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Submarine ferromanganese deposits from the Mariana and Volcano volcanic arcs, West Pacific

1987; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr87281

2332-4899

James R. Hein, C.L. Fleishman, L.A. Morgenson, Sherman H. Bloomer, Robert J. Stern,

Geomagnetism and Paleomagnetism Studies

than mid-plate deposits.Also, ferromanganese oxides form cement and stratabound layers within sedimentary rocks in volcanic arcs, and contain a different suite of enriched trace metals (including Mo, Ni, Ba, and Zn) than those of mid-plate or spreading axes deposits. Sample Collection and DescriptionsEighty-four dredge attempts were made (Table 2; Figs.123), 76 with circular chain-bag dredges, 3 with a pipe dredge made on board ship after all the chain-bag dredges were lost, and 5 with a rectangular chain-bag dredge also made on board ship.Eighteen dredge attempts (21%) either resulted in an empty bag or lost dredge.Of the 66 dredges that recovered rocks, 18 (27%) contained significant ferromanganese deposits and 22 (33%) contained some ferromanganese oxides (Table 2).Thus, 60% of the dredge recoveries contained ferromanganese deposits.Substrate rocks consist of intermediate to mafic volcanic rocks, volcaniclastic rocks, volcanic breccia, and minor limestone (Table 2).Volcaniclastic rocks are dominantly sandstone but range from mudstone to conglomerate.Benthic organisms, dominantly corals and silicosponges, were recovered in several dredges (Table 2).Ferromanganese deposits occur in five ways (Figs.4567): 1) Yellow-brown to black ferromanganese encrustations occur on rocks.Some of these rocks are poorly indurated mudstone (Fig. 4).The crusts are friable, layered, or laminated, and have dominantly botryoidal or granular to smooth surfaces.These deposits will be referred to as crusts', 2) Brown, grey, and black ferromanganese oxides cement volcaniclastic sandstone, volcanic breccia, and conglomerate (Figs. 5, 6).Sandstone is most commonly the host for these earthy, friable, black to brown cements.Rarely, the cement in breccia is grey with a metallic luster.These deposits will be referred to as manganiferous sandstone; 3) Manganese oxide occurs as stratiform, stratabound layers that are pale to dark grey, rarely brownish-grey, with a submetallic luster (Fig. 7); rarely the luster is metallic or glassy.These deposits occur as discrete beds or lenses, millimeters to centimeters thick, within the manganiferous sandstone.The manganese oxides are friable to dense, massive to laminated, and brittle.Pebble and cobble size chunks of these beds were recovered in several dredge hauls with only minor amounts of the associated manganiferous sandstone.This separation was due to the presence of thick manganese beds and the apparent loss of much of the friable sandstone in the process of dredging.These deposits will be referred to as stratabound manganese; 4) One dredge haul, D3, recovered cobbles of dark grey submetallic, irregularly shaped masses of manganese (Fig. 7A).Chemically these deposits are similar to the stratabound manganese, but differ in their texture, irregular morphology, and possibly host rock.Other than the manganese, only a small amount of red mud was recovered in D3.This manganese probably formed in much the same way as the stratabound manganese, but hydrothermal fluids debouched onto the seafloor or manganese precipitated within soft, water-saturated mud, rather than between sandstone layers within the sediment section.These deposits are considered equivalent to stratabound manganese; 5) Dredge D12, taken in the submerged caldera of Maug volcano, recovered two cobbles composed of magnetite, siderite, and goethite.In air, fresh surfaces of these black cobbles altered within days to yellow-brown goethite, reflecting the very fine grain size of the presumably hydrothermal magnetite.Chemically, the crusts and cemented sandstones are ferromanganese oxide deposits, commonly with iron oxide more abundant than manganese oxide, whereas the stratabound manganese contains little iron and the magnetite-siderite cobbles contain little manganese. MineralogyThree dominant manganese minerals occur, S-MnO2, birnessite [(Ca,Na)Mn7O14 and todorokite [(Na,Ca,Mn)2Mn5O12 3H2O] (Table 3).5-MnO2 is defined by two X-ray reflections at about 2.40 A and 1.42 A and has also been referred to as vernadite.Bimessite has two reflections, near 7.1 A and 3.5 A, and todorokite also has two reflections, near 9.6 A and 4.8 A, but in several samples todorokite has a third reflection at 3.3 A. These X-ray patterns occur in other marine ferromanganese deposits, except for the 3.3 A todorokite peak.However, this third todorokite peak has been reported for samples that occur on land (ASTM file).Birnessite and todorokite that occur in abyssal manganese nodules and in on-land deposits commonly contain more X-ray reflections than those that occur in the Mariana arc samples (Bums and Bums, 1977).Ferromanganese crust samples are predominantly 5-MnO2.Only three of the analyzed samples contain todorokite and one contains birnessite as subordinate constituents.In contrast, the manganiferous sandstones and stratabound manganese are composed of a widely varying mixture of the three manganese minerals.In four samples todorokite occurs alone, and in one breccia 5-MnO2 occurs alone.5-MnO2, birnessite, and todorokite in the Mariana-Volcano arc samples display a wide range of crystaUinities or grain size as indicated by the sharpness of the X-ray reflections.In general, the crystallinity (or grain size) of the manganese minerals is greatest in the submetallic stratabound manganese.These deposits are also the only ones that contain todorokite with three X-ray reflections.Some samples were X-rayed a second time after 9 months of exposure to the atmosphere.In some of these samples , much of the