The coticule rocks (spessartine quartzites) of the Venn-Stavelot Massif,Ardennes, a volcanoclastic Metasediment?

Thin spessartine-quartzite layers (coticules) are interstratified with Ordovician (Salmian) shales of the Venn-Stavelot Massif, Ardennes. These coticules indicate sudden interruptions in the sedimentation process of the shales. The lower contact of the coticules represents an abrupt change in the chemical composition from the underlying shales. In contrast, the upper limit of the coticules is chemically more diffuse. Phase relations of the phengitebearing spessartine-quartzites including paragonite, chlorite, and chloritoid or kaolinite as subordinate phases give evidence for a manganese-montmorillonitic source material of the coticules. This is in good agreement with the internal structures observed in the coticule layers (swelling and sliding effects, Liesegang structures). Since there is a positive correlation between the oxidation ratio of the enclosing shales and the chemical composition of the coticules, it is proposed that the source material of the coticules developed in situ by halmyrolysis out of tuffs. High oxidation ratios of the shales with iron fixed in the trivalent state but with divalent and thus mobile manganese led to the formation of coticule starting material, dominantly a manganese-montmorillonite. Halmyrolysis products formed under low oxidation ratio conditions of the enclosing shales are characterized by hydrosilicates rich in iron. The Obrochishte manganese deposit, Bulgaria, is discussed as a nonmetamorphic equivalent of the coticules of the Ardennes.     

A hydrogeochemical survey of Kilimanjaro (Tanzania): implications for water sources and ages

Kilimanjaro, Tanzania, the highest mountain in Africa, has undergone extensive hydrologic changes over the past century in an area where water resources are critical. A hydrochemical and isotopic synoptic sampling program in January 2006 is used to characterize hydrogeology, hydrology, and water quality of the area. Samples were collected from the summit and southern side of Kilimanjaro and the Moshi region (Tanzania). Sample sources included four glaciers, seven groundwater wells, 12 rivers, 10 springs, precipitation, and a lake. Analyses included major ion chemistry, stable isotopes of water (¹⁸O and D); in addition, seven samples were analyzed for tritium. The samples generally have good water quality with the exception of three samples with elevated fluoride concentrations (>3 mg/L) and elevated nitrate concentrations (>2.5 mg/L NO₃ as N). There is a strong elevation control on stable isotopes, with an apparent elevation effect of – 0.1 ‰ δ¹⁸O per 100 m rise in elevation (R ² = 0.79). The results, including the tritium values, show that the hydrogeologic system is comprised of both local and regional flow systems, and that regional rivers are receiving significant inflow from shallow groundwater, and at very high elevations the hydrologic system is derived from groundwater, precipitation, and glacial melt water.