Geochemistry of Fe-Mn sediments

Korotkina O.A., Kogarko L.N, Bazilevskaya E.S, Kubrakova I.V.

The analysis of 17 Fe-Mn deposits from three regions of the Atlantic Ocean was performed. The samples were collected near Elena Seamount, Strakhov Depression, and Cape Verde Seamount in 16^th voyage of “Academician Ioffe” ship and 23^rd voyage of “Academician Nicolay Strakhov” scientific vessel in 2006. The analysis for major componenets as well as for Ni, Co, Cu, P, S etc was performed by -ICP with the utilization of the spectrometers of the latest generation. REE were measured by the same method after the ion-exchange separation of matrix components. BM was separated from the matrix with the utilization of sorbents and analyzed by the ETAAS method.

The diagram Fe-Mn-(Ni+Co+Cu)*10 is commonly used for the geochemistry of Fe-Mn formations. The sum of Ni+Co+Cu is called polymetallic index. Our data plotted onto this diagram fall into hydrogeneous field (fig. 1). The prevailing mineral of hydrogeneous deposits is ferruginous vernadite. The rate of the growth of hydrogeneous deposits varies from 1 to 15 mm per Ma. Their composition is characterized by Mn/Fe ratio close to 1, high concentrations of Co and other microelements including REE.

Fig 1 Diagram Fe-Mn-(Ni+Co+Cu)*10

Rare earths

REE exhibit similar patterns in Fe-Mn deposites which is particularly obvious when normalized by the composition of sheet (fig.2)

The following features of REE patterns may be noted:

The predominance of LREE over HREE.

Fig 2 REE contents in Fe-Mn crusts normalized by the composition of sheet

Distinguishable positive Ce anomaly caused by the oxidation of Ce3+ to Ce4+. Tetravalent Ce is actively accumulated during the precipitation of the Fe hydroxides. Ce content in the investigated crust is 3.5 times that of the mean concentration in Atalantic seawater.

Rare elements contents in Fe-Mn deposits

The investigations demonstrated that the distribution of rare elements in the Fe-Mn deposits of the studied regions is variable. The elements like As, Mo, Ba,Co, Pb, Sc, Sr, V, Y, Zn are concentrated to a larger degree inh the Fe-Mn deposits of Cape Verde Seamount (table 1).The maximum contents of Cu (2580 ppm), Ni (54 ppm) É Li (500 ppm) are in the vicinity of Elena Seamount (fig. 3).

The lowest Li (3,3 ppm) and Cu (220 ppm) contents were found at the Cape Verde Seamount ground. Near the Elena Seamount As (158 ppm) and Y (78 ppm) show the lowest values. Other elements (Ba, Co, Ni, Mo, Pb, Sc, Sr, V, Zn) attain minima in the neighbourhood of the Strakhov depression (table 1).

Table 1 Composition of Fe-Mn deposits

Fig 3 Rare elements contents in Fe-Mn deposits

Distribution of small elements in depth

Some regularity in distribution of a set of elements was observed in various depths from Strakhov Depression (north, 5640 m) to Cape Verde Seamount (south, 1722 m). In small depths (Cape Verde Seamount) content of As, Pb, Sr, óÏ, and Pt is 2 times larger in comparison with northern regions. Content of Fe, V, Y, Zn, Ba, and Mo is also larger (fig. 4‑5). Concentration of the other elements is less. For example, content of Cu and Li is 2 times less in comparison with the Cu-Li content in the regions with small depth. Thus there is a regularity connected with As, Pb, Sr, óÏ, and Pt accumulation in shallow regions, and accumulation of Cu and Li in deep-water regions.

Figure 4. Dependence of small elements concentration on depth

Figure 5. Dependence of Cu and Li concentration on depth

Noble metals

Concentration of Pt in the investigated samples is quite different. The largest concentrations of platinoids were observed in Fe-Mn deposits of Cape Verde Seamount (0.2 ppm), the smallest concentrations of Pt were obtained in the samples collected in the depth more than 5000 m near Strakhov Depression (0.04 ppm).

In the investigated samples of Fe-Mn deposits the Au concentration is almost constant. Maximum value of Au concentration is 0.2 ppm near Strakhov Depression, minimum value is less than 0.1 ppm in the samples from Cape Verde Seamount.

Platinoids accumulation occurs mainly in upper layers (mountains), while concentration of platinoids in depression is minimal.

Distribution of gold and platinum depends on depth. The deeper, the larger accumulation of gold and less platinum (figure 6). Platinum concentration reduction connected with oxygen absorption processes that more active in the depths more than 2000 m.

Figure 6. Dependence Pt , Au on depth