title: Rare earth element mineralisation in Australia
Creators: name: Chandler, Ross
Publisher: name: The Australian National University
Year: 2025

description: Australia hosts a variety of Rare Earth Element (REE) deposits. Whilst carbonatite-associated deposits are the only style currently mined, some peralkaline volcanic and Iron Oxide Copper Gold (IOCG) systems also contain economic concentrations of REE. Despite growing interest in REE, the ore genesis of all three deposit styles in Australia remains understudied, hampering effective utilisation of the nation's Critical Metal resources. This thesis addresses this knowledge gap first by assessing known occurrences, followed by microanalytical and geochemical studies of REE mineralisation nationwide. Firstly this thesis presents a compilation of the geological and isotopic characteristics of Australia's carbonatites. The 16 known carbonatites range from Neoarchean to Jurassic in age, are all hosted within Proterozoic (or older) crust and are extremely geologically diverse. They range from multiphase intrusive complexes dominated by carbonatite (e.g. Gifford Creek and Mt Weld, WA) or silicate rocks (e.g. Cummins Range and Cundeelee, WA), to small volume carbonatite dykes (e.g. Yungal dykes, WA). The diversity is reflected in radiogenic isotopic signatures (Rb/Sr and Sm/Nd), which indicate derivation from a variety of mantle sources. REE and Nb mineralisation is hosted within both primary carbonatite and overlying carbonatite-derived regolith, with dolomite carbonatite displaying greater increases in REE grade during weathering, compared to siderite carbonatites. The second part of this thesis focusses on the Mt Weld carbonatite (WA), which hosts a large REE deposit within the regolith overlying a c. 2.06 Ga carbonatite complex. Samples are taken from an exploration drill hole, and reveal an intrusive architecture of inner REE-mineralised magnesio- to ferrocarbonatite (c. 600m diameter) surrounded by an annulus of REE-poor calciocarbonatite (c. 1.2km thick). Monazite is the main host of REE, with textures and chemistries suggestive of a period of late hydrothermal mineralisation. A comparison of carbonatite and regolith geochemistry suggests minimal horizontal migration of ore elements during regolith formation, displaying a ~5x upgrade in REE during weathering. Thirdly, REE mineralisation within the Bingie Grumble peralkaline trachyte (NSW) is investigated. REE are hosted within magmatic eudialyte that is variably pseudomorphically replaced by intergrown amorphous zircon, hydroxylbastnasite-(Nd) and an array of Ce silicate (Mn) phases. 40Ar/39Ar geochronology confirms a c. 17.1 Ma age for the trachyte. Using mineral and whole-rock chemistries, a model for the deuteric (autometasomatic) alteration of the trachyte is presented, where hydrothermal fluids from the de-volatilisation of the erupting lava interact with eudialyte, causing pseudomorphic replacement by secondary mineral assemblages. The deuteric fluids efficiently oxidised CeIII to CeIV and in some cases, transported trivalent LREE significant distances within flows, forming Ce-deficient hydroxylbastnasite-(Nd), some of the most Nd-rich naturally occurring minerals reported to date (up to 34% Nd2O3). Finally, this thesis presents a study on the Cu-Au-U systems of the eastern Mt Isa Inlier (QLD) which identifies two distinct styles of REE-enriched deposits: F-Ba rich IOCG deposits and U-Cu-Au skarn deposits. The first is characterised by a single phase of REE and Cu mineralisation, and the latter by early REE and U-rich skarn formation followed by a late Cu-Au overprint. REE patterns systematically vary between deposit types, with IOCG deposits characterised by positive Eu anomalies in mineral and whole rock, while skarns have negative Eu anomalies. These observations suggest distinctly different mechanisms of ore formation. The contrasting behaviour of Eu between the two deposit types may reflect the metal source in each deposit type, supported by an association of the skarns with evolved intrusives, and the F-Ba rich IOCGs with regions of albitisation.