257 :t 125 Ma, besides its intrusive character in SHGG and PLMVSS. NT is represented by intrusive bodies concordant with the mineraIized horizons. Its mineraIogy suggests greenschists facies metamorphism and its rare earth elements pattem with a positive Eu anomaIy indicates that this rock derived trom a differentiation processo 1:20.000 geological mapping was done in an area where a large number of deposits occur. The ore bodies consist of quartz, pyrite and gold, and the hydrothermaI alteration zone contain quartz, sericite, pyrite (altered to limonite), and magnetite (altered to hematite). Furthermore chalcopyrite, galena and sphalerite occur only in the Onça deposito Gold deposits occur in two main areas, the Santa Elina sector, with 2 deposits and Pontes e Lacerda sector, with 23 deposits. Mineralization is associated with a 200 km long shear zone related to the Aguapei evento Tectonics involves oblique overthrusting (trom NE to SW) which led to formation ofrecumbent folds and thrusts, upright folds and faults with dominant strike-slip component. These unconfommities are potential sites for .mineraIization, and provided pathways for the mineralizing fluids. Hydrothermal aIteration at Pontes e Lacerda region consists dominantly of sericitization, sulfidization and silicification. The hydrothermal processes were responsible for enhanced concentrations of K20, Rb, Ba and Fe203 and losses in CaO, Sr, MnO and FeD. The Zr, Y, Cr, Ab03 Si02 and Ti02 contents remained unchanged during the processo Increasing rare earth elements contents in the altered volcanic rocks may be due to a probable magmatic contribution to the fluids, which is also indicated by positive Ce anomaly in altered basalts and high contents of Bi, Se and Te in sulfides and gold. The K/ Ar dating of hydrothermal sericites ftom gold deposits veins yielded ages in the range 960 to 920 Ma which may indicate the age of original crystallization of sericite. Pb-Pb dating in galenas yielded model ages in the range 1000 to 800 My for the Onça deposit, in agreement with K/ Ar ages. Pb isotopic ratios indicate a high U/Pb and low Th/Pb for the Pb source in Upper Crust before incorporation in galena crystals. Thus, the Pontes e Lacerda gold deposits yelded ages in correlaion to Aguapei event and they were probably originated during a Proterozoic extentional tectonic period in SW part of Amazon Craton, which may characterize an important metallogenic epoch in the Pontes e Lacerda region. The present study comprising 11 gold deposits led to a regional exploration model which includes the following ore controls: structural (thrust zone due to the Aguapei event), stratigraphic (pLMVSS and AG geological contact), lithologic (mineralization associated to PLMVSS, AG and tonalites), mineralogical (sericite, magnetite and sulfides), geochemical (loss and gain of elements and oxides) and chronostratigraphic (age in the range of 1000 to 800 Ma)

ASSUNTO(S)

sul ouro - mato grosso prospecção geoquimica - mato grosso sul

ACESSO AO ARTIGO

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