Formation of Fe–Si–Mn oxyhydroxides at the PACMANUS hydrothermal field, Eastern Manus Basin: Mineralogical and geochemical evidence

Abstract

Samples of Fe–Si–Mn oxyhydroxides were collected from the PACMANUS hydrothermal field, which lies in a young back-arc setting in the Eastern Manus Basin. The purpose of the study was to understand the origin and characteristics of Fe–Si–Mn oxyhydroxides associated with massive sulfides in a back-arc basin. The PACMANUS Fe–Si–Mn oxyhydroxides are composed of Fe oxyhydroxides and Mn oxyhydroxides with opal-A and nontronite; they have very low concentrations of trace elements (except for Ba, Mo, V and U) and rare earth elements, and they show REE distribution patterns with positive Eu anomalies and slight enrichments of LREEs. The Fe–Si–Mn oxyhydroxides appear to be precipitated mainly from hydrothermal fluid with limited seawater contamination, and scavenged trace metals are predominantly from the ambient seawater. The differences in the REE distribution patterns between the Fe-oxyhydroxide fraction and Mn-oxyhydroxide fraction originate from diagenetic processes. There are diverse filamentous microtextures resembling unique microbial populations, suggesting microbially-mediated mineralization during the precipitation of the Fe–Si–Mn oxyhydroxides.A possible genetic scenario for the formation of Fe–Si–Mn oxyhydroxides in the PACMANUS hydrothermal field is proposed: (1) precipitation of silica by the mixing of hydrothermal fluid with seawater at a diffuse vent, promoted by Fe-oxidizing bacteria and microbial mineralization; (2) rapid precipitation of Fe-oxyhydroxide from the hydrothermal fluid due to Fe2+ oxidation; (3) growth of Mn-oxyhydroxide partially encasing Fe-oxyhydroxide. Microbes act through the whole scenario. The Fe–Si–Mn oxyhydroxides have undergone changes as a consequence of fluctuating hydrothermal conditions and subsequent diagenetic degradation.