Control of melt structures on Cu-An mineralization in basic-ultrabasic complexes of northern China

Based on systematic analyses of 72 samples of different basic-ultrabasic ro cks, the present paper discusses the relationship between melt structure an d Cu and Au mineralization. It is found that if the NBO/T, NBO, M2+, FeO an d 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 m etallization. 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 Aut ions in basic-ultrabasic mel t and the Au+ ions constitute tetrahedral sites together with Fe3+ ions. Th erefore, 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 th e 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 basi c-ultrabasic rocks are favourable to Cu or Au mineralization or they are ju st ore-free rocks by analysing integrated diagrams of the characteristic va lues of the magmatic melt structure.