Abstract
The aim of this study is to provide a detailed description of the geology of the Imaloto Basin, in South-Western Madagascar. It also gives an account of the petrological characterization of the Imaloto coals, and undertakes an evaluation of the coal resources of the Basin. Data was collected from historical drilling conducted by the company Lemur Holdings between 2009 and 2012, as well as additional drilling from July to December 2019, which was undertaken for a social and environmental impact assessment (SEIA). A thorough mapping of the Imaloto Basin was undertaken to construct a detailed geological map at a scale of 1/5000. Mapping was initiated by a satellite imagery interpretation in 2008, followed by ground-based detailed mapping between February 2011 and May 2012 during the Lemur Holdings field campaigns. The final map was completed by another phase of ground-based detailed mapping during the 2019 SEIA drilling program, followed by another satellite image interpretation in 2022 as field work was interrupted by the COVID-19 pandemic of 2020 to 2021. The detailed geological map was delineated, and gave the opportunity to describe the major lithological units within the Sakoa Group, especially on the “Coal Measures”, which is the lowermost coal-bearing sedimentary rocks of the Basin. The axial zones of the Dwyka glacial valleys, as well as the positions of the major faults that traverse the Basin were identified. W-E and N-S cross-sections were constructed along selected drill lines within the areas drilled, using borehole core as well as surface geology as aides, to portray the continuity of various lithologies, especially the seam continuity and the impact of faults in the Basin. Coal seam thicknesses combined with coal quality parameters were used to evaluate the coal resources of the Imaloto Basin. The gross tonnage in-situ that has been estimated over the Lemur Holdings permit areas is 135.737 Mt (GTIS). Of this, 91.613 Mt have been classified into the
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measured resource confidence level, 31.497 Mt were classified into the indicated resource confidence level and 12.267 Mt were classified into the inferred resource confidence level, according to the JORC code (2004). Geological models of seam distribution, resource blocks and seam parameters were constructed using the geological software Surfer (Version10.7.972). For the current study on Imaloto coal quality, twelve coal samples were collected from three economic coal seams (namely: Main Seam, Upper Seam and Top Seam) of seven boreholes drilled across the Imaloto Basin during the 2019 SEIA drilling program, and characterized using petrography, geochemistry, and mineralogy. Petrographic analysis of the coal samples revealed an abundance of inertinite macerals in the Main Seam. The average inertinite content (mmf) of the Main Seam is 63 vol. % (ranging from 52 vol.% to 71.3 vol.%), 41.7 vol. % for the Upper Seam (ranging from 23 vol.% to 61 vol.% 2138) and 34 vol. % for the Top Seam (ranging from 33 vol.% to 35 vol.%). Vitrinite macerals are more abundant in the Upper Seam, and Top Seam samples, with an upward increase in abundance. The average vitrinite content (mmf) of the Main Seam is 21.1 vol. %, that of the Upper Seam is 39.7 vol. % and that of the Top Seam is 50.3 vol. %. This suggests that the Main Seam was formed in a more oxidizing environment with fluctuating water table during peatification. The Upper Seam and the Top Seam were formed in more an-aerobic telmatic and lower deltaic depositional environments in mires that were rapidly covered by sediments or water, preventing from severe biochemical alteration through oxidation. Vitrinite reflectance measurements place the Imaloto coals between the Sub-Bituminous A and Bituminous Medium D. Minerals in the Imaloto coal samples include clays, quartz (silicates), pyrite (sulphides), carbonates, with the silicates (clays and quartz) being the most abundant ones. The average clay content in the Main Seam is 11.06%, 23.70% in the Upper Seam and 28.90% in the Top Seam.
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The clays occur either in bands alternating with various macerals, as disseminated fine-grained mineral matter (probably syngenetic) or as small lenses, probably in-filling cell cavities or replacing cell structures. Pyrite is the most common sulphide mineral identified in the Imaloto coal samples. The average pyrite content in the Imaloto coal samples is 9% in the Main Seam, 9.5% in the Upper, and 7.1% in the Top Seam. The samples with higher pyrite contents tend to have a higher TS %.
Results of XRF revealed high Na (possibly organic) in some samples of the Top seam, in which XRD also determined high abundance of the mineral analcime. This study has expanded the knowledge base of the Imaloto Basin and coal quality, petrology, geochemistry, and mineralogy of the Sakoa Coalfields in Madagascar. The findings herein would be of fundamental aid for future exploration, mine design and exploitation, as well as to coal marketers and users.