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 us e 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 numbe r of the analyte vs. the matrix mean atomic number. Recently, the asse ssment of sub-trace Au contents in lepidolites from the Goncalo lithiu m mine, northern Portugal, using SXRFA was described, and the optimal excitation energy established at 13 keV. New results from muscovite an d lepidolite micas from various localities in Portugal are now reporte d. Experiments were performed at the LURE (Laboratoire pour l'Utilisat ion du Rayonnement Electro-magnetique), DCI (Dispositif de Collision s ous Igloo) storage-ring with a Si(Li) detector, using local data handl ing and processing programs. A complex population of trace elements gi ves rise to interferences on the diagnostic Au-L(alpha) line, namely f rom 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 p rocessing. Unfortunately, the lack of suitable standards currently hin ders the presentation of reliable absolute figures for Au contents.