Experimental Equilibrium and Fractional Crystallization of a H2O, CO2, Cl and S-Bearing Potassic Mafic Magma at 1.0 GPa, With Implications for the Origin of Porphyry Cu (Au, Mo)-Forming Potassic Magmas.

This article focuses on experimental investigations into the origin and volatile evolution of intermediate to felsic potassic magmas associated with porphyry Cu (Au, Mo) deposits in the Sanjiang region of southwestern China. Using piston-cylinder experiments at 1.0 GPa and 1150–850 °C with a novel zircon capsule liner to prevent Fe and S loss, the study demonstrates that these magmas can form solely by differentiation of mantle-derived mafic potassic magmas without requiring crustal melting. The experimental melts closely match natural Sanjiang magmas in major, trace element, and volatile (H2O, Cl, S) contents, suggesting that porphyry-forming magmas likely contained 8–13 wt% H2O, 0.37–0.90 wt% Cl, and 0.07–0.29 wt% S, with sulfur contents buffered by sulfide and sulfate phases during high-pressure differentiation. These findings imply that the mineralization potential of porphyry magmas in the region is controlled more by differentiation conditions than by source composition, with implications for understanding potassic magma evolution in diverse tectonic settings.