The Geology of shear-zone hosted gold deposits in Northeast Brazil

Abstract

Mesothermal gold-bearing quartz veins are widespread in the Borborema Province in N.E. Brazil and well exposed in two active mines: São Francisco mine and Cachoeira de Minas mine. Although the Borborema Province shows a complex crustal evolution, the general structure consists of a mosaic of Archaean-Early Proterozoic massif terranes surrounded by Proterozoic fold belts. The major tectonic features are the development of crustal-scale shear zones and the emplacement of batholiths and stocks of granitoid related to the Brasiliano-Pan African thermal-tectonic event (0.9-0.5 Ga). The province is a medium to high-grade metamorphic zone cut by two E-W crustal-scale lineament systems or first order shear zones, about 150 km apart. These structures developed a complex anastomosing network of thrust faults and subsidiary strike-slip shear zones. Deformation is predominantly in the plastic regime, although plastic-brittle deformation also occurs. Many subsidiaries N to NE trending structures are characterised as second-order shear zones, which allowed the migration of metal-bearing hydrothermal fluids and provided sites for mineralization. Mesothermal lode gold deposits occur either within the shear zones, on the limbs of folds commonly associated with thrust faults and strike-slip faults, or in close association with major shear zones.....There is a large scatter in the δ18O quartz values (7.0 to 14.5 0/00), suggesting mixing of fluids from different sources, including that of meteoric water. On the basis of these studies the Borborema Province is shown to have had a complex geological evolution. The model for mineralization invokes several sources for the fluids and the ore components; the importance of granitic magmatism and convecting meteoric fluids is highlighted. Further exploration should be concentrated in fold belts which represent former back-arc basins, and particularly where these contain subsidiary shear zones