Sorption of metallic compounds on activated carbon: application to exploration for concealed deposits in southern Spain

Metal transfer from three buried volcano-sedimentary massive sulphide depos its to the soil atmosphere has been studied using integrative collectors co ntaining specific activated carbon products. The experimental sites are loc ated in the southwest Iberian Pyrite Belt, where three orebodies (Los Frail es, Sierrecilla, Herrerias) Lie beneath either overburden or several tens o f metres of schist and volcanic country rocks. Soil-gas geochemistry profil es (CO2, He, and Rn) were mapped at each site, and gas anomalies revealed t he presence of fractures. It was possible to distinguish soil-gas anomalies related to fractures cross-cutting the orebodies at all three sites, where wide helium concentration anomalies and more narrow Rn and CO2 anomalies t raced the occurrence of the orebodies at depth. Integrative collectors cont aining specific activated carbon were set up for about 100 days along 400- to 600-m-long profiles spaced 10 to 40 m apart. After retrieval of the coll ectors, the metals sorbed on the activated carbon were eluted and analyzed by ICP-MS for Cu, Pb, Zn, As, Ag, Cd, Cr, Ni and Co. Anomalous amounts of a dsorbed metallic components were observed in places for all these metals ab ove the orebodies. Metal anomalies were coincident with carbon dioxide anom alies, indicating that metal transfer from the orebody to the soil atmosphe re occurs through the cross-cutting fractures. Where metal anomalies were o bserved, measurements indicate that over the 100-day integration period, a natural flux of 10 l CO2 passes through the collector cross-section. No cor relation was found between the metal content eluted from the activated carb on and the metal content of the soil from the same locality. This suggests that the metallic compounds sorbed onto the gas collectors are not simply d erived from a highly mobile metallic fraction of the soil, but are solely r elated to a soil-gas flux. One profile located 400 m above an orebody which lies below the water table, revealed volatile metallic compound anomalies associated with a soil Hg anomaly, despite there being only a minimal CO2 a nomaly at this site. The lateral dispersion of metallic compound anomalies appears to be controlled by soil matrix characteristics. In deeply develope d soils, anomalies range from 30 to 60 m wide, whereas in the poorly develo ped soils which overlie schists and volcanic rocks, some anomalies are only 10 m wide. Results obtained from the same site during the rainy and dry se asons are fairly reproducible. The results obtained in this study suggest t hat metallic compounds in the soil atmosphere, associated with gas anomalie s, are related to sulphide mineralization at depth. This polymetallic signa l can be measured using specific activated carbon gas collectors. The use o f integrative collectors to trap metals transferred with the gas phase has potential as a tool for detecting buried or hidden mineralization. (C) 1999 Elsevier Science B.V. All rights reserved.