Re-Os geochronological constraints on the Dabolava mesothermal gold occurence, Madagascar: Implications for the Ikalamavony sub-domain deposition age
Résumé
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 I, 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, 30-50 cm-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 I pyrite and electrum assemblage yields a Palaeoproterozoic age (1961 +/- 79 Ma) whereas the pyrite and electrum assemblage of discordant type 2 veins yields a Pan-African age (533 +/- 23 Ma). 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 +/- 79 Ma 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 (similar to 1800 Ma) 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.