Abstract
The Dulong Sn-Zn-In polymetallic deposit in the Yunnan province, SW China, hosts a reserve of 5.0 Mt Zn, 0.4 Mt Sn, and 7 Kt In. It is one of the most important polymetallic tin ore districts in China. Granites at Dulong mining area include mainly the Laojunshan granite (third phase), which occurs as quartz porphyry or granite porphyry dikes in the Southern edge of the Laojunshan intrusive complex. Granites of phases one and two are intersected at drill holes at depth. There are three types of cassiterite mineralization developed in the deposit: cassiterite-magnetite ± sulfide ore (Cst I), cassiterite-sulfide ore (Cst II) within the proximal skarn in contact with the concealed granite (granites of phases one to two and three), and cassiterite-quartz vein ore (Cst III) near porphyritic granite. Field geology and petrographic studies indicate that acid neutralising muscovitization and pyroxene reactions were part of mechanisms for Sn precipitation resulting from fluid-rock interaction. In situ U–Pb dating of cassiterite samples from the ore stages of cassiterite-sulfide (Cst II) and Cassiterite-quartz vein (Cst III) yielded Tera-Wasserburg U–Pb lower intercept ages of 88.5 ± 2.1 Ma and 82.1 ± 6.3 Ma, respectively. The two mineralization ages are consistent with the emplacement age of the Laojunshan granite (75.9–92.9 Ma) within error, suggesting a close temporal link between Sn-Zn(-In) mineralization and granitic magmatism. LA-ICPMS trace element study of cassiterite indicates that tetravalent elements (such as Zr, Hf, Ti, U, W) are incorporated in cassiterite by direct substitution, and the trivalent element (Fe) is replaced by coupled substitution. CL image shows that the fluorescence signal of Cst I–II is greater than that of Cst III, which is caused by differences in contents of activating luminescence elements (Al, Ti, W, etc.) and quenching luminescence element (Fe). Elevated W and Fe but lowered Zr, Hf, Nb, and Ta concentrations of the three type cassiterites from the Dulong Sn-Zn-In polymetallic deposit are distinctly different from those of cassiterites in VMS/SEDEX tin deposits, but similar to those from granite-related tin deposits. From cassiterite-magnetite ± sulfide (Cst I), cassiterite-sulfide ore (Cst II), to cassiterite-quartz vein ore-stage (Cst III), high field strength elements (HFSEs: Zr, Nb, Ta, Hf) decrease. This fact combined with cassiterite crystallization ages, indicates that Cst I–II mainly related to concealed granite (Laojunshan granites of phases one and two) while Cst III is mainly related to porphyritic granite (Laojunshan granites of phase three).
Highlights
Most of the worldwide tin mineralization is considered to be related to hydrothermal processes, which are associated with highly fractionated granites [1,2,3]
Due to the cassiterite, particles are too small (Figure 4a), and U–Pb dating for cassiterite-magnetite ± sulfide ore (Cst I) failed
LA-ICP-MS cassiterite U–Pb isotopic analyses were performed on two cassiterite samples (Cst-II: DL18-153 and Cst-III: DL18122)
Summary
Most of the worldwide tin mineralization is considered to be related to hydrothermal processes, which are associated with highly fractionated granites [1,2,3]. Many trace elements, including Mn, Fe, Ti, Nb, Ta, Sc, V, Ga, Cr, Zr, Hf, W, Sb, and U can be accommodated in the structure of cassiterite [4,5,6] The enrichment of these elements in cassiterite is related to chemical conditions and the geological processes in the ore precipitation environment, ore-forming processes, and the nature of associated ore-forming fluids. These trace element abundances in cassiterite may be used to record the physicochemical information of ore-forming fluid and reflect upon the precipitation environment of cassiterite. With the development of analytical techniques, reliable results have been obtained by LA-(MC)-ICPMS U–Pb dating on U-bearing minerals such as zircon, rutile, monazite, perovskite, titanite, allanite, columbite-tantalite, calcite and cassiterite [6,9,10,16,17,18,19,20]
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