Alteration Processes of Paleoproterozoic Manganese Protores of the North Téra:Mineralogy and Geochemistry(West Africa)

摘要

The manganese deposit of North Téra is located in the western side of the Diagorou-Darbani greenstone belt in the Niger Liptako Province(Northeastern part of Man Schield,West African Craton).Manganese mineralization bearing rocks are mainly gondites(a kind of garnet-rich quartzite of the spessartine type)whose borders consist of a high concentration of Mn oxide concretions.The methodological approach implemented consisted of a field study followed by a polarizing microscopic analysis in transmitted and reflected light and a geochimical analysis.North Téra gondites originate from Birimian manganese-rich sediments metamorphism in amphibolite facies.The supergene alteration,more or less important,has developed on the protore of gondites,a saprolitic profile whose setting up conditions were recently described in our previous work.The macroscopic analysis of the manganese deposits samples combined with the metallographic microscopic and geochemical analysis confirms that manganese mineralization originated from weathering processes.The first oxide that forms around the garnets is nsutite,while the mesostasis is invaded by pyrolusite with lenses of lithiophorite destabilizing in favor of pyrolusite.In the botryoidal concretions,the latter forms an alternation with the psilomelane rich in Ba(8,101ppm)in the uppermost levels.The abundance diagram shows that the normalize Rare Earths compared to PAAS derived from a plagioclase-rich quartzitic source.This observation is corroborated by a positive anomaly in Eu.Otherwise,the high levels of transition elements(Ni,V,Co and Cr)highlight the involvement of a basic source.The contents of major elements show a significant leaching,with an increase in those of Mn and Al during the increasing weathering.The CIA and PIA indices move in the same direction towards high values leading to the Al2O3 pole on the Al2O3-CaO*+Na2O-K2O diagram.This indicates an increasing alteration along the profile,according to an alteration path comprising plagioclase-smectite-kaolinite and illite to kaolinite.