The Key Controls on Making Rare Earth-Rich Carbonatites

Frances Wall1, Sam Broom-Fendley1,2, Vicky Do Cabo3, Emma Dowman4, Dora Kavecsanszki1, Ed Loye1 and Kate Moore1,2, (1)University of Exeter, Camborne School of Mines, Penryn TR10 9FE, United Kingdom, (2)British Geological Survey, Keyworth, Nottingham, United Kingdom, (3)Geological Survey of Namibia, Economic Geology, Windhoek, Namibia, (4)Kingston University, School of Geography, Geology and the Environment, London, United Kingdom

Contact First Author: Frances Wall; wall.sweeney@googlemail.com

Previously Published Material: Gathers together and analyses results from PhD and MSc theses, which are being prepared for various separate publications and have been presented as conference papers

Abstract ID#: 34590

 

English Abstract:
Deposits related to carbonatites are still the most important resource for rare earth elements (REE). Levels of light rare earths (La, Ce, Pr, Nd, Sm) can reach >10 wt% in some carbonatites, e.g. in the carbonatite mines at Mountain Pass, USA and Mount Weld, Australia, and although enrichment in the higher atomic number ‘heavy’ REE is rare, it does occur, and the deposit under exploration at Lofdal, Namibia is an example.

Results from our studies at Songwe, Chilwa Island and Kangankunde, Malawi, at Lofdal, Namibia; Mountain Pass and Mount Weld help towards a better understanding of the key controls needed to produce high enough levels of REE, in large enough quantities, to be of economic interest.

Carbonatites originate from small degree partial melts in the mantle. The most important control on production of large REE-rich carbonatites may be geodynamic environment or a changing mantle through geological time.

Whatever the source, it is hard to envisage creating a REE-rich carbonatite without fractionation of magma during the journey into the upper crust. Apatite is the most important single mineral control on fractionation of REE in carbonatite magmas.

Many REE-rich carbonatites form in transition environments and show pegmatoid textures. All have evidence of substantial fluid activity, including fluid metasomatic aureoles around the carbonatite, explosion breccias and subsolidus fluid activity within the carbonatite intrusions. REE-bearing mineralizing fluids follow earlier fenite fluids out into the metasomatic aureole. Mantle source is the ultimate control on the heavy REE-enriched carbonatite at Lofdal but hydrothermal processes are key to precipitation of the ore minerals.

Subsequent alteration can provide further upgrading, for example the concentration of insoluble components during weathering at Mount Weld and the wholesale reworking of probable earlier carbonatite dykes during metamorphic and metasomatic alteration at Bayan Obo, China.

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