Characterization of the diagenesis, alteration and Cu-mineralization in the Cretaceous sedimentary rocks of the Tordillos deposit, Neuquén Basin, Argentina

JOSEFINA PONS; FRANCHINI MARTA; LARRY MEINERT; ADOLFO GIUSIANO; AGNES IMPICCINI,; DANIEL BEAUFORT; PATRICIA PATRIER; ANA LAURA RAINOLDI

Hobart

Conferencia; SEG-CODES Conference; 2015

SEG 2015 Organizing Committee

The Neuquén Basin hosts 21 red-bed type stratiform copper deposits where mineralization is related to basin-scale fluid migration. The Tordillos deposit is one example, located in the northern sector of the Dorsal Huincul and hosted in the red-bed succession of theCretaceous Neuquén Group. The Tordillos deposit has an estimated resource of 9.5Mt at 0.42% Cu, with U (135-251 ppm) and V (250-980 ppm) anomalies. The mineralization occurs in fluvial channel facies of the Huincul Formation where the originally red sandstones and conglomerates have been bleached and contain bitumen impregnations. The red layers are preserved only in the less permeable rocks of the floodplain facies. This presentation characterizes the alteration and mineralization of the Tordillos deposit to elucidate the relationship between diagenetic processes, hydrocarbons and Cu-rich fluid migration, and their implication in the origin of Cu mineralization.Andean tectonics during the Miocene triggered the breakdown of seals and upward flow of multiple fluid pulses from underlying oil reservoirs through existing structures of the Huincul High into the host Huincul Formation. Redox reactions occurred when hydrocarbons arrived into the Huincul Formation, causing dissolution of grains and early authigenic minerals (hematite-barite1-calcite-1), and precipitation of new cements (calcite-2, clays, pyrite, and calcite-3), and smectite-hematite toward the redox front. Mixing of Ba-rich basinal water that migrated along with hydrocarbons, with interstitial sulfate-rich water caused the precipitation of barite-2. Late input of Cu chloride brines precipitated Fe-Cu and Cu sulfides at the expense of pyrite and BSR of sulfates.