CONICET | Buscador de Institutos y Recursos Humanos

The Tordillos sediment- hosted Cu deposit is located in the north-central sector of the Huincul High structure, in the Neuquén Basin, Argentina.It is hosted in sedimentary rocks of the Huincul Formation, an Upper Cretaceousredbed sequence deposited in a fluvial environment. Field mapping, petrography,and chemical analysis of the authigenicclay, sulfate, carbonate, and sulfide minerals at Tordillos have identified systematic variation in alteration and paragenesis withtime and with respect to the main feeder zones. The alteration associated with Cumineralization provides evidence of a multistage infiltration of fluids spatiallyassociated with paleo-hydrocarbon migration driven by Tertiary tectonism.Early diagenesisformed hematite, kaolinite, and calcite1 that have been preserved in the reddishbrown mudstones. Later influx of hydrocarbons and warm, reduced formationwaters from deeper reservoir rocks is associated with dissolution of earlycements and clasts and precipitation of new minerals. In the thickestpaleo-channels of the Huincul Formation, dissolution of early cements (hematite,kaolinite, calcite1 and barite1) and clasts locally increased the porosity of sandstonesup to 24% and destroyed hematite that changed sandstones and conglomerates fromred to yellowish-grey. Bitumen impregnations are abundant along with calcite2, 3,and 4, pyrite, minor montmorillonite, quartz overgrowth, and coarse baritecrystals with primary organic fluid inclusions at the contact with hydrocarbontubes. The MnO enrichment of calcite in stages 3 and 4 reflects theirprecipitation under reduced conditions. Grey and greenish grey sandstones withmontmorillonite, V-hematite, smectite-chlorite group minerals and pyrite,respectively are exposed between the reddish-brown mudstones and the yellowish-greysandstones and conglomerates. The grey and greenish-grey horizons arerepresentative of abrupt changes in Eh-pH conditions, between regional early oxidizingsolutions responsible for the formation of early diagenetic redbeds and laterinfiltration of reducing fluids responsible for alteration of sedimentary rocks.Copper mineralization is located inside the altered sandstones andconglomerates zones, forming halos surrounding vertical tubes containing bitumen.These tubes are interpreted as the conduits through which migrated hydrocarbons, formation waters, and subsequent chlorideand metal-rich fluids. In these places, pyrite, calcite,and barite cements of the altered rocks, have been dissolved and replaced by copper-richsulfides and then by late supergene Cu minerals.Interconnectionamong stacked permeable paleo-channels in the mid-section of the HuinculFormation facilitated the migration of several generations of fluids duringearly diagenesis, hydrocarbon up flow, and exhumation of the Neuquén Group,resulting in the development of alteration halos and Cu mineralization that canbe used, both in outcrop and drill core, during exploration for sediment-hostedCu mineralization.

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