Bulk microanalysis of assemblages of small fluid inclusions by LA-ICP-MS: Methodology and application to orogenic gold systems

摘要

Microanalysis of individual fluid inclusions by laser ablation inductively coupled mass spectrometry (LA-ICP-MS) is a powerful tool for reconstructing the composition of hydrothermal fluids, but it demands a sample quality that is unattainable in many cases. In orogenic gold deposits, the need for direct fluid microanalysis has been present for several decades, but due to the high fluid flux and prolonged hydrothermal and tectonic history that typifies these systems, most samples do not meet the criteria of fluid inclusion size and distribution that allow LA-ICP-MS analysis of individual isolated fluid inclusions. To overcome this difficulty, a method has been developed and tested, whereby areas of quartz densely populated with fluid inclusions (e.g., growth zones, secondary planes) are analyzed along a single continuous laser ablation profile; the generated signals are subsequently converted to time-slice datasets and plotted as element ratios in ternary diagrams to reconstruct specific major-and trace-element ratios. The estimated fluid compositions are in good agreement with previous analytical results of the same material (microthermometry, evaporate mound and conventional LA-ICP-MS) and are shown to be geologically viable on a boarder scale when compared to literature data from similar ore environments. The method has high spatial and chemical resolution, which allows the reconstruction of microscale fluid chemical changes, such as significant fluctuation in the relative element concentration (e.g., K, Rb, Ba, Sr and V versus Na, Li and As) during crystal growth, as observed in one of the test samples. The significance of this bulk LA protocol is that it allows the quick, easy and cost-effective microanalysis of samples that are typically inundated with fluid inclusions, such as those in orogenic and epithermal systems, which would otherwise not be amenable for further quantitative analysis to constrain fluid chemistry.