Magnetic anomaly and lithogeochemical investigations of Cr–Ni mineralization related to the mafic-ultramafic rocks of Kettara sill, Variscan central Jebilet, Morocco

Abstract

The hercynian Kettara sill located at the central part of the central Jebilet is known for its differentiated mafic-ultramafic rocks. In the southwest part of this massif, there is a magnetic anomaly associated with the mafic-ultramafic rocks. Susceptibility measurements indicate that these rocks are magnetic (5 to 40.1 10−3 SI). These hercynian mafic-ultramafic rocks constitute a tholeiitic series that includes voluminous plagioclase wehrlite, troctolite, olivine gabbro and leucogabbro, and thin veins of dolerite and trondhjemite. Petrographical study of these mafic-ultramafic bodies shows that the olivine, plagioclase, and clinopyroxene are commonly altered or metamorphosed to serpentine, sericite, Mg-chlorite, epidote, tremolite, and magnetite. Minor amounts of pyrrhotite, chalcopyrite, and pentlandite are present. The magmatic minerals and alteration-derived phases represent the mineralogy of Cr–Ni mineralization. Furthermore, results from inductively coupled mass spectrometry analyses show a high concentration of Cr (up to 3900 ppm) and Ni (up to 1400 ppm). The compilation of the magnetic, mineralogical, and geochemical data and their interpretation allows us to advance that (i) the magnetite, ilmenite, Cr spinel, pyrrhotite, and high content of nickel and chromium could explain the magnetic anomaly associated with the mafic-ultramafic rocks and (ii) the existence of pentlandite and Cr spinel is a strong argument that allows us to suggest that the mafic-ultramafic Kettara sill represent the host rock body of the Cr–Ni mineralization. The results of this study shed a light on the origin and the importance of some magnetic anomalies that were usually neglected by previous geophysical investigations since they are associated with the outcrops of mafic or ultramafic rocks. In that regard, this study has important implication in mineral exploration in the central Jebilet massif.