Abstract
Proximal and distal garnets from skarns at Jiaduobule, Tibet, are used to demonstrate how variation in the fluid composition and parameters such as salinity, pH, fO2, and \({\text{X}}_{{\rm CO}_{2}}\) will impact on rare earth element (REE) distribution in garnets, and also to constrain skarn evolution across the orefield from proximal (Fe mineralization) to distal (Cu mineralization). These garnets display a diversity from proximal to distal skarn which is expressed in mineral assemblages, textures, major to trace element contents, and particularly, chondrite-normalized REE fractionation trends. The empirical variation among REE fractionation trends, determined from laser ablation inductively coupled-plasma mass spectrometry data, can be numerically modelled in terms of variable fluid compositions and physicochemical parameters, among which the key determining factors are salinity, pH, \({\text{X}}_{{{\text{CO}}_{{2}} }}\) and Ca content buffered from the rock-fluid reaction with carbonate rocks. Modelling REE trends in skarn garnet is shown to be valuable for constraining conditions during garnet formation and a useful tool for monitoring the evolution of complex skarn deposits.
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Acknowledgements
This work was carried out while J. X. was a visiting scholar at the University of Adelaide. We thank staff at Adelaide Microscopy for assistance with microanalytical work and gratefully acknowledge discussion and advice from Sasha Krneta and Martin Smith. We also acknowledge Aqueous Solutions LLC for granting a student licence for Geochemist’s Workbench®12 and the kind support of Joel Brugger and research group (Monash University) for access to updated databases. The constructive comments from two reviewers are appreciated and assisted us to improve this article.
Funding
We acknowledge support from the National Key R&D Program of China (2018YFC0604104). J. X. acknowledge support from the National Natural Science Foundation of China (41802098). CLC acknowledges support from the ‘FOX’ project (Trace elements in iron oxides), supported by BHP Olympic Dam and the S.A. Mining and Petroleum Services Centre of Excellence.
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Xu, J., Ciobanu, C.L., Cook, N.J. et al. Numerical modelling of rare earth element fractionation trends in garnet: a tool to monitor skarn evolution. Contrib Mineral Petrol 175, 30 (2020). https://doi.org/10.1007/s00410-020-1670-7
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DOI: https://doi.org/10.1007/s00410-020-1670-7