Textural and chemical variations in cassiterite from the Yuanlinzi Sn-Cu deposit, NE China: Insights into ore-forming processes

Abstract

Microtextural, U–Pb isotopic, and trace element analyses were conducted on magmatic and hydrothermal cassiterites from the Yuanlinzi Sn–Cu deposit (NE China) to determine the timing of mineralization and investigate the ore-forming process. Magmatic cassiterites are disseminated in pegmatite, while hydrothermal cassiterites can be classified into three types: cassiterite-bearing stockworks in pegmatite, disseminated cassiterite in wall rock, and cassiterite-quartz-feldspar veins. U–Pb dating of the four types of cassiterite yields a similar age of ∼ 139 Ma, which is consistent with the ages of regional tin mineralization and granites within the mining district. These results suggest a strong association between Early Cretaceous magmatism and tin mineralization. The four types of cassiterite exhibit comparable textural features, characterized by alternating oscillatory CL zonation and dark homogeneous bands, with the latter commonly enriched in W, Nb, Ta, and U. The direct substitution of tetravalent ions, including Zr, Hf, Ti, and V, with Sn4+ and the coupled substitutions of Fe3+ + OH– = Sn4+ + O2– or Fe3+ + H+ = Sn4+, 2Sb3+ + U6+ = 3Sn4+ and Sc3+ + V5+ = 2Sn4+ are responsible for the incorporation of these elements in cassiterite. The enrichment of W, Nb, Ta, and U in the dark domains of cassiterite is related to the coupled substitution of W + U and Nb + Ta, while the excess of Nb and Ta in the oscillatory-zoned domains may be attributed to the substitution of 4(Nb, Ta)5+ + □ = 5Sn4+. The trace element variations observed in the four types of cassiterite, particularly Al, Sc, Ti, V, and Fe, reflect the ongoing enhancement of the water–rock reaction during the mineralization process. The examination of redox-sensitive elements such as Fe, U, and V suggested that tin mineralization in the Yuanlinzi deposit occurred in an oxidizing environment where oxygen fugacity gradually increased. Additionally, we proposed that the Sc/V ratio of cassiterite could serve as an indicator of the redox state of the associated melt/fluid.