The Gbongogo gold prospects, northern Cote d’Ivoire: constraints on ore genesis and structural evolution
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Date
2021
Authors
Stapley, Thomas
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Abstract
The Palaeoproterozoic Baoule-Mossi domain of the West African Craton hosts numerous economic and world-class gold ´
deposits within Birimian volcano-sedimentary belts. The Mankono Prospect Area is located at the southern extent of the
junction between the regional Boundiali-Bagoe and Senoufo greenstone belts in northern Cote d’Ivoire. The Gbongogo Main ˆ
Tonalite, host to an approximate 1 Moz at 2 g/t gold endowment, occurs within the Mankono Prospect Area. This work explores
the geological framework and structural setting of the Mankono Prospect Area, investigating an atypical style of intrusion-hosted
gold-telluride mineralisation preferentially hosted by the Gbongogo Main Tonalite intrusive body.
The Mankono Prospect Area is characterised by two lithologically and geochronologically contrasting meta-volcanosedimentary
domains, the Western Domain (ca. 2180-2135 Ma) and Eastern Domain (ca. 2100-2090 Ma), having been juxtaposed along the
deep-seated Gbongogo Thrust. Four deformation events are locally recognised in the Mankono Prospect Area, termed D1 - D4,
active during tectonic events of the Eburnean Orogeny (2130-1980 Ma). The bulk of the deformation observed in the Mankono
Prospect Area, and cogenetic greenschist metamorphism, occurred during D2 wherein NE-SW trending structures formed
broadly under NW-SE directed shortening at ca. 2100 Ma, folding early D1 structures. The D3 event has been subdivided
into two discrete periods of deformation, D3A and D3B, during largely a change in compressive shortening from NW-SE (D2)
to E-W (D3). D3A has been associated with a period of brittle-ductile deformation in which the juxtaposition of the Eastern
and Western domains took place. In contrast, D3B deformation is brittle and associated with the development of mineralised,
sub-vertical, E-W trending veins forming preferentially within rheologically competent units. A final deformation event, D4, is
recognised by chlorite shear zones cross-cutting the gold-telluride mineralised veins within the Gbongogo Main Tonalite.
The timing of the gold mineralisation event remains poorly constrained. However, cross-cutting relationships indicate
mineralisation succeeds emplacement of the Gbongogo Main Tonalite at 2094 ± 4 Ma. Mineralisation is spatially and
temporally associated with quartz-tourmaline and tourmaline veins preferentially cross-cutting the Gbongogo Main Tonalite
as semi-sheeted, subvertical, E-W trending veins. Gold is concentrated within the Gbongogo Main Tonalite largely due to
competency contrasts, having acted as a favourable structural site along which hydrothermal fluids could easily migrate. Native
gold occurs intimately associated with a co-precipitated suite of Bi-rich tellurides, forming late in the mineralisation paragenesis
as fracture-filled veinlets, void infillings of pre-existing pyrite, and concentrating preferentially along pyrite crystal boundaries.Mineralisation styles and assemblages, similar to that forming within the Gbongogo Main Tonalite, are additionally present
within quartz-tourmaline veins cross-cutting regionally competent units including the mafic sills and meta-rhyodacites.
Fluid inclusion studies conducted on the mineralised quartz-tourmaline, gold-bearing veins show co-existing carbonic-aqueous
populations of low-salinity (<7 wt % NaCl equiv), H2O-CO2-NaCl fluid compositions, and with P-T conditions of fluid
inclusion entrapment estimated at approximately 180-350◦C and 1-3 kbars. Fluid inclusion characteristics associated with
mineralised veins in the Gbongogo Main Tonalite have demonstrated similarities to that of orogenic, intrusion-hosted deposits
in the Baoule-Mossi domain, strongly contrasting that of the fluid-systems associated with more commonly noted shear- or ´
fault-hosted gold mineralisation in the West African Craton. Analyses conducted on the regional quartz-tourmaline vein system
indicated comparable fluid inclusion and tourmaline trace element characteristics to that of the mineralised veins cross-cutting the
Gbongogo Main Tonalite. This may suggest the presence of a more regionally-extensive, gold-telluride mineralisation system.
Description
A dissertation submitted in partial fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2021