Biotite composition constrains Tongling skarn Cu–Au mineralization, eastern China

Abstract

Skarn Cu, Au deposits are most important deposit-type in the Tongling region, eastern China, where skarn Au deposits are closely associated with pyroxene monzodiorite, and skarn Cu deposits are genetically related to quartz monzodiorite and granodiorite. However, the causes on the Tongling skarn Cu and Au mineralization differences remain poorly constrained. This study systematically presents petrographic, mineralogical, in-situ geochemical analyses of primary magmatic biotite from the Tongling skarn Cu and Au deposits. Biotite from the pyroxene monzodiorite has higher FeO, TiO₂, F and Cl contents, and lower SiO₂, MgO and MnO contents than that in the quartz monzodiorite and granodiorite. Compared with the quartz monzodiorite and granodiorite, biotite in the pyroxene monzodiorite exhibits lower Rb–Co–Ni–Cs but higher Nb–Ta–Li contents. Moreover, the content of total rare earth elements (REE) in biotite decreases from the pyroxene monzodiorite to quartz monzodiorite and then to granodiorite. These element variations are consistent with the magma fractionation trend. A slightly correlation of Rb content with Cu and Au contents in biotite from the pyroxene monzodiorite to the quartz monzodiorite and granodiorite suggests that magma fractionation is critical to Cu, Au mineralization in the Tongling region. The higher biotite IV(F) and IV(Cl) values of the granodiorite than the pyroxene monzodiorite indicate F and Cl depletion during the fractional crystallization, which may be caused by fluid exsolution. The lower biotite log (f HF/f HCl) values (−2.26 to −1.88, avg. −2.07) of the granodiorite than the pyroxene monzodiorite (−2.09 to −1.71, avg. −1.91) indicate more Cl-enriched fluid in the granodiorite. Therefore, we inferred that the differences of Cu and Au mineralization in the Tongling region are probably magma fractionation process, halogen fugacity in magma or fluid, and Cl-enriched degree of fluid.