LA–ICP–MS U–Pb Dating, Elemental Mapping and In Situ Trace Element Analyses of Cassiterites from the Gejiu Tin Polymetallic Deposit, SW China: Constraints on the Timing of Mineralization and Precipitation Environment

Abstract

As a major constituent in magmatic–hydrothermal ore deposits, cassiterites, with moderate amounts of U and low Pb, can be dated with U–Pb geochronology. The tetragonal lattice structure makes cassiterites capable of incorporating dozens of elements within its crystal lattice (e.g., Fe, Ti, W, Zr, Hf, Ta, Nb, Mn, Sc, V, and Sb). Variations of these elements record information of potential elemental substitution mechanisms and precipitation environments of cassiterites. In this study, we collected cassiterite grains from four different ore styles of the Gejiu tin polymetallic deposit to perform LA–ICP–MS U–Pb dating, multiple element mapping, and in situ trace element analysis on these cassiterites. Systematic U–Pb dating yielded Tera–Wasserburg lower intercepted ages at around 85 Ma, coinciding with zircon U–Pb ages of regional Late Yanshanian granitoids, within their respective analytical uncertainties. Such age coincidence, combined with the spatial association, suggests that tin mineralization may be genetically related to the Late Cretaceous granitic magmatism. Multielemental mapping shows that the distribution of Nb, Ta, and Ti in the cassiterite grains correlates well with the regular oscillatory zoning patterns in cathodoluminescence (CL) images. The relatively high Sb, Fe, W, Ga, and U concentrations control the dark luminescing domains in these cassiterite grains. The systematic variations in chemical compositions suggest that trace elements such as Sc, V, Fe, and Ga incorporate in cassiterites via coupled substitutions of Sc3+ + V5+ ↔ 2 (Sn, Ti)4+, Fe3+ + Ga5+ ↔ 2 (Sn, Ti)4+ and Fe3+ + OH– ↔ Sn4+ + O2– or Fe3+ + H+ ↔ Sn4+. The covariation of redox sensitive elements such as W, U, Fe, and Sb indicates that the ”tin-granite” type of cassiterites were formed under an oxidized state whereas cassiterites from skarn, massive sulfide, and oxidized ore styles were precipitated in a reducing environment.