Episodic Subseafloor Hydrothermal Activity Within the Eastern Manus Back-Arc Basin Determined by Uranium-Series Disequilibrium in Barite

摘要

The geochronology of seafloor hydrothermal activity from several mineralized, active and inactive vent fields at Suzette and Pual Ridge, eastern Manus back-arc basin, Bismarck Sea, Papua New Guinea, has been established using the Th-228/Ra-228, Ra-228/Ra-226, and Ra-226/Ba uranium-series disequilibrium methods for barite-rich samples. These hydrothermal systems underwent episodes of hydrothermal fluid circulation that produced barite over the last similar to 6,000 years. Precipitation of hydrothermal barite, disseminated within the host rock and lining fractures, vugs, and conduits, was controlled by decreasing temperatures and mixing of hydrothermal fluids with oxidized seawater. Repeated fluctuations in hydrothermal fluid temperature and chemistry resulted in complex sequences of barite and sulfide minerals lining vugs and conduits, whereas rapid cooling following fracture events led to precipitation of only barite and some Fe oxide coatings on barite crystals. A wide range in ages of barite within drill cores (i.e., 380-5,990 yrs before present (BP)) reflect repeated pulses of hydrothermal fluids through the altered volcanic and mineralized rocks that make up the hydrothermal mound structures, with the younger ages (i.e., 34-220 yrs BP), representing more recent barite-precipitating fluid circulation accessing fractures, veins, and vugs. Barite from chimney samples is mostly young (i.e., 23-1,680 yrs BP), with the majority precipitated between 23 to 41 yrs BP; reflecting the structural stability (standing life) of the chimneys and possible influences from varying hydrothermal fluid chemistry or reactivation of vent fields. Three generalized age groupings, representing major episodes of barite-precipitating hydrothermal fluid circulation, are resolved from the Suzette area (19-45, 810-1,900, and 3,740-4,710 yrs BP, including 1 sigma errors); with the most recent ages recorded from both the Suzette and Pual Ridge areas. Fast-spreading rates within the Manus basin, an abundance of faults and variable magmatic inputs are major factors controlling the frequency and longevity of these episodes of hydrothermal activity. Mineral resource models and comparisons indicate that the Cu-Zn-Ag-Au-rich Suzette deposit is small; however the measured age range is comparable to ages given for modeled (typical) seafloor massive sulfide deposits from back-arc and arc environments. We suggest that the scale of the Suzette deposit may have been limited during formation by a low hydrothermal fluid flux and/or inappropriate physical and chemical conditions for sufficient metal precipitation from hydrothermal fluids. Further exploration will determine the full extent of the resource at depth.