Sulfur Isotope Characteristics of Mesothermal Gold Deposits: Results of an Investigation of the Rio Itapicuru Region, Bahia, Brazil

摘要

Stable isotope analyses were carried out on sulfides from three mesothermal gold deposits of the Rio Itapicuru greenstone belt (Fazenda Brasileiro, Maria Preta, and Ambrosio mines) to constrain the source of the hydrothermal solutions. The ores are hosted in a volcanic-sedimentary sequence that evolved in an Archean cratonic area reworked during the Transamazonic event (ca. 2.1 Ga). The ore bodies were deposited by hydrothermal solutions in polycyclic shear zones generated by Paleoproterozoic dynamothermal metamorphism. In the Fazenda Brasileiro deposit, delta~(34)S values for arsenopyrile from mineralized quartz veins range from 5.5 per thousand to +1.0 per thousand; pyrites from Au-rich mineralized veins from the Maria Preta deposit yielded delta~(34)S values ranging from +8.9 per thousand to +6.9 per thousand, in agreement with published delta~(34)S data (+0.1 per thousand to +6.6 per thousand). A narrow spread in the delta~(34)S values (or Au-rich sulfides from both deposits indicates that gold deposition occurred under conditions involving a small range of fO_2 and fH_22O. A barren, galena-rich vein from the Ambrosio deposit yielded delta~(34)S values in the -1 .Woo to -9.9 per thousand range, probably reflecting mixing of different sources. These results, compared to stable sulfur isotope modeling of lode gold deposits, indicate that the studied deposits formed from hydrothermal solutions of metamorphic origin followed by duluite solutions of magmatic origin.