Le gisement manganifère latéritique de Biniomi (Gabon)

International audienceLe gisement de manganèse (Mn) de Biniomi (Gabon) est situé au sein de plateaux latéritiques, à 75 km à l'est-sud-est du gisement de Moanda (qui à lui seul représente 25 % des réserves mondiales de minerai de Mn). Si le site de Moanda a été bien étudié, celui de Biniomi ne compte actuellement que peu de travaux visant à le caractériser. Il est localisé dans le Bassin de Franceville, lequel se compose de séries sédimentaires paléoprotérozoïques non métamorphisées, déposées en discordance sur un socle cristallin Archéen [1]. Les séries carbonatées enrichies en manganèse, intercalées dans des black shales [2], vont constituer le protore à l'origine des concentrations manganésifères secondaires aujourd'hui exploitées. Ces enrichissements sont induits par le haut potentiel d'oxydo-réduction du manganèse, qui se retrouve ainsi concentré dans les formations sédimentaires dès la phase d'oxygénation de la Terre [3]. Le Mn, essentiellement précipité sous forme de carbonates et de silicates, est repris par l'altération supergène pour former des oxydes et oxyhydroxydes de Mn (III/IV) [4][5]

Les isotopes de l'oxygène, un traceur des origines gèologique et/ou gèographique des gemmes

The geological study of gem deposits of high economic value from current or former mines allows fot both the proposition of a genetic model and the development of a mineralogical and isotope database used for the authentification of gems. The geological particularities of Colombian emeralds, rubies from central and southern Asia, and tsavorites and tansanites from East Africa explain their 18O/16O isotopic ratios and their mineralogy used for their geological and or geographical origins

¹⁸O/¹⁶O and V/Cr ratios in gem tsavorites from the Neoproterozoic Mozambique metamorphic belt: a clue towards their origins?

The combination of oxygen isotope composition with V–Cr–Mn trace element concentrations of V-bearing garnets (tsavorites) originating from the main deposits of the Neoproterozoic Mozambique Metamorphic Belt is reported for the first time. The database enables the identification of the geological and geographical sources of the main productive areas from northern and southern Tanzania, Kenya, and Madagascar. Three consistent sets of δ<sup>18</sup>O values between 9.5‰ and 11.0‰, 11.6‰ and 14.5‰, and 15.5‰ and 21.1‰ have been recognized for primary deposits hosted in graphitic gneisses related to the Neoproterozic metasedimentary series. The δ<sup>18</sup>O value of tsavorite is a good tracer of the environment of its formation; the δ<sup>18</sup>O of the fluid in equilibrium with tsavorite was buffered by the host rock during metamorphism and fluid-rock interaction. This study is a first step in characterizing the geochemistry of gem tsavorite from most of the deposits and occurrences worldwide

The role of evaporites in the formation of gems during metamorphism of carbonate platforms: a review

The mineral and fluid inclusions trapped by gemstones during the metamorphism of carbonate platform successions are precious markers for the understanding of gem genesis. The nature and chemical composition of inclusions highlight the major contribution of evaporites through dissolution or fusion, depending on the temperature of formation from greenschist to granulite facies. The fluids are highly saline NaCl-brines circulating either in an open system in the greenschist facies (Colombian and Afghan emeralds) and with huge fluid-rock metasomatic interactions, or sulphurous fluids (ruby, garnet tsavorite, zoisite tanzanite and lapis-lazuli) or molten salts formed in a closed system with a low fluid mobility (ruby in marble) in the conditions of the amphibolite to granulite facies. These chloride-fluoride-sulphate ± carbonate-rich fluids scavenged the metals essential for gem formation. At high temperature, the anions SO42−, NO3−, BO3− and F− are powerful fluxes which lower the temperature of chloride- and fluoride-rich ionic liquids. They provided transport over a very short distance of aluminium and/or silica and transition metals which are necessary for gem growth. In summary, the genetic models proposed for these high-value and ornamental gems underline the importance of the metamorphism of evaporites formed on continental carbonate shelves and emphasise the chemical power accompanying metamorphism at moderate to high temperatures of evaporite-rich and organic matter-rich protoliths to form gem minerals

Evidence of evaporites in the genesis of the vanadian grossular 'tsavorite' deposit in Namalulu, Tanzania

The Namalulu tsavorite (vanadian grossular up to 0.80 wt.% V2O3) deposit in northeastern Tanzania occurs in metasomatized graphitic gneiss overlain by a dolomitic marble unit. These rocks belong to the Neoproterozoic metamorphic Mozambique Belt. The graphitic gneiss is composed of quartz, V-bearing kyanite (up to 0.4 wt.% V2O3), V-bearing muscovite (up to 1.0 wt.% V2O3), V-bearing rutile (up to 1.9 wt.% V2O3), and graphite. This rock is affected by calcic metasomatism precipitating calcite and mobilizing the elements for tsavorite crystallization in quartz-calcite veins. The dolomitic marble unit hosts an anhydrite-gypsum-dolomite lens (lens I) and a calcite-scapolite-diopside-sulfides-graphite lens (lens II). This last unit is characterized by the presence of F-bearing minerals (tremolite, phlogopite, tainiolite, titanite), with up to 9.4 wt.% F for tainiolite, and Ba-bearing minerals (feldspar, phlogopite), with up to 7.5 wt.% BaO for feldspar. Lithium (up to 2.0 wt.% Li2O) and boron (up to 110 ppm) are also present in tainiolite, as well as in F-bearing tremolite. This is the first description of metamorphic tainiolite. The protoliths of the Namalulu rocks correspond to organic-rich black shales, pure and magnesian carbonates, and evaporites. They were presumably deposited in a marine coastal sabkha located at the eastern margin of the Congo-Kalahari cratons bounded by the Mozambique Ocean during early Neoproterozoic. Tsavorite formed in the Ca-metasomatized gneiss during amphibolite facies retrograde metamorphism at 5.6-6.7 kbar and 630 ± 30 °C

Lithostratigraphic and structural controls of 'tsavorite' deposits at Lemshuku, Merelani area, Tanzania

The first study of the Lemshuku 'tsavorite' mining district is presented. From bottom to top, the lithostratigraphic column corresponds to a metasedimentary sequence composed of quartzite, fine-grained graphitic gneiss, kyanite-graphite gneiss, biotite-almandine gneiss, metasomatized graphitic gneiss and dolomitic marble. 'Tsavorite' occurs in quartz veins and rarely as nodular concretions. Two factors control mineralization: (1) lithostratigraphy, with 'tsavorite' in association with pyrite and graphite confined to quartz veins within the metasomatized graphitic gneiss: and (2) structure, with the mineralized veins characteristically controlled by tight isoclinal folds associated with shearing