Abstract. Reprocessing of regional-scale airborne electromagnetic
data is used to build a 3D geological model of the Nasia sub-basin,
northern Ghana. The resulting 3D geological model consistently integrates
all the prior pieces of information brought by electromagnetic data,
lithologic logs, ground-based geophysical surveys, and geological
knowledge of the terrain. The geo-modeling process is aimed at defining the
lithostratigraphy of the area, chiefly to improve the stratigraphic
definition of the area, and for hydrogeological purposes. The airborne
electromagnetic measurements, consisting of GEOTEM B-field data, were
originally collected for mineral exploration purposes. Thus, those B-field
data had to be (re)processed and properly inverted as the original survey
and data handling were designed for the detection of potential mineral
targets and not for detailed geological mapping. These new geophysical
inversion results, compared with the original conductivity–depth images,
provided a significantly different picture of the subsurface. The new
geophysical model led to new interpretations of the geological settings and
to the construction of a comprehensive 3D geo-model of the basin. In this
respect, the evidence of a hitherto unexposed system of paleovalleys could
be inferred from the airborne data. The stratigraphic position of these
paleovalleys suggests a distinctly different glaciation history from the
known Marinoan events, commonly associated with the Kodjari formation of the
Voltaian sedimentary basin. Indeed, the presence of the paleovalleys within
the Panabako may be correlated with mountain glaciation within the Sturtian
age, though no unequivocal glaciogenic strata have yet been identified.
Pre-Marinoan glaciation is recorded in rocks of the Wassangara group of the
Taoudéni Basin. The combination of the Marinoan and, possibly, Sturtian
glaciation episodes, both of the Cryogenian period, can be an indication of
a Neoproterozoic Snowball Earth. Hence, the occurrence of those geological
features not only has important socioeconomic consequences – as the
paleovalleys can act as reservoirs for groundwater – but also from a
scientific point of view, they could be extremely relevant as their presence
would require a revision of the present stratigraphy of the area.
Gold-bearing granitoid deposits have recently been discovered in the Birimian of Ghana but their mode of formation and ore genesis remain enigmatic. This study presents petrographic, and geochemical characteristics of mineralised and unmineralised (gold grade >0.05 and <0.05 g/t respectively) granitoids (now gneisses) and schists (metasedimentary) to evaluate their petrogenesis/provenance, and relationship to gold mineralisation in the Abansuoso area. The unmineralised rocks comprise biotite-and hornblende-biotite gneisses, sericite-quartz, carbonate-sericite, and biotite-quartz schist. The mineralised varieties are biotite-, muscovite gneiss, iron-carbonate-sericite, carbonate-sericite-quartz, chloritecarbonate, and biotite-carbonate schist. The mineralised and unmineralised gneisses are both metaluminous and peraluminous. Both mineralised and unmineraised gneiss and schist show Nb-Ta trough, depleted LILE and enriched HFSE although widespread overall trace element concentrations for the mineralised rocks on UCC-normalised multi-element diagram, suggestive of their formation in an arc setting. This suggests coeval granitic plutonism and sedimentation with subduction-accretion during the Eburnean orogeny, hence, mineralisation may be orogenic-type.
Abstract. Re-processing of regional-scale airborne electromagnetic data is used in building a 3D geological model of the Nasia Sub-Basin, Northern Ghana. The resulting 3D geological model consistently integrates all the pieces of information brought by the electromagnetic data, lithologic logs, ground-based geophysical surveys and the prior geological knowledge of the terrain based on previous research. The geo-modelling process is aimed at defining the lithostratigraphy of the area, chiefly to improve the stratigraphic definition of the area as well as for hydrogeological purposes. The airborne electromagnetic measurements, consisting of GEOTEM B-field data, were originally collected for mineral exploration purposes. Thus, those B-field data had to be (re)processed and properly inverted as the original survey and data handling were designed for the detection of potential mineral targets and not for detailed geological mapping. These new geophysical inversion results, compared with the original Conductivity Depth Images, provided a significantly different picture of the subsurface. The new geophysical model led to new interpretations of the geological settings and to the construction of a comprehensive 3D geomodel of the basin. In this respect, the evidences of a hitherto unexposed paleovalley could be inferred from the airborne data. The stratigraphic position of these paleovalleys suggests a distinctly different glaciation history from the Marinoan events, commonly associated with the Kodjari formation of the Voltaian sedimentary basin. Indeed, their presence may be correlated to mountain glaciation within the Sturtian period though no unequivocal glaciogenic strata have yet been identified. This pre-Marinoan glaciation is recorded in rocks of the Wassangara group of the Taoudeni basin. The combination of the Marinoan and, possibly, Sturtian glaciation episodes, both of the Cryogenian period, can be an indication of a Neoproterozoic Snowball Earth. Hence, the occurrence of those geological features, do not only have an important socio-economic consequences – as the paleovalleys can act as reservoirs for groundwater – but, also from a scientific point of view, could be extremely relevant – as their presence would require a revision of the present stratigraphy of the area.
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