Geochemistry of the Puncoviscana and Cachi Formations - Sierra de Cachi, Salta

Abstract

The small bodies and dikes of La Poma trondhjemite belt (La Poma Department, Salta province) contain tantalum-niobium (Cachi Formation). The country rocks are metasedimentary rocks of Puncoviscana Formation. The trondhjemite bodies are barren in ore-forming elements against the metasedimentary rocks, with higher contents in Ta, Nb, Li and Be. The mineralization was formed by melting in the aureole contact. The trondhjemitic magma raised the temperature of the country rocks, as observed in a thermal aureole around the plutons changing the original rocks to greenshists (granulite facies). When the rocks were partially melted, pegmatite lenses contain higher values of tantalium and niobium concentrated over ten times higher in the country rocks than in the trondhjemites. The pegmatite bodies have originated by partial melting of the metasedimentary rocks. Due to the process of melting, high contents of the lithophile elements, such as Ta, Nb, U, Th, W, Li and Be have been moved from the metasediments to the melted zones. The trondhjemitic magma origin should be related to an active subduction zone during the evolution of the Famatinian arc (480 - 460 Ma). The Narich magmatic arc testifies a probable change in the arc configuration, leading to melt the basic lower crust or the subducted young oceanic crust (< 25 Ma) or a change in the subduction angle resulting in the partial melting of the already accreted lower crust. Based on these observations a new exploration for tantalium and niobiun will be planned for this area.