Origin of the ore-forming fluids of the Tongchang porphyry Cu–Mo deposit in the Jinshajiang–Red River alkaline igneous belt, SW China: Constraints from He, Ar and S isotopes

Abstract

The Jinshajiang–Red River alkaline igneous belt with abundant Cu–Mo–Au mineralization, in the eastern Indian–Asian collision zone, is an important Cenozoic magmatic belt formed under an intra-continental strike-slip system in southwestern (SW) China. The Tongchang deposit is a representative porphyry Cu–Mo deposit in southern segment of the Jinshajiang–Red River alkaline igneous belt, with 8621t Cu @ 1.24wt.% and 17,060t Mo @ 0.218wt.%. In this study, He, Ar and S isotopic compositions of the Tongchang deposit were determined. He and Ar isotopic compositions suggest that the ore-forming fluids, with 3He/4He ratios varying from 0.17 to 1.50Ra and 40Ar/36Ar ratios from 299.1 to 347.3 for the deposit, are a mixture between a crust-derived fluid (MASW) with near atmospheric Ar and crustal He, and a mantle-derived fluid. However, the δ34S values of the hydrothermal pyrite samples ranging from 1.0‰ to 1.5‰ with an average of 1.2‰, indicate that the sulfur in the ore-forming fluids of the Tongchang deposit was primarily derived from the magma or indirectly mantle-derived without assimilation of crustal sulfur. In combination with previously published He and Ar isotopic data of the Yulong and Machangqing deposits in northern and central segments of the Jinshajiang–Red River alkaline igneous belt, respectively, the ore-forming fluids of the Yulong and Machangqing deposits are obviously richer in 3He and 40Ar, and poorer in 36Ar in comparison with the Tongchang deposit, implying that more mantle-derived fluids were involved in the ore-forming fluids of the Yulong and Machangqing deposits than those for the Tongchang deposit. This might be one of the most important factors producing larger scales of mineralization in the Yulong and Machangqing deposits than the Tongchang deposit.