DIAGENESIS OF MIDDLE ORDOVICIAN K-BENTONITES FROM THE CENTRAL ARGENTINE PRECORDILLERA

Whole rock chemistry and mineralogy of Middle Ordovician K-bentonites from Cerro Viejo, San Juan province, Argentina was investigated to determine the conditions of diagenesis of these beds. Clay mineralogy of the beds consists of R1 illite / smectite (I/S)and kaolinite. Kaolinite increases up section in relative proportion to R1 I/S from 2-5% at the base of the section to 50% at the top of the section. This gradation corresponds directly with the lithologic transition of the limestone of the San Juan formation at the base of the section to the black shales of the Cerro Viejo and Los Azules formations at the top of the section. This gradation is attributed to early kaolinization resulting from organic material associated with the adjacent black shales. Whole rock chemistry indicates that metals often associated with brines or MVT deposits are elevated with concentrations of Ba (3000-5300 ppm), Zn (52-320 ppm), Sr (100-312 ppm), and Cu (6-40.8 ppm). Other elements associated with hydrothermal fluids are also presnt such as As (5.3-310 ppm) B, (120-360 ppm)and Cl (<100-11500 ppm). Concentrations of these elements covary regardless of stratigraphic position indicating that they were introduced from diagenetic processes operating on all of the K-bentonite beds and are not likely the result of primary volcanic chemistry. Thermal history data from previous studies indicate that areas west of Cerro Viejo experienced burial temperatures in the range of 300 to >350 C. Fluid derived from this region of low grade metamorphism could be responsible for the observed chemistry at Cerro Viejo. These data indicate that there may have been large scale fluid migration in the Argentine Precordillera. These fluids may be signifcant factors controlling mineral and petroleum distribution in the Argentine Precordillera.