Garnierite-bearing serpentinite from the Central Eastern Desert of Egypt: A signature of paleo-weathering in the Arabian Nubian Shield?

Abstract

This paper records the garnierite in Wadi Dubur Neoproterozoic serpentinites from the Central Eastern Desert of Egypt. Garnierite (Ni-Mg hydrous silicates) occurs in two types (Type I and Type II) of infillings in the weathered serpentinite that was displaced sinisterly by NNW-SSE trending strike-slip faulting and dextrally with NE-trending shearing. All recognized garnierites are serpentine-like (népouite/pecoraite) existing in mixtures with the hosting serpentines. The Type I garnierite is Fe-richer [Mg0.26-0.71 Ni1.22-1.79 Fe1.28-3.05 Al 0.0-0.02 Si1.11-1.73 O5(OH)4] showing zoning in a collomorph texture, while the Type II garnierite is homogenous with more Si and Mg [Mg0.54-0.94 Ni1.38-2.04 Fe0.38-1.29 Al 0.0-0.01 Si1.66-2.10 O5(OH)4]. Raman spectra indicate that the substitution of Ni for Fe formed a poorly crystalline goethite. The serpentinization of the Dubur peridotite protolith and the associated alteration of chromite and Ni-sulfides were important for the early individualization of Ni. It is proposed that the unweathered serpentinites represent the bedrock, while the altered garnierite-bearing ones represent the saprock. The early weathering of serpentinites contemporaneous with a faulting activation caused in situ precipitation of Ni and Fe in veins (Type I garnierite). With the faulting repetition and progressive alteration maintenance Ni, Si and Mg were transferred to the percolating meteoric solutions, and moved downward through the weathering profile precipitating the Type II garnierite in gouges. Although the studied Ni-rich rocks represent a narrow mineralogical occurrence, they are remnants of a former lateritic regolith most probably formed by tropical or sub-tropical weathering during Cretaceous at the expense of Wadi Dubur Neoproterozoic serpentinites. The formed regolith has been subsequently largely eroded, buried and recently exhumed in arid climate.