Trace-element signatures of apatites in granitoids from the Mt Isa Inlier, northwestern Queensland

Abstract

The concentrations of trace elements in apatite from granitoid rocks of the Mt Isa Inlier have been investigated using the laser-ablation inductively coupled plasma-mass spectrometry (ICP-MS) microprobe. The results indicate that the distribution of trace elements [especially rare-earth elements (REE), Sr, Y, Mn and Th] in apatite strongly reflects the chemical characteristics of the parental rock. The variations in the trace-element concentrations of apatite are correlated with parameters such as the SiO2 content, oxidation state of iron, total alkalis and the aluminium saturation index (ASI). The relative enrichment of Y, HREE and Mn and the relative depletion of Sr in the apatites studied reflect the degree of fractionation of the host granite. Apatites from strongly oxidised plutons tend to have higher concentrations of LREE relative to MREE. Manganese concentrations are higher in apatite from reduced granitoids because Mn2+ substitutes directly for Ca2+. The La/Ce ratio of apatite is wellcorrelated with the whole-rock K2O and Na2O contents, as well as with the oxidation state and ASI. Because apatite trace-element composition reflects the chemistry of the whole rock, it can be a useful indicator mineral for the recognition of mineralised granite suites, where particular mineralisation styles are associated with granitoids that have specific geochemical fingerprints.