CARACTERÍSTICAS GEOQUÍMICAS E ISOTÓPICAS DAS ZONAS DE ALTERAÇÃO ASSOCIADAS COM DEPÓSITOS DE SULFETO MACIÇO VULCANOGÊNICOS METAMORFISADOS À ALTO GRAU, E SUAS APLICAÇÕES EM EXPLORAÇÃO MINERAL

Palavras-chave: alteração hidrotermal, metais base, depósitos vulcanogênicos, petroquímica, isótopos de oxigênio, exploração mineral

University of Toronto, UofT, Toronto, Canadá

DATA DE DEFESA: 1996
ÁREA DE CONCENTRAÇÃO: Geologia
ORIENTADOR: Steven D Scott

RESUMO
 

GEOCHEMICAL AND ISOTOPIC CHARACTERISTICS OF ALTERATION ZONES IN HIGHLY METAMORPHOSED VOLCANOGENIC MASSIVE SULFIDE DEPOSITS AND THEIR POTENTIAL APPLICATION TO MINERAL EXPLORATION

Key words: hydrothermal alteration, base metals, volcanogenic deposits, petrochemistry, oxygen isotope, mineral exploration

University of Toronto, UofT, Toronto, Canadá

DATE OF ORAL PRESENTATION: 1996
TOPIC OF THE THESIS: Geology
SUPERVISOR: Steven D Scott

ABSTRACT

 The Archean Geco, Canada and the Proterozoic Palmeirópolis, Brazil volcanogenic massive sulfide deposits are within tholeiitic bimodal volcano-sedimentary sequences. Both deposits were subjected to amphibolite facies metamorphism.
Metamorphic conditions in the alteration zones of the Palmeirópolis deposits are estimated to have been 550 to 625o and 2 to 5.5 kbar. These conditions are very similar to estimates for the Geco deposits by previous work.
The Geco and Palmeirópolis deposits are associated with large volumes of hydrothermally altered volcanic rocks. The Geco alteration zones are characterized by anthophyllite-cordierite assemblages and by muscovite-sillimanite assemblages. Based on trace and rare earth elements geochemistry, these assemblages are interpreted to be hydrothermally altered mafic and felsic metavolcanic rocks, respectively. Alteration zones at Palmeirópolis are composed of a complex mixture of anthophyllite-biotite, biotite-plagioclase and sillimanite-quartz assemblages. Lenses of amphibolite within the alteration zones are very common. This study suggests that the alteration zones in Palmeirópolis consist of a mixture of mafic and felsic components but with a strong predominance of the mafic ones.
The oxygen isotope signature of the anthophyllite-cordierita rocks from the Geco alteration zones is similar to that of primary chlorite alteration zones from non-metamorphosed deposits, and can be easily distinguished from the isotope signature of the host rocks. No isotopic distintion exists between host and altered rocks at Palmeirópolis and rehomogenization by pervasive fluid flow during metamorphism is suggested. The difference of isotope behaviour between the two deposits may reflect the volume of altered rocks present. The small size of the alteration zone and the close spatial association of non-altered and altered rocks at Palnmeirópolis facilitated isotopic reequilibration. At Geco, the much larger alteration zone was an effectively closed isotopic system which retained its distinctive ?18O signature through high-grade metamorphism. Oxygen isotopes may be a useful exploration tool in highly metamorphosed terrains if used with caution. Large areas affected by hydrothermal alteration and, consequently, large ore bodies such as Geco, may be identifiable using this technique. However, areas characterized by small hydrothermal cells, such as Palmeirópolis, are likely to have undergone isotopic reequilibration during high grade metamorphism and would be undetectable.