GEOCHEMICAL SIGNATURE AND GEODYNAMIC SIGNIFICANCE OF AN Ag-Hg MINERALIZED DYKE SWARM IN THE NEOPROTEROZOIC INLIER OF IMITER - ANTI-ATLAS (MOROCCO)

Abstract

The Cryogynian basement of the Imiter Inlier and its volcanic cover are crosscut by an important swarm of mafic and felsic dykes. In the volcanic series, the dykes are oriented N-S to N70°E and emplaced within a E-W tensional stress field, whereas in the basement they are parallel to pre-existing planes (S0//S1) which vary in trend from N70°E to N90°E. Silver mineralization is closely associated with mafic dykes that behaved as mechanical traps, either due to their brittle structural behaviour or by behaving as a barrier to fluid flow. Two petrogenetic groups have been recognised in dyke rocks: the first group consists of basalts and basaltic-andesites, the second one corresponds to andesites, quartz-microdiorites, and keratophyres. Both groups are calc-alkalic and display geochemical features comparable to those of magmatic rocks from modern active continental margins and island arcs: large ion lithophile (LIL) element enrichments, high field strength (HFS) element depletion, and weakly fractionated rare earth element (REE) (La/Yb)N = 3.7 to 6.6) patterns. Constraints of the regional geology suggest that these rocks were associated with post-orogenic, post-Panafrican rifting and preclude any subduction activity at that time. Their nature and their chemical signature can be related to: (i) the existence of fragments of an older subducted crust, (ii) partial melting of a mantle contaminated during the orogenic Panafrican stages or (iii) contamination during ascent of mantle derived magmas by African crust.