The Cooley and Ridge deposits at McArthur River are a group of small deposits located immediately to the east of the giant H.Y.C. stratiform Ag-Pb-Zn deposit. Two styles of mineralization occur in the deposits, discordant and concordant.Discordant mineralization resembles closely Mississippi Valley-type mineralization. It occurs in brecciated Cooley and Emmerugga Dolomite in the Cooley I, Cooley II, Cooley III, and Ridge I deposits and in the eastern part of the Ridge II deposit. The sulfide and gangue minerals comprising the discordant deposits are relatively coarse grained, they occur in veins and in disseminated to massive patches in the host breccias, and they were deposited under open-space filling conditions. The sulfide and gangue minerals were deposited sequentially both in time and space. In time, the sequence of mineral deposition was (from early to late) pyrite + marcasite + barite + dolomite --> Cu-bearing sulfide minerals --> galena + sphalerite. In space, the Cu-bearing sulfide minerals occur mostly in the Cooley II deposit next to the Emu fault, with galena and sphalerite becoming progressively more abundant away from the Cooley II deposit. It is not known if mineral deposition overlapped in time and space, or only in space. The characteristics of the discordant deposits provide strong evidence that they formed epigenetically when metal- and sulfate-rich mineralizing solutions, emanating from the Western and/or Emu faults flowed into, and reacted with, the adjacent dolostone breccias.The concordant mineralization comprises fine-grained pyrite, galena, and sphalerite, concentrated in certain horizons of the H.Y.C. Pyritic Shale. It occurs in the western part of the Ridge II deposit and is, in most respects, the same as the mineralization in the nearby H.Y.C. deposit. As in the case of the discordant mineralization, the sulfide minerals in the concordant mineralization were also deposited sequentially. Following deposition of the H.Y.C. Pyritic Shale, laminae and framboids of pyrite grew during early diagenesis. This earlier pyrite (Py 1 ) was subsequently overgrown by later pyrite (Py 2 ) which, in turn, was followed by the deposition of galena and sphalerite. The closeness of the concordant Ridge II mineralization to the discordant mineralization, as well as the similarities between the sulfide-mineral parageneses of the two types of mineralization, suggest that they had a common origin and that the concordant mineralization formed epigenetically when the metal- and sulfate-rich mineralizing solutions that formed the discordant deposits flowed farther westward into the H.Y.C. Pyritic Shale. The model best explaining the concordant mineralization is one in which organic matter in the shale is oxidized and sulfate in solution is concurrently reduced.The resemblance between the H.Y.C. and the concordant Ridge II deposits raises the interesting possibility that the H.Y.C. deposit also formed epigenetically by sulfate-reducing reactions, and such a possibility is not inconsistent with the characteristics of the H.Y.C. deposit.