GOLD ASSESSMENT IN MICAS BY XRF USING SYNCHROTRON-RADIATION

The detection of sub-trace concentrations (ng g(-1)) of heavy elements in geological samples by X-ray fluorescence may be achieved by the use of synchrotron radiation (SXRF). Minimum detection limits range from a few mu g g(-1) to tenths of ng g(-1), depending on the atomic number of the analyte vs. the matrix mean atomic number. Recently, the assessment of sub-trace Au contents in lepidolites from the Goncalo lithium mine, northern Portugal, using SXRFA was described, and the optimal excitation energy established at 13 keV. New results from muscovite and lepidolite micas from various localities in Portugal are now reported. Experiments were performed at the LURE (Laboratoire pour l'Utilisation du Rayonnement Electro-magnetique), DCI (Dispositif de Collision sous Igloo) storage-ring with a Si(Li) detector, using local data handling and processing programs. A complex population of trace elements gives rise to interferences on the diagnostic Au-L(alpha) line, namely from K-lines of lighter elements ( Zn-K-alpha, Ge-K-alpha, Ga-K-alpha) and from the L-spectra of Ta and W, which were accounted for in data processing. Unfortunately, the lack of suitable standards currently hinders the presentation of reliable absolute figures for Au contents.