Precipitation of Fe III oxyhydroxide deposits from shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea

Ž . Previous research on sea floor Fe III oxyhydroxide deposits has focused primarily on deep-sea, hydrothermal systems found along volcanically active portions of the mid-ocean ridges and on hydrogenetic deposits formed in deep basins and along continental shelves. There is, however, not much known about their formation in shallow-water settings associated with volcanic islands. The hydrothermal system at Ambitle Island, Papua New Guinea provides an excellent opportunity to Ž . study the formation of Fe III oxyhydroxides in a shallow-water setting. Precipitation from the hydrothermal solution is caused by mixing with seawater. Based on a SrrSr mixing model, the calculated minimum and maximum seawater Ž . fractions are approximately 11 and 57%, respectively. Thus, precipitation of Tutum Bay Fe III oxyhydroxides takes place at Ž . a temperature range between approximately 60 and 938C. The chemical composition shows low Mn contents FerMn)600 , Ž . and elements that are usually enriched in Fe III oxyhydroxides, such as Co and V are below crustal abundance and well below their concentrations in island-arc volcanics. Arsenic concentrations, on the other hand, are by two orders of magnitude Ž . higher than those in other marine deposits. Rare earth element REE concentrations reflect their concentration in the hydrothermal fluids rather than seawater. The crystallinity of the deposits increases with age, as protoferrihydrite is apparently altered to Fe-smectite and hematite, and As-bearing minerals are formed. Contact with seawater, and therefore oxidizing conditions, seems to be the factor increasing the crystallinity. q 1999 Elsevier Science B.V. All rights reserved. Ž .