OPHIOLITE-HOSTED VOLCANOGENIC MASSIVE-SULFIDE DEPOSITS: A VIEW IN 2003

Volcanogenic massive-sulfide (VMS) deposits, important sources of Cu and Au, are found in approximately 50 of the >200 ophiolite complexes recognized throughout the world. Although interpretation of the lava chemistry of some ophiolites (e.g., Troodos, Cyprus, and Semail, Oman) remains controversial, the data suggest that intercalated VMS deposits formed in nascent oceanic arcs or backarcs (“suprasubduction” settings) comparable to volcanic-arc environments of the modern western Pacific Ocean. VMS deposits in other ophiolite suites (e.g., Ergani, Turkey) are hosted within thick accumulations of clastic sediment penetrated by diabasic intrusions; these deposits may be analogous to massive sulfides on sediment-covered oceanic ridges (e.g., Middle Valley, Escanaba Trough). Ophiolite-hosted VMS deposits typically have root zones extending to the base of the sheeted-dike section where water-rock reactions at temperatures up to 450°C formed epidosites (epidote-quartz rocks) and leached metals and sulfur from the wallrock. Fluid upwelling along fault-controlled pathways in subsea-floor volcanic rocks resulted in zoned alteration "pipes" with chlorite-quartz-pyrite-chalcopyrite-rich cores and broader halos of clay alteration and hematite precipitation. At the Aarja deposit, Oman, the hydrothermal alteration pattern is complicated by thermal effects of subvolcanic “plagiogranite” intrusions. Sulfide-bearing stockwork zones grade upward through intensely-altered basalt into conformable lenses of massive to brecciated, pyritic sulfide containing zoned Cu and Zn mineralization and minor concentrations of Au and Ag. Gold may be concentrated by several orders of magnitude in ferruginous cappings on sulfide mounds. VMS deposits frequently occur at the top of distinct volcanic cycles with siliceous Fe-Mn or sulfide-bearing sedimentary deposits containing both hydrothermal and pelagic components. Features (e.g., high Na/K ratios in vent fluids, zone refining of metals, formation of anhydrite, and distribution of Fe oxyhydroxides) recognized in actively-forming modern analogs such as the TAG sulfide mound on the Mid-Atlantic Ridge have contributed to our understanding of VMS deposits in ophiolites.