Applications of the combined portable XRF-benchtop SEM methodology to PGE exploration

Abstract

Portable XRF (pXRF) and benchtop scanning electron microscopes equipped with energy dispersive systems (bSEM-EDS) are two real-time analytical techniques that can be combined to collect on-site lithogeochemical and mineral chemical results at dramatically reduced cost and time compared to traditional analytical methods. The Coldwell Complex, northwestern Ontario, is used as a case study on how the combination of these techniques can be applied to mineral exploration. Our results show that whole-rock Ba, V/Ti, Cu/S determined by pXRF, and plagioclase, olivine, and clinopyroxene compositions measured by bSEM-EDS can be applied to exploration for PGE deposits. Cu/S ratios for disseminated sulfides are a proxy for metal tenor, which can be applied as a vector towards higher grade mineralization. Potential magma recharge zones are identified by inspecting down-hole variations in plagioclase, olivine, and clinopyroxene compositions. Fluctuating down-hole variations in these parameters are interpreted to reflect episodic magma recharge. Whether rocks contain PGE mineralization can initially be assessed by documenting the variability of mineral chemistry, i.e., the greater variability of mineral chemistry could be used to vector higher grade PGE mineralization, particularly if the mineralization is conduit-related. Lastly, geochemical mapping of the igneous stratigraphy enables identification of favorable hosts for mineralization. Down-hole whole-rock variations in Ba and V/Ti from pXRF, and mineral chemical profiles from bSEM-EDS are used to discriminate the mineralized Marathon Series from the barren Layered Series throughout the Coldwell Complex. Whole-rock Ba and V/Ti from pXRF and mineral chemical results from bSEM-EDS could prove useful in other settings of the Midcontinent Rift (e.g., the Duluth Complex, Eagle and Eagle East intrusions, Tamarack Intrusive Complex, and the Sonju Lake intrusion) and elsewhere (e.g., the Stillwater Complex) to identify mineralized host rock types. An additional important application is a rapid interpretation of petrogenesis. Down-hole variations in olivine-clinopyroxene Mg-Fe exchange coefficients determined by bSEM-EDS analyses show that olivine-clinopyroxene pairs in the Marathon Series units, particularly for those within PGE mineralized zones, have compositions that are closer to chemical equilibrium (constant) compared to units of the barren Layered Series (highly variable), and thus could be a tool for interpreting petrogenetic processes active in the magma chamber or during the mineralization forming event.