GENERAL FEATURES OF THE GOLD DEPOSITS IN THE RIO ITAPICURU GREENSTONE BELT (RIGB, NE BRAZIL), DISCUSSION OF THE ORIGIN, TIMING AND TECTONIC MODEL

Abstract

Anomalous concentration of gold is associated with relatively narrow shear zones in the supracrustal sequence of the Rio Itapicuru Greenstone Belt (RIGB). Both geological mapping and geochemical / geophysical surveys led to the discovery of several gold mines. Among them, the Fazenda Brasileiro mine (1401 Au) is the largest one. This paper draws the general features of the RIGB gold deposits and puts constraints on the relationship between mineralization and the structural evolution of the belt. Mineralization is essentially associated (direct or indirectly) with carbonaceous lithologies and is represented by quartz veins/veinlets that define the main orebodies. In the main mines, Au is almost always related to the occurrence of arsenopyrite, appearing as inclusions, within microfractures or attached to the crystal faces. Hydrothermal effects have been recognized within the host-rocks. Recent structural and geochronological data show that gold precipitated late (2,083-2,031 Ga) with respect to the regional tectonic events (Ca - 2,100-2,080 Ga). A coupling between thrusting and late stage of granitic magmatism to account for the generation of the mineralizing fluids is suggested. By leaching Au from the volcanic pile, the metamorphic and pluton-related fluids were driven along early planar and linear anisotrophies. Hence this pathway within the shear zones led to a geometric disposition of the oreshoots in which their long axes are parallel to the stretching lineation. Quartz microfabrics and