The rare-earth elements (REE) are a group
of seventeen speciality metals that have unique and diverse chemical, magnetic, and luminescent properties that make them strategically important in a number
of high-technology industries. Consequently, the REE are increasingly becoming more attractive commodity targets for the
mineral industry. This paper presents a comprehensive review
of the distribution, geological characteristics and resources
of Australia's major REE
deposits. REE in Australia are associated with igneous, sedimentary, and metamorphic rocks in a wide range
of geological environments. Elevated concentrations
of these elements have been documented in various heavy-
mineral sand
deposits (beach, dune, marine tidal, and channel), carbonatite intrusions, (per)alkaline igneous rocks, iron-oxide breccia complexes, calc-silicate rocks (skarns), fluorapatite veins, pegmatites, phosphorites, fluviatile sandstones, unconformity-related uranium
deposits, and lignites. The distribution and concentration
of REE in these
deposits are influenced by various rock-forming processes including enrichment in magmatic or hydrothermal fluids, separation into
mineral species and precipitation, and subsequent redistribution and concentration through weathering and other surface processes. The lanthanide series
of REE (lanthanum to lutetium) and yttrium show a close
genetic and spatial association with alkaline felsic igneous rocks, however, scandium in laterite pr
ofiles has a closer affinity with ultramafic/mafic igneous rocks.
A mineral-system approach has been used in this review to classify the major Australian REE deposits according to various mineralising criteria and/or associated geological events. This hierarchical classification framework has the advantage over more traditional descriptive classifications in that it has the benefits of attempting to understand those geological processes considered critical to the formation of a particular deposit type, and also it has a more predictive capacity for identifying potential new areas and types of REE mineralisation. The highest level of the classification comprises four general ‘mineral-system association’ categories, regolith, basinal, metamorphic, and magmatic, which in turn contain sixteen ‘deposit type’ members, namely: regolith — carbonatite-associated; ultramafic/mafic rock-associated; basinal — heavy mineral sand deposits in beach, high dune, offshore shallow marine tidal, and tidal environments; phosphorite; lignite; unconformity-related; metamorphic — calc-silicate; and magmatic — (per)alkaline rocks; carbonatite; pegmatite; skarn; apatite and/or fluorite veins; and iron-oxide breccia complex.
The review summarises the geological factors considered important for the formation of REE deposits. These factors can be used to identify mappable features in databases, which in turn can be applied to produce mineral potential/prospectivity maps.