Modes of occurrence of noble and nonferrous metals in ferromanganese formations in the central part of the Sea of Okhotsk

Abstract

Ferromanganese formations are among the major solid mineral resources in the World Ocean. They are characterized by high contents of Fe, Mn, and often nonferrous and noble metals. The issue of the mode of occurrence and genesis of noble and nonferrous metals in the FMS long ago attracted the attention of researchers. However, this issue remains debatable. According to some researchers, the metals mentioned above are extracted from seawater by manganese and iron hydroxides. Other researchers believe that the metals are derived from postmagmatic and hydrothermal solutions. Proponents of the extraterrestrial origin have suggested that platinum group elements (PGE) and some other heavy metals could be incorporated as cosmogenic spherules during the growth of ferromanganese formations. However, all these hypotheses remain controversial. We investigated ferromanganese crusts from the Kashevarov Bank, Deryugin Basin, and Akademiya Nauk Rise in the central part of the Sea of Okhotsk (Fig. 1). Ore crusts from this region are thin (up to 2 cm), usually homogeneous, and vaguely layered (Fig. 2a). The slope of the Kashevarov Bank also contains oolitic crusts related to the cementation of sandy sediment by ferromanganese hydroxides. However, the predominant crust type is characterized by a monolithic upper layer (3 to 4 mm thick) and an oolitic lower layer (Fig. 2b). Data on the chemical composition showed that the crusts contain almost equal contents of Mn and Fe. The total content of ore elements in them is as much as 30 wt %. The Mn content varies from 5.6 to 16.0%; the Fe content, from 3.9 to 17.7%; and the Mn/Fe ratio, from 0.18 to 1.42. Contents of nonferrous metals are very low (0. n ‐0.0 n %) relative to the oceanic ore crust. The total content of Ni, Co, Cu, Pb, and Zn varies from 0.03 to 0.46%. The content of Au and Ag is 0.01 and 0.24 g/t, respectively. In order to elucidate the mode of occurrence of noble and nonferrous metals, we prepared eight polished samples and examined them with an electron microscope and microprobe analyzer (JXA8100, Jeol, Japan). The results show that all samples contain inclusions of noble and nonferrous metals of different compositions. Native silver is the major constituent. Small grains of gold, iron, copper, tungsten, Fe‐Sb oxides, Zn‐Ag‐Cu sulfides, phosphates of rare earth elements, and intermetallic (Cu‐Zn, Cu‐Ni, Sn‐Pb‐Cu‐Ni, Ni‐ Cr‐Mo, Fe‐Cr‐Ni, and Au‐Cu‐Ag) compounds are the subordinate components (Tables 1, 2; Fig. 3).