Petrogenesis of lithium-rare metal pegmatites along the eastern margin of the Leinster Granite, southeast Ireland

 

PhD Candidate: Renata Barros

Supervisors: Dr. Julian F. Menuge, Mr. John Harrop

Funded by: CAPES/Science Without Borders

Abstract

Pegmatites are known for their wide chemical and mineralogical diversity, giving them great economic potential for many industrial minerals and strategic metals. The processes that bring pegmatites to extreme enrichment in rare elements are still in debate. The most popular idea is that pegmatitic fluids represent the residual phase after the crystallisation of large granitic bodies, concentrating elements such as lithium that are incompatible in granitic minerals. This hypothesis is supported by the frequent spatial association between pegmatites and granitic batholiths and many studies have successfully demonstrated the link between parent granite and pegmatites, including examples in Canada (?erný et al. 2012) and Spain (Roda-Robles et al. 2012). However, the lack of adjacent parental granite and age and/or geochemical incompatibilities in many pegmatite occurrences (as documented by. Walker et al. 1989 and Muller et al. 2015, for example) lead to a second hypothesis that they represent separate partial melts from rare element-rich sources.

In southeast Ireland, lithium pegmatites that intrude the margins of the S-type two-mica Leinster Granite were targeted by exploration campaigns in the 1960s-1980s, but not enough resource was found to attract extracting companies. Nowadays with the increasing demand for elements used in technology and with the growing potential of Li-batteries as an alternative to carbon-based fuels in vehicles, the Irish deposits are back in the spotlight and being actively drilled again for lithium.

Pegmatites and granite have indistinguishable emplacement ages and previous research suggested pegmatitic fluids were formed after extensive crystallisation of the Leinster Granite (Whitworth & Rankin 1989, O’Connor et al. 1991, Whitworth 1992), but results were inconclusive. The fundamental questions that remained unsolved and that I now address are: what are the age and conditions of emplacement of pegmatites? What is the origin of Li-rich fluids? Are there other metal-bearing ore minerals? Can the granite-pegmatites relationship be better determined? Methods that are being used to answer these questions include: petrography, ICP-MS whole-rock geochemistry, geochemical modelling, SEM, EPMA and LA-ICP-MS mineral chemistry, in situ Rb-Sr dating and possibly Sm-Nd and/or U-Pb dating of garnet and cassiterite, respectively.

References:

Muller, A, Ihlen, P.M., Snook, B., Larsen, R.B., Flem, B., Bingen, B. & Williamson, B.J. (2015) The chemistry of quartz in granitic pegmatites of southern Norway: petrogenetic and economic implications. Economic Geology 110, 1737-1757.

O’Connor, P.J., Gallagher, V. & Kennan, P.S. (1991) Genesis of lithium pegmatites from the Leinster Granite margin, southeast Ireland: geochemical constraints. Geological Journal 26, 295-305.

Roda-Robles, E., Pesquera, A., Gil-Crespo, P. & Torres-Ruiz, J. (2012) From granite to highly evolved pegmatite: a case study of the Pinilla de Fermoselle granite-pegmatite system (Zamora, Spain). Lithos 153, 192-207.

Walker, R.J., Hanson, G.N. & Papike, J.J. (1989) Trace element constraints on pegmatite genesis: Tin Mountain pegmatite, Black Hills, South Dakota. Contributions to Mineralogy and Petrology 101, 290-300.

Whitworth, M.P. (1992) Petrogenetic implications of garnets associated with lithium pegmatites from SE Ireland. Mineralogical Magazine 56, 75-83.

Whitworth, M.P. & Rankin, A.H. (1989) Evolution of fluid phases associated with lithium pegmatites from SE Ireland. Mineralogical Magazine 53, 271-284.