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

Abstract

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 magmatichydrothermal 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.

Formation Conditions and 40Ar/39Ar Age of the Gem-Bearing Boqueirão Granitic Pegmatite, Parelhas, Rio Grande do Norte, Brazil

“…Fluid inclusion assemblages, composed of melt inclusions and aqueous and CO 2 -bearing fluid inclusions in minerals from Zone II of the gem-bearing Boqueirão granitic pegmatite, indicate the magmatic-hydrothermal transition. Similar phenomena have been recorded in other granitic pegmatites worldwide [50,51]. Melt inclusions represent entrapped remains of the silicate-rich melt, whereas fluid inclusions contain the fluid phase exsolved during the crystallization process.…”

The Distribution of Rare Metals in the LCT Pegmatites from the Giraúl Field, Angola

“…According to experimental studies, during this separation Nb and Ta are partitioned into the magma [11,15,90]. However, according to [91], magmatic fractionation alone cannot explain the formation of leucogranites with high Ta/(Ta + Nb) ratios.…”

“…Usually, the tendency followed in granitic pegmatites ranges from compositions rich in Nb and Fe to compositions rich in Ta and Mn [7]. However, anomalous behaviour of this trend can be produced by coeval crystallisation of other mineral phases that host some Nb and Ta, such as micas [8][9][10][11]. Therefore, a strong knowledge of the processes that generate the source rocks of these rare elements is necessary to understand the mechanisms of transport and accumulation of Nb-Ta oxides.…”

“…Niobium and tantalum concentrate during the magmatic crystallisation by fractionation processes [12,13]. However, in the late processes of pegmatite evolution, such as those that form albitites, a Ta-enrichment is usually produced together with a high increase in the amount of Nb-Ta oxide minerals [11]. This enrichment has been explained considering strictly magmatic processes by extreme fractionation during crystallisation process [2,9,[14][15][16][17].…”

Tools and Workflows for Grassroots Li–Cs–Ta (LCT) Pegmatite Exploration

“…Over the last four decades research into rare metal Li-Cs-Ta (LCT) pegmatite mineralisation has predominantly focused on the understanding of late-stage magmatic fractionation and hydrothermal alteration processes enriching incompatible elements of potential economic interest in felsic peraluminous melts. In particular, the mineralogy and geochemistry of LCT pegmatites have been studied in great detail and are presented in case studies from Canada and the US [1][2][3][4], Ireland [5,6], the European Variscides [7][8][9][10][11][12], the Sveconorwegian orogeny [13,14], Central-East Africa [15][16][17][18] and Australasia [19][20][21][22]. These case studies have resulted in the definition of tectonic, mineralogical and geochemical factors controlling the location of mineralisation and zonation within the roof of peraluminous S-type granite plutons, pegmatitic offshoots into metasedimentary country rock or anatectic melts with no established link to S-type granites.…”