CONICET | Buscador de Institutos y Recursos Humanos

Stratiform copper deposits in Neuquén Basin are hosted in Late Cretaceous sedimentary rocks of the Neuquén Group (Cenomanian-Campanian). The mineralization occurs in bleached sandstone that is interlayered with barren red beds. Petrographic studies were integrated with X-ray and geochemical analyses to determine the behavior of the oxides during the alteration of the red beds and subsequent mineralization. When compared with the original red strata, the bleached facies has the following characteristics: (1) absence of hematite coatings, (2) lithic fragments, feldspar grains, nonferrous calcite cement, and quartz overgrowths are partially dissolved, (3) lithic fragments and feldspar grains are altered to illite and kaolinite; these phyllosilicates also occur as grain coatings and intergranular/intragranular pore fillings, (4) porosity enhancement, (5) ferrous and magnesic calcite cement fills secondary porosity, and (6) bitumen occurs as disseminated specks or pipes. The style of the mineralized copper zones varies from discontinuous, sheet-like disseminations, with extensive horizontal dimensions, to cylindrical pipes with limited horizontal dimensions; these pipes cut across or are concordant with primary bedding. Other styles include mineralized fractures, shear zones, and contacts between sandstone and silicified woods. The mineralization consists of Cu (V-U-Ag) with chalcocite, pyrite, chalcopyrite, bornite, and supergene covellite-digenite. Oxidation of the Cu-bearing sulfides resulted in minor copper mobilization, creating thick sequences of disseminated, low-grade mineralization composed of malachite, chrysocolla, brochantite, hematite, goethite, azurite, volborthite, cuprite, tenorite, turquoise, atacamite, and carnotite. Mass balance calculations during the alteration of red beds registered mass losses up to 22%, immobility of SiO2 and K2O, and removal of FetO3, Al2O3, Na2O, and MgO; CaO and CO2 were added or removed depending on the rate between dissolution of authigenic calcite in red beds and precipitation of secondary calcite during alteration. Later mineralization of the bleached sandstone produced mass gain up to 30% and addition of Cu-V-Ag-U, CO2, FetO3, and MgO, whereas SiO2, Al2O3, Na2O, and K2O remained immobile. Oxides lost during alteration correlate with Fe coating removal, feldspar dissolution, and carbonitization vs. decarbonitization; immobility of SiO2 and K2O can be attributed to illitization. Sulfidation and carbonitization during mineralization produced the enrichment in Cu (V-U-Ag) CO2, FetO3, and MgO. The Neuquén Basin is famous because of its proliferous history in hydrocarbon resources, and hydrocarbons have played a critical role in red-bed alteration and copper sulfide precipitation. The organic acids solubilized in the formation waters migrated with hydrocarbons and dissolved carbonate and aluminosilicates, enhancing the secondary porosity of the red beds. They also caused feldspar and lithic fragment alteration, and illitization/kaolinization. Later hydrocarbons reacted with hematite coatings, reducing Fe3+ to Fe2+, and mobilized Fe2+ due to its solubility in these low pH conditions. Copper required for sulfide precipitation could have been transported by oxidized and saline basinal ground water. The interaction of these brines with hydrocarbons may have favored Cu sulfide precipitation; later weathering of sulfides by descending cool meteoric waters resulted in the widespread oxidation blanket.

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