Chemical composition of ferromanganese deposits from the Mariana and Volcano volcanic arcs

During R/V T. Thompson cruise TT-192 around the the Mariana and Volcano Volcanic Arcs 60% of the dredge recoveries contained ferromanganese deposits. Substrate rocks consisted of intermediate to mafic volcanic rocks, volcaniclastic rocks, volcanic breccia, and minor limestone. Volcaniclastic rocks were dominantly sandstone but ranged from mudstone to conglomerate. Ferromanganese deposits occur in five ways: Yellow-brown to black ferromanganese encrustations on rocks; brown, grey, and black ferromanganese oxides cementing volcaniclastic sandstone; stratiform, stratabound Mn oxide rich layers that are pale to dark grey, rarely brownish-grey, with a submetallic luster; cobbles of dark grey submetallic, irregularly shaped masses of manganese (Mn nodules ?) and weatehered black cobbles rich in magnetite. A selection of these deposits was chemically analysed including major and minor elements, rare earth elements, and platinum-group elements. The chemistry determination was done by Induction Coupled Plasma analyses done by J. G. Crock, and K. Kennedy at the U.S. Geological Survey laboratories in Reston, Denver and Menlo Park respectively. Water and CO2 determinations were done by wet chemical techniques.