Multi-stage calcite precipitation during the diagenesis ofthe Huincul FormationinLos Chihuidos High, Neuquén Basin, Argentina: C and O isotopes in the characterization of the fluids source

ANA LAURA RAINOLDI; FRANCHINI MARTA; ADRIAN BOYCE; JOSEFINA PONS; CESARETTI NORA

San Pablo

Simposio; 9th South American Symposium on Isotope Geology; 2014

Institute of Geoscience-University of San Pablo

The interaction of hydrocarbons with metal-charged basinal brines has resulted in the formation of several copper deposits hosted in the Upper Cretaceous red beds of the Neuquén Basin. Field, petrographic, and mineralogical observations indicate that hydrocarbon migration predatedthe mineralization stage. Carbonate precipitation records multiple stages, which span: i) early diagenesis, ii) burial diagenesis-hydrocarbon upflow, and, iii) latediagenesis-copper mineralization. C and O isotopes from each of these carbonates allowed the characterization of fluidsand C sources.C and O isotopes range from -3.2 to -6.9? and -8.5 to -10.2?in the earliest diagenetic calcite, and become significantly lower in later stages: -7.3 to -10 ? and -10.8 to -14.4?in burial calcite and -9.4 to -11.2?and -17.4 to -18.3?in latecalcite.These values suggest an increasingly significant influence of organic carbon into the diagenetic environments, whereas variable O isotope could reflect increasing temperatures with time, and/or an evolution of the fluids through increasing water-rock interaction: both are conceptually reasonable.This evolution isechoed in the isotopic variation of zoned concretions, with isotopically heavy cores and lighter rims. Abundant primary and secondary organic-rich fluid inclusions and bitumen impregnations mainly hosted in burial diagenetic calcite witness the former presence of hydrocarbons and related fluids asa potential source of the 13C- and 18O-depleted isotopes.