Relationship between cathodoluminescence response and trace element characterization of cassiterite from the Yunling tin deposit in western Yunnan, China: Implications for substitution mechanism and ore genesis

Abstract

Cathodoluminescent (CL) imaging, combined quantitative geochemical analyses using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), was employed to study the cassiterite crystals from the Yunling tin deposit in western Yunnan Province, China. This investigation aimed to establish a connection between CL response with trace element compositions. The CL imaging of a well-formed centimeter-scale euhedral single crystal of cassiterite revealed two types of primary growth domains, namely oscillatory and sector zoning. High spatial resolution LA-ICP-MS spot analysis along three centimeter-scale profiles revealed that the oscillatory zoning patterns were primarily defined by the concentrations of Nb and Ta. The CL response associated with the oscillatory zoning was attributed to the interplay between activators such as Ti, Nb, Ta, and V, and quenchers like Fe. On the other hand, sector zoning exhibited significantly high W content, reaching three orders of magnitude greater than that in the oscillatory zoning. Consequently, the sectors displayed a dark CL response. Furthermore, U showed a sectoral preference, with contents within the sector zoning examined in this study reaching up to 10 ppm. The systematic variations in geochemical compositions suggest that the incorporation of trace elements in Yunling cassiterite involves a coupled substitutional mechanism of pentavalent and trivalent elements (e.g., Sc3+ + V5+ ↔ 2Sn4+) with H+ potentially serving as a compensating ion for the Fe3+ excess.It is worth noting that the trace element enrichment and potential substitution in the sector and non-sector regions of the single crystal may differ significantly. Therefore, the use of cassiterite trace elements as indicators of the mineralization environment requires careful consideration, particularly when analyzing randomly selected particles obtained from crushing. In such instances, it is recommended to utilize cassiterite samples that exhibit a preserved crystal texture extending from the core to the rim, such as cassiterite in thin sections or morphology-integral cassiterite separates. Moreover, the contrasting W contents observed between the sector and non-sector zones may provide an explanation for two trends in the Fe–W diagram of cassiterite associated with granite that were previously-unspecified.