Diagenesis of Tertiary playa sandstones of Northern Chile; implications for Andean uplift and metallogeny

The Oligo-Miocene Pacencia Group of Antofagasta Province, northern Chile consists of over 2 km of molassic sediments deposited in a series of alluvial fan and playa sub-environments. Sandstones of the Artolla Member, exposed around San Bartolo, represent playa-marginal sandflat deposits and locally host stratiform copper mineralization. A detailed study of these sediments has revealed a complex sequence of diagenetic modifications to the arkosic detrital mineral assemblage. Early dissolution of unstable ferro-magnesian minerals and calcic feldspar resulted in the release of ions into intra-stratal solution and the subsequent formation of a suite of eogenetic authigenic minerals. These include early hematite coatings on framework grains, pore-lining zeolite and pore-filling calcite, gypsum and celestite cements. Syntaxial overgrowths of albite, quartz and less common K-feldspar are well developed. Following early oxidative intrastratal conditions, the influx of acidic fluids derived from mudrocks during mesogenesis resulted in an important secondary porosity generation and the establishment of an acidic, reducing intrastratal environment. This facilitated the subsequent introduction of low temperature mineralizing solutions and formation of the San Bartolo copper deposit. A return to oxidative diagenetic conditions is recorded by late stage overgrowths of K-feldspar and quartz and the oxidation of native copper/chalcopyrite cements to cuprite/tenorite and chalcocite/covellite assemblages. This final telogenetic modification to the mineral assemblage is related to flushing by meteoric waters and provides independent evidence for an important phase of Andean deformation and uplift in the middle to upper Miocene.