Quantification of hydrothermal alteration in Co-Ni-rich mineralized structures along fault segments crossing the Aït Ahmane and Ousdrat plutons, Bou Azzer El-Graara inlier, Central Anti-Atlas: Implications for genesis and mineral exploration

Abstract

Within the Bou Azzer El Graara inlier, Morocco, the analysis of hydrothermal alteration along the N120 and N040 composite faults hosting CoNi mineralization is of major importance in evaluation of petrogenesis and exploration potential. This research aims to address this through the use remote sensing data followed up with fieldwork linked with detailed sampling with follow-up petrographic and geochemical analyses. The combination of these approaches reveals a strong relationship between brittle deformation and hydrothermal alteration and mineralization, with zonation from core to fault margin; these variably mineralized faults cross-cut the syn- to late-collisional Cryogenian Aït Ahmane and Ousdrat plutons. The mineralization and alteration is also visible at the nodes between the N120 and N040 faults. The Chlorite-Carbonate-Pyrite Index (CCPI) is used to generally geochemically characterize the alteration associated with the CoNi mineralization in Bou Azzer. The moderately low CCPI and high Alteration Index (AI) support the observations that sericite, propylitic, and chlorite alteration types are spatially associated with these mineralized veins. As a mineral exploration tool, we have adopted appropriate methods by calculating the Molar Element Ratios (MER) (precisely: PER and GER diagrams) and mass changes. These MER diagrams show that the samples from faulted zones in two quartz diorite plutons are the traps of K-Mg-Ca-Si metasomatism linked to gains of these elements with the probable release of others elements, such as Fe and Mg. Closer to the vein core, this metasomatism intensifies and can be correlated with some base and strategic metals (Co-Ni-As-Cu-Pb-Zn-Ag) forming an ore assemblage with rammelsbergite, skutterudite, nickeline, cobaltite, loellingite, and other sulfide species, etc. This ore assemblage is probably formed during the cycling of the ductile-brittle transition during epidodic faulting and fluid flow. The examination of the distribution and abundance of propylitic, chloritic, sericitic, and carbonate alteration and the analysis of mass change exhibit a gain of K, Ca, and Mg and significant loss of Si and Fe linked to those alteration types; these alteration features are regionally extensive, possibly considered as a geochemical 6. In addition, the intermediate plutons that host alteration-mineralization have changed from Mg-Ca-Fe to K-Mg-Si alteration assemblages; these are linked to the cogenetic mineralization episode (along the NE-veins), and triggered by the reactivation of NW-SE contact zones, with remobilization, and the generation of NE-SW faults. Following these findings, we propose that these faults, which were generated during the late Pan-African and the early Cadomian orogenies, respectively, are a good structural and geochemical metallotect. As such, they deserve in-depth study within a framework of regional exploration in the Bou-Azzer El Graara inlier and throughout the Anti Atlas.