Stable isotope-based modeling of the origin and genesis of an unusual Au–Ag–Sn–W epithermal system at Cirotan, Indonesia

The Pliocene Cirotan low-sulphidation epithermal gold deposit, western Java, Indonesia, is characterized by complex polymetallic assemblages and progressive enrichment in Sn–W and Au–Ag in the late stages of mineralization. Five distinct ore/alteration stages are distinguished: (1) wallrock silicification; (2) siliceous breccia; (3) cockade breccia; (4) high-grade veins; and (5) late drusy quartz. The δ34S values of the vein sulphides are very homogeneous and lie mostly between +2.5 and +5.4‰, with a mean of +3.8±0.9‰. These values closely match values from recent arc lavas and volcanic gases in the Sunda–Banda island arc, indicating direct incorporation of magmatic sulphur into the epithermal fluid system. By contrast, δ18O values of quartz/chalcedony decrease systematically from siliceous breccia (+8.5±0.4‰) to cockade breccia (+8.1±0.5‰), high-grade veins (+7.4±0.2‰), and drusy quartz (+6.9±0.4‰). This isotopic shift cannot be explained by cooling or boiling, but is satisfactorily modelled by mixing of andesitic magmatic water with heated local meteoric water. Rhodochrosite, which formed only during a discrete episode at the onset of cockade breccia formation, has unusually low δ13C (−18.3 to −14.8‰) and rather high δ18O (+22.7 to +26.2‰) values. These data are consistent with a short-lived episode of open-system boiling. Based on the data presented in this paper and other isotopic data sets for arc volcanics and current models of plate-tectonics in the region, we propose a metallogenic model, which involves mixing of magmatic and meteoric waters, and explains the unusual and progressive enrichment in Sn and W at Cirotan by increased recycling of slab-derived sedimentary material during Pliocene subduction.