Tracing the ore-formation history of the shear-zone-controlled Huogeqi Cu–Pb–Zn deposit in Inner Mongolia, northern China, using H, O, S, and Fe isotopes

Abstract

The original ore-fluid of the Huogeqi Cu–Pb–Zn deposit in Inner Mongolia, northern China, was enriched in heavy oxygen isotopes with δ18O values ranging from 9.9 to 11.4 per mil, which is characteristic of the metamorphic devolatilization of pelitic rocks. The δD values determined by direct measurement of syn-ore hydrothermal tremolite range from −116 to −82 per mil, lying between the domains of typical metamorphic fluid and meteoric water, which is in equilibrium with organic matter. Oxygen and hydrogen isotope ratios indicate that the ore-fluid was derived from deep-sourced metamorphic fluid and interacted with organic-rich shale during fluid migration, which is consistent with the fluid evolution history revealed by a previous fluid inclusion study. Sulfides in the deposit are characteristically enriched in heavy S isotopes, with an average δ34S value of 13.4±6.2 per mil (1σ, n=103). The S-isotope ratios are identical to stratabound sulfides generated through the non-bacterial reduction of Neoproterozoic marine sulfate (with δ34S values of ~17 per mil). Previous studies on lead isotopes of sulfides revealed that the ore-forming metals (Cu, Pb, and Zn) at the Huogeqi deposit were also remobilized from a stratabound source. This source was syngenetically elevated in its Cu-, Pb-, and Zn-sulfide content as a result of submarine hydrothermal activities forming sulfide-rich layers within a rift tectonic setting. The Fe isotope ratios for sulfides are consistent with those of an intercalated iron-formation within the ore-hosting rocks, suggesting that the Fe in the sulfides was derived from local host rocks during sulfide precipitation and the Fe-rich rocks are favorable lithological units for high-grade mineralization. The heterogeneous sources of ore-fluid, S, ore-forming metals, and Fe are explained by a multistage genetic model, which is supported by the geological characteristic of the deposit. The enriched sulfides were subsequently remobilized and enriched by metamorphic devolatilization during the Permian and Triassic periods. The metamorphic ore-fluid ascended along a shear zone and interacted with organic-rich shale. Sulfides eventually precipitated within the shear zone at a shallower crustal level, especially where the shear zone intersected Fe-rich host rocks. This multistage genetic model has implications for mineral exploration. Greenschist to amphibolite facies terranes containing thick Neoproterozoic rift sequences are ideal regions for potential Cu–Pb–Zn mineralization. In particular, intercalated volcanic rocks within the rift sequences are indicative of high heat-flow and are ideal for the development of submarine hydrothermal systems. The primary structures hosting mineralization and ore shoots in the Huogeqi area are jogs in the shear zones. In addition, Fe-rich lithological units, such as iron-formations, are ideal hosts for high-grade ore.