Strong metal anomalies in stream sediments from semiarid watersheds in Northern Chile: when geological and structural analyses contribute to understanding environmental disturbances

Abstract

We present data from a reconnaissance geochemical environmental survey (stream sediments) in the Limarí watershed (northern Chile), and include information from the neighboring Elqui Basin for a combined analysis. Given that the region has a long historical record of mining activities, important environmental disturbances were expected. However, one of the rivers in the Limarí watershed that was chosen to serve as a baseline, as no mining activities had ever taken place along the valley, showed one of the largest geochemical anomalies. The sampled stream sediments of the Hurtado River are highly enriched in Cu (50-1,880 μg g-1), Zn (65-6,580 μg g-1), and Cd (130-31,350 ng g-1). The river system is sourced in the high-altitude domain of the Andes, and drains important Miocene hydrothermal alteration zones. The Coipita zone (El Indio gold belt) appears to be the most likely candidate to have originated the metal anomaly. The study of Landsat images suggests that the belt of alteration zones is located within a large (400+ km long, ~150 km wide) NW-SE dextral fault zone. This highly fractured domain may have conditioned the rapid unroofing of epithermal ore deposits in Miocene time, contributed to important circulation of meteoric waters, and eventually, to subsequent strong oxidation, leaching, and dispersion of metals, thus contributing to major metal dispersion in the Elqui and Limarí fluvial systems.