Alteration, Cu mineralization and bitumen in the Barda González prospect, Neuquén

Abstract

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 Cumineralization occurs 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 andshow total to partial dissolution of their cement and their feldspar and volcanic lithics that are also altered to clay minerals (illite>kaolinite). The copper minerals 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 pyriteand traces of psilomelane. Mineralized sandstone samples with the highest Cuconcentration (16%) have Mo (810 ppm), U (9.43 ppm), Th (40 ppm) and V (127-221ppm) anomalies. The geochemistry of bitumen indicates that it is a residue of hydrocarbon migration generated from a Jurassic marine rocks (Los Molles or Vaca Muerta Formations). Upwelling chlorine-rich brines that accompanied the hydrocarbon migrations could have stripped Cu from the thick red bed sequenceand S from the evaporites within the basin; S could have been also transportedby hydrocarbons. The migration of these fluids from the source rocks to the reservoir through the faults and permeable layers would have played animportant role in copper extraction from leaching iron oxides and hydroxides ofthe red beds, its transport and alteration of the sandstone (PortezueloFormation) with the resulting increase of permeatibility and the copper-ironsulfides precipitation in the water/hydrocarbon interface.