Mineralogy and origin of Mesozoic skarn deposits in Southeast China

Abstract

The Mesozoic geology of Southeast China is characterized by extensive and widespread magmatism, associated with large-scale mineralization that formed many world-class W-Sn and smaller Fe skarn deposits. These deposits are clustered in the Nanling, Yangtze River, and Coastal regions. In the southern Nanling region, the Tengtie Fe skarn deposit is associated with a Mesozoic granitic intrusion, which is composed of the late Jurassic porphyritic and the early Cretaceous fine-grained biotite granites. Orebodies are hosted in the late Paleozoic and Mesozoic carbonates. Interaction of the magma with limestone and dolomitic limestone formed calcic and magnesian skarns, respectively. The prograde skarn stage has an anhydrous mineral assemblage. The retrograde stage is represented by hydrous minerals. The sulfide stage has a mineral assemblage of chalcopyrite and pyrite. Magnetite ores occur in all the three stages. Besides, there is subordinate zircon in both the prograde and retrograde stages. Magnetite of the prograde stage has the highest Co, but that of the sulfide stage is extremely poor in Co. Magnetite of magnesian skarns contains more Mg, Mn, and Al than that of calcic skarns. Besides, magnetite from calcic and magnesian skarns contains 6-185 ppm and 61-1,246 ppm Sn, respectively. Regionally, intensive Mesozoic Sn mineralization indicates that concurrent magmatic-hydrothermal fluids may be rich in Sn and contribute to the formation of high-Sn magnetite. On the other hand, zircon in skarns has relatively low (Sm/La)N ratios and high La contents with flat LREEN and subdued flattening of HREEN. Variable Eu anomalies and slightly positive Ce anomalies recorded in the zircon grains indicate that they may have crystallized from highly heterogeneous, but generally reducing fluids. They have low δ18O values (–5.1 to –2.7 ‰), suggesting the involvement of meteoric fluids. Dating of zircons indicates a duration of less than several million years for skarnization. In the eastern Nanling region, the Baoshan W skarn deposit is associated with a Jurassic granitic intrusion, and scheelite orebodies are hosted in the Carboniferous carbonates. Skarns are zoned outwards mineralogically from garnet, through pyroxene, to wollastonite, and overprinted by retrograde hydrous minerals. Scheelite in both the prograde and retrograde stages has complex zonation under BSE and CL images. They contain Mo ranging from 54 ppm to 16 wt.% and total REEs from 11.6 to 321.2 ppm. REE concentrations and REEN profiles vary with the distribution of major elements. Mo-rich scheelite displays a negative Eu anomaly, whereas Mo-poor one shows a positive Eu anomaly. We proposed a modified LSM equation, which was used to calculate REE patterns in the fluids. The fluids show similar concaved-up patterns in all stages, but evolved from strong enrichment in LREE and depletion in MREE in the early stage to weaker enrichment in LREE and less depletion in MREE in the late stage. Our study demonstrates that geochemistry of skarn minerals can serve as sensitive indicators for wall-rock interactions and fluid sources of skarnization, and further could be used to quantitatively reconstruct the changing physicochemical conditions during ore deposition.