REE GEOCHEMISTRY SYSTEMATICS OF SCHEELITE FROM THE ALPS USING LUMINESCENCE SPECTROSCOPY - FROM GLOBAL REGULARITIES TO LOCAL-CONTROL

Scheelite [CaWO4] is a common accessory mineral in various kinds of ro cks and ore deposits. Rare earth elements (REE) geochemistry of ore mi nerals may give information about the source of the ore-bearing fluid, and about the physico-chemical conditions of ore transport and/or pre cipitation. Luminescence spectroscopy is able to detect several of the REE3+ ions substituting for Ca2+ in the scheelite lattice; however, d ifficulties in the interpretation of the luminescence spectra in terms of quantifying the REE prohibited the extensive use of luminescence a s an analytical method for REE in scheelite. This paper describes a ne w luminescence method, named thermal X-ray excited luminescence spectr oscopy (XLT), which drastically increases the peak/background ratio in the luminescence spectra of scheelite. This method allows to analyse even small samples (single grains < 0.2 mm), is sensitive only to REE3 + ions replacing Ca2+ in the scheelite lattice (avoiding contamination e.g., by mineral inclusions), and is well suited for the analysis of large sample sets. Our study concentrates on 70 samples of scheelite f rom the Alps, but draws on a yet unpublished database of more than 200 0 scheelite spectra from 350 occurrences world-wide. This work demonst rates that luminescence spectra are a powerful tool in the discussion of diverse metallogenic problems, even if they are not converted to RE E concentrations. The luminescence spectra of scheelite exhibit severa l characteristic features which are diagnostic to the genetic type of the mineralisation. In particular, it is possible to distinguish schee lites from skarn/calcsilicate rocks, from molybdenite-veins, from stra tabound metamorphic deposits, or from hydrothermal deposits. Scheelite from hy drothermal Au-deposits also often display distinctive charact eristics. Different case studies illustrate the ability of the XLT met hod to recognise hydrothermal events at the scale of an ore province. Applied to the study of single grains from heavy mineral concentrates obtained from fluvial sediments, the XLT method enables the genetic ty pe of the lode deposit to be recognised, thus proving to be a promisin g prospecting tool.