In situ H2O and δD in andradite from the Kara skarn, Tasmania: Insights into fluid evolution and tungsten transport

Abstract

Metamorphic fluids concentrate economically valuable elements in many ore deposits. In the case of metasomatic deposits such as skarns, hydrothermal fluids also drive reactions that form calc-silicate minerals such as garnet, which in turn host such ores. This measures in situ H2O, δD, major, and trace element chemistry in traverses across two garnet crystals from the Kara iron- and tungsten-skarn in Northwest Tasmania, Australia, to evaluate how hydrothermal fluids control skarn formation and metal endowment. The andradite-rich grains (>0.93 Xand) exhibit water contents of up to 7,000 μg/g within the grain cores that decrease to about 1,000 μg/g in the grain rims. The water within these garnet crystals is structurally bound OH. A strong correlation between molar concentration of hydrogen and fluorine suggest that the two elements may be coupled to one another in a 3:1 ratio of hydrogen to fluorine. The concentration of tungsten in garnet peaks at approximately 360 μg/g and displays a strong correlation with hydrogen. Values of δD across the crystals range between –210 and –150‰, suggesting meteoric fluids were present in the system that formed the Kara skarn. The two δD profiles display variable trends which could be explained by a diffusive process, such as intra-crystalline diffusion or diffusion-limited uptake. This study is one of the first to explore in situ variations in H2O and δD within natural garnet crystals using Fourier Transform Infrared spectroscopy and Nanoscale Secondary Ion Mass Spectrometry. The findings of this work have implications for the transport of tungsten throughout Kara skarn formation.