The firstly-discovered gold minerals in sediment-hosted stratiform copper (SSC) deposits: a case study in central Yunnan province, China

As an important Cu-Fe resource base in China, the Kangdian Cu-Fe metallogenic province on the southwestern edge of the Yangtze Block mainly developed marine volcanic rocks-hosted iron oxide-copper-gold deposits (IOCG) and sediment-hosted stratiform copper deposits (SSC). For a long time, it is considered by many that gold mineralization in this province is related to IOCG deposits, while there have been few reports on the development of gold mineralization in SSC deposits. This work took a SSC deposit in central Yunnan as an example, through in-depth and detailed mineralogical research, discovered gold mineral, such as electrum, for the first time in this type of deposit. It occurs as interstitial gold or inclusion gold in mineral fractures or grain boundaries such as bornite and cobaltite. The gold grade in the original ores is relatively low and undetectable, with individual silver grade of 25.4 g/t. The Au and Ag grades in the copper concentrate powder range from 0.18 to 0.22 g/t and 38 to 42 g/t, respectively. The electron probe elemental surface mapping results show that the distribution of Au and Ag in the gold mineral is highly consistent, confirming that gold appears in the form of Ag-Au alloy. The spatial distribution of Au-Ag elements does not completely overlap with the spatial distribution of Cu, Co, As and S elements, further proving that Au-Ag alloy is an independent gold mineral, rather than a simple solid solution existing in gold bearing sulfides. Based on mineralogical observations, this study suggests that gold mainly exists in the form of independent mineral phases and has spatial correlation with Co-As elements. Its mineralization characteristics are different from those of gold in typical IOCG deposits. In addition, it is highly likely that the gold in the SSC deposit was formed by later gold-rich fluids filling and encapsulating early formed bornite and cobaltite. The first discovery of gold minerals in the SSC deposits in the research area not only enhances the understanding on the genesis of SSC deposits, but also provides a new direction for gold exploration in central Yunnan and even the entire Kangdian region.