The Ambatolampy Group, central Madagascar, a Neoproterozoic rift-basin sequence?

The crystalline basement of central Madagascar is composed of the Neoarchaean, high-grade metamorphic Antananarivo Domain, made up of granulite to upper-amphibolite orthogneisses and paragneisses, and intruded by Tonian igneous rocks of the Imorona-Itsindro suite (Archibald et al. 2016). Along its southern, western and northern margins several terranes were accreted between the Paleoproterozoic and the Neoproterozoic (Tucker et al. 2014) before Madagascar was affected by the collision of East- and West-Gondwana at the end of the Ediacaran.

Within the Antananarivo Domain, a more than 700 km long and up to 80 km wide belt of supracrustal amphibolite-facies rocks forms te Ambatolampy Group. It is characterized by abundant monotonous biotite schists and gneisses that are locally migmatised. The schists contain biotite, sillimanite, garnet and locally thick graphite-rich layers. Associated paragneisses are also biotite-rich and commonly carry sillimanite or hornblende. White quartzites ranging from thick-bedded ridge-forming units to fine, cm-scale interbeds are coarse-grained and contain often sillimanite. Dark quartzites rich in magnetite and heavy minerals occur as cm-thin layers throughout the whole group. Small bodies of pyroxenite, pyroxene-amphibolite, amphibolite ±garnet, and pyroxene gneiss are common, especially close to the base of the group.

The age of the Ambatolampy Group is highly controversial. A group of researchers from BGS and USGS reported a youngest detrital zircon age of 1054 Ma, whereas Archibald et al. (2016) assumed a Mesoproterozoic age, based on their youngest zircons of roughly 1.8 Ga. We present new near-concordant U-Pb detrital zircons ages as young as 800 Ma, indicating a sedimentary input from igneous rocks of the Imorona-Itsindro suite. Sedimentation must have ceased before 630 Ma which is constrained by the U-Pb zircon age of an intruding leucogabbro.

About half of Madagascar’s known 1050 gold occurrences are lying within the Ambatolampy Group. Fine-grained disseminated gold appears to be concentrated within relatively narrow stratigraphic intervals of the Ambatolampy Group, defined by the occurrence of boudinaged or fractured magnetite quartzite. In general, the gold grades in fresh rocks are below economic interest, the highest gold tenors were recorded in an up to 30 meter thick laterite zone above the basement. Another important commodity related to the Ambatolampy Group is graphite which had seen a mining boom in the 1910s and 1920s. The graphite is flaky with crystal diameters between 0.5 and 5 mm and contents of graphitic carbon between 6 and 15 %. Individual seams are up to 12 m wide and can be tracked for several kilometers.

We interpret the Ambatolampy Group as a mainly siliciclastic fill of a continental rift basin during a phase of crustal extension occurring contemporaneously with the intrusion of the Imorona-Itsindro Suite. The gold mineralization is most likely related to fluvial deposits from surrounding gold-bearing Archean basement.

 

References

Archibald, D.B. et al. 2015. Tectonophysics 662, pp. 167-182.

Archibald, D.B. et al. 2016. Precambr. Res. 281, pp. 312–337.

Tucker, R.D. et al. 2014. J. African Earth Sci. 94, pp. 9-30.