GEOTHERMAL-TYPE ALTERATION IN A BURIAL METAMORPHOSED VOLCANIC PILE, CENTRAL CHILE

The Upper Cretaceous and Tertiary volcanic sequences in the Chilean An des are affected by burial metamorphism. For example, in central Chile (between 32-degrees-30' and 35-degrees-S), the Abanico Formation, a f olded upper Cretaceous to Palaeogene unit composed of basic lavas, tuf fs and ash flows of intermediate composition and volcaniclastic sandst ones, is characterized by heulandite- to laumontite-bearing zeolite fa cies assemblages in its upper part passing with depth to prehnite-pump ellyite facies assemblages. However, at c. 33-degrees-30'S in the Aban ico Hill area, located just east of a graben at Santiago (the longitud inal Central Valley), the alteration pattern is unrelated to stratigra phic depth. It is characterized by a lateral increase in grade defined by assemblages with yugawaralite (reported for the first time in the Andes) +/- laumontite, then wairakite +/- epidote, and finally with ab undant epidote successively closer to the graben boundary. This patter n was formed in a palaeogeothermal system with a high-T and a very low -P gradient. Geothermal alteration has also been inferred from the bor der zone of a Neogene caldera in the Abanico Hill area, and from the w estern fault boundary of the graben. These expressions of geothermal a lteration, together with the occurrence of caldera structures, the hug e volume of ignimbrites and numerous epithermal precious metal deposit s of late Cretaceous and Tertiary age in central and northern Chile, s uggest that fossil geothermal systems of this age are probably common features in the Chilean Andes.