Mineral chemistry of magnetite and its constraints on ore-forming processes of the Dulong Sn-Zn-In polymetallic deposit, southeastern Yunnan Province

The Dulong Sn-Zn-In-polymetallic deposit lies in Laojunshan ore-concentrated area of the famous W-Sn polymetallic metallogenic zone in southeastern Yunnan. Mineralization in this deposit is closely related to the large-scale Cretaceous granite magma intrusion. It is a giant skarn (magmatic hydrothermal) mineralization system, whose skarn minerals include garnet, diopside, tremolite, actinolite, magnetite, wolframite, cassiterite, marmatite, molybdenite, chalcopyrite, pyrite, sphalerite and galenite. Based on field observation and study, the magnetite may be classified into two mineralization stages and types, including the early saccular (or banded) magnetite ( I-Mag) in the skarn and the later vein magnetite (II-Mag ) in the host rocks or early ore bodies. The analytical results show that from I-Mag to II-Mag, the concentrations of Si, Ca, Mn, Sigma REE, Pb, Zn and Ti increase, whereas those of Mg, Sn, W, In, V, Cr and Ga decrease, and REE patterns are changed from slightly flat to right-dipping, with negative Eu anomalies. Magnetite genetic classification diagrams of TiO2 vs. Al2O, vs. MgO + MnO, Ti + V vs. Ca + Al + Mn, Ni/Cr vs. Ti, Ti + V vs. NV(Cr + Mn) reveal that the magnetite is magmatic hydrothermal and skarn genetic type. The concentrations of Ti, V, Zr, Hf, Nb and Ta and the values of Y/Ho, Ni/Co, Ti/V and Hf/Zr show linear correlations. All of them show that the two types of magnetites have the same source with the Laojunshan granite intrusion, and they are products at different stages of the same mineralizing processes. The REE characteristics of II-Mag are similar to the Laojunshan granite intrusion, indicating that magnetite is genetically related to the Laojunshan granite intrusion. Plotting of Cr vs. V, Ti + V vs. Al + Mn, Ga vs. Mg and Ga vs. Sn for two types magnetite indicate that the two types of magnetites are formed in an ore-forming fluid with higher oxygen fugacity and temperature (nearly 300 degrees C ), which show increasing temperatures with decreasing oxygen fugacity.