GEOLOGY AND GEOPHYSICS AS SUCCESSFUL TOOLS IN THE DISCOVERY OF THE LAGOA-SALGADA OREBODY (SADO TERTIARY BASIN-IBERIAN PYRITE BELT), GRANDOLA, PORTUGAL

Gravimetry was the main exploration method used in this high risk area to the north of the Grandola fault, in the Iberian Pyrite Belt (IPB). It showed several targets, among which the Valverde-Lagoa Salgada are a stood out. Here, in August 1992, a polymetallic massive sulphide ore body was found, named Lagoa Salgada. According to the geological chara cteristics of the area and to the nature of the surveyed targets, othe r exploration techniques were used, namely: geology, magnetometry, res istivity, electrical sounding, magneto-telluric, seismic and mechanica l drilling. The deposit was found under a Tertiary cover, 128 m in dep th, complete with a 15 m thick gossan, caused by paleoalteration, with a supergene enrichment zone. This gossan gave way to massive sulphide s, with intersections: at times, exceeding 60 m. They are limited towa rds the footwall by an important fault with strong associated kaoliniz ation, which is interpreted as a reverse fault, placed on the inverse limb of an anticlinal structure. Throughout the deposit, the chemical composition of the mineralization showed great variation reaching sign ificant values of Zn, Pb, Sn, Cu, As, HE, Sb, Cd, Au and Ag. In the in itial phase the deposit has an estimated tonnage of 5 Mt. It is relate d to a large gravity anomaly with the general orientation NW-SE, which was investigated by 15 boreholes made by the Institute Geologico e Mi neiro. The drilling cut an important volcanic centre of an acid to int ermediate nature, around which intense hydrothermal activity was assoc iated with contemporaneous sulphide mineralization. The gravity anomal y has two nuclei, a NW and a central one, about 450 m apart, where a p yrite orebody with polymetallic sulphides and stockwork sulphide miner alization are developed. This discovery led to an extensive exploratio n project that confirmed the mining potentialities of the NW sector of the IPB, to the north of Grandola fault, where previously no polymeta llic sulphide occurrences were known.