ABSTRACT
A model is presented of fluid evolution for a type of deposit that substantially differs from typical porphyry systems. The model is based on geochemical, fluid inclusion and stable isotope data. The mineralisation is developed predominantly in the apical, porphyritic, part of a Neogene subvolcanic granodiorite pluton. Extensive low-temperature hydrothermal alteration and K-metasomatism is associated with the mineralisation in the intrusion, while the overlying andesites are affected by high sulphidation alteration. Escaping magmatic vapor, resulting from fluid immiscibility, is responsible for the acid alteration. The mineralisation and alteration in granodiorite is the product of a large convective hydrothermal system. This circulated via a network of fractures, induced by the rapid cooling of the apical part of the intrusion and by probable penetration of hot supercritical magmatic fluids. These fluids are related to the continued crystallisation of granodiorite magma further at depth.
