A study of a suite of 250 polished surfaces of ores from the Mount Isa mine, Queensland, Australia, reveals, at high magnifications, unusual and interesting replacement structures. The ore minerals are remarkably stratified in sheared and altered slate of pre-Cambrian age. The common minerals are pyrite, sphalerite, galena, pyrrhotite, arsenopyrite, marcasite, quartz, and carbonate in various combinations. Several uncommon minerals are present. Five varieties of pyrite are recognized, of which three were deposited before crushing and shearing of the host rock; the differences are probably due to varying proportions of FeS. Marcasite is derived from pyrrhotite. Oxidation and supergene enrichment processes and products are discussed. The mineralization sequences are: (I) early deposition of pyrite in the slate; (2) crushing and faulting of the pyritic slates and deposition of sulphides of iron, zinc, copper, and lead; (3) oxidation and supergene sulphide enrichment yielding chalcocite, covellite, anglesite, cerussite, jasper, goethite, lepidocrocite. A syngenetic origin for the ore is excluded and an origin by metasomatic replacement is proposed.