The effects of K-metasomatism in the Bahía Laura Volcanic Complex, Deseado Massif, Argentina: Petrologic and metallogenic consequences

The Deseado Massif is a 60 000 km2 geological region located in southern Argentinean Patagonia. The most important geological feature of this area is an extensive plateau characterized by complex and long-lived Middle to Upper Jurassic volcanism, composed of intermediate to acid volcanic rocks of the Bahía Laura Volcanic Complex. This voluminous volcanism originated in a widespread extensional environment that affected all of Patagonia, related to the opening of the Atlantic Ocean. The volcanic rocks are genetically related to Jurassic epithermal deposits. Consequently the region is an important gold and silver producer with four operational mines (Cerro Vanguardia, Mina Martha, Manantial Espejo and San José) and more than 50 epithermal occurrences undergoing different levels of exploration. Potassic metasomatism identified in the Bahía Laura Volcanic Complex is difficult to recognize in hand specimen, but is characterized by K2O/Na2O ratios higher than 2.5 and L.O.I. values lower than 3 wt.%, and affects only rocks with silica contents over 68 wt.%. The metasomatism produced an intense geochemical change in the affected rocks, with significant additions of K2O, Rb, Au, Sb, W and a minor enrichment in Cs, Cu, As, and probably Ag and Hg. Strongly depleted elements are MgO, CaO, Na2O, P2O5, MnO, Sr and U, whereas FeOt, TiO2, L.O.I., Pb and Zn are moderately depleted. Finally, SiO2, Al2O3, Mo and most trace elements and REE remained almost invariant during increasing metasomatism. Strong similarities in lithologies and geochemical signatures observed in K-metasomatized areas around the world, regardless of their geological settings and fluid origins, suggest that volcanic glass probably played an important role as an ion exchanger during alteration. Due to the high mobility of elements that occurs during K-metasomatism, no classification or discrimination diagram based on major elements, TiO2 or Rb are considered to be valid for the chemical characterization of volcanic rocks based on metasomatized samples. The precious and base metals enrichment/depletion pattern of K-metasomatized rocks is identical to the geochemical signature observed in the upper parts of low sulfidation environments from the Deseado Massif. This geochemical signature, along with spatial relations observed between K-metasomatized rocks and epithermal manifestations, implies that hydrothermal fluids were responsible for the metasomatism observed in the Bahia Laura Volcanic Complex.