Application of portable X-ray fluorescence analyses to metabasalt stratigraphy, Plutonic Gold Mine, Western Australia

Stratigraphy, structure and host-rock chemistry are dominant controls on the location of Au in Archaean greenstone-hosted Au deposits, but the stratigraphy in such deposits is seldom obvious due to the monotonous nature of the host rocks or pervasive alteration associated with Au mineralisation. Portable, hand-held, X-ray fluorescence (pXRF) spectrometry provides a method to rapidly collect large amounts of whole-rock geochemical data that can yield new insights into both stratigraphy and Au localisation. Here we present results of pXRF analyses of samples from a representative section through Au-mineralised amphibolite-facies metabasaltic rocks at Plutonic Gold Mine, Western Australia. These data illustrate a geochemical stratigraphy in which individual lava flows can be identified on the basis of element concentrations. The most evolved basalts are at the structural base of the succession, and the least evolved at the top of the sequence, confirming previous geochemical interpretations and textural evidence that the sequence is overturned, and demonstrating for the first time that the presented section does not involve significant structural repetition. In conjunction with Au assay data, the pXRF data reveal that Au commonly occurs along basalt flow boundaries. The elemental concentration data clearly demonstrates for the first time the stratigraphic control on Au mineralisation that is not readily apparent at the macroscopic level. The methods described in this paper are readily applied, and have the potential to enhance the understanding of otherwise unclear stratigraphy and its control on mineralisation in many different types of deposits worldwide. ► Portable, hand-held, X-ray fluorescence (pXRF) spectrometry provides a method to rapidly collect large amounts of whole-rock geochemical data. ► Element concentration data obtained by pXRF reveals, for the first time, the stratigraphy of basalt flows in an amphibolite-facies metamorphosed greenstone belt sequence at Plutonic Gold Mine, Western Australia. ► Gold is shown to be localised along basalt flow boundaries. ► The pXRF data support previous interpretations that the sequence in the study area is overturned, and show that the basalts become increasingly evolved down-section, without significant structural repetition. ► The methods described here are readily applied, and have the potential to enhance the understanding of otherwise unclear stratigraphy and its control on mineralisation in many differe