In situ Rhenium (Re) quantification by pXRF in Molybdenite from porphyry-epithermal deposits (Thrace, NE Greece)

Even if natural occurrence of Re can be found as rheniite (ReS₂), most Re substitutes to Mo in molybdenite (MoS₂), which explains why Re is usually a by-product of Cu-Mo deposits. In Thrace region (NE Greece), molybdenite can be enriched up to 4.7 wt% in Re in porphyry-epithermal deposits (Voudouris et al., 2013). Now, spectroscopic portable tools (e.g. pXRF) allows to directly detect Re in the field. First qualitative results obtained by pXRF show that is possible to know in which deposits from Greece the molybdenite is the most enriched without the necessity of long and costly laboratory measurements (EDS-SEM or EPMA). The X-250 pXRF (SciAps) used in this study do not include Re in its quantification program. Moreover, the spot diameter (4 mm) is generally larger than the molybdenite size in this area. Hence, Re cannot be quantified and even if it were, the value would be that of Re in the analytical spot and not in molybdenite only. The aims of this study is to (1) directly quantify Re with the pXRF and (2) determine the concentration in Re within the molybdenite.

In Energy-dispersive XRF, there is an interference between the Zn-Kα emission line (8.6389 keV) and the Re-Lα emission line (8.6524 keV). If Zn is quantified and Re not included into the analytical program, the Re signal will be interpreted as Zn quantities. That is the case with our X-250 pXRF. In Thrace region, molybdenite occurs in quartz veins sometimes associated with few feldspar or pyrite but no Zn-bearing minerals. When measuring molybdenite-bearing veins, all the Zn quantitatively measured by the pXRF corresponds to a Re signal. That effect can be corrected by applying a correction factor on the Zn value to convert it into a Re quantity in situ by using calibration curves. A specific user method can also be easily implemented into the tool. In case Zn-bearing minerals are also found in the molybdenite-bearing veins, the in situ method requires a multilinear correction of signal obtained from the ROIs of Zn and Re. That is more difficult to implement and in the meanwhile, one can obtain signals from Zn and Re separately by proceeding to spectral decomposition using the PyMCA software (Solé et al., 2007). Once separated, these signals can be converted into concentrations of each elements with calibration curves. These curves have been built from the measurement of reference samples consisting in chosen proportions of SiO₂ (considered as the matrix), MoS₂, Re and Zn powders. That enabled to evaluate the impact of each parameter on detection limit, precision and accuracy of the Mo, Re and Zn concentrations. The calibration curves were tested by the use of a set of validation samples.

In our case study, Re and Mo are only within molybdenite. The quantity of Re in the analysed area is mainly induced by the quantity of molybdenite, thus Mo, in the same area. The effect of Re-enrichment in molybdenite appears as a second order phenomenon. With these developments, Re-enrichment in molybdenite becomes a mapable parameter.