A thermodynamic reconstruction of the supergene alteration at the Afton copper mine, south-central British Columbia, demonstrates that the dominance of native copper and the lack of copper enrichment in the supergene zone are related to the relatively mafic composition of the wall rocks and the absence of abundant hypogene sulfides. Supergene alteration of porphyry copper deposits in general can be interpreted in terms of a plot. Two arbitary "limiting" curves characterized by extreme values of acidic and basic pH can be drawn. A reacting fluid evolving near the acidic limiting curve will produce a well developed leached cap with an enriched supergene ore blanket underneath. In contrast, a reacting fluid evolving near the basic limiting curve will lead to a spectrum of copper oxides and native copper with little enrichment, as typified by Afton.