Matrix-Matched Reference Materials for the Analysis of Tungsten-Minerals for Trace Elements with Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Scheelite (CaWO4) and wolframite-group minerals (Fe,Mn)WO4 represent the economically most important W-ore minerals. In order to constrain the processes responsible for the formation and overprint of W-ores knowledge of trace element concentrations including Mo, Sr, As, Nb, Ta and REE are necessary. In some cases, these ores are also associated with Au-mineralization.

Accuracy of quantitative analysis with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is improved by the use of matrix matched reference materials (RMs). In addition to matrix effects, the analysis of W-minerals involves analytical challenges such as abundance sensitivity of W on Ta and Re and molecular interferences of W-oxides and -nitrides on Au.

A gem-quality scheelite (Scheelite S) from a late-stage alpine vein in the Felbertal tungsten mine, Austrian Alps, is free of Ta and Re and therefore provides the opportunity to determine correction factors for abundance sensitivity of W on Ta and Re. For an exemplary set of samples from the Felbertal tungsten mine, the uncorrected data yield fairly constant Re concentrations of ~5-6µg/g and 0.99-2.14 µg/g for Ta respectively. After correction for abundance sensitivity, Re is below the lower limit of quantification and Ta ranges from 0.07-1.09 µg/g.

For Au the uncorrected concentrations range from 0.14 to 0.38 µg/g. After the correction for molecular interferences by W-oxides and -nitrides the concentrations varied between 0 and 0.32 µg/g.

Preliminary data from wolframite-group minerals indicate similar effects of the abundance sensitivity of W on Ta and Re, as well as the molecular interferences of W-oxides an –nitrides on Au.

Our study demonstrates significantly improved analyses of some important trace elements in W-minerals when matrix-matched RMs are used for the correction of the abundance sensitivity and molecular interferences.