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

A regional geochemical reconnaissance survey of the Cordillera Occidental o f Ecuador was initiated in 1995 as a sub-component of a wider Mining Develo pment and Environmental Control Technical Assistance Project (PRODEMINCA) i n Ecuador. The 36,000 km(2) survey area encompasses oceanic and continental -margin volcanosedimentary terranes with known occurrences of porphyry-styl e Cu-Mo, exhalative massive sulphide, epithermal Au and mesothermal polymet allic mineralisation. A survey sample medium of < 177 mu m stream sediments was selected following an orientation study in the vicinity of known porph yry Cu mineralisation. In the 2-4 degrees S sector of the Cordillera Occide ntal 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 tr ace elements (Au, Ag, Cu, Pb, Zn, Mo, Ni, Co, Cd, Bi, As, Sb, Fe, Hg, Mn, T e, Pa, Cr V, Sn, W, La, Al, Mg, Ca, Na, K, Sr, Y, Ga, Li, Nb, Sc, Ta, Ti, Z r). A stringent quality-control procedure included the systematic analysis of certified reference samples, field duplicates and replicates, data for w hich were used to calculate analytical precision, temporal drift and practi cal 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 continen tal volcanics of the Saraguro Group and the acid and intermediate lavas ext ruded from the Late Miocene to the Quaternary. Regional geochemical images for Au and associated pathfinder elements are dominated by anomalies relati ng to known mines and prospects. New exploration targets, often inconspicuo us at the regional scale, have however been identified through the normalis ation of data for individual lithological units against their respective ge ochemical backgrounds. In addition to mineral exploration, the drainage geo chemical dataset for the Cordillera Occidental provides an unparalleled env ironmental baseline against which the impacts of future anthropogenic activ ities (including mining) may be assessed. A basis for the formulation of pr agmatic sediment quality criteria and for the identification of natural geo chemical hazards is also provided. (C) 2000 Elsevier Science Ltd. All right s reserved.