The mechanism of magma mixing in Ni-Cu sulfide-bearing mafic-ultramafic rocks: A case study of Huangshandong deposit

Mafic-ultramafic cumulates are characterized by a high proportion of mafic mineral crystals and a limited intergranular melt fraction, so the mixing between mafic-ultramafic crystal mush is not a wide range of mixing. So far, researches on mixing mechanism have still not been well undefined. In this paper, geochemistry of whole rock and plagioclase studies and numerical simulation for transition zone between mafic rock and ultramafic rock are carried out to investigate the mixing process between mafic and ultramafic crystal mush. In drill ZK+10-1, we identified that there are two transition zones with different lengths between pyroxenite, hornblende gabbro and lherzolite. The length of the upper transition zone is longer, and the contents of plagioclase xenocrysts and sulfide increased with increasing depth; while the lower transition zone is shorter and the content of plagioclase and sulfide does not significant change. The geochemical characteristics of rocks and minerals show that the injection of ultramafic magma can lead to the increase of Cr2O3 content in the transition zone, and makes the plagioclase in transition zone producing a reverse zoning. However, the different change trends of SiO2, CaO and Al2O3 represents different mixing methods. When the pyroxene crystal mush intrudes into the gabbro crystal mush from above, gravitational forces cause the denser sulfides within the pyroxene crystal mush to sediment, but these sulfides are unable to filter the gabbroic crystal mush below due to its higher viscosity, which is correspondent to the results of numerical simulations. Furthermore, heating of the upper part of the gabbroic crystal mush by the overlying pyroxene crystal mush reduces its own viscosity. As a result, plagioclase experiences greater buoyancy, leading to upward migration and the formation of plagioclase xenocrysts. In contrast, when the dense ultramafic crystal mush invades the mafic crystal mush from below, it results in only simple and mechanical mixing. Therefore, the mixing between mafic-ultramafic crystal mush is mainly due to the difference of density leading to the exchange of minerals between the two crystal mushes instead of large-scale melt mixing, while viscosity and porosity have a certain influence on the mixing process.