The Origin of Spodumene Pegmatites Associated with the Leinster Granite in Southeast Ireland

Rare-element pegmatites have diverse chemical signatures and are important sources of strategic metals such as Li, Cs, and Ta. The two main hypotheses to explain rare-element pegmatite formation are (1) residual magmas from the crystallization of granitic rocks, and (2) partial melts from a relatively rare-element-rich source. In southeast Ireland, spodumene and spodumenefree pegmatite dikes occur along the eastern margin of the S-type Leinster Granite batholith. With indistinguishable emplacement ages around 400 Ma, the origin of the Li-rich pegmatitic fluids has been suggested to have resulted from extreme fractional crystallization of Leinster Granite granodiorite magma. To test this hypothesis, we used whole-rock geochemistry of pegmatite and granodiorite samples from drill cores and geochemical modeling of in situ crystallization and batch melting to investigate which process better explains the formation of the pegmatites. Chemical signatures of the pegmatites and granodiorite do not indicate a direct comagmatic relationship, as the granodiorite has higher concentrations of many incompatible elements than the pegmatites (e.g., concentrations of Zr, Ti, and Y). Concentrations of Li, Rb, Cs, Sr, and Ba show no clear fractionation trends from granodiorite to pegmatite. The in situ crystallization model using the average granodiorite composition as the initial magma generates a range of compositions that does not include pegmatites, so it is unlikely that they represent residual granitic magmas. Modeling of partial melting indicates that the Leinster Granite granodiorite and pegmatite magmas could have formed in separate events and from chemically different source rocks, with the pegmatite magmas presumably formed in a younger event because the pegmatites intrude the granodiorite.European Commission - European Regional Development FundScience Foundation Irelan

The impact of iron sulfide on lead recovery at the giant Navan Zn-Pb orebody, Ireland

It has been proposed that blending of Navan Conglomerate Group Ore (CGO) with Pale Beds Ore (PBO), the latter which floats well in isolation, results in sub-optimal Pb recovery with increased abundance of pyrite reporting to the concentrate and increased abundance of galena reporting to the tails. QEMSCAN data indicate that poor liberation of galena particles is not the primary cause of sub-optimal recovery of galena. Rather, principal component analysis (PCA) of time of flight secondary ion mass spectrometry (ToF-SIMS) reveals that chemically altered surface species interfere with the selectivity and recovery of froth flotation indicating that there is some poisoning of galena mineral surfaces particularly with Fe-containing species possibly leading to loss of recovery. Analysis of the Pb-circuit cleaner tails indicates Pb-species association with sphalerite surfaces, as was observed for the flotation feed and rougher tails but with insufficient induced hydrophobicity by Pb–collector interaction for flotation. A small population of relatively clean galena surfaces is also observed which may result from low bubble–particle collision efficiency or insufficient liberation. Fine-grained framboidal pyrite is the main diluting phase in the cleaner concentrate and is likely present due to entrainment. The presence of framboidal pyrite in CGO is of particular significance as its large surface area increases the rate of galvanic interaction with other metal sulfide minerals. We propose that increased abundance of refractory framboidal pyrite in CGO is the critical factor affecting the performance of the Navan Pb flotation circuit rather than purely high pyrite abundance.Irish Research CouncilBoliden Tara Mines Ltd Enterprise Partnership Scheme PhD scholarshi

Controls on chemical evolution and rare element enrichment in crystallising albite-spodumene pegmatite and wallrocks: Constraints from mineral chemistry

Internal differentiation and consequent geochemical evolution in pegmatites are significant processes in the development of economically viable deposits of metal-bearing minerals. Albite-spodumene pegmatites, which represent important resources of Li and Ta worldwide, challenge the general rules of pegmatite petrogenesis as these are nearly homogeneous bodies with little or no intrusion-scale pegmatite zonation. Bulk intrusion concentrations of Li are in the uppermost range obtained by magmatic enrichment experiments, around 2 wt% Li2O, and extensive volumes of saccharoidal or platy albite are present. In Leinster, southeast Ireland, weakly zoned to homogeneous albitised spodumene pegmatites and their wallrocks were studied to compare mineral chemistry variations and understand the internal evolution of pegmatites, characteristics linked to the poor development of zonation, and links between internal evolution and pegmatite-wallrock interactions. Leinster pegmatites present mineralogical, textural and geochemical characteristics coherent with Li-saturation, and possibly supersaturation, prior to crystallisation. Weak border to centre zonation in the thickest bodies can be attributed to geochemically evolved initial melt, likely leading to nearly contemporaneous crystallisation throughout the intrusion and resulting in limited internal geochemical fractionation. Increased abundance of minerals bearing highly incompatible elements (e.g. columbite-group minerals and cassiterite) and network modifiers (e.g. phosphates) in albitite indicates it is a fractionation product from pegmatite crystallisation. Enrichment in incompatible elements B, Li, Rb, Cs and F in spodumene pegmatite exocontacts in different country rock types suggests unmixing of a hydrous fluid from the residual melt after the crystallisation of main pegmatitic assemblages, and that the H2O-rich component was mobilised into country rocks.European Commission - European Regional Development FundScience Foundation IrelandCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Preliminary paragenetic studies of the high grade Island Pod Zn-Pb orebody, Lisheen

