Sediment- hosted copper deposits in the Cretaceous rocks of Neuquen basin, Argentina

The Tordillos and the Sapo Sur prospects are two examples of sediment-hosted Cu (V-U) deposits located in the Dorsal de Huincul and Dorso de los Chihuidos Highs, respectively. Both deposits are located near hydrocarbon fields and are hosted in the paleochannels red bed sequences of the Huincul Formation (Late Cenomanian-Early Turonian, Neuquen Group). During the eodiagenesis the precipitation of hematite gave the red coloration to the sedimentary rocks. With progressive burial precipitated pore-filling kaolinite and quartz and albite overgrowths following by calcite, barite and gypsum cements. The Cu mineralization is hosted in these pervasively bleached rocks impregnated with bitumen. The bleaching was produced by the interaction of the reducing hydrocarbons with the authigenic minerals. The synchronic oxidation of hydrocarbons locally produced carbonates precipitation. Copper mineralization is interstitial, occludes the primary and secondary porosity of the rocks and fractures. It consists of chalcocite (Cu1.8-2S)>chalcopyrite and bornite partially replaced by supergene minerals (chrysocolla > brochantite> malachite > azurite traces > covellite (Cu1-1.014S) > complex oxides of Cu, Mn and Ba-copper wad > cuprite > tenorite, volborthite, and traces of U-V minerals, carnotite, franchevillite?). Hypogene sulfurs selectively replace early interstitial barite cement. This new information has enabled an interpretation of the source of sulphur and the hydrothermal fluid-flow processes that favoured the formation of these deposit.