Geochemistry and origins of lacustrine ferromanganese nodules from the Malawi Rift, Central Africa

Ferromanganese nodules recovered from 100-130 m depth near Likoma Island, eastern Lake Malawi, have been analysed for some thirty-four elements by DC-arc optical emission spectrometry. The concretions routinely hold in excess of 50% Fe + Mn, although actual Fe Mn ratios appear to vary inversely with nodule size. Subcrustal values are recorded for Mg, Al, Si, K, Ca and Ti. The abundance of these major elements is considered to primarily reflect the amount and mineralogy of detrital impurities within the nodule structures. Of the twenty-six analysed trace elements, Zn, Co, Pb, Ba, Y, La, V, Zr, Ag, Be, and Nb are present at levels exceeding their average crustal abundances, while subcrustal or subdetection limit values are recorded for Sr, Ni, Cu, Cr, Sc, Rb, Ga, Li, B, Mo, Cd, Bi, Sn, Ce, and Nd. The high enrichment factor noted for Ba (21.95), the limited enrichment of Co (5.53) and Zn (2.06) and depletion of Ni (0.62) and Cu (0.09) are characteristic of most lacustrine ferromanganese deposits and adequately distinguish the Lake Malawi nodules from their deep-ocean counterparts. While the ferromanganese deposits of the Malawi rift are predominantly of diagenetic origin, hydrothermal exhalations may significantly control the supply of elements such as Fe, V, and Be. This implies a need to extend existing classification systems to include nodules formed through the simultaneous precipitation of metals from two or more sources. The nodule sequences in the vicinity of Likoma Island are physically suited to economic exploitation, but fail to meet prescribed chemical criteria for Mn or Ni-Co-Cu ores.