MINERALIZATION, ANTIFORMS AND CRUSTAL EXTENSION IN ANDESITIC ARCS

The distribution and stratigraphic position of porphyry copper and epi thermal gold deposits in andesitic arcs of the western Pacific and eas tern Europe suggest that porphyry copper and epithermal vein deposits of adularia-sericite type develop successively under different stress regimes in an evolving arc, rather than being genetically related as c ommonly supposed. Absence of coeval high-level stocks in the root zone s of many adularia-sericite deposits suggests that circulation of the dominantly meteoric hydrothermal fluids is not driven by shallow intru sions. The location of several world-class deposits on basement geanti clines, and on more localized antiforms of which at least one has been interpreted as a metamorphic core complex, implies that elevation of the arc, emplacement of magmatic sills at depth and adularia-sericite type gold mineralization are genetically related to subduction-induced crustal extension. Ascent of deep hydrothermal fluids, predominantly meteoric but with a metamorphic or magmatic component, may be controll ed by regional low-angle structures at depth, analogous to those infer red for some mesothermal gold deposits. Mineralization at shallow (epi thermal) depths in high-angle structures largely reflects the high geo thermal gradient and mixing of deep fluid with cool meteoric water in or at the base of the permeable volcanic cover. Andesitic magmatism ma y resume following porphyry copper mineralization, adularia-sericite e pithermal gold mineralization, or continued extension to form a 'back arc' spreading system, depending on the relative plate motion.