Regional geochemical reconnaissance of the Cordillera Occidental of Ecuador: economic and environmental applications

A regional geochemical reconnaissance survey of the Cordillera Occidental of Ecuador was initiated in 1995 as a sub-component of a wider Mining Development and Environmental Control Technical Assistance Project (PRODEMINCA) in Ecuador. The 36,000 km(2) survey area encompasses oceanic and continental-margin volcanosedimentary terranes with known occurrences of porphyry-style Cu-Mo, exhalative massive sulphide, epithermal Au and mesothermal polymetallic mineralisation. A survey sample medium of < 177 mu m stream sediments was selected following an orientation study in the vicinity of known porphyry Cu mineralisation. In the 2-4 degrees S sector of the Cordillera Occidental for which data are presented, 4850 drainage samples were collected at an average density of 1 per 2.57 km2. All were analysed for 36 major and trace elements (Au, Ag, Cu, Pb, Zn, Mo, Ni, Co, Cd, Bi, As, Sb, Fe, Hg, Mn, Te, Pa, Cr V, Sn, W, La, Al, Mg, Ca, Na, K, Sr, Y, Ga, Li, Nb, Sc, Ta, Ti, Zr). A stringent quality-control procedure included the systematic analysis of certified reference samples, field duplicates and replicates, data for which were used to calculate analytical precision, temporal drift and practical detection limits. Results for this part of the cordillera highlight the contrasting lithogeochemical signatures of the ocean-floor basalt terrane (Pallatanga Unit), the island-arc terrane of the Macuchi Unit, the continental volcanics of the Saraguro Group and the acid and intermediate lavas extruded from the Late Miocene to the Quaternary. Regional geochemical images for Au and associated pathfinder elements are dominated by anomalies relating to known mines and prospects. New exploration targets, often inconspicuous at the regional scale, have however been identified through the normalisation of data for individual lithological units against their respective geochemical backgrounds. In addition to mineral exploration, the drainage geochemical dataset for the Cordillera Occidental provides an unparalleled environmental baseline against which the impacts of future anthropogenic activities (including mining) may be assessed. A basis for the formulation of pragmatic sediment quality criteria and for the identification of natural geochemical hazards is also provided. (C) 2000 Elsevier Science Ltd. All rights reserved.