Rare element enrichment processes in LCT pegmatites and implications for geochemical exploration

 

PhD Candidate: David Kaeter

Supervisor: Dr Julian Menuge 

Funded by: iCRAG (Science Foundation Ireland under grant number 13/RC/2092, co-funded under the European Regional Development Fund and by iCRAG industry partners)

 

Abstract:

Spodumene pegmatites and related rocks constitute the LCT (Li, Cs, Ta) class of rare metal pegmatites [1] that provides all global caesium production, around a third of lithium [2] and a significant proportion of tantalum [3], along with economic potential for Sn, Rb and Be. The use of lithium in Li-ion batteries is likely to lead to continuing rapid growth in demand [2]. Tantalum is used in electronic components, especially high specification capacitors where it cannot readily be substituted, and demand for the metal is likely to rise [3]. Although a growing proportion of the world’s lithium is extracted from Andean salars, LCT pegmatites remain exploration targets for security of supply reasons and because of the potential for economic Ta, Cs and other rare metals.

A belt of LCT spodumene pegmatites follows the eastern margin of the Tullow Lowlands pluton of the Leinster Granite batholith. The LCT pegmatites scarcely outcrop, but boulder fields reveal their subcrop [5]. Following initial exploration in the early 1970s [4], current exploration by International Lithium Corp. began on licences taken out in 2009 on a continuous belt extending from Borris, Co. Carlow north-eastwards to Shillelagh, Co. Wicklow. Ongoing drilling at Aclare, Moylisha and Monaughrim has extended known spodumene pegmatite occurrences and revealed others, and geochemical soil sampling has been conducted along the ~40 km length of the licence areas. Pegmatite bodies up to tens of metres thick show ore grade Li over thicknesses of several metres [e.g. 4] and tantalum minerals have been seen in thin section; pegmatites’ strike-parallel dimensions are at least hundreds of metres [5]. Based on drilling and boulder distribution, including rare exposed contacts, the pegmatites were emplaced variously into the Tullow Lowlands pluton and approximately coeval minor granite intrusions and into Ordovician metasedimentary and metavolcanic rocks [6]. The Tullow Lowlands pluton, minor granites and pegmatites, were emplaced syntectonically into the East Carlow Deformation Zone (ECDZ) around 400 Ma [6]. Recent petrogenetic modelling suggests that pegmatite magma batches did not form by fractional crystallization of Tullow Lowlands magma, but as separate crustal melts [7].

The project will commence with an optical and SEM / MLA search for rarer minerals in spodumene pegmatite intrusions from well-characterised localities in the Leinster LCT suite, followed by mineral LA-ICP-MS major and trace element analysis. Major and trace element variations within zoned crystals; between crystals of the same mineral representing successive crystallisation stage in the same pegmatite; and in different pegmatites will be determined. The focus will be on minerals that record variation in Ta, Nb, Cs, Rb, and in Mn/Fe ratio which accompanies rare metal fractionation: feldspars, white micas, garnet, spodumene, beryl, cassiterite, tourmaline, zircon and Ta minerals. Many are likely resistate minerals and their exploration usefulness in this context will be assessed. The mineral chemistry data will be used to test a variety of petrogenetic models of pegmatite crystallization, testing for Rayleigh fractionation, constitutional zone refining and immiscibility, focusing on the causes of rare metal enrichment and depletion. Nd-Sr-O isotopic data will be obtained on a subset of the mineral samples, to seek evidence for separate magmatic pulses and magma mixing (Nd-Sr isotopes) and arrival of hydrothermal fluids (O isotopes). The possible significance for rare metal concentration and crystallization of deformation in the ECDZ will be explored. The magmatic relationships between spodumene and barren pegmatites will also be examined to determine whether they represent different, unrelated magma batches, or parts of a sequence of crystallization. If barren pegmatites and aplites carry signatures indicative of their relationship to spodumene pegmatites, this will have major implications for exploration strategy. Field comparison with the closely analogous and well exposed Brazil Lake LCT pegmatite, Nova Scotia, will assist in interpretation of the Leinster pegmatites.

 

References:

London, D. (2008) Canad. Mineral., Spec. Publ. 10, 347 pp

Evans, K. (2014) In: Gunn, G. (ed.), Wiley, pp. 230-260

Linnen, R., Trueman, D.L. and Burt, R. (2014) In: Gunn, G. (ed.), Wiley, pp. 361-384

Steiger, R. and von Knorring, O. (1974) J. Earth Sci. Leeds Geol. Assoc. 8, 433-443

Steiger, R. (1977) Inst. Mining Metall. and Geol. Surv. Finland Symposium, Helsinki, pp. 14-24

O’Connor, P.J., Gallagher, V. and Kennan, P.S. (1991) Geol. J. 26, 295-305

Barros, R. and Menuge, J.F. (2016) Canad. Mineral. (in press)