THE ROLE OF MAGMATIC BRINE AND CL- AND S-RICH MINERALS IN GENERATING CU-MO-AU MINERALIZATION OF THE TONGCHANG MINE, DEXING PORPHYRY DEPOSIT, SE CHINA

The Cu-Mo-Au mineralization at Dexing represents the largest porphyry deposit in eastern China. This deposit consists of 3 individual mineralized plutons including the Tongchang orebody. Prior research has determined the petrogenesis, tectonic setting, and magmatic evolution of the constituent plutons, but the role of volatiles in magma differentiation and mineralization is poorly understood.

We investigated silicate melt inclusions, apatite, mica, amphibole, and anhydrite of 5 porphyritic granodiorite samples spanning the potassic-alteration zone of the Tongchang pluton to determine their volatile contents. The edenitic amphiboles, biotites, and apatites are moderately to strongly enriched in Cl relative to most other porphyry Cu systems; these phases represent fluid-mineral±melt interactions occurring at magmatic to subsolidus conditions. Chlorine ranges from 0.18 to 0.32 wt.% in amphiboles and 0.22 to 0.35 wt.% in biotite. The log (ƒHCl/ƒHF) of the Tongchang hydrothermal fluids, computed from biotite data, exceeds those of numerous other Cu porphyries. Chlorine in apatite shows a bimodal distribution of either 0.19 to 1.35 or 1.48 to 3.73 wt.%. Regarding magmatic S, anhydrite inclusions are variably distributed in plagioclase phenocrysts of some granodiorite samples but not in all. The crystallized silicate melt inclusions in quartz of the granodiorites were rehomogenized to glass and analyzed.

The mineralizing fluids were variably enriched in Cl and oxidized S species. The melt inclusions are chemically evolved to granitic compositions containing 0.15 to 0.44 wt.% Cl. For comparison, the computed solubilities of Cl in brine-saturated granitic melts, with these melt inclusion compositions, range from 0.36 to 0.53 wt.% for 50 MPa and from 0.42 to 0.62 wt.% at 200 MPa. Thus, the elevated Cl contents of some melt inclusions indicate that fractions of the granitic melt equilibrated with brine during the differentiation of fluid-saturated granodioritic magma. In addition, the SO₃ contents of apatite correlate positively with Cl. Our interpretation is that magmatic anhydrite was variably dissolved by the Cl-enriched fluids during magma differentiation, and this process generated S- and Cl-enriched fluids that were highly efficient in dissolving, transporting, and depositing Cu-Mo-Au.