Abstract
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.
Highlights
The gold content in the earth’s crust varies from 1 to 6 ppb [1]
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
Morphology ferromanganese crust (Figure recovered the with northern struction,The and150-mm-thick movement, followed by the resumption of3a) its was formation
Summary
The gold content in the earth’s crust varies from 1 to 6 ppb [1]. The sources of gold deposits are rocks with gold concentrations exceeding those of the earth’s crust contents by 2–3 times or more. Co-rich crusts contain on average 30 ppb of gold, and its concentration increases from the Atlantic to the Indian Ocean, reaching a maximum value in the Pacific Ocean (6, 21, and 35 ppb, respectively) [12]. The bulk sample of the crusts from the HB-9 guyot of the Markus Wake seamounts (Pacific Ocean) contains 75 ppb of gold [23]. Prime Zone (CPZ) area, which is characterized by the maximum average gold content in Co-rich crusts for the World Ocean (55 ppm) [12]. The given values of gold content in nodules and crusts determine the small size of their particles, as well as an extremely low probability of detection. We determined anomalous gold concentrations (up to 1390 ppb) in ferromanganese Co-rich crusts from the NW Pacific. This work is devoted to the study of this phenomenon
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