Lithium-caesium-tantalum pegmatites in Archean of the Superior Province in Quebec:  towards a better understanding of their genesis

The Cisco Project, located in the Frotet-Evans greenstone belt of the Opatica Subprovince, hosts an extensive LCT-type pegmatite system. The pegmatite bodies are hosted within a mafic volcano-sedimentary sequence intruded predominantly by gabbroic rocks and form a dyke swarm along an approximately 1.8 km-long NE-SW-trending corridor. Drilling indicates that several dykes observed at surface converge at depth into a larger body, while others display pronounced variations in thickness. Textural and mineralogical variability is also observed, ranging from aplitic to coarse-grained facies and including several spodumene textures.

This presentation provides a progress update on the ongoing work. It describes the observed textural and mineralogical variability and examines whether these variations are reflected in the geochemistry. Structural measurements are used to constrain the orientation and geometry of the pegmatite bodies. The results are preliminary, and the objective is to better constrain the internal architecture of the system, open discussion on possible emplacement mechanisms, and place Cisco within the context of large pegmatite systems of the Superior Province.

This presentation will focus on the results of three CONSOREM projects involving lithium in the Eeyou Istchee James Bay territory. This work combines geochemical, mineralogical, lithological, structural, statistical and biogeochemical approaches in order to develop vectoring tools to optimize exploration for lithium-cesium-tantalum (LCT) pegmatites in Québec.

On a regional scale, a spatial relationship exists between protocraton paleosutures and/or subprovince contacts (the presence of transcrustal faults that very likely acted as conduits), tourmaline fields and fertile pegmatites. On a local scale, Cs-, Rb- and Sn-enriched haloes exist in mafic and ultramafic host rocks near mineralized pegmatites. Concentrations of these elements increase toward the pegmatite–host rock contact, and enrichment factors may be related to, or even proportional to, the thickness of the intrusions. Arsenic and gallium are also two interesting pathfinder elements in the context of lithium-bearing mineralization. The first element is statistically closely associated with the Cs-Li-Rb-Sn-Ta suite (e.g. Greenbushes mine, Australia) and strongly mobilized in these settings, highlighting the hydrothermal activity associated with regional transcrustal faults. In the context of a link between the emplacement of orogenic gold deposits and lithium-bearing pegmatites, the use of As indirectly targets the most favourable structures. Multivariate statistical analysis of geochemical data from pegmatites at the Shaakichiuwaanaan project indicates that gallium behaves similarly to lithium. The use of this element also overcomes the problem of analytical method heterogeneity for lithium. The local geochemical distribution of data is strongly controlled by the mineralogy of the pegmatites. Indeed, a wide array of primary (magmatic) and secondary (hydrothermal) minerals are related to lithium pegmatites, which adds to the complexity of these mineralizing systems and necessarily prompts reflection on the metallurgical processing of the ore. The presentation will also address the analysis of physical, morphological and chemical parameters of lakes to optimally and rigorously evaluate anomalies from lake bottom sediment sample analyses.

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.

The Adina lithium deposit located in the Eeyou Istchee James Bay region has indicated mineral resources of 61.4 Mt grading 1.40% LiO₂. Spodumene is the main lithium-bearing mineral present in granitic pegmatite dykes hosted in an assemblage dominated by metabasalts, metagabbros, and sheeted porphyritic felsic rocks. This sequence is affected by D1–D2 deformation associated with a regional dextral transpression event, including E–W and NE–SW-trending shear zones. The metabasalts show two foliations, S1 and S2, whereas the metagabbros and porphyritic felsic intrusions exhibit only the S2 foliation. The latter exhibits a pronounced subvertical L2 stretching lineation, coaxial with P2 fold axes, which feature NE–SW-trending S2 axial planes with a steep dip (≥70°) to the NW. These relationships are consistent with a SE-verging sheath fold geometry. Double boudinage, parallel and perpendicular to L2, also affects the foliation and the metagabbros. The pegmatites, whose geochemical signature suggests a pelitic source, occur either as dykes formed in pre-existing shear planes subparallel to S2, or as moderately SE-dipping (~20°) lenses intercalated between metagabbro boudins. The apparent lack of a nearby granitic pluton and the spatial distribution of the pegmatites suggest a metasedimentary source located north of the deposit. Transpressive deformation is interpreted to have promoted partial melting of this lithology and controlled the emplacement of the pegmatites.