WEATHERING OF BASALTIC ROCKS FROM THE FRENCH MASSIF CENTRAL: ORIGIN AND FATE OF Ni, Cr, Zn AND Cu

Abstract

We have examined the behavior of Cr, Ni, Cu and Zn during the weathering of basaltic rocks in a moderate temperate climate. Samples of unweathered basaltic rocks, moderately weathered and strongly weathered equivalents were studied at six sites from the Massif Central of France. Unweathered basaltic rocks and basanites are rich in Cr (248–455 mg/kg) and Ni (126–237 mg/kg); all four elements are present in similar amounts in all three categories of sample. The location of these trace elements was investigated by optical and electron microscopy (EMPA, SEM–EDS). In unweathered basaltic rocks, Cr bearing phases are: chromian spinel (4–42 wt%), Fe–Ti spinel (0.12–1.24 wt%) and titanian augite (200–8000 mg/kg), whereas nickel is incorporated into forsterite (830–3600 mg/kg) and the spinel-group minerals (0.11–0.14 wt%). Copper and Zn are found at lower concentrations, but regularly distributed in the phases studied. In weathered samples, authigenic phases fi ll fractures and also replace the primary minerals. They are composed of mixtures of clays, iron oxides and organic compounds. Authigenic phases contain lower amounts of those trace elements than primary minerals. Under conditions of moderate weathering, only 20 to 40% of the total content in Cr, Ni, Cu and Zn appears to be mobile during the weathering of the basaltic rocks. As spinel-group minerals are not affected by strong weathering under a temperate climate, only the fraction of Cr, Ni and Zn present in the silicates of the unweathered basaltic rocks is released.