Lower to Middle Jurassic volcanism and Au–Ag mineralization at Cerro Moro District, Deseado Massif, Argentine Patagonia

Abstract

In the Cerro Moro Mine area, a horst block exposes a volcanic sequence with ages ranging from 191 Ma to 167 Ma, extending the Jurassic volcanic history of the Deseado Massif back to the Lower Jurassic (Sinemurian-Pleinsbachian boundary). Three volcanic sequences are recognized: a lower volcanic complex comprising welded tuffs, rhyolitic volcanic breccias, andesite lavas and intercalated continental sediments from 191 to 186 Ma, an intermediate sequence of welded ignimbrites, block-and-ash flow deposits and epiclastic sediments with ages ranging from 182 to 181 Ma, and an upper sequence of rhyolitic domes and welded ignimbrites from 180 to 167 Ma. Epithermal vein mineralization was formed in a series of pulses beginning at 180 Ma and culminating in a bonanza event at 171 Ma, coincident with the eruption of the upper volcanic sequence. Increasing grades of precious and base metals with time leading up to the bonanza event are interpreted as the result of repeated fertilization of the hydrothermal system by successive fluid pulses. Detrital, entrained and inherited zircons in the Jurassic rocks have ages ranging from the Triassic to Paleoproterozoic.