The Co-Ni arsenide ore deposits at Bou Azzer, Morocco, represent an uncommon type of Co mineralization. Several Co-Ni-As-Au-Ag mines are active in Bou Azzer and adjacent areas. The Co-Ni arsenide ores are structurally controlled and concentrated mainly within quartz-carbonate veins along the boundaries of Neoproterozoic serpentinized mantle peridotites, quartz diorite, and Precambrian volcanic rocks. They have varying shapes and sizes; however, all deposits have the same general mineralogy in decreasing order of abundance: arsenides, sulfarsenides, and sulfides. Arsenide minerals include skutterudite (CoAs3), safflorite (CoAs2), loellingite (FeAs2), nickeline (NiAs), rammelsbergite (NiAs2), and S-rich Ni-Co diarsenide. Co-rich ores are predominant over Ni-rich ones in almost all deposits. Arsenopyrite and, to a lesser extent, gersdorffite are the main sulfarsenide minerals in almost all samples studied. Bornite, an unidentified Cu-Zn sulfide, chalcocite, chalcopyrite, pyrite, pyrrhotite, sphalerite, and tetrahedrite, in decreasing order of abundance, are the main sulfide minerals present in Bou Azzer ores. Gold-silver alloys, with about 15 wt percent Ag on average, are commonly associated with Ni-Co arsenides and/or disseminated in quartz veins. In the Ni-rich assemblage pure nickeline with high Ni content (~48 wt %) and very low Co content (≤ 0.1 wt %) grades outward to compositions with less Ni (~20 wt %) and very high Co (up to 18 wt %). This suggests a depositional sequence from Ni monoarsenide (nickeline) to Ni-Co diarsenide to cobalt triarsenides (i.e., a progressive increase in Co and As). The ore-forming elements (Co, Ni, Fe, and As) were originally leached from serpentinites by acidic magmatic fluids under moderately reducing conditions at high fluid/rock ratios. Deposition of ore minerals occurred in response to increasing pH and decreasing oxygen fugacity of the mineralizing fluids caused by mixing between the magmatic brines and meteoric water. The predominance of Co over Ni arsenide minerals in the Bou Azzer mineralization despite the high Ni/Co ratio of the serpentinites may be attributable to the different solubilities of Ni and Co in the hydrothermal system.