Cassiterite as a record of Sn mineral system processes

Cassiterite (SnO2) is associated with mineral systems enriched in the 'critical' metals W, Nb, Ta, Li and In (Chakhmouradian et al. 2015) and is a potential multi-process recorder of these mineralised systems. Cassiterite is known to display complex cathodoluminescent microstructures, however detailed physicochemical models for their formation are yet to be established. Here we present a panchromatic CL, hyperspectral CL and quantitative X-ray element map study of these microstructures in cassiterite, along with observations from EPMA and LA-ICP-MS trace element datasets.

We show that sector zoning imparts a significant control on W and Fe distribution within cassiterite and that CL imaging alone does not reveal the full microstructural history with high Fe, Ta, Nb or W contents in cassiterite. We also show that the previously reported coupled substitution mechanisms for W, Nb and Ta into cassiterite cannot account for the high contents observed in some samples, and that different sectors within the same crystal show different preferred substitutional stoichiometries. Furthermore, we discuss unusual Nb/Ta and Zr/Hf ratios which imply the existence of fractionation processes during hydrothermal-metasomatic Sn mineralisation that can drive the Nb/Ta ratio to large values in excess of chondritic ratios (Nb/Ta >> 100).