Geochemical Characterization of Neogene Magmatism linked to Skarn Mineralization, SW Mendoza, Argentina

PONS M. J., FRANCHINI M. B. Y MEINERT L.

Florence, Italy,

Congreso; en 32th International Geological Congress,; 2004

Subcomité Organizador del 32th International Geological Congress

During the Tertiary, Southwestern Mendoza province was affected by several tectonic events associated with intense volcanic activity, represented by two Eruptive Cycles: Molle (L-M Miocene) and Huincan (M-U Miocene). The igneous rocks were emplaced into Mz-Cz sedimentary sequences along the main thrust faults (N-S), as stocks, dikes and sills of gabbrodiorite to diorite composition and their volcanic equivalents: basalts, andesites and trachyandesites. Near contacts with plutons, sedimentary units are altered to zoned metamorphic aureoles with biotite, pyroxene and amphibole hornfels, recrystallized limestone and superposed banded Fe-skarns (i.e. Hierro Indio (HI), Vegas Peladas (VP) and El Kaiser (EK)) and Cu-skarns (i.e. Co La Virgen). Some of these skarns have anomalous precious metal contents and/or are related to porphyry Cu-Mo-Au mineralization (i.e. Co Amarillo). All these mineralized skarns of SW Mendoza seem to be associated with the least differentiated facies of the Huincan Fm. (MEC).Geochemically, the plutons are granitoids of intermediate composition, metaluminous, subalkaline with a calc-alkaline affinity. They have evolved from the fractional crystallization of their main mineral components, pl-px-amph-mg-sph. Their trace element and REE contents are similar to rocks of the Neogene Volcanic Arc, to the Co. Nevazón (CN) (37.5° SL, in the Paleogene Volcanic Arc of NW Neuquén), and other Andean Quaternary Volcanic rocks of the TSVZ (34.5°-37° SL), for which a relatively thin continental crust has been defined. This suggests that these rocks formed from a calc-alkaline, metaluminous magma, derived from a similar magmatic source, with little or no residual garnet in the source rocks.The variability in the Sm/Yb and La/Yb ratios points to small increases in crustal thickness from south (30 km; NW Neuquén) to north (50 km SW Mendoza). The variable Cs contents of the HI and CN rocks, suggests some degree of crustal contamination. However, the former are distinguished by a lack of the reduced character shown by CN igneous rocks. The Rb vs. Y+Nb contents of HI are similar to igneous rocks associated with Fe-skarns, whereas VP and EK igneous rocks show more dispersion. VP, EK and HI igneous rocks are associated with Fe skarns, and in the case of VP, to Pb-Ag-Zn polymetallic veins, as well. CN plutons are associated with Au skarns.