Numerical modelling of rare earth element fractionation trends in garnet: a tool to monitor skarn evolution

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.