An Igneous Application For In-situ Strontium Isotopic Laser Ablation

Amanda Stoltze, Ian Campbell, Wolfgang Muller, Malcom McCulloch

The Neptune Laser Ablation Inductively Coupled Plasma Mass Spectrometry for in-situ isotope analyses has been applied to the Wallaby gold deposit an Archaean Greenstone hosted deposit from the Yilgarn Craton. The problem was to determine whether a suite of igneous intrusions adjacent to mineralisation were the source of gold forming fluids. One method of determining a link is by radiogenic isotopic comparison between the intrusion suit and mineralisation. This is the first use of this technique on igneous and gold associated minerals, and provides a powerful tool for the study of Sr isotopes in systems that are prone to element mobilisation after mineral formation. By choosing minerals that have very high Sr concentrations and negligible Rb and rare earth elements, it is possible to exclude Rb mobility from the Sr isotope results. The data acquired with the Laser technique has been compared to that acquired using conventional isotope dilution techniques to confirm the accuracy of this technique for the current application.

Conventional isotope dilution techniques were applied to whole rock and mineral separates and the results covered a wide range of values 87Sr/86Sr = 0.69715±1 to 0.70849±1 but with the majority of samples occupying a very small range of 87Sr/86Sr near 0.70147. The large range in Sr isotopes suggests that post crystallization mobility of Rb and/or Sr has occurred as the Sr isotope range is larger than would be expected for a suite of related igneous intrusions. A change in the Rb/Sr ratio after crystallization will disrupt the correction applied to the data and result in a departure from the true initial ratio. If the Rb/Sr ratio has increased after crystallization the correction applied to the measured ratio is too large resulting in the estimated initial ratio being too low, and lowering the Rb/Sr ratio will increase the initial isotopic ratio after correction. On close inspection it was noted that all sample that fell below the common value contained mica and all samples that were above the common value contained feldspar altered to sericite (fine grained white mica). This information suggests post-crystallization Rb and/or Sr movement is facilitated by mica in a sample. It is therefore necessary to exclude any contamination of micaceous minerals from a Sr isotope sample, which is best done by in-situ analysis.

In this study only minerals with Rb/Sr ratios approaching 0 were used, these included calcite, dolomite, and scheelite. The in-situ nature of the analyses allows minerals to be chosen from thin sections and rock chips providing excellent geological control. The results obtained are displayed in figure 1 and highlight the effectiveness of this technique in systems that have experienced post-crystallization element mobility. The results also show that intrusions and ore have the same Sr isotopic ratio, suggesting the same source for Sr. This can be used as evidence for a genetic link between the igneous units and ore at the Wallaby gold deposit.

Figure 1. Results of 87Sr/86Sr using isotope dilution on whole rock and mineral separates, and in-situ minerals using LA ICP-MS. Tridacna was used as a standard during analyse, note the low variation in the standard.