In-situ scheelite LASS-ICPMS reconnaissance Sm-Nd isotope characterisation and prospects for dating

Abstract

Scheelite (CaWO4) is a useful mineral in determining the paragenesis of mineralising systems. While it is well-established that scheelite grains contain trace element budgets indicative of the mineralising fluid, scheelite grains within the same rock commonly display a wide range of inter-element ratios such as Sm/Nd caused by crystal-melt/fluid fractionation. With time, the varying Sm/Nd leads to variability in Nd isotope ratios, opening the possibility of using the 147Sm-144Nd decay system within a single thin section to 1) establish the initial Nd isotopic composition of the mineralising fluid and 2) date scheelite crystallisation. Here, we provide the first assessment of rapid in-situ Sm-Nd scheelite characterisation by simultaneous acquisition of Sm-Nd isotopes and trace elements (REE + Sr + Mo) using laser ablation split-stream (LASS) inductively coupled plasma mass spectrometry (ICPMS) for scheelite mineralisation associated with the 368 Ma Barrytown Granite in New Zealand. The resulting data plot as linear arrays on isochron diagrams that yield robust regression ages of 307 + 76/−66 Ma and 281 + 190–140 Ma that overlap the age of the host granite and have initial 143Nd/144Nd compositions of εNdi (368) = −4.4 ± 0.7 and εNdi (368Ma) = −5.0 ± 1.3 overlapping those of age-corrected host rocks. Analysis of Archean scheelite from the Young-Davidson Gold Mine in Abitibi Greenstone Belt, Canada yields an isochron age that overlaps with the existing age constraints of the scheelite mineralisation (2.47–2.6 Ga) and confirms the viability of the method. Despite the large errors, the in-situ Sm-Nd scheelite isochron method therefore has the potential to be a very powerful tool in mineral exploration and paragenetic studies because it 1) enables characterisation of the initial Nd isotope composition of the hydrothermal fluid on a microscale; 2) is useful for placing broad age constraints on a deposit; and 3) could be used for assessing detrital scheelite provenance. Constraints on the method include a minimum concentration of Nd in scheelite of at least 100 ppm and necessary variability of Sm/Nd within a sample. Ultimately these factors limit the current application of this approach to geochronology.