Base and precious metals geochemistry of rock units of the mainland Aysén region, Chilean Patagonia

Abstract

In order to evaluate the applicability of regional rock geochemistry as an aid in mineral exploration, over 1000 rock chip samples of the Aysén region, Chile, were taken during the period 1993–1995. All samples were analyzed at commercial laboratories for 30 elements by induced coupled plasma atomic emission spectrometry (ICP–AES), and in addition, Au was determined by atomic absorption spectrometry (AAS). Rock samples were classified into two broad groups: (1) unaltered unmineralized rocks; and (2) altered and mineralized rocks. The geologic–tectonic setting of the area is a segment of the active continental margin of South America where, during the Late Jurassic and Cretaceous, a magmatic arc developed accompanied by an easterly marine back-arc basin. The basement is formed of Paleozoic metamorphic rocks that are interpreted as sedimentary wedges accreted to the Gondwana continent. The back-arc basin was filled by the end of the Mesozoic, and Tertiary volcanic and terrestrial sedimentary rocks that represent local basins and within-plate volcanism lie uncomformably on older units. The main mineralization in the region is coeval with Late Jurassic–Cretaceous magmatism and this is consistent with the geochemical data presented in this paper. Younger rocks show low geochemical values, suggesting that the change of tectonic regime by the end of the Mesozoic resulted in limited mineralizing processes during the Tertiary in the region. The regional rock geochemistry shows that unaltered unmineralized rock units of this region are well within global mean ranges for similar rock types, excepting As, which exhibits a conspicuous positive anomaly for most rock types. Altered and mineralized rock geochemistry and statistical treatment of data suggest potential for polymetallic mineralization in the region, the most prospective rock units being the volcanic Mesozoic rocks.