Developing techniques and reference materials for LA-ICP-MS U-Pb geochronology of Sn-W minerals

Abstract

Tin-tungsten magmatic-hydrothermal deposits are sources of critical raw materials (Sn, W, Nb, Ta, Li), key to the development of technologies involved in the green transition. However, the current and projected supply of many of these mineral commodities is often dominated by entities whose practices or geopolitical setting may raise issues from a social, political, or environmental standpoint. To meet a steadily increasing demand, new responsible mineral extraction projects must therefore be developed. Successful exploration and economic appraisal of newly identified mineral deposits require (1) an understanding of the ore-forming processes to build an exploration model, and (2) an early estimate of the deposit size to facilitate well-targeted investments. One key parameter that helps to achieve both goals is the knowledge of absolute timing and duration of the mineralisation process.We present new analytical developments in U-Pb dating of strategic Sn-W ore minerals (cassiterite, wolframite, scheelite) using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We used a suite of Sn-W mineral specimens to characterise U/Pb downhole fractionation behaviour and polyatomic interference patterns for these three matrices, allowing the optimisation of ablation and ICP-MS settings. In parallel with technical developments, we compiled a large library of potential primary and secondary cassiterite, wolframite, and scheelite reference materials (RMs) which we characterised for major and trace elements. To further our understanding of geochemistry of Sn-W phases, we also performed high-resolution compositional mapping of key trace elements (e.g. U, Pb, REE) with an ultra-fast washout laser ablation system.Promising RM candidates will be developed into primary RMs with a careful characterisation of compositional homogeneity and precise age determination by isotope dilution-thermal ionisation mass spectrometry (ID-TIMS). Thus characterised RMs and a set of analytical best practices will be made available to laboratories wishing to test and further develop such methods. The ultimate goal of this effort is to build a set of community shared materials and techniques that will allow precise and accurate temporal characterisation of Sn-W mineralisation.