Spandler, and two co-authors, reported in mid-2020 that Australia is host to a diverse range of rare earth element (REE) ore deposits, and therefore is well placed to be a major supplier of REE into the future. This paper presents a review of the geology and tectonic setting of Australia's hard-rock REE resources. The deposits can be classified into four groups:
1. Carbonatite associated;
2. Peralkaline/alkaline volcanic associated;
3. Unconformity related, and;
4. Skarns and iron-oxide‑copper‑gold (IOCG) related.
With the exception of the unconformity related deposits, all of these deposit groups are directly or indirectly related to continental alkaline magmatism. Extensive fractional crystallization and/or igneous accumulation of REE minerals were essential ore-forming processes for carbonatite-associated and peralkaline/ alkaline volcanic-associated deposits, while hydrothermal transport and concentration of REE sourced from basement rocks was responsible for producing ore in unconformity-related, skarns and, potentially, IOCG deposits. The economic potential of many deposits has also been enhanced by supergene alteration processes.
All of Australia's REE deposits formed in an intracontinental setting in association with crustal-scale fault zones or structures that acted as transport conduits for ore-forming magmas or fluids. Most deposits formed in the Mesoproterozoic under conditions of relative tectonic quiescence. There is little evidence for the involvement of mantle plumes, with the exception of the Cenozoic peralkaline volcanic systems of eastern Australia, and possibly the IOCG deposits. Instead, ore productive magmas were generated by melting of previously-enriched mantle lithosphere in response to disruption of the lithosphere-asthenophere boundary due to fault activation.
REE minerals in many deposits also record episodes of recrystallisation/resetting due to far-field effects of orogenic activity that may significantly postdate primary ore formation. Therefore, REE orebodies can be effective recorders of intracontinental deformation events.
In general, Australia's inventory of REE deposits is similar to the global record. Globally, the Mesoproterozoic appears to be a particularly productive time period for forming REE orebodies due to favorable conditions for generating ore-fertile magmas and favorable preservation potential due to a general lack of aggressive continental recycling (i.e., active plate tectonics).
The authors concluded that:
• Australia hosts a diverse array of hard-rock REE ore deposits and is well placed to be a major REE producer into the future. Most deposits are related to alkaline igneous rocks, although hydrothermal processes were crucial to the formation of many deposits, most notably the unconformity-related REE deposits. REE enrichment via supergene alteration is also important for the economic viability of carbonatite-hosted REE deposits.
• Most deposits formed in an anorogenic setting in isolation of plate margins or mantle plumes. Ore–producing alkaline magmas were sourced from melting of enriched mantle sources, with extensive fractional crystallization being a crucial process leading to ore formation. All deposits are closely associated with lithospheric-scale structures that likely act as conduits for transporting REE-bearing fluids and/or melts within the crust.
• A majority of Australia's deposits are Mesoproterozoic in age, although many preserve records of REE mineral recrystallisation due to orogenic or deformation events subsequent to ore formation. REE ore minerals can therefore be useful archives of geological events that affect continental interiors.
• The complex geology of most deposits, coupled with their remote locations, presents challenges for ore processing and economic viability, but also provides optimism that there may be many other REE orebodies within the Australian crust awaiting discovery.
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