Rare earth element geochemistry of feldspars: examples from Fe-oxide Cu-Au systems in the Olympic Cu-Au Province, South Australia

Abstract

Rare earth element (REE) fractionation trends in feldspars are reported from Olympic Dam (including Wirrda Well and Phillip’s Ridge) and Cape Donington (Port Lincoln), for comparison with two other igneous-hydrothermal terranes within the eastern Gawler Craton: Moonta-Wallaroo and Hillside. The case studies were selected as they represent ~ 1590 Ma Hiltaba Suite and/or ~ 1845 − 1810 Ma Donington Suite granites, and, aside from Cape Donington, are associated with Mesoproterozoic iron-oxide copper gold (IOCG)-type mineralization. Both plagioclase and alkali feldspar were analyzed within selected samples with the purpose of constraining and linking changes in REE concentrations and fractionation trends in feldspars to local and whole-rock textures and geochemistry. Two unique, reproducible fractionation trends were obtained for igneous plagioclase and alkali feldspars, distinguished from one another by light rare earth element enrichment, Eu-anomalies and degrees of fractionation (e.g. La/Lu slopes). Results for hydrothermal albite and K-feldspar indicate that REE concentrations and fractionation trends are generally inherited from igneous predecessors, however in some instances, significant amounts of REE appear to have been lost to the fluid. These results may have critical implications for the formation of world-class IOCG systems, in which widespread alkali metasomatism plays a key role by altering the physical and chemical properties of the host rocks during early stages of IOCG formation, as well as trapping trace elements (including REE).