Hydrothermal alterations of the Bou Azzer East Co-Ni-As-Au deposits (central Anti-Atlas, Morocco)

Abstract

The polymetallic Co, Ni, Cu, Zn, Au and Ag deposits of Bou Azzer East is linked to Bou Azzer ophiolite located in the Central Anti Atlas. The ultramafic rocks are strongly deformed and altered by serpentinization, carbonatization and silicification. The mineralization is hosted by quartz, quartz-carbonate and/or chlorite veins, intimately associated with the serpentinite-quartz diorite contact. The description of alteration phenomena as mineralizing events, especially in Co-Ni arsenides and precious metals Au-Ag is realized. The petro-mineralogical and geochemical studies of the mineralized structures and their surrounding host rocks reveal several types of alterations that developed a lateral zoning at the serpentinite level, with formations of silica-, talc-, carbonate-, and chlorite-rich serpentinites. Nearly all primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. The presence of antigorite dominated serpentinites indicates that the Bou Azzer ultramafic sequence was hydrated at relatively high temperature between 200 and 350 °C. In agreement with fluid inclusions study and the estimated temperatures from the chemical composition of chlorites (220–320 °C), which showed an evolution from ferrous chlorites (pychnochlorite and ripidolite) to magnesian chlorites (penninite and clinochlore). The altered zone is characterized by high metal contents of As, Co, Mo, Ag and Au in the silica-rich serpentinite and Cr, Cu, Zn and Pb in the carbonate-rich serpentinite. The enrichment in these elements result from the action of brines with a high Si content and fluids rich in Ca, Mg and CO2 on serpentinized peridotite, respectively. The chloritite breccia developed mainly in shear zones also shows elevated levels in As, Co, Mo, Ag and Au, buffered by the interaction between the fluid and the surrounding rocks.