Both amorphous and crystalline ferromanganese deposits have been dredged from depths between 1,400 and 2,250 m on the flanks of Welker, Miller, Murray, and Patton Seamounts in the Gulf of Alaska (53-55°N, 140-150°W). Prominent 1 to 11-cm thick massive crusts, consisting largely of black amorphous oxide and poorly crystalline ^dgrMnO₂, occur as rounded multishelled coatings on the surfaces of alkali-basalt pillows and volcanic breccia. These crusts are characterized by a simple internal stratification constructed from isotropic oxide microlaminations in alternating colloform and columnar aggregates. Detrital fragments of quartz, plagioclase, palagonite, and mafic volcanic rock are concentrated along cusps or channels within crenulated oxide layers. Bul -chemical analyses of the massive amorphous crusts yield Mn/Fe ratios of 1.5 to 2.5 and relatively high Ni (0.26 to 0.65%), Co (0.23 to 0.66%), and Cu (0.03 to 0.12%). The occurrence and composition of these amorphous crusts suggest that they are authigenic deposits with a growth mechanism similar to that for the top surfaces of Pacific deep-sea manganese nodules.

Thin (1 to 10 mm) subparallel crusts, interconnecting veinlets, and nodular infillings associated with friable tuffaceous sediment are composed of well-crystallized todorokite and cryptomelane; ^dgrMnO₂ and birnessite(?) are minor constituents. Complex textural variations are characteristic, but broad colloform bands of variably anistropic radiating oxide fibers, and massive zones of very coarse grained (as much as 1 mm long) strongly anisotropic acicular todorokite or cryptomelane crystals, are common. These massive anisotropic oxide zones contain abundant recrystallized radiolarian tests. Ferromanganese samples from Patton Seamount have a third association: undulating bands of columnar or nodular todorokite-rich oxide and volcanic detritus (mostly palagonite) occurring in a matrix of microcrystalline phosphorite (carbonate-apatite). Crystalline ferromanganese oxide deposits have bulk-chemical compositions similar to those of the amorphous oxides but with somewhat higher Mn/Fe ratios, higher Ni, and lower Co. In contrast to the amorphous crusts, crystalline ferromanganese accumulations on Gulf of Alaska seamounts are analogous to the bottoms of deep-sea nodules; that is, the formation of these accumulations is closely related to diagenetic modification of the associated sedimentary substrate.

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