Magma and fluid sources in an intracontinental porphyry system: A case study of the Relin Mo-W(-Cu) deposit, southern Yidun terrane, SW China

The Relin Mo-W (-Cu) deposit in the northern Sanjiang area is bound to a Late Cretaceous intracontinental porphyry showing variable alteration. Here we present whole-rock chemistry and Sr, Nd, Pb, Li, and B isotope data to constrain the sources of ore-magmas and to understand how magmatic-hydrothermal processes mobilize the ore elements and alter the magmatic rocks. Chemical variations indicate the ore-bearing porphyries reflect two processes: fractional crystallization and late-magmatic alteration. Fresh and weakly altered porphyries are metaluminous to weakly peraluminous, showing I-type affinity. Chemical variation among these rocks can be explained by fractional crystallization. Most of these rocks show narrow ranges of isotopic compositions with -8.6 to -6.6 for epsilon Nd80, 0.70660 to 0.71028 for 87Sr/86Sr80, and high 207Pb/204Pb80 (15.57-15.66) and 208Pb/204Pb80 (39.21-39.51) values at 206Pb/204Pb80 values of 17.37 to 18.96. The chemical and isotopic compositions of these rocks indicate that the porphyries represent mantle melts that mixed with partial melts from the Paleoproterozoic crust. Fresh and weakly altered porphyries have uniform 87Li (-2.3 to 1.5 %o) and 811B (-8.0 to -12.0 %o). The strong sericite alteration of the porphyries resulted in the loss of Na2O and Sr (breakdown of feldspar) and the strong enrichment of the ore elements Cu, Mo, W, and Sn. Porphyries with varying degrees of alteration show large ranges of 87Li (-6.0 %o to 11.4 %o) and 811B (-8.0 to -29.2 %o). The anomalously high 87Li and low 811B values of the altered rocks indicate that the intrusions drove the flow of external fluids that altered the magmatic rocks and leached the ore elements W, Mo, Cu, and Sn from the porphyries and possibly the local wall rocks.

Automated summary

The study indicates that the Relin Mo-W (-Cu) deposit in the northern Sanjiang area is mainly derived from mantle melts that mixed with partial melts from the Paleoproterozoic crust. The formation of the deposit is mainly influenced by fractional crystallization and late-magmatic alteration, with strong sericite alteration resulting in enrichment of ore elements.