Zeolitic hydrothermal metamorphism in laterites of western Senegal

Abstract

In western Senegal, Tertiary “phosphatic lateroids” of sedimentary origin and volcanics belonging to oceanic alkaline basalts experienced a hydrothermal metamorphism related to later volcanic activities. This hydrothermal event is mainly characterized by a natrolite + chlorite + calcite paragenesis in both types of rocks. The natrolites in the sedimentary phosphatic lateroids have an average formula, calculated on the basis of 10 oxygens, as following: (Na 1.078Ca 0.050) (Al 2.055Si 3.161) O 10 Those of the volcanic bodies contain also K, and have different NaCa substitutions. The 37 Sr 36 Sr ratios of the zeolites and calcite, as well as those of the different host-rocks support the influence of a late hydrothermal event with values, between 0.70310 ± 0.00030 and 0.70479 ± 0.00023, characteristic of a mantle origin. Therefore, the Cenozoic sedimentary rocks of western Senegal have undergone successively two types of transformations: (1) a supergene lateritic weathering which induced a loss of alkali and alkaline-earth elements and produced the phosphatic lateroids; and (2) a hypogene hydrothermal alteration which again enriched the rocks in the alkali elements and produced a real chemical retromorphosis.