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

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 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 87Sr/86Sr mixing model, the calculated minimum and maximum seawater fractions are approximately 11 and 57%, respectively. Thus, precipitation of Tutum Bay Fe(III) oxyhydroxides takes place at a temperature range between approximately 60 and 93°C. The chemical composition shows low Mn contents (Fe/Mn>600), 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 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.