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

RAINOLDI ANA; MARTA FRANCHINI; ADRIAN BOYCE; ADOLFO GIUSIANO; CESARETTI NORA; JOSEFINA PONS; FRANCISCO JAVIER RÍOS

MINERALIUM DEPOSITA

SPRINGER

Lugar: Berlin; Año: 2019 vol. 54 p. 415 - 436

0026-4598

Los Chihuidos and El Porvenir Cu deposits of the Neuquén Basin, the leading producer of hydrocarbons in Argentina, are examples of sediment-hosted deposits generated by interaction of hydrocarbons with host red-beds and metal-charged basinal brines. They are hosted in the Upper Cretaceous red beds of the Neuquén Group, where mineralization occurs in bleached interconnected fluvial paleochannels; barren fine-grained facies remain red, indicating a strong influence of porosity and permeability in ore formation. Sulfides from the chalcocite-spionkopite series replace previous pyrite and chalcopyrite-bornite or are directly precipitated in pore space. Hypogene ore is altered to supergene minerals.Stable isotope and fluid inclusion analysis record a complex formation history beginning with the early diagenesis of red beds, in which kaolinite precipitated around 60ºC from meteoric waters, after hematite formation. Calcite I (δ13C -8 to -3.3per mil) and barite (δ34S +4.1per mil) then precipitated with increasing temperature(75-85ºC) from a more interacted fluid (δ18Ofluids +0.9 to +2.7per mil).During the Tertiary Andean orogeny, hydrocarbon migrated and reacted with the red beds resulting in the dissolution of early cements and precipitation of smectite (δ18Ofluids -2.2? and δDfluids of -73.7per mil), chlorite-smectite mixed-layer (δ18Ofluids -6.9 and δDfluids -84per mil), pyrite (δ34S +10.2per mil), and calcite II (δ13C -12.9 to -7.4per mil) as by-product of the redox reactions. This 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 slightly higher temperatures (125-145ºC) at El Porvenir deposit. During subsequent uplift, metal-charged basinal brines flowed into the bleached sandstones andprecipitated sulfides characterized by an overall progressive and marked lowering of δ34S from chalcopyrite-bornite (δ34S +12.3 per mil) to chalcocite-spionkopite (δ34S +4.1 to -64 per mil). The same trend is observed in the C isotope compositions of calcites IIIa and IIIb accompanying Fe-Cu and Cu sulfides, respectively, that range from -6.8 to -31.4 per mil. These calcites formed from saline fluids (up to 21.3 wt % NaCl equiv) at temperatures of 135-160ºC and 75-100ºC for calcites IIIa and IIIb in El Porvenir deposit, and of 70-90ºC for calcite III in Los Chihuidos deposit. The δ18O values (+5.6 to +11.4 per mil) of fluids in equilibrium with calcite IIIa are similar to oilfield and basinal brines whereas fluids in equilibrium with calcite IIIb show much lighter δ18O (-0.2 to +2.8per mil), which together with the significant lowering of δ13C (and δ34S), suggest the involvement of methane as the main reductant. We thus hypothesizes that mineralization in the Neuquén SSC deposits involved Fe and Fe-Cu stages dominated initially by thermochemical sulfate reduction with a final Cu stage, dominated by biogenic reduction of aqueous sulfate that migrated with hydrocarbons and metal-charged basinal brines.