WEATHERING-RELATED METALS MOBILITY: SUPERGENE PROCESSES AND COPPER ENRICHMENT SINCE LATE PALEOZOIC TIME

The weathering of sulfide-bearing mineral deposits has produced economically important ore deposits since at least late Proterozoic time, although metals mobility from probable non-sulfide sources has engendered enhanced metals accumulations since at least early Proterozoic time (e.g., Garlick, 1961; Meyer, 1980). Nonetheless, preservation of supergene copper deposits of economic significance is represented from early Paleozoic time, with most ore deposits of Cretaceous age or younger. Albeit any sulfidic copper occurrence may serve as a protore metals source for weathering solutions, a number of geologic, geochemical, (paleo-)climatic, and local tectonic factors influence strongly the probability of supergene copper ore deposit genesis and preservation. Because geochemically significant copper mobility is restricted to weathering solution pH values of less than about 5 (unless halogens such as chloride ion are present; Rose, 1967), source rocks for copper are characterized by the presence of necessary pyrite in addition to copper sulfides. In general terms, source region copper and reduced sulfur contents, alteration mineralogy, and geochemical reactivity determine the nature of weathering solution compositions and the capacity of such solutions to transport metals away from source rocks; sink region geochemical reactivity, permeability, tectonic-climatic history determine the amount of metals that may be accumulated and preserved. Consideration of regional and global climate changes, resulting in favorable weathering profile development and consequent exhumation of sulfidic copper protoliths, shows that many supergene copper ore deposits were engendered during the late Mesozoic (Cretaceous age examples of southern Mongolia) and during numerous, well-defined (e.g., in western South America; see Mote and Brimhall, 2001) to poorly-defined (Sonora and southwestern U.S.; e.g., Cook, S., pers. comm..; Enders, 2000) intervals during middle to late Tertiary time. Geochemically and economically important supergene copper enrichment is found in active tectonic regions as young as late Miocene (e.g., Sarchesmeh and Meduk, southeastern Iran) to Pliocene (central Chile) time.