New geochronological tool for Cu-Au mineralisation in the Arabian Shield, Saudi Arabia: Testing in-situ Rb-Sr dating via LA-ICP-MS/MS

Abstract

In-situ Rb-Sr dating by a laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS/MS) has been recently developed for geoscience applications, taking advantage of a reaction cell capability of new generation ICP MS/MS instruments to resolve isobaric interferences specifically for beta-decay geochronometers. This development allows a rapid and interference free analysis of ⁸⁷Rb and ⁸⁷Sr isotopes via the reaction/collision cell filled with a reactive gas such as N₂O and or SF₆ to separate more reactive Sr ions from inert Rb species. The other advantage of the collision cell ICP-MS/MS instruments is their compatibility with a LA system which in tandem open up in-situ Rb-Sr dating applications of various geological materials at micro-scale level and directly from ‘solid’ samples or minerals of interest. Yet, the main drawback or limitation of a successful application of the LA-ICP-MS/MS systems for Rb-Sr dating are currently poorly quantified and unconstrained ‘elemental fractionation phenomena’ and ‘matrix effects’, as well as the general lack of suitable chemically/mineralogically well characterised reference materials (i.e., mineral-specific standards) that are homogeneous at micro-scale level. Thus, this PhD thesis aims to fill some of these knowledge gaps by investigating and quantifying the impact of the elemental fractionation and matrix effects on the accuracy and precision of in-situ Rb-Sr dating of selected K-rich silicate minerals (micas, feldspar and clays), including phlogopite, biotite, K-feldspar and glauconite (Mica-Mg, Mica-Fe, FK-N and GL-O). These minerals were formed as nano-powders and fused glasses and analysed by the LA-ICP-MS/MS approach. Results are assessed and discussed in terms of suitability of the above materials/mineral standards for the in-situ Rb-Sr dating applications. Overall, results suggest that nano-powder mineral standards represent suitable reference materials for reliable in-situ Rb-Sr dating applications, but ‘chemical matching’ between unknown sample/mineral and nano-powder standard is important to minimize the above-mentioned ‘matrix effects’ and their impact on the precision and accuracy of acquired Rb-Sr data. Finally, our validated in-situ Rb-Sr dating approach (using Mica-Mg-NP as a standard) was also used on a case study to constrain the timing and origin of a Cu-Au mineralisation in the Mount Ablah region in Saudi Arabia (Arabian Shield), based on the analysis and in-situ dating of selected micas and feldspars associated with the above mineralised system linked to a regional shear zone. The results and acquired in-situ Rb-Sr ages indicate that the development of the Umm Farwah shear zone occurred at 651 ± 20 Ma, followed by the emplacement of Mount Ablah pegmatite dated at 625 ± 19 Ma. Subsequent greisenisation of local igneous rocks took place between 613 Ma and 589 Ma, followed by a younger reactivation event(s) - dated between 580 Ma and 530 Ma - that are believed to form the Cu-Au mineralisation in the Mount Ablah region.