Adakites have been originally proposed to be generated by partial melting of hot and young ( c.a. 25 Ma) subducting oceanic crust at convergent margin. They are intermediate-felsic, usually amphibole-rich volcanic or plutonic rocks displaying high Sr and La, but low Y and Yb signatures that result in high Sr/Y and La/Yb ratios. Adakites are not only found in the tectonic settings of forearc region, main volcanic arc, back-arc, with subducting oceanic crust as a necessary component, but also in intraplate regions, where partial melting of thickened or delaminated continental crust happened. They may have high MgO contents and Mg#, high Cr, Ni contents, reflecting interaction between pristine slab melts and mantle peridotite.
Given that the basaltic crust has Cu concentrations several times higher than the lower continental crust and the mantle wedge, ‘primitive’ adakites produced by oceanic slab melting should contain significantly higher Cu contents than adakites derived from the continental crust, as well as normal arc andesites. A globally compiled dataset shows that Cu concentrations in adakites are generally lower than that in normal arc rocks. We attribute this low copper content to loss of magmatic fluids as a result of sulphate reduction during adakitic magma differentiation, in turn induced by the crystallization of Fe–Ti oxides, essential to mineralization. Therefore, the underflow of oceanic-slab-derived adakites that can release larger amounts of Cu-Au by crystal fractionation leads to higher potential for Cu–Au mineralization along convergent margins, usually associated with ridge subduction. Such basaltic slab melts initially have considerably higher Cu contents and thus play a crucial role particularly in the relatively closed magma system responsible for generating porphyry Cu deposits.
Ours case studies from both Shaxi and Atlas porphyry Cu-Au deposits from China and Philippines show that the Cu-Au mineralization related to diorites characterized by high oxygen fugacities (DFMQ +2), indicating quite a good potential for porphyry Cu-Au mineralization.
Keywords: adakite; Cu–Au ore deposit; slab melting; plate subduction; Shaxi and Atlas porphyry Cu-Au deposits
*This study is supported by the National Natural Science Foundation of China (Nos. 41673040, 41373007).