The age of supergene manganese deposits in Katanga and its implications for the Neogene evolution of the African Great Lakes Region

Abstract

International audienceSupergene manganese deposits commonly contain K-rich Mn oxides with tunnel structure, such ascryptomelane, which are suitable for radiometric dating using the 39Ar–40Ar method. In Africa, Mn depositshave been dated by this method for localities in western and southern parts of the continent, whereas onlysome preliminary data are available for Central Africa. Herewe present new39Ar–40Ar ages for Mnoxide samplesof the Kisenge deposit, in southwestern Katanga, Democratic Republic of the Congo. The samples represent supergeneMn oxide deposits that formed at the expense of primary Paleoproterozoic rhodochrosite-dominatedcarbonate ores. Main phases of Mn oxide formation are dated at c. 10.5 Ma, 3.6 Ma and 2.6 Ma for a core thatcrosses a mineralized interval. The latter shows a decrease in age with increasing depth, recording downwardpenetration of a weathering front. Surface samples of the Kisenge deposits also record a ≥c.19.2 Ma phase, aswell as c. 15.7 Ma, 14.2 Ma and 13.6 Ma phases. The obtained ages correspond to distinct periods of paleosurfacedevelopment and stability during the Mio-Pliocene in Katanga. Because Katanga is a key area bordered to theNorth by the Congo Basin and to the East by the East African Rift System, these ages also provide constraintsfor the geodynamic evolution of the entire region. For the Mio-Pliocene, the Kisenge deposits record ages thatare not systematically found elsewhere in Africa, although the 10.5–11 Ma event corresponds to a roughly simultaneousevent in the Kalahari Manganese Field, South Africa. The rest of the Katanga paleosurface record differssomewhat fromrecords for other parts of Africa, forwhich older, Eocene ages have been obtained. This differenceis most probably related to the specific regional geodynamic context: uplift of the East African Plateau, with associatederosion, and the opening of the East African Rift System at c. 25 Ma are events whose effects, in thestudy area, interfere with those of processes responsible for the development of continent-wide paleosurfaces

Critical elements in non-sulfide Zn deposits: a reanalysis of the Kabwe Zn-Pb ores (central Zambia)

“…Although a late Eocene-Oligocene age has been produced for the Kabwe descloizite (∼20-37 Ma; N.J. Evans, unpubl., in Boni et al, 2007), by analogy with other supergene deposits occurring in this region (e.g. supergene Cu-Co and manganese deposits in the Katanga region; Dewaele et al, 2006;Decrée et al, 2010;Decrée et al, 2015;De Putter et al, 2015) and, more in general, in the southern African craton (Pack et al, 2000;Boni et al, 2007;Gutzmer et al, 2012;Arfèet al, 2017, Smithsonite 1 smithsonite replacing the host rock −6.2 9.1 21.5 S109 and references therein), the ∼300-500 m deep (Kamona and Friedrich, 2007) supergene alteration profile at Kabwe could have formed during a period longer than the single descloizite age, possibly starting in the Late Cretaceous-early Eocene and extending until the Mio-Pliocene. In this timeframe, a tropical-humid climate persisted in the region and laterite profiles developed in the Katanga region (Giresse, 2005).…”

Supergene manganese ore records 75 Myr-long Campanian to Pleistocene geodynamic evolution and weathering history of the Central African Great Lakes Region – Tectonics drives, climate assists

“…As recalled in detail by De Putter et al (2015), even though cryptomelane (K(Mn 4+ 7 Mn 3+ )O 16 ), which is the K-rich (>5%) end-member of a group of Mn oxides with tunnel structure (coronadite group, hollandite supergroup according to Biagioni et al, 2013), is the best candidate for 40 Ar/ 39 Ar dating, the reality imposed by sampling, the extent of the possible combinations between the various end-members of the coronadite group (K, Na, Pb, Ba, Sr, etc. in tunnel structure) during the development of supergene systems and the possible presence of other frequently observed Mn oxides such as pyrolusite (MnO 2 ) or lithiophorite ((Al,Li)(Mn 4+ ,Mn 3+ ) 2 O 2 (OH) 2 ) leads most often to the analysis of composite materials which cannot be formally designated as cryptomelane.…”

“…Due to the complexity of obtained results, their processing must go beyond a simple visual examination of an age spectrum as often done for most geochronological studies. De Putter et al (2015) presented some visualization and processing tools, three of them quite classical:…”

Cenozoic lateritic weathering and erosion history of Peninsular India from 40Ar/39Ar dating of supergene K–Mn oxides

“…The inverse isochron approach also is particularly useful to detect different contamination from various excess or inherited argon reservoirs, which may result from atmospheric argon incorporated in the less retentive site of the grain, or excess 40 Ar released from older K-bearing minerals. For some authors (De Putter et al, 2015), best inverse isochrons exclude the first heating steps, which are usually dominated by trapped atmospheric Ar in the less retentive crystalline sites. These ages are derived from best-fitted inverse isochrons that should have mean square weighted deviation (MSWD) as close as possible to 1 with regard to the distribution of points and their absolute error, but this value is not a limitation to derive an acceptable inverse isochron.…”

“…When the age spectra present an obviously convex hump shape (Fig. 9a) the youngest and oldest apparent ages may result from a mixing between gases released from two phases (e.g., Ruffet et al, 1996;Hautmann and Lippolt, 2000;Vasconcelos and Conroy, 2003;Beauvais et al, 2008;De Putter et al, 2015). Possible argon loss from less retentive intercrystalline site could also affect these degasing patterns (Fig.…”