Origin of hydrothermal deposits related to the Emeishan magmatism

Abstract

In the western Yangtze Block, southwest China, there are many hydrothermal native copper ore deposits and Zn–Pb ore deposits. All of these hydrothermal ore deposits are spatially associated with the Permian Emeishan flood basalts (EFBs), although they are much younger than the EFB. The maximum time interval between the hydrothermal ore deposits and the EFB is over 100Ma. During this time interval, there is no documented magmatism in this region, and many studies indicate that magmatic fluids played important roles in the formations of these deposits. Thus, the origin of these hydrothermal deposits has long been controversial. In this study, we present a model that considers the underplated Emeishan basalts at the base of the crust as the main source of the ore-forming metals and fluids and present thermal simulation results of the evolution of the underplated Emeishan basalt. The results indicate that the underplated basalts begin to release metal-bearing fluid at 30Ma after the onset of the underplating, consistent with age data of the oldest hydrothermal deposits in this region, such as the Huize Pb–Zn and native Cu ore deposits. The simulation results also indicate that the releasing ore-forming fluid from the crystallising underplated basalts can last over 100Ma, which almost covers the entire age data available for the hydrothermal deposits, and thus successfully demonstrates the lack of temporal association between the hydrothermal deposits and the EFB. The model is developed primarily for the origin of the hydrothermal mineralisation in the SYG province, but it has general applicability to other sediment-hosted Pb–Zn deposits around the world.