Stream Sediment Geochemistry and Automated Mineralogy as Exploration Tools for LCT-Pegmatite and Granite-related Mineralisation: Case studies from the Leinster Granite (Ireland) and the Vosges Mountains (France)
Date: 26 April 2021
University of Exeter
PhD in Geology by Publication
This PhD thesis consists of four original papers and a synthesis chapter investigating the applicability of geochemical pathfinders and automated mineralogical analysis to regional (1 sample per 4 km2) stream sediment surveys. In the context of exploration for rare metal granite and Li-Cs-Ta (LCT) pegmatites, the principal aim of this ...
This PhD thesis consists of four original papers and a synthesis chapter investigating the applicability of geochemical pathfinders and automated mineralogical analysis to regional (1 sample per 4 km2) stream sediment surveys. In the context of exploration for rare metal granite and Li-Cs-Ta (LCT) pegmatites, the principal aim of this study was to examine and establish the essential links between geological, geochemical and mineralogical factors and to develop robust tools for commercial stream sediment geochemistry- and mineralogy-based exploration targeting. The concept of applying granite and pegmatite mineralisation-related pathfinder elements and ratios to stream sediments, developed originally for mineral (Kfeldspar, mica, Nb-Ta oxides) and bulk litho-geochemical exploration using outcrop and drill core samples, was tested in a case study using the Geological Survey of Ireland (GSI) stream sediment dataset for the Leinster Granite and adjacent Caradocian Volcanic Belt. Firstly, this study has demonstrated that incompatible (K/Rb) and immobile (Nb/Ta, Zr/Hf) trace element ratios, along with ore-forming elements (Li, Cs, Ta, Nb, Sn, W), provide useful pathfinders in exploration at 1:500,000 scale. Secondly, they can be used to delineate catchment areas within the Leinster Granite that bear evidence of increased magmatic fractionation and hydrothermal alteration characteristic of LCT pegmatite mineralisation. Prospective catchments have been identified in areas with known mineralisation (e.g., eastern flank of the Leinster Granite in the Blackstairs and Northern Units) and where none had been previously detected (e.g., Borris-Fennagh area). Distinctive values of K/Rb (<150), Nb/Ta (<7), Zr/Hf (28-47 ppm), Cs (12-47 ppm, Ta <7.5 ppm, W <10 ppm and Sn up to 50 ppm in samples located in the Blackstairs, Tullow and Northern units of the Leinster Granite, imply that fractional crystallisation and magmatic-hydrothermal alteration locally altered the chemistry of the Leinster granite and led to enrichment of incompatible elements, most importantly Li, Ta, and Sn. Additional examination of the applicability of geochemical pathfinder ratios for rare metal granite and LCT pegmatite exploration was conducted on nearly 200 stream sediment samples from the Northern and Central regions of the Vosges Mountains, NE France. The Vosges Mountains, which represent a Variscan basement complex on the western flank of the Rhine Graben, are relatively underexplored in terms of mineralisation, with the last regional mineral reconnaissance campaign conducted in the early 1980s. A similar approach was used to that for the Leinster Granite, to produce: (1) a geochemical classification of principal regional lithological units; (2) an assessment of magmatic fractionation and hydrothermal alteration processes in underlying rocks; and (3) the delineation of new exploration targets in the Grosse Goutte, Hergauchamps, Grand Rombach, Agigoutte and Barembach areas. In addition, the combination of detailed geological observations in the catchment areas, as well as the application of automated mineralogy using QEMSCAN®, improved the characterisation of signatures in stream sediments related to magmatichydrothermal mineralisation. This included the identification of mineral associations characteristic of tourmaline-muscovite-chlorite (greisen) alteration and the presence of cassiterite, wolframite, ilmenorutile and columbite minerals. These mineral associations suggest the presence of critical metal mineralisation associated with late-stage hydrothermal alteration affecting S- and hybrid I-S-type source granites, along major regional lineaments and shear zones of the Vosges Mountains. Overall, the results of this study demonstrate that the application of geochemical magmatic fractionation pathfinder elements and ratios can be used in surface geological materials affected by secondary dispersion, if the catchment geology and mineralogy is taken into account and integrated into the geochemical interpretation. The knowledge gained from this research was summarised in a review paper providing a synthesis of LCT pegmatite genesis (Steiner, 2019a), development of applicable exploration techniques (Steiner, 2018, 2019b; Steiner et al., 2019), a catchment-prospectivity map for the Sainte Marie-aux-Mines area (Steiner, 2019b), and a systematic ‘cookbook’ approach to commercial exploration targeting and related investigations (Steiner 2019a).
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