Alteration halos in the Tordillos sediment-hosted copper deposit of the Neuquén Basin, Argentina

PONS, JOSEFINA; FRANCHINI, MARTA; GIUSIANO, ADOLFO; PATRIER PATRICIA; BEAUFORT DANIEL; IMPICCINI, AGNES; RAINOLDI, ANA LAURA; MEINERT, LAWRENCE

ORE GEOLOGY REVIEWS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2017 vol. 80 p. 691 - 715

0169-1368

The Tordillos sediment-hosted Cu deposit is located in the north-central sector of the Huincul High structure, inthe 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 theauthigenic clay, sulfate, carbonate, and sulfide minerals at Tordillos have identified systematic variation in alterationand paragenesis with time and with respect to the main feeder zones. The alteration associated with Cumineralization provides evidence of a multistage infiltration of fluids spatially associated with paleo-hydrocarbonmigration driven by Tertiary tectonism.Early diagenesis formed hematite, kaolinite, and calcite1 that have been preserved in the reddish brown mudstones.Later influx of hydrocarbons and warm, reduced formation waters from deeper reservoir rocks is associatedwith dissolution of early cements and clasts and precipitation of new minerals. In the thickest paleochannelsof the Huincul Formation, dissolution of early cements (hematite, kaolinite, calcite1 and barite1) andclasts locally increased the porosity of sandstones up to 24% and destroyed hematite that changed sandstonesand conglomerates from red to yellowish-grey. Bitumen impregnations are abundant along with calcite2, 3,and 4, pyrite, minor montmorillonite, quartz overgrowth, and coarse barite2 crystals with primary organicfluid inclusions at the contact with hydrocarbon tubes. The MnO enrichment of calcite in stages 3 and 4 reflectstheir precipitation under reduced conditions. Grey and greenish-grey sandstoneswithmontmorillonite, V-hematite,smectite-chlorite group minerals and pyrite, respectively are exposed between the reddish-brown mudstonesand the yellowish-grey sandstones and conglomerates. The grey and greenish-grey horizons arerepresentative of abrupt changes in Eh-pH conditions, between regional early oxidizing solutions responsiblefor the formation of early diagenetic redbeds and later infiltration of reducing fluids responsible for alterationof sedimentary rocks. Copper mineralization is located inside the altered sandstones and conglomerates zones,forming halos surrounding vertical tubes containing bitumen. These tubes are interpreted as the conduitsthrough which migrated hydrocarbons, formation waters, and subsequent chloride and metal-rich fluids. Inthese places, pyrite, calcite, and barite cements of the altered rocks, have been dissolved and replaced by copper-rich sulfides and then by late supergene Cu minerals.Interconnection among stacked permeable paleo-channels in the mid-section of the Huincul Formation facilitatedthe migration of several generations of fluids during early diagenesis, hydrocarbon up flow, and exhumation ofthe Neuquén Group, resulting in the development of alteration halos and Cu mineralization that can be used, bothin outcrop and drill core, during exploration for sediment-hosted Cu mineralization.