Geophysical Case History of the Discovery of the Aguas Tenidas East Massive Sulphide Deposit, SW Spain

The Aguas Tenidas East volcanogenic deposit was discovered after several years of geophysical surveying of the Iberian pyrite belt by Billiton Espanola in joint venture with Promotoras de Recursos Naturales. The discovery followed others made in the belt in recent years, in particular that of the large Neves Corvo deposit in 1977. The Aguas Tenidas East deposit is typical of the Pyrite Belt, comprising predominantly massive pyrite with a few percent sphalerite, chalcopyrite and galena, in a host sequence of sediments and felsic to intermediate volcanics. As of 1987 it contained 27 million tonnes at 1.1% Cu, 2.8% Zn, 0.8% Pb. The discovery in 1985 followed from TEM surveying along a prospective mineralised horizon extending from a smaller, exhausted surface mine. A strong conductor with a long decay was detected along strike and subsequently drilled. However the drill-hole was stopped after it passed the interpreted depth of the source without intersecting any mineralisation. Subsequent downhole TEM surveying lead to the drilling of another hole which intersected massive sulphides at a vertical depth of 330 metres. Other geophysical methods, such as INPUT, magnetics, gravity, IP and Max-Min were tried either before or after the discovery, but generally were either ambiguous or not cost-effective. Downhole techniques using TEM and mise-a-la-masse were very useful in delineating the deposit. Initial modelling of the TEM data assumed only an inductive response and failed to explain the observed anomalies. Subsequent modelling of the known mineralisation clearly indicates that the galvanic response (due to current gathering) is of considerable importance. Preliminary 2.5D numerical modelling allowing for current gathering appears to account for most the large Aguas Tenidas East EM response. Also of major significance in this discovery was the early, misleading extrapolation of surface geology to depth. Not only did the steep surface dips incorrectly constrain the geophysical model (since the deposit is sub-horizontal), but also the mineralised horizon at the nearby small open-cut mine, which was considered the prospective one, appears to be unrelated stratigraphically to the deeper Aguas Tenidas East host horizon. Without down-hole EM this situation may have resulted in the deposit not being discovered.