Simandla and Paradis reported in 2018 that most carbonatites were emplaced in continental extensional settings and range in age from Archean to recent. They commonly coexist with alkaline silicate igneous rocks, forming alkaline-carbonatite complexes, but some occur as isolated pipes, sills, dikes, plugs, lava flows, and pyroclastic blankets. Incorporating cone sheets, ring dikes, radial dikes, and fenitization-type halos into an exploration model and recognizing associated alkaline silicate igneous rocks increases the footprint of the target. Undeformed complexes have circular, ring, or crescent-shaped aeromagnetic and radiometric signatures.
Carbonatites can be effectively detected by soil, till, and stream-sediment geochemical surveys, as well as biogeochemical and indicator mineral surveys Carbonatites and alkaline-carbonatite complexes are the main sources of rare earth elements (REE) and Nb, and host significant deposits of apatite, vermiculite, Cu, Ti, fluorite, Th, U, natural zirconia, and Fe. Nine per cent of carbonatites and alkaline-carbonatite complexes contain active or historic mines, making them outstanding multi-commodity exploration targets.
Mineralized carbonatites and alkaline-carbonatite complexes are highly sought after, multi-commodity, but poorly understood exploration targets (Verwoerd 1986; Mariano 1989a, 1989b; Richardson and Birkett 1996a, 1996b; Pell 1996; Birkett and Simandl 1999). They are the main source of niobium (Nb; Mackay and Simandl 2014a; Simandl et al. 2018) and rare earth elements (REE; Simandl 2014; Verplank et al. 2016), which are considered critical metals for key economic sectors in industrialized countries (European Commission 2017), and have become popular exploration targets for mining companies worldwide. Alkaline-carbonatite complexes are significant sources of Cu, apatite, fluorite, vermiculite, and other commodities. Most modern studies on carbonatites address their origin and aim to improve our understanding of the Earth’s mantle (Bell and Tilton 2001, 2002; Bell and Simonetti 2010; Rukhlov et al. 2015).
Carbonatites and alkaline-carbonatite complexes were considered controversial ever since A.G. Högbom began detailed geological work at the Alnö Island alkaline-carbonatite complex in 1889. The history of carbonatite research up to 1966 is well summarized in the 1st and 2nd editions of the widely available benchmark publication entitled ‘The Geology of Carbonatites’ by Heinrich (1980). A number of additional reviews have been performed since that time, including the book edited by K. Bell (1989) entitled ‘Carbonatites: Genesis and Evolution’. However, the most important review from an exploration geologist’s point of view is probably the compilation of Wooley and Kjarsgaard (2008a) entitled ‘Carbonatite occurrences of the word; map and database’, available free of charge for download from the Geological Survey of Canada website.
The highly descriptive and factual nature of this publication avoids traps and controversies associated with the genesis of carbonatites, and takes into consideration some of the key suggestions of Mitchell (2005) who attempts to improve currently accepted classification of Le Maître (2002) by making it more relevant to geological mappers and exploration geologists. The descriptive approach, in combination with the availability of data as spreadsheets, will make the Wooley and Kjarsgaard (2008a) compilation a valuable source of information for years to come, since spreadsheets can be easily updated and customized by the user.
Herein, we review the definition and classification of carbonatites and summarize information pertinent for carbonatite exploration such as tectonic setting, morphology, carbonatite-related alkali silicate rock associations, accompanying alteration, and temporal distributions. We also discuss carbonatites and alkaline-carbonatite complexes in terms of resources, exploration methods, and prospectivity.
The four main objectives of this paper are to:
- i) alert the exploration community to the multi-commodity exploration potential of carbonatites and alkaline-carbonatite complexes,
- ii) highlight the relevance of tectonic setting, carbonatite-related alkali metasomatism (fenitization type metasomatism), and alkaline-carbonatite complex
morphology to the exploration geologist, - iii) present relevant advances in exploration methods, and
- iv) bridge the gap separating carbonatite-related academic researchers and mineral explorers.
[Outstanding paper.]
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