Geochemical anomalies in northern Chile as a surface expression of the extended supergene metallogenesis of buried copper deposits

Abstract

ABSTRACT Supergene enrichment played a critical role in making northern Chile the most productive copper province of the world. This occurred over a long period from 45 to 9 Ma in a semi-arid climate. Subsequently, many deposits were covered by thick alluvial gravels, such that exploration is now focused on deposits buried beneath these gravels. Well-defined saline+metal anomalies are present at the gravel surface above deposits. Work, starting in 1999, characterized these anomalies and ascribed their origin to pumping of mineralized saline waters to the surface during earthquakes. Atacamite, a copper hydroxychloride, and an important ore mineral, had previously been considered to be of primary supergene origin, i.e. 45 to 9 Ma. However, it is readily soluble in meteoric water and could not have been stable either during supergene enrichment by meteoric water, or later, when stream waters carrying alluvial gravels penetrated oxide zones. Atacamite must be younger than 9 Ma. Studies at the University of Chile have established that: (a) atacamite has been found at the surface along faults, 300 m above copper mineralization; (b) salinities of fluid inclusions in atacamite are the same as local saline groundwater; (c) 36 Cl analyses of atacamite show that it could not have formed prior to 1.5 Ma; (d) U-Th disequilibrium dating of gypsum from atacamite-gypsum intergrowths give Pleistocene ages that range from 237 ka for Chuquicamata in the east to 75 ka for Michilla on the Pacific coast. We propose that anomaly formation at the surface and saline metasomatism of supergene oxides were parts of the same process, which occurred after the climate became hyper-arid. Tectonic de-watering of the forearc basin sent saline waters through oxide zones to replace pre-existing minerals with atacamite and continued to the surface to create geochemical anomalies; these waters, having first modified the deposits, were rich in indicator elements. Thus, although perceptions are that geochemical anomalies above thick gravel sequences should be weak, selective leach anomalies in northern Chile have clear anomaly/background contrast.