Apatite halogens and Sr-O and zircon Hf-O isotopes: recycled volatiles in Jurassic porphyry ore systems in southern Tibet

In-situ analysis of minerals, such as zircon hafnium (Hf) and oxygen (O) isotopes, has been pivotal in investigating magmatic evolution and related ore systems. In order to better constrain the petrogenesis of Cu-porphyries and related ore-forming processes in southern Tibet, we used a combined application of both zircon by Hf-O isotopes and apatite by developing geochemical indicators involving Sr-O isotopes and volatile (chlorine and sulfur) concentrations. Apatite has several advantages over zircon: it carries ore-related volatiles, is present in less-evolved magmas and is sensitive to magmatic processes. Data on major- and trace elements and Sr-O isotopes in gem-like apatite can supplement information from zircon Hf-O isotopes. We have applied these techniques to granitoids from the Jurassic arc in southern Tibet to illustrate their use in tracing volatile evolution and related genesis of porphyry systems. We demonstrate that (1) robust apatite Sr-O isotopes combined with zircon Hf-O isotopes can record recycled oceanic components in the sources of Gangdese Jurassic porphyries; (2) volatile-enriched magmas metasomatised hydrous mantle wedge to form the Jurassic mineralised rocks; (3) the ore-forming magma with higher contents of water, Cl and S, favoured the transfer of metals and volatiles to the upper crust and to form the Jurassic porphyry Cu-Au deposits.