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Centre for Earth and Environmental Science Research

Postmagmatic Mineralisation and Fluid Processes in Carbonatite systems

Executive Summary
ResearchersEmma Dowman, CEESR
Prof. A. Rankin, CEESR
Dr P. Murphy, CEESR
Professor Frances Wall, NHM London and CSM Exeter University
Funding Body/SourceKingston University
Duration2005 -
Project Summary

Introduction and Background

Carbonatites are rare igneous rocks that contain more than 50 % primary carbonates. They are known to occur in shallow, deep-seated and extrusive environments on all continents, except Antarctica (Woolley, 1999). Intrusive carbonatites typically form part of alkalic igneous complexes. Most are surrounded by a zone of metasomatically-altered silicate rock, (fenites), believed to have formed by reaction with Na-K-rich fluids expelled from carbonatite or undersaturated alkaline magma during emplacement.

One of the characteristic features of carbonatite complexes is their geochemical enrichment. Besides CO2, Ca, Mg and Fe (and Na and/or K within metasomatised country rocks), these include the rare earth elements (REE), Ba, Sr, F, P, Nb, and U. Sometimes these elements are so enriched in carbonatites and/or associated rocks that they reach ore grade, providing the bulk of the world`s supply of REE and Nb (Notholt et al., 1990). An understanding of the processes responsible for this enrichment, notably the REE, is of fundamental scientific importance to geochemists and petrologists as well as economic geologists and ore geneticists (Jones et al., 1996; Moller, 1999).

Paragenetic studies on individual carbonatite complexes frequently reveals a multistage mineralization process whereby late-stage hydrothermal or carbothermal mineralization associated with carbonatites overprints earlier orthomagmatic mineralization. (Rankin, 2005) The importance of magmatically-derived alkali chloride-carbonate brines has been established by various authors based on fluid inclusion and experimental studies. Recently, Buhn and Rankin (1999), applying these techniques to the Kalkfeld carbonatite complex, proposed that during evolution of an orthomagmatic, alkali-rich, carbonatitic fluid, the elements Ca, Nb, REE, Ba, Sr, F are likely to precipitate first, whereas more soluble elements such as Cu, Pb, U and Zr (plus Na &K) are preferentially retained and should only precipitate when the fluid permeates further into the country rocks forming zones of fenitization. This also appears to be the case with the Amba Dongar carbonatite complex of India (Buhn et al., 2003)

This research project is aimed at establishing whether the above model can be extended to other carbonatites, especially those highly enriched in REEs.

Methodology and Study Area

The Kangangkunde carbonatite complex of Malawi. is of considerable scientific and economic importance due to the widespread occurrence of REE (and Sr) minerals within and around the complex (Holt, 1965). However, much of the REE mineralization appears to be late-stage, in contrast to predictions of the above model. Furthermore, large volumes of alkali-rich fluids (perhaps with a meteoric fluid component) are also evident in the nearby Chilwa Island carbonatite complex of Malawi (Wooley, 1969). Hence, both complexes afford a valuable opportunity to evaluate the role of magmatic and postmagmatic fluid processes during the development of REE-mineralization and fenitizatiion associated with carbonatites.

Current research is being carried out on extensive and well documented sample suites from these two areas from the collections of the Natural History Museum in London.

The initial phase of the work has involved a systematic mineralogical, petrological and fluid inclusion study of samples suites from Kangankude to establish the mineral and fluid paragenesis and paragentic sequence for the REE mineralization. In parallel, studies are being carried out to establish the fluid inclusion, geochemical and mineralogical characteristics of fenitisation at Kangangkunde and more importantly in the Chilwa Island samples. The main techniques employed are optical and scanning electron microscopy, cathodoluminesence, fluid inclusion microthermometry, Laser-Raman spectroscopy. Laser ICP-MS and Electron Probe Microanalysis.

Initial studies show that, as predicted by Buehn and Rankin (1999), Zr, Ti in addition to the REEs are very mobile in late stage carbothermal fluids with notable enrichments extending out from the complex into the surrounding country rocks (Dowman et al., 2007).

Publications

Buhn B., and Rankin, A.H., 1999, Composition of natural, volatile-rich Na-Ca-REE-Sr carbonatitic fluids trapped in fluid inclusions: Geochimica et Cosmochimica Acta, v. 63, p. 3781-3797.

Buhn, B., Rankin, A.H., Schneider, J., and Dulski, P., 2002, The nature of orthomagmatic, carbonatitic fluids precipitating REE, Sr-rich fluorite: Fluid-inclusion evidence from the Okorusu fluorite deposit, Namibia: Chemical Geology, v. 186, p. 75-98.

Dowman, E., Rankin, A.H. and Wall, F. 2007. Fluid and solid inclusion evidence for late-stage mobility of zirconium, titanium and REE in carbonatite systems. ECROFI - XIX, Berne, Switzerland, July 17-20, Program with abstracts

Holt, D.N., 1965. The Kangankunde Hill Rare Earth Prospect. Geol. Surv. Malawi Bulletin No. 20, 130pp.

Jones A P, Wall F and Williams C T (Eds.), 1995. Rare Earth Minerals; Chemistry, origin and ore deposits. Mineralogical Society Special Series, (Chapman and Hall, London), 372 pp.

Mariano, A.N., 1989, Nature of economic mineralization in carbonatites and related rocks: in Bell, K., ed., Carbonatites, Unwin Hyman, London, p. 149-176.

Moller, P., 1989, REE(Y), Nb, and Ta enrichment in pegmatites and carbonatite-alkalic rock complexes: in Moller, P., Cerny, P., and Saupé, F., eds., Lanthanides, Tantalum and Niobium, Springer-Verlag, Berlin, Heidelberg

Notholt, A.J.G., Highley D.E., and Deans, T., 1990, Economic minerals in carbonatites and associated alkaline igneous rocks: Institute of Mineralogy and. Metallurgy, Transactions, v. 99, p. B59-B80.

Rankin, A.H. 2005. Carbonatite-Associated Rare Metal Deposits: Composition and Evolution of Ore-Forming Fluids - the Fluid Inclusion Evidence. In: Rare Metal Geochemistry and Ore Deposits (Samson, I. and Linnen, R., Eds.). GAC-MAC Short Course Series V.17, 299-314.

Woolley, A. R. 1969. Some Aspects of Fenitization with Particular Reference to Chilwa Island and Kangankunde, Malawi Bulletin of the British Museum (Natural History) Mineralogy, v. 2 no. 4. 191-220.

Woolley, A.R., 1989, The spatial and temporal distribution of carbonatites. in Bell, K., ed., Carbonatites: Genesis and Evolution, Unwin Hyman, London, p. 15-37

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