Control of Melt Structures on Cu–Au Mineralization in Basic‐Ultrabasic Complexes of Northern China

AbstractBased on systematic analyses of 72 samples of different basic‐ultrabasic rocks, the present paper discusses the relationship between melt structure and Cu and Au mineralization. It is found that if the NBO/T, NBO, M2+, FeO and MgO values are relatively high and the T, M3+, Fe2O3 and CaO values are low the basic‐ultrabasic melt will be favourable to Cu (Ni) mineralization, but if the former are low and the latter are high it is favourable to Au metallization. Cu ions occupy dominantly octahedra in basic‐ultrabasic melt and the higher the NBO/T, NBO and M2+ values, the more the octahedra in the melt. Au element mainly takes the form of Au+ ions in basic‐ultrabasic melt and the Au+ ions constitute tetrahedral sites together with Fe3+ ions. Therefore, low M2+ and high Fe3+, i.e. high oxygen fugacity, can promote the enrichment of Au+ ions and Au mineralization. Components M+ (other than Au+), Al2O3 and SiO2 in basic‐ultrabasic melt have no effect on metallogenetic species. As mentioned above, in relevant diagrams distribution areas of the characteristic values of ore‐free melt and those of ore‐forming melt are overlapped in different degrees, which possibly indicates that not all the magmas have mineralizing ability. It can be well distinguished whether basic‐ultrabasic rocks are favourable to Cu or Au mineralization or they are just ore‐free rocks by analysing integrated diagrams of the characteristic values of the magmatic melt structure.