Diffuse flow hydrothermal manganese mineralization along the active Mariana and southern Izu‐Bonin arc system, western Pacific

Abstract

[1] Abundant ferromanganese oxides were collected along 1200 km of the active Izu-Bonin-Mariana arc system. Chemical compositions and mineralogy show that samples were collected from two deposit types: Fe-Mn crusts of mixed hydrogenetic/ hydrothermal origin and hydrothermal Mn oxide deposits; this paper addresses only the second type. Mn oxides cement volcaniclastic and biogenic sandstone and breccia layers (Mn sandstone) and form discrete dense stratabound layers along bedding planes and within beds (stratabound Mn). The Mn oxide was deposited within coarse-grained sediments from diffuse flow systems where precipitation occurred below the seafloor. Deposits were exposed at the seabed by faulting, mass wasting, and erosion. Scanning electron microscopy and microprobe analyses indicate the presence of both amorphous and crystalline 10 Å and 7 Å manganate minerals, the fundamental chemical difference being high water contents in the amorphous Mn oxides. Alternation of amorphous and crystalline laminae occurs in many samples, which likely resulted from initial rapid precipitation of amorphous Mn oxides from waxing pulses of hydrothermal fluids followed by precipitation of slow forming crystallites during waning stages. The chemical composition is characteristic of a hydrothermal origin including strong fractionation between Fe (mean 0.9 wt %) and Mn (mean 48 wt %) for the stratabound Mn, generally low trace metal contents, and very low rare earth element and platinum group element contents. However, Mo, Cd, Zn, Cu, Ni, and Co occur in high concentrations in some samples and may be good indicator elements for proximity to the heat source or to massive sulfide deposits. For the Mn sandstones, Fe (mean 8.4%) and Mn (12.4%) are not significantly fractionated because of high Fe contents in the volcaniclastic material. However, the proportion of hydrothermal Fe (nondetrital Fe) to total Fe is remarkably constant (49-58%) for all the sample groups, regardless of the degree of Mn mineralization. Factor analyses indicate various mixtures of two dominant components: hydrothermal Mn oxide for the stratabound Mn and detrital aluminosilicate for the Mn-cemented sandstone; and two minor components, hydrothermal Fe oxyhydroxide and biocarbonate/biosilica. Our conceptual model shows that Mn mineralization was produced by hydrothermal convection cells within arc volcanoes and sedimentary prisms that occur along the flanks and within calderas. The main source of hydrothermal fluid was seawater that penetrated through fractures, faults, and permeable volcanic edifices. The fluids were heated by magma, enriched in metals by leaching of basement rocks and sediments, and mixed with magmatic fluids and gases. Dikes and sills may have been another source of heat that drove small-scale circulation within sedimentary prisms.

Molecular evidence for microorganisms participating in Fe, Mn, and S biogeochemical cycling in two low‐temperature hydrothermal fields at the Southwest Indian Ridge

“…The geochemical analysis results were consistent with the mineralogical analysis results. Minerals (e.g., Opal‐A, ferrihydrite, birnessite, and nontronite) that are common in other low‐temperature hydrothermal environments (e.g., those reported by Boyd and Scott [], Hein et al ., [], Dekov et al ., [], and Sun et al ., []) were present in both hydrothermal deposits. We did not detect Fe sulfides, anhydrites, or other metal sulfides (e.g., chalcopyrite and sphalerite), which form in high‐temperature hydrothermal environments.…”

Epithermal Manganese Mineralization, Kimolos Island, South Aegean Volcanic Arc, Greece

“…These elements are probably partitioned among all those phases, but predominantly with an aluminosilicate phase. The high Na content may suggest involvement of seawater (Hein et al, 2008); Mg does not correlate with any other element and may also originate from seawater. The Ca content probably reflects a carbonate sediment source.…”

“…Hydrothermal enrichment of Zn in manganese deposits has been mentioned from the Galapagos area (Moore and Vogt, 1976, Cronan, 1986, Rogers, 1996, the Mariana-Bonin arc (Hein et al, 2008) and the Tonga-Kermadec Ridge and Lau Basin (Rogers et al, 2001). According to Cronan (1986), Hein et al (2000), and Rogers et al (2001), Zn enrichment in manganese deposits may represent the proximity of sulfide deposits that may be located near or below the Zn rich zones.…”

Rapid Shifts in Chemical and Isotopic Compositions of Sediment Pore Waters in the Amami Sankaku Basin in Response to Initial Arc Rifting in the Mid‐Oligocene

“…The reactivity of Mg in hydrothermal systems or during the alteration of volcaniclstic sediments in general can be represented by the following reaction (Reeves et al, ): where talc (Mg 3 Si 4 O 10 (OH) 2 ) is used to represent clay minerals (hydrated magnesium silicate) (Seyfried, ). As a result, low concentration of Mg in pore waters can be indicative of a proximal subseafloor hydrothermal deposit (Hein et al, ). Mg is enriched in more distal deposits when hydrothermal fluids mix with pore fluids and seawater (Mottl, ).…”

“…where talc (Mg 3 Si 4 O 10 (OH) 2 ) is used to represent clay minerals (hydrated magnesium silicate) (Seyfried, 1987). As a result, low concentration of Mg in pore waters can be indicative of a proximal subseafloor hydrothermal deposit (Hein et al, 2008). Mg is enriched in more distal deposits when hydrothermal fluids mix with pore fluids and seawater (Mottl, 1983).…”