Re–Os geochronological constraints on the Dabolava mesothermal gold occurence, Madagascar: Implications for the Ikalamavony sub-domain deposition age

Abstract

New Re–Os ages for early concordant veins and later discordant veins in the Ikalamovony sub-domain of west-central Madagascar require respectively a Paleoproterozoic age for metasedimentary rocks and a Pan-African age for the orogenic type occurrences. Indeed, this paper focuses on Re–Os geochronology of Dabolava gold occurrences, located in the Ikalamavony sub-domain, western central part of Madagascar. Two types of gold veins have been identified in this region; (i) type 1, centimetre-thick quartz veinlets, with diffuse contacts and concordant to the main regional foliation of the host amphibolites, part of the upper-Group of the Ikalamavony sub-domain; (ii) type 2, 30–50cm-thick quartz veins variously transposed in shear zones affecting the Dabolava granodioritic orthogneisses, part of the Dabolava magmatic suite. These two types of veins with Au–Py assemblages have been sampled for Re–Os geochronology. The type 1 pyrite and electrum assemblage yields a Palaeoproterozoic age (1961±79Ma) whereas the pyrite and electrum assemblage of discordant type 2 veins yields a Pan-African age (533±23Ma). These ages record two main gold mineralization events that affected this crustal segment. Assuming that the Re–Os systematics were not perturbed by more recent events, the 1961±79Ma age obtained in this study on type 1 gold vein concordant in amphibolites favours a Palaeoproterozoic deposition age for the whole Ikalamavony sub-domain, both lower- and upper Groups. This age contrasts with the previously proposed Mesoproterozoic age for the deposition of the upper Group of the Ikalamavony sub-domain but is consistent with the Paleoproterozoic age (∼1800Ma) deduced from detrital zircon ages in the Ikalamavony quartzites located in the Ikalamavony lower Group. The Pan-African age obtained on the type 2 gold deposits is much younger than the emplacement age of the host granodiorite and precludes a porphyry-type model for this gold mineralization and rather favours an orogenic type with a deposition during the Pan-African orogeny.