Linking carbonate-hosted Zn[sbnd]Pb deposit to deep mantle activity: Evidence from in situ U[sbnd]Pb geochronology of calcite from the world-class Huayuan Zn[sbnd]Pb ore field in South China

Abstract

Carbonate-hosted ZnPb deposits are a major source of zinc, lead, and some critical raw metal resources worldwide (e.g., germanium and gallium). However, the timing and geodynamics setting of their mineralization remain enigmatic, leading to ongoing debates about their origins. To shed light on these issues, we utilized LA-ICP-MS in situ UPb dating of ore-related calcites to determine the timing and geodynamic setting of mineralization events in the world-class Huayuan carbonate-hosted ZnPb ore field in South China. The petrographic and cathodoluminescence features, fluid inclusion characteristics, and the Pb isotope data indicate that the tested calcites were formed from the same ore-forming fluids as the intergrown sphalerite and were not overprinted by possible later hydrothermal fluids. Our results show that the calcite UPb ages of 510–495 Ma refute the previously proposed Mississippi Valley-type (MVT) model, which suggested that the younger Caledonian orogeny (460–420 Ma) played a significant role in the genesis of the Huayuan ZnPb ore field. Instead, our findings link the formation of this ore field to crustal extension and high heat flux resulting from mantle upwelling events in northeast Gondwana during the Cambrian. The findings indicate that deep mantle processes can trigger shallow crustal fluid circulation to create a giant carbonate-hosted ZnPb ore field in a long-dormant rift.We present a genetic model proposing that the extensive carbonate platform of the Qingxudong Formation was likely a “brine factory” that supplied the brines to the Proterozoic basement through reactivated syn-rift faults. The high heat flux from upwelling mantle not only heated these brines to high temperatures but also caused early maturation of organic matter in the lower Cambrian Niutitang black shales to generate hydrocarbons. The hydrocarbons migrated into the limestone of the Qingxudong Formation. Meanwhile, the hot brines leached Zn and Pb from the Proterozoic basement rock. These ZnPb bearing brines ascended to the Qingxudong Formation where they mixed with the hydrocarbons. These hydrocarbons acted as reducing agents for ZnPb sulfide precipitation, forming the Huayuan giant ZnPb ore field. Furthermore, UPb dating results of syn-mineralization calcite, together with the relationships between the ores and host rocks, indicate that ZnPb mineralization occurred over timespans during or shortly after host-rock deposition and continuing during lithification for several million years in this ore field. The similar prolonged duration of mineralization may be relatively prevalent in other carbonate-hosted ZnPb deposits/ore fields. We therefore recommend that reliable age dating for all mineralization stages is necessary to establish genetic models for carbonate-hosted ZnPb deposits.