STABLE ISOTOPE AND FLUID INCLUSION STUDY OF STRATIFORM COPPER DEPOSITS FROM THE NEUQUÉN BASIN, ARGENTINA

RAINOLDI A. L. ; FRANCHINI M. B.; BOYCE A.; GIUSIANO A.; CESARETTI N.; PONS M. J.; RÍOS J. F.

MINERALIUM DEPOSITA

SPRINGER

Lugar: Berlin; Año: 2019

0026-4598

LosChihuidos and El Porvenir Cu deposits of the Neuquén Basin, the leadingproducer of hydrocarbons in Argentina, are examples of sediment-hosted Cu depositsgenerated by interaction of hydrocarbons with host red-beds and metal-chargedbasinal brines. They are hosted in the Upper Cretaceous red beds of the NeuquénGroup, where mineralization occurs in bleached interconnected fluvial palaeochannels;barren fine-grained facies remain red, indicating a strong influence ofporosity and permeability in ore formation. Sulfides from thechalcocite-spionkopite series replace previous pyrite and chalcopyrite-borniteor are directly precipitated in pore space. Hypogene ore is altered tosupergene minerals.Stableisotope and fluid inclusion analysis record a complex formation historybeginning with the early diagenesis of red beds, in which kaoliniteprecipitated around 60ºC from meteoric waters, after hematite formation.Calcite I (δ13C-8 to -3.3?) and barite (δ34S +4.1?) thenprecipitated with increasing temperature (75-85ºC) from a more evolved fluid (δ18Ofluids+0.9 to +2.7?).Duringthe Tertiary Andean orogeny, hydrocarbon migrated and reacted with the red bedsresulting in the dissolution of early cements and precipitation of smectite (δ18Ofluids-2.2? and δDfluids of -73.7?), chlorite-smectite mixed-layer (δ18Ofluids-6.9? and δDfluids-84?), pyrite (δ34S +10.2?), andcalcite II (δ13C -12.9 to -6.8?) as by-product of the redoxreactions. Calcite II formed from low-salinity fluids (0.4 to 5.9 wt % NaCl equiv)at temperatures between 80-105ºC at Los Chihuidos deposit, and at slightlyhigher temperatures (125-145ºC) at El Porvenir deposit.Duringsubsequent uplift, metal-charged basinal brines flowed into the bleachedsandstones and precipitated sulfides characterized by an overall progressiveand marked lowering of δ34S from chalcopyrite-bornite (δ34S+12.3?) to chalcocite-spionkopite (δ34S -64 to +4.1?). The sametrend is observed in the C isotope compositions of calcites IIIa and IIIbaccompanying Fe-Cu and Cu sulfides respectively, which range from d13C-31.4 to -10.1?. These calcites formed from saline fluids (up to 21.3 wt % NaClequiv) at temperatures of 135-160ºC and 75-100ºC for calcites IIIa and IIIb inEl Porvenir deposit, and of 70-90ºC for calcite III in Los Chihuidos deposit.The δ18O values (+5.6 to +11.4?) of fluids in equilibrium withcalcite IIIa are similar to oilfield and basinal brines whereas fluids inequilibrium with calcite IIIb show much lighter δ18O values (-0.2 to+2.8?), which together with the significant lowering of δ13C (and δ34S),suggest the involvement of methane as the main reductant and possibly meteoric fluidinteraction. We thus hypothesize thatmineralization in the Neuquén sediment-hosted, stratiform copper depositsinvolved Fe and Fe-Cu stages dominated initially by thermochemical sulfatereduction with a final Cu stage, dominated by biogenic reduction of aqueoussulfate that migrated with hydrocarbons and metal-charged basinal brines. Themost likely source of sulfur in Los Chihuidos and El Porvenir deposits would bethe evaporite successions below the mineralized rocks.