The distribution and substitution mechanisms of trace elements in cassiterite

Abstract

The trace element records of cassiterite offer important insights into magmatic and hydrothermal processes in a wide variety of mineralizing environments, however, controls on trace element variability in cassiterite is poorly understood. We investigate the trace element geochemistry of cassiterite via a dataset of 656 Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analytical points, collected from 26 samples across 20 localities, including cassiterite from pegmatite, greisen, skarn and vein-hosted Sn-bearing deposits. Fifty-three trace elements are measured, including the geochemical pairs Zr-Hf and Nb-Ta. The ratios of these elements show considerable fractionation in cassiterite and may be used to track hydrothermal processes during Sn mineralization. The Zr/Hf and Nb/Ta ratios also clearly discriminate cassiterite from magmatic vs hydrothermal systems, allowing for the identification of the primary source of alluvial cassiterite with unknown paragenesis. The intracrystalline geochemical behavior of trace elements in cassiterite is also investigated via LA-ICP-MS maps, which highlight the concentric growth and sector zonation behavior of each element. Uranium shows strong sector zonation, with W-rich sectors that tend to be dark in cathodoluminescence imaging and display U contents an order of magnitude higher than other sectors of the crystal. The substitution mechanisms by which these trace elements are accommodated into the cassiterite lattice are examined via lattice strain modelling. Some elements (such as the Rare Earth Elements and chalcophile elements) are predicted to partition favorably into cassiterite, but their concentrations remain low (<1 ppm). The reasons for their low abundance in cassiterite are due to geochemical behaviors not considered by the lattice strain model and the presence of other minerals into which these elements are preferentially partitioned. The low REE contents in cassiterite are favorable to in situ Hf isotopic analysis, due to low Yb and Lu concentrations. Analytical difficulties for the LA-ICP-MS analysis of a Sn-rich substrate, including polyatomic Sn-Ar and doubly charged Sn interferences, are noted for the first time. Of the 53 elements measured in this contribution, 18 are highlighted for future routine trace element analyses of cassiterite, due to their high concentrations and concentrically zoned characteristics, making them suitable for use in the interpretation of the complex growth histories of cassiterite crystals. This study provides a foundation for future trace element investigations of cassiterite and demonstrates that Zr/Hf and Nb/Ta ratios in cassiterite track paragenetic processes across multiple mineral systems.