Low and intermediate-sulfidation epithermal deposits

Low-sulfidation deposits have been well-recognised for a long time. They have characteristic textures and mineralogy, relatively short vein lengths, and mostly low fluid salinities and temperatures. They mainly occur in distal volcanic settings and their relationship to magmatism, apart from the common volcanic host rocks, is mostly speculative. Compared to low-sulfidation deposits, intermediate-sulfidation deposit textures are mostly less diverse, mineralogy more diverse, their veins can be very long, fluid salinities vary widely and are commonly higher, consistent with their higher base metal contents. Some intermediate-sulfidation deposits are closely associated with porphyry deposits, where, if lithocaps are present, they sit below the lithocap and related mineralisation. Their distribution and chemistry strongly suggests they are related to the spent fluids left after porphyry Cu mineralisation has deposited – variable Cu and Au, elevated Zn, Pb, Ag and Mn, with fluctuating fO2. But other intermediate-sulfidation deposits appear to have no relationship to porphyry systems, and their mineralogy and metal zoning suggests connection to magmas of different compositions. Many of the biggest examples have little direct or obvious association with magmas, and many are hosted in subvolcanic basement, yet their composition and mineralogy leave little doubt that a magmatic source was a crucial part of their genesis, but a different magmatic source compared to those related to porphyry systems; potentially less oxidised I-type or even S-type plutons at depth. There need to be more comprehensive studies of the diversity of intermediate-sulfidation deposits before we can claim to understand them.