Petrogenesis of Miocene basic shoshonitic lavas in the Bolivian Andes and implications for hydrothermal gold, silver and tin deposits

Abstract

Miocene (16.4 to 7.4 Ma) basic to intermediate shoshonitic lavas and intrusives are located in the inner arc of the Central Andes in Bolivia and were erupted from major fissures in an active thrust belt during arc broadening. They are spatially and temporally linked with intermediate to felsic volcanic and intrusive rocks of the High K calc-alkaline suite which host major ore deposits of silver, tin and gold. Their petrography, mineral and whole rock geochemistry and Sr, Nd and O isotopes indicate that the lavas were formed by fractionation of OIB-like alkali basalt combined with assimilation of continental crust. Eruption of alkali basalt correlates with an aseismic ridge-trench collision. Sr and Nd isotopes indicate that the High K calc-alkaline suite evolved from shoshonitic magmas that underwent further fractionation and assimilation, possibly in response to a change from tectonic quiescence to active compression. The unusual richness of the mineralization may be related to a combination of structural, magmatic and tectonic factors which created optimum conditions for magma-crust-fluid interactions to occur. All of these factors may be controlled by the aseismic ridge-trench collision.