Gold in Ferromanganese Deposits from the NW Pacific

Ferromanganese crusts from four different areas of the North-West Pacific Ocean—the Detroit (northern part of the Imperial Ridge) guyot, the Zubov (Marshall Islands) guyot, the “Gummi Bear” seamount (an intraplate volcano near the Krusenstern FZ), and Belyaevsky volcano (the Sea of Japan)—were studied. Samples from the Detroit and Zubov guyots and the “Gummi Bear” seamount have similar chemical and mineral compositions of hydrogenetic cobalt-rich ferromanganese crusts. Crust from the Sea of Japan seems to reflect a hydrothermal influence. The gold content in most samples from the Detroit guyot was 68 ppb and from the Zubov guyot varied from 180 to 1390 ppb, which is higher than the average for the Pacific crusts (55 ppb). Gold content in two other samples was less than 10 ppb. Based on the electron microscopic studies, aggregation of gold particles with a size of 680 μm were identified in the Detroit guyot crust. The sizes of the Au particles are up to 10–15 μm, which has not been previously noted. Gold particles similar in morphology and size were also found in the Zubov guyot crust, which is located far from the Detroit guyot. The largest particle of gold (≈60 μm), represented by electrum, was found in the clay substrate from the “Gummi Bear” seamount. The lamellar, rudaceous morphology of the gold particles from the Detroit and Zubov guyots reflects their in situ formation, in contrast to the agglutinated, rounded with traces of dragging gold grain found in the substrate of the sample from the “Gummi Bear” seamount. Three-component (Ag-Au-Cu) gold particles were found in the hydrothermal crust from the Belyaevsky underwater volcano. Grains similar in composition were also found in Co-rich crust. The research results show that the gold was probably added to by hydrothermal fluid in the already-formed hydrogenetic ferromanganese crusts during rejuvenated volcanic stages. Biogeochemical processes may have played a major role in the formation of submicron solid-phase gold particles.

Hydrothermal Ore Genesis in the Sea of Japan

—The Sea of Japan is a tectonically active region with rift-related destruction of the Earth’s crust and numerous volcanic edifices on the seafloor. Since the 1970s, numerous zones with ferromanganese crusts (FMCs) and phosphorite and barite ore occurrences have been discovered during the repeated expeditions of the Pacific Oceanological Institute, Vladivostok. Analysis of the distribution of these ore occurrences showed that all of them are confined to tectonically active zones of the seafloor: submarine volcanoes, tectonic scarps, or fault zones. In some zones, phosphorites occur together with FMCs, and in one zone, together with FMCs and barites. Ferromanganese hydroxides, phosphorites, or barites are found in the pores of basalts composing submarine volcanic edifices in the Sea of Japan. These data indicate that the ore matter in all zones is supplied with postvolcanic gas-hydrothermal fluids or hydrothermal solutions circulating along deep faults during the destruction of the continental crust in the southern and eastern parts of the sea. Thus, ferromanganese, phosphate, and barite ore occurrences in the Sea of Japan are related to low-temperature hydrothermal-sedimentary processes.