Magmatic control on orebody distribution of porphyry-skarn gold-copper deposit: A case study of Beiya deposit from Sanjiang metallogenic belt in the southwest China

The giant Beiya porphyry–skarn-type gold (Au)–copper (Cu) deposit in the Sanjiang domain is marked by the presence of majority of the ore bodies in the wall rock, which is distinct from the fact that the worldwide porphyry–skarn Cu–Au deposits develop abundant Cu–Au ore bodies within ore-forming porphyries; however, the formation mechanism of this peculiar phenomenon remains to be investigated. Here, we take the contemporaneous ore-forming porphyry and postmineralization lamprophyre in the Beiya ore district as object of study and gather new and early data of chronology, mineralogy and geochemistry to elucidate the aforementioned issue. The ore-forming quartz syenite porphyry has been dated at 35.8–36.9 Ma, slightly earlier than the lamprophyre (ca. 34.9 Ma). The lamprophyre has high Ni (55.7–187 ppm), Cr (137–459 ppm), Sr (286–1012 ppm), and Ba (964–2228 ppm) contents; high Ba/La (16.6–33.2) ratio; low Hf/Sm (0.66–1.27) and Zr/Nb (14.9–22.3) ratios; and enriched Sr–Nd isotopes (0.7059–0.7080 and −1.30–4.95, respectively), indicating its origin in an enriched mantle metasomatized by pelagic sediment-related slab fluids. The trace and platinum-group elemental characteristics further demonstrate that the lamprophyre underwent crystal fractionation and sulfide liquation during magmatic evolution. It is clear that the lamprophyre has low Au and Cu concentrations owing to sulfide liquation; therefore, it is unlikely that the ore-forming porphyry evolved from a mantle-derived magma. The quartz syenite porphyry has enriched Sr (0.7067–0.7092), Nd (−2.40 to −6.00), and Hf (−7.40 to 4.90) isotopes, which are similar to the Neoproterozoic crustal materials with high Au (6–16 ppb) and Cu (383–445 ppm) abundances; therefore, we infer that the ore-forming porphyry stemmed from melting of the Neoproterozoic juvenile lower crust. Furthermore, the whole-rock data combined with in situ analysis of zircon and magnetite indicated that the quartz syenite porphyry underwent mafic magma replenishment and crystal fractionation during its evolution. Thus, we can infer that the lower-crustal remelting related to the intracontinental orogenic environment and subsequent biotite fractionation resulted in the lack of mafic minerals for the Beiya ore-forming porphyry, distinct from the worldwide porphyry–skarn Au–Cu deposits that are sourced from enriched mantle wedge and develop a large number of amphiboles and biotites in the ore-forming porphyries. Because of this petrogenetic model, the oxidation–reduction reaction in a porphyry ore-forming system, which is expressed as Fe²⁺ in the magma being oxidized to Fe³⁺ along with SO₄²⁻ being reduced to S²⁻, could only occur in the surrounding rock at the top of the exocontact zone of the ore-forming porphyry via the upward migration of Fe²⁺ in the forms of gas phase, thus providing abundant S²⁻ for the formation of Au–Cu ore bodies. This peculiar magmatic–metallogenic mechanism also highlights prospecting direction of the Beiya deposit in the future.