The Annual Irish Geological Research Meeting, Trinity College Dublin, Ireland, 3-5 March 2017Irish-type deposits are a series of Zn-Pb orebodies which formed from the carbonate replacement of Lower Carboniferous limestone, triggered primarily by fluid mixing. This project aims to use isotopic (Zn-Cu-S and clumped O-C) techniques to identify geochemical halos and increase our understanding of hydrothermal fluid processes in these deposits

The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

The processes involved in the magmatic–hydrothermal transition in rare-element pegmatite crystallization are obscure, and the role of hydrothermal mechanisms in producing economic concentrations of rare elements such as tantalum remains contentious. To decipher the paragenetic information encoded in zoned minerals crystallized during the magmatic–hydrothermal transition, we applied SEM-EDS and LA-ICP-MS chemical mapping to muscovite- and columbite-group minerals (CGM) from a rare-element pegmatite of the albite-spodumene subtype from Aclare, southeast Ireland. We present a three-stage model for the magmatic–hydrothermal transition based on petrography, imaging and quantification of rare-element (Li, B, Rb, Nb, Sn, Cs, Ba, Ta, W, U) zoning, integrated with geochemical modeling and constraints from published literature. Stage I marks the end of purely magmatic crystallization from a peraluminous granitic melt. In stage II, polymerized silicic melt and depolymerized alkaline aqueous melt coexist as immiscible media, both of which influence muscovite and CGM crystallization. Stage II also marks the onset of phyllic alteration of primary mineral assemblages. Hydrothermal fluid release causes further resorption of primary minerals and eventual precipitation of fine-grained albite in stage III. Muscovite and CGM both exhibit trace-element zoning, while CGM also show major-element zoning. Petrographic relationships and geochemical markers such as Ta# (=Ta/[Ta + Nb]) of individual mineral zones reveal that both mineral species crystallized contemporaneously over all three stages. Furthermore, Rayleigh fractional crystallization of muscovite is efficient in fractionating Ta from Nb. Tantalum and Nb are additionally fractionated by halogen-rich aqueous media, which remobilize both elements, but redeposit preferentially Ta-enriched oxides. Columbite–tantalite mineralization is therefore both magmatic and hydrothermal. Albite associated with stage-III muscovite and CGM was likely precipitated from a hydrothermal fluid when pH changed due to hydrolysis of primary minerals. The complex zoning and resorption of minerals indicates that bulk analyses and conventional LA-ICP-MS spot ablation analyses of muscovite in rare element pegmatites may lead to erroneous modelling of element partitioning and fluid evolution. Combined petrographic and high-resolution geochemical analysis of two mineral species (which co-crystallize and incorporate the same elements of interest) is an effective tool to assess the complex processes of crystal–melt–fluid interaction. Our three-stage model may also be applicable to the still not well understood magmatic–hydrothermal transition from fertile granitic melts to formation of Sn-W veins and greisens or porphyry-type deposits.European Commission - European Regional Development FundScience Foundation IrelandiCRAG industry partners24 month embargo - A

Constraining fluid mixing processes at the Irish-type Lisheen and Navan Zn-Pb orebodies: preliminary evidence from clumped C-O isotopes

