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

ANA L. RAINOLDI; M. FRANCHINI; A. BOYCE; M. PONS; N. N. CESARETTI

Sao Paulo

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

i nstitute of g eosciences/University of s ão Paulo

  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 predated the mineralization stage. Carbonate precipitation records multiple stages, which span: i) early diagenesis, ii) burial diagenesis-hydrocarbon upflow, and, iii) late diagenesis-copper mineralization. C and O isotopes from each of these carbonates allowed the characterization of the fluids and 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 late calcite. 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 is echoed in the isotopic variation of zoned concretions, with isotopically heavy cores and lighter rims. Abundant organic-rich fluid inclusions and bitumen impregnations witness the former presence of hydrocarbons and related fluids as a potential source of the 13C- and 18O-depleted isotopes.