Linking two types of breccias to sediment-hosted stratiform copper (SSC) and iron oxide-copper-gold (IOCG) deposits in the Kangdian metallogenic belt, SW China

Abstract

Voluminous breccias, collectively known as the Yinmin Breccia, are widely distributed in the Kangdian metallogenic belt, Southwest China. Both sediment-hosted stratiform copper (SSC) and iron oxide-copper-gold (IOCG) deposits occur in the belt and exhibit close spatial association with these breccias. However, the origin of the Yinmin Breccia and its relationship to the two types of ore deposits remain enigmatic. Here, we integrate petrological studies with new age constraints and stable isotopic systematics to address these issues. Detailed field mapping and petrological observations allow for the classification of the conventional Yinmin Breccia into two types, namely the monomict and polymict breccias. The monomict breccia contains clasts predominantly of fragments of the Yinmin Formation and cements of quartz, carbonates, and locally sulfides, a mineral assemblage similar to the components of SSC mineralization. In contrast, the polymict breccia contains clasts of the monomict breccia, rocks of the host sequences, and dolerite intrusions, all of which are altered and cemented by IOCG-related hydrothermal mineral assemblages. Moreover, the IOCG-type alteration is observed to locally overprint and postdate the SSC mineralization and monomict breccias. A dolerite intrusion crosscutting the SSC orebody and monomict breccia yields a zircon U-Pb age of 1698 ± 29 Ma, providing a lower limit on the timing of the monomict brecciation and SSC mineralization. B-C-O-S isotopic data suggest that the monomict breccia resulted from dissolution and collapse of evaporites, whereas the polymict breccia formed due to over-pressurizing of magmatic-hydrothermal fluid during the IOCG mineralization process. We hence propose that the SSC and IOCG deposits in the Kangdian region are products of two discrete mineralization events which coincidentally occurred in the same location. This work highlights the critical linkage between hydrothermal breccias and regional mineralization.