International Geological Congress, Cape Town, South Africa, 27th August - 04th September 2016Our research has applied the clumped O-C isotope technique to the Irish Zn-Pb ore field. Preliminary clumped C-O data will be presented from the Lisheen Zn-Pb orebody (22.3 Mt mined at 11.7% Zn and 2.0% Pb) [4] along with fluid inclusion data . Samples were analysed from all main carbonate generations across the deposit , including: regional dolomite (D 1), dark grey to black pre-ore hydrother mal dolomite (D 2 ; also known as black-matrix breccia), medium-to-coarse-grained ore-stage white ferroan dolomite (D 3 ; white-matrix breccia) , late veins of ferroan dolomite (D4) and white calcite (C4), post-ore crosscutting pink saddle dolomite (D 5), and post-ore white blocky calcite (C 6). We will discuss existing metallogenetic models for the Lisheen deposit and the potential for clumped C-O analysis to constrain fluid flow pathways and mixing processes, and as a tool for mineral exploration .European Commission - European Regional Development FundScience Foundation Irelan

The magmatic–hydrothermal transition in rare-element pegmatites from southeast Ireland: LA-ICP-MS chemical mapping of muscovite and columbite–tantalite

The processes involved in the magmatic–hydrothermal transition in rare-element pegmatite crystallization are obscure, and the role of hydrothermal mechanisms in producing economic concentrations of rare elements such as tantalum remains contentious. To decipher the paragenetic information encoded in zoned minerals crystallized during the magmatic–hydrothermal transition, we applied SEM-EDS and LA-ICP-MS chemical mapping to muscovite- and columbite-group minerals (CGM) from a rare-element pegmatite of the albite-spodumene subtype from Aclare, southeast Ireland. We present a three-stage model for the magmatic–hydrothermal transition based on petrography, imaging and quantification of rare-element (Li, B, Rb, Nb, Sn, Cs, Ba, Ta, W, U) zoning, integrated with geochemical modeling and constraints from published literature. Stage I marks the end of purely magmatic crystallization from a peraluminous granitic melt. In stage II, polymerized silicic melt and depolymerized alkaline aqueous melt coexist as immiscible media, both of which influence muscovite and CGM crystallization. Stage II also marks the onset of phyllic alteration of primary mineral assemblages. Hydrothermal fluid release causes further resorption of primary minerals and eventual precipitation of fine-grained albite in stage III. Muscovite and CGM both exhibit trace-element zoning, while CGM also show major-element zoning. Petrographic relationships and geochemical markers such as Ta# (=Ta/[Ta + Nb]) of individual mineral zones reveal that both mineral species crystallized contemporaneously over all three stages. Furthermore, Rayleigh fractional crystallization of muscovite is efficient in fractionating Ta from Nb. Tantalum and Nb are additionally fractionated by halogen-rich aqueous media, which remobilize both elements, but redeposit preferentially Ta-enriched oxides. Columbite–tantalite mineralization is therefore both magmatic and hydrothermal. Albite associated with stage-III muscovite and CGM was likely precipitated from a hydrothermal fluid when pH changed due to hydrolysis of primary minerals. The complex zoning and resorption of minerals indicates that bulk analyses and conventional LA-ICP-MS spot ablation analyses of muscovite in rare element pegmatites may lead to erroneous modelling of element partitioning and fluid evolution. Combined petrographic and high-resolution geochemical analysis of two mineral species (which co-crystallize and incorporate the same elements of interest) is an effective tool to assess the complex processes of crystal–melt–fluid interaction. Our three-stage model may also be applicable to the still not well understood magmatic–hydrothermal transition from fertile granitic melts to formation of Sn-W veins and greisens or porphyry-type deposits.European Commission - European Regional Development FundScience Foundation IrelandiCRAG industry partners24 month embargo - A

A comparison between clumped C-O and fluid inclusion temperatures for carbonates associated with Irish-type Zn-Pb orebodies

6th International Clumped Isotope Workshop 2017, Paris, France, 10-12 August 2017Ireland hosts the greatest concentration of discovered zinc per square kilometre on Earth, with past and current production from five Irish-type carbonate-hosted deposits, including the giant Navan deposit. Clumped C-O isotope analysis of carbonate phases offers a powerful new technique to deliver accurate fluid temperatures and fluid O isotope compositions, refining evolving genetic models and developing new tools for exploration

Controls on the formation of a large Zn-Pb Irish-type deposit: evidence from the Navan halos

IAEG Annual Conference 2017, Sligo, Ireland, 12-14 May 2017The mainly epigenetic Navan orebody is hosted by the Lower Carboniferous shallow water carbonate Pale Beds. Subordinate mineralization, hosted by submarine debris flows and the lowermost overlying basinal rocks, formed during seafloor exhalation and reaches ore grade in the Conglomerate Group Ore