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Lithium-caesium-tantalum pegmatites in Archean of the Superior Province in Quebec: towards a better understanding of their genesis
Wednesday, November 18, 2026
Since the boom in lithium exploration, knowledge of lithium–cesium–tantalum (LCT) pegmatite deposits has progressed rapidly worldwide. Advances now span key aspects of their genesis, including metal sources, the mechanisms of element transfer within the ancient continental crust, the timing of emplacement relative to deformation, the structural controls on emplacement, and their geochemical signature and petrogenetic processes.
This session will review recent advances in LCT pegmatite geoscience through integrated thematic studies and specific case studies. It will highlight examples from the Superior Craton in Quebec to compare geodynamic settings, mineralization styles, and emplacement models. The goal is to promote an up-to-date and comparative understanding of the processes controlling the formation of these strategic deposits.
10:10 a.m.
The impact of the magmatic-hydrothermal transition on the evolution of the Mirage albite-spodumene pegmatite, James Bay, Québec
Conference details
The Mirage pegmatite field in the James Bay region of Québec hosts several tourmaline-rich albite-spodumene pegmatites that lack conventional macroscopic mineral zonation. Here, we interpret the evolution of these dykes using petrography and mineral chemistry from Dyke 6, which extends ~410 m along strike and ~417 m down dip and averages 20 m in thickness, reaching 37 m locally. Spodumene occurs throughout the dyke to within centimetres of both contacts. In contrast to the characteristically coarse-grained textures of many pegmatites, Dyke 6 contains interspersed aplitic and pegmatitic domains, with grain sizes ranging from tens of microns to several centimetres.
The near margin-to-margin distribution of spodumene indicates that the emplaced melt was already highly evolved and reached spodumene saturation early in its crystallization history. Spodumene-bearing aplitic pods and spodumene-free aplitic layers occur throughout the dyke and commonly form sharp contacts with the surrounding coarser-grained pegmatite. We interpret these aplitic textures as the product of two episodes of fluid exsolution. The first episode, associated with magma emplacement, is thought to have produced the spodumene-bearing pods, whereas a later episode, near the magmatic–hydrothermal transition, produced the spodumene-free layers. Tourmaline displays growth zoning consistent with progressive fractional crystallization, characterized by increasing Mn, Zn and Al contents and decreasing Mg content. The transition to hydrothermal conditions is expressed by widespread albitization of K-feldspar, together with the replacement of spodumene by muscovite and cookeite.
Taken together, these observations support a model in which magma fractionation, mineral growth and episodic fluid release jointly controlled the development of the internal textures and the evolution of the mineral chemistry of the Mirage pegmatites.
