Various distinctive nickel-rich minerals are commonly encountered in barren and weakly mineralized ultramafic rocks. The minerals include awaruite, heazlewoodite, pentlandite, millerite, and violarite but few, if any, iron sulfides. Using the Dumont ultramafic body in northern Quebec as an example, it is demonstrated that certain assemblages of these opaque minerals tend to be spatially associated with particular types of alteration, namely, serpentinization and talc-carbonate alteration. An analysis of nickeliferous opaque mineral assemblages on the one hand and alteration reactions on the other leads to a general model in which the former are controlled by the latter through Fe-related redox mechanisms. The model predicts that incipient serpentinization which generates H 2 should be accompanied by reduced assemblages characterized by low-sulfur minerals such as awaruite and heazlewoodite, while carbonate alteration which generates O 2 should be accompanied by oxidized assemblages characterized by high-sulfur minerals such as millerite. The general applicability of the model was tested by compiling bulk mineral assemblages from many locales, as reported by numerous investigators; the results showed a gratifying confirmation of the model.Extension of the model to consider higher sulfur contents was explored in order to explain the common assemblages found in conventional nickel deposits. These sulfide-rich occurrences, in contrast to the barren or weakly mineralized ultramafic rocks, consist mainly of iron sulfides (pyrrhotite + or - pyrite) in addition to pentlandite, and lack awaruite and heazlewoodite. The difference in assemblages can be explained by the model. Because of greater sulfide content and hence higher sulfur, (1) the sulfide assemblages are self-buffered with respect to oxygen and sulfur, therefore remaining largely unaffected by alteration reactions; and (2) the assemblages are required by compatabilities at high S:Ni ratios to include iron sulfides.An important consequence of the model is that serpentinization, by converting some silicate nickel into a sulfide form, may be economically significant in the case of low-grade deposits.