Comparison of Modern Zn-Ba-Pb Ore Deposits at Convergent Plate Margins and Fe-Cu-Zn Deposits at Divergent Plate Margins

At divergent plate margins, black smoke forms immediately on contact of ascending hydrothermal solutions with sea water. The black smoke, consisting mainly of black ore (BO) and barite ore (BaO), is rapidly dispersed in seawater leaving behind a dominantly yellow ore (YO). At convergent plate margins, on the other hand, zinc sulfides and associated chalcophilic elements start depositing within the hydothermal mound at mesothermal temperatures, and are largely trapped there to form a mixed yellow, black and barite ore (YO-BO-BaO). The abundance of individual chalcophilic elements within the mixed ore varies by more than four orders of magnitude, with Zn-Pb-Ba-Cu-As-Sb-Cd being the elements most enriched in these deposits. The differing patterns of element deposition in these two tectonic settings is related to the temperature of the hydrothermal fluids, which is a function of water depth and therefore of tectonic setting (mid-ocean ridge vs volcanic arc). In highly sedimented basins, such as the JADE hydrothermal field in the Okinawa Trough, Pb and other chalcophilic elements are leached from the sediment by slowly migrating hydrothermal fluids to form a mixed black-yellow-barite ore (BO-YO-BaO). This type of deposit is the most enriched in Zn-Pb-Ba-As-Sb-Ag in decreasing order of element abundance. These results indicate that black ore (sphalerite plus galena) is most enriched in sedimented-hosted deposits such as the JADE deposit, is moderately enriched in submarine hydrothermal mineral deposits from convergent plate margins and is poorly enriched in submarine hydrothermal mineral deposits from divergent plate margins.