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

Abstract

Metal transfer from three buried volcano-sedimentary massive sulphide deposits to the soil atmosphere has been studied using integrative collectors containing specific activated carbon products. The experimental sites are located in the southwest Iberian Pyrite Belt, where three orebodies (Los Frailes, Sierrecilla, Herrerias) lie beneath either overburden or several tens of metres of schist and volcanic country rocks. Soil-gas geochemistry profiles (CO 2, He, and Rn) were mapped at each site, and gas anomalies revealed the 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 CO 2 anomalies traced the occurrence of the orebodies at depth. Integrative collectors containing 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 collectors, 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 adsorbed metallic components were observed in places for all these metals above the orebodies. Metal anomalies were coincident with carbon dioxide anomalies, indicating that metal transfer from the orebody to the soil atmosphere occurs through the cross-cutting fractures. Where metal anomalies were observed, measurements indicate that over the 100-day integration period, a natural flux of 10 l CO 2 passes through the collector cross-section. No correlation was found between the metal content eluted from the activated carbon 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 derived from a highly mobile metallic fraction of the soil, but are solely related 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 CO 2 anomaly at this site. The lateral dispersion of metallic compound anomalies appears to be controlled by soil matrix characteristics. In deeply developed soils, anomalies range from 30 to 60 m wide, whereas in the poorly developed 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 seasons are fairly reproducible. The results obtained in this study suggest that metallic compounds in the soil atmosphere, associated with gas anomalies, are related to sulphide mineralization at depth. This polymetallic signal can be measured using specific activated carbon gas collectors. The use of integrative collectors to trap metals transferred with the gas phase has potential as a tool for detecting buried or hidden mineralization.