todorokite had transformed to birnessite.Substrate rocks are dominantly composed of plagioclase and pyroxene.Some rocks also have amphibole and quartz.Secondary minerals include smectite, chlorite, illite, zeolites, goethite, calcite, and others (see Table 3).The magnetite cobbles from Maug caldera contains magnetite, siderite, goethite, aragonite, sanidine, gypsum, and lepidocrocite. Chemical CompositionThe three main types of ferromanganese deposits are chemically distinct (Tables 567891011) and can be classified according to Fe/Mn ratios.Fe/Mn ratios for crusts, manganiferous sandstones, and stratabound manganese are >1, 1.0 to 0.1, and <0.1, respectively (Tables 5,11).Rare exceptions are noted to this classification for dredges from the Cross-Chain Seamounts and dredge D8 from Poyo Seamount.In addition, crusts are relatively rich in P, Co, Ni, Cu, Pb, V, Zn, As, Ba, and REE, and are best characterized by high Fe/Mn, P, Co, Ni, As, and REE contents (Tables 5,6, 11).Manganiferous sandstones are relatively enriched in Si and other terrigenous (aluminosilicate) elements, and moderately enriched in Cu, Ni, V, Zn, and Ba.REE concentrations are mostly between those of crusts and stratabound manganese, although they overlap in part with the latter.Manganiferous sandstones are best characterized by moderate Fe/Mn and high Si, Al, and Cu (Tables 5,11).Stratabound manganese is strongly enriched in Mn, Mo, and Ba, moderately enriched in Ni, V, and Zn, and strongly depleted in Si, Fe, and REE.It is best characterized by low Fe/Mn, Fe, and REE and by high Mn, Mo, and Ba (Tables 5, 6, 11).Co and Ni in the crusts are correlated with both the Fe and Mn phases, in contrast to both abyssal nodules and seamount crusts (Table 10).Fe in Mariana-Volcano arc crusts is positively correlated with Co, Ni, Pb, V, Ti, P, As, Sr, Y, Pt, and REE.Mn is positively correlated with Co, Ni, V, Sr, Cu, Mo, Zn, Cd, and Ba.This indicates that whereas Co, Ni, Sr, and V are partitioned between the Fe and Mn phases, the other elements are selectively absorbed or substituted in one of the major oxide phases.Pacific crusts typically have Co, Ni, Pb, and Zn positively correlated with the Mn phase and Cu correlated with the Fe phase (Hein et al. t 1987).Pt is positively correlated with the same elements as Fe in addition to Pb and is negatively correlated with the aluminosilicate elements.In manganiferous sandstone K, Mo, and Ba are positively correlated with Mn and Ti, P, As, Co, Cu, Pb, Zn, and Pt with Fe (Table 9).Pt is correlated with Fe, Ti, P, As, Ca, Ni, Pb, and Sr.In stratabound manganese, Mo and some REE are positively correlated with Mn, and P and Si with Fe (Table 8).The high Mo contents are also positively correlated with As and the high Ba with Pb, Sr, and V. Pt has a perfect positive correlation with Pb and Eu and is also positively correlated with P and Pd.Pd is positively correlated with Cu, Ni, Pb, Sr, and PL Three of the six platinum group elements (Pt, Pd, and Rh) vary from below their limits of detection to maximum values of 0.190, 0.015, and 0.0046 ppm, respectively (Table 5).A Pt content of 0.2 ppm is equivalent to the lowest values obtained for ferromanganese crusts from the central Pacific, which range from 0.2 to 1.2 ppm (Hein et al., 1987).Crusts contain two to three and a half times as much Pt as manganiferous sandstone and stratabound manganese, whereas, stratabound manganese has the greatest Pd content.Detectable Rh occurs only in the crusts.These relations suggest that Pt and Rh enrichments over crustal averages result from hydrogenetic (scavenging) processes and the enrichment of Pd over crustal averages results from hydrothermal (leaching) processes.Some elements vary consistently in the ferromanganese deposits with latitude along the arc (~15-27°N) and with depth of water (range 530-3921 m).Manganiferous sandstone and stratabound manganese show positive correlations with latitude for K and Mg respectively and negative correlations with Fe, Pb, and V and As, respectively.In contrast, crusts show a negative correlation with latitude in the aluminosilicate elements and a positive correlation with Pt, all REE, and many other metals (Table 10).Through the processes of erosion and leaching, the manganiferous sandstone and stratabound manganese reflect the arc volcanic rocks.The volcanic rocks collected during the cruise also show compositional changes with latitude, where enriched incompatible elements, especially K, Rb, and Ba, increase from 20° to 24.6° latitude, and then decrease farther to the north (P.-N.Lin et al., written communication, 1987).In crusts, the aluminosilicate elements increase and P, Co, Mo, Ni, Pt, As, Pb, Sr, V, Zn, Y, and REE decrease with increasing depth of water (Table 10).These results are consistent for most elements with results for hydrogenetic crusts from the central Pacific (Hein et al., 1987).Chondrite-normalized rare earth element (REE) plots also distinguish the three main types of ferromanganese deposits (Table 6;.Crusts typically have a very small negative Ce anomaly, negative Eu and Dy anomalies, in some samples a small positive Gd anomaly, and are depleted in the heavy REE relative to the light REE in the crusts.Stratabound manganese patterns are characterized by many anomalies, but an especially strong positive Tb anomaly, and depletion in light and heavy REE relative to the other ferromanganese deposits.Manganiferous sandstone patterns are depleted in both the heavy and light REE, but more so in the heavy REE.No particular pattern of anomalies is recognized.REE patterns for all three types of ferromanganese deposits contrast with patterns for Mariana arc volcanic rocks, which lack significant anomalies, and are relatively depleted in all REE (Figs. 11, 12).The stratabound manganese patterns are most comparable to the volcanic rocks in total REE abundance.