Geochemical Tracers of Processes Affecting the Formation of Seafloor Hydrothermal Fluids and Deposits in the Manus Back-Arc Basin

Paul Craddock, Ph.D., 2009
Margaret Tivey and Wolfgang Bach, Advisors

Systematic differences in trace element compositions (rare earth elements (REE), heavy metals, metalloids) of vent fluids and deposits from Manus back–arc basin hydrothermal systems are used to investigate processes that affect their formation. Processes responsible for observed differences include (a) fluid–rock interaction, (b) magmatic acid volatile input and, (c) local seawater entrainment and mixing with hydrothermal fluids, sulfide precipitation and metal remobilization. REEs in vent fluids and deposits in the Manus Basin exhibit a range of patterns that contrast with relatively uniform patterns observed in mid-ocean ridge hydrothermal systems. The data indicate that REEs in fluids and deposits can be used as indicators of styles of magmatic volatile input in back-arc hydrothermal systems. Vent fluid heavy metal and metalloid concentrations also exhibit considerable differences, related to differences in pH owing largely to variable input of magmatic volatiles. Metal concentrations may be locally affected by dissolution of previously deposited sulfide. Sulfide deposits in the Manus Basin exhibit a wide range of mineral compositions. The formation of Zn- versus Cu-rich deposits reflects differences in metal concentrations and conditions of sulfide precipitation. Metal and metalloid distributions in massive sulfide deposits can be used as indicators of these conditions.