Abstract
Different challenges lie ahead to evaluate and produce mineral resource estimates for spodumene pegmatite deposit as they are structurally controlled, geochemically zoned, and mostly hosted by mafic rocks. In addition to these aspects, metallurgical process requirements in terms of mineralogy, grain size, density, Li2O and Fe2O3 content need to be considered. This presentation outlines the key elements to be considered when estimating these deposits and their main differences with more traditional estimates (e.g. in gold).
Sample representativeness is key, ensuring that samples and Li2O grades are truly representative of lithologies (e.g. pegmatite (Li from spodumene) vs host rocks (Li from holmquistite)) and from intrusion sub-facies (e.g. aplite, core zone, intergrowth, etc.). Collecting representative samples from pegmatites can be difficult due to their coarse-grained nature and zoned mineral assemblages, resulting in different grades of valuable minerals (e.g., spodumene, tantalite, lepidolite).
Pegmatites occur within different host rocks. Domaining accounts for the interaction between the pegmatite and its host rock. From district to deposit scale, several element ratios, as K/Rb, K/Cs, are used as geochemical indicator of the fractionation and mineral potential of rare-metal granites and pegmatites. At the district scale, they may help to reveal regional zonation and thus point toward the most promising ground. At the deposit scale, fractionation ratios may help to decipher the emplacement of pegmatite bodies as for example if they crystallized from single- or multi-stage events with different generations of dykes. Finally, at the pegmatite body scale, these ratios allow to characterize both mineralogical and textural zonation in link with primary undercooling and secondary alteration processes. Accurate domaining in pegmatite deposits involves delineating distinct zones based on mineralogy, physical characteristics, and lithological variations. Pegmatites exhibit diverse shapes, including sheets, dykes, and veins, domaining considers the geometry of these bodies.
Indeed, a major obstacle spodumene concentrate producers face is converter plant specifications of >6 wt% Li2O and <1 wt% Fe2O3 allowing the production of battery-grade lithium chemicals. This implies to characterize the iron content through the deposit, considering the enclaves and/or the contact with the host rocks to avoid any contamination. Moreover, the Li grade is not homogeneous distributed as most LCT pegmatite bodies show a distinctive internal zonation featuring different zones: border, wall, intermediate (where lithium, cesium, and tantalum are generally concentrated), and core. This zonation is expressed both in cross section and map view; thus, what may appear to be a common pegmatite may instead be the edge of a mineralized body. The LCT pegmatites also tend to be enriched in Be, B, F, P, Mn, Ga, Rb, Nb, Sn, and Hf.
In summary, estimating mineral resources for lithium spodumene pegmatites requires multidisciplinary expertise, robust data, and careful modeling. Understanding their complexities is crucial for accurate resource estimation and sustainable mining practices.