Characterization of the Diagenesis, Alteration, and Cu Mineralization in the Cretaceous Sedimentary Rocks of the Tordillos Deposit, Neuquén Basin, Argentina

PONS M.J.; FRANCHINI M.; MEINERT L.; GIUSIANO A.; IMPINCCINI A.; BEAUFORT D.; PATRIER P.; RAINOLDI A. L.

Hobart

Conferencia; SEG2015 Conference; 2015

SEG, CODES

The Neuquén Basin hosts 21 red-bed type stratiform copper deposits in which mineralization is related to basin-scale fluid migration. The Tordillos deposit is one example, located in the northern sector of the Dorsal Huincul and hosted in the redbed succession of the Cretaceous Neuquén Group. The Tordillos deposit has an estimated resource of 9.5 Mt at 0.42% Cu, with U (135-251 ppm) and V(250-980 ppm) anomalies. The mineralization occurs in fluvial channel facies ofthe Huincul Formation, where the originally red sandstones and conglomerateshave been bleached. Bitumen impregnations are common in these bleached rocks.The red layers are preserved only in the less permeable rocks of the flood plain facies. Pink, fine-grained sandstones owe their color to hematite coatings on quartz grains and micro inclusions of hematite inside calcite-1 cement. These rocks also contain kaolinite and thin barite (barite-1) rims on some sandstone grains. In contrast, in the bleached rocks hematite coatings are absent, bitumen coats detrital grains, authigenic minerals like kaolinite and calcite-I are partially to completely dissolved, and porosity is considerably enhanced  (20  vol. %). New mineral precipitationincludes:  quartz overgrowths, smectite ±chlorite coatings that also replace detrital clasts, and early kaolinite, pore-filling pyrite and late poikilitic, white and gray calcite (2 and 3). Inthe western area of the deposit, Ba anomalies up to 2% correlate with the presence of coarse and tabular barite-2 crystals that fill the primary and secondary porosity and are cut by veinlets of barite-3. Calcite-2 and 3 show δ13CPDB (-11 to -7) and δ18OPDB (-13 to -10) values typical of an organic source. Quartz overgrowths, barite-2, and calcite-3 host abundant organic fluid inclusions with bluish and yellowish fluorescence. Aqueous fluidinclusions associated with organic fluid inclusions in quartz homogenized to temperatures between 91 and 120°C(n=19), their initial melting temperatures (between -35 and-9 °C) indicate the presence of FeCl2 or NaCl?MgCl (n=4) and KCl (n=10),respectively, and their final melting temperatures ranged between -7 to -0.3°C.The mineralizationis epigenetic and fills the secondary porosity of the permeable and bleached rocks of the Huincul Formation. It consists of minerals of the chalcocite group with relict chalcopyrite and bornite, and supergene Cu-V-U minerals (covellite,brochantite > malachite, copper wad-tenorite and cuprite, chrysocolla, Cu-KBa vanadates and urovanadates) always in contact with impregnations of bitumen.  Chalcocite-spionkopite minerals have δ34Svalues that range from -7.3 to -17.8. Peripheral to the mineralized layers,sandstones and conglomerates are cemented by Cu-smectite ± V-hematite. Thefollowing features document a complex history for the formation of thisdeposit: 1) redox reactions occurred when the Huincul Formation acted as a petroleum conduit, causing dissolution of grains and early authigenic minerals(hematite-barite1-calcite-1),  and precipitation of new cements (calcite-2, clays, pyrite, and calcite-3),  2) the mixing of Ba-rich basinal water that migrated along with hydrocarbons, with interstitial sulfate-rich water caused the precipitation of barite-2, and 3) the late input of Cu chloride brinesprecipitated Fe-Cu and Cu sulfides at the expense of pyrite and BSR ofsulfates. Andean tectonics during the Miocene triggered the breakdown of seals and upward flow of these multiple fluid pulses from underlying oil reservoirs through existing structures of the Dorsal de Huincul into the host HuinculFormation.