ALTERACIÓN, MINERALIZACIÓN DE Cu Y BITUMEN EN

JOSEFINA M. PONS; MARTA FRANCHINI ; ADOLFO GIUSIANO; AGNES IMPICCINI; MARTA GODEAS

Revista de la Asociación Geológica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 2009 vol. 64 p. 501 - 513

0004-4822

Barda González is a stratiform sedimentary copper prospect hosted in the Cretaceous rocks of the Portezuelo Formation (Neuquén Group) and located in the Dorsal de Huincul fault zone, near the Barda González, Bajo Barda González and Puesto Espinosa oil fields. The Cu mineralizationoccurs in coarse sandstone and conglomerate layers along fluvial paleochannels 600 m wide by 2000 m long with a NNE strike. These rocks are bleached, altered and very permeable. They lack iron oxides and hydroxides and show total to partial dissolution of their cement and their feldspar and volcanic lithics that are also altered to clay minerals (illite>kaolinite). The copperminerals are disseminated, in tubes or nodules always in contact with bitumen. They consit of chalcocite, covelline, trace of chalcopyrite and a variety of supergene minerals (malachite with chrysocolla, brochantite, azurite, turquoise, atacamite, volborthite).There are also disseminated pyrite and traces of psilomelane. Mineralized sandstone samples with the highest Cu concentration (16%) have Mo (810 ppm), U (9.43 ppm), Th (40 ppm) and V (127-221 ppm) anomalies. The geochemistry of bitumen indicates that it is a residue of hydrocarbon migration generated from a Jurassic marine rocks (Los Molles or VacaMuerta Formations). Upwelling chlorine-rich brines that accompanied the hydrocarbon migrations could have stripped Cu from the thick red bed sequence and S from the evaporites within the basin; S could have been also transported by hydrocarbons.The migration of these fluids from the source rocks to the reservoir through the faults and permeable layers would have played an important role in copper extraction from leaching iron oxides and hydroxides of the red beds, its transport and alterationof the sandstone (Portezuelo Formation) with the resulting increase of permeatibility and the copper-iron sulfides precipitation in the water/hydrocarbon interface.