CAUSES OF THE SEPARATION OF IRON AND MANGANESE.
When iron and manganese ores occur in more or less separate deposits, it becomes necessary to suppose the action of influences different from those which cause the deposition of both together, and such influences are to be found in the different modes of precipitation, under certain conditions, of the two metals. It has been shown by Fresenius[28] that certain warm springs, on reaching the surface, first deposit hydrous sesquioxide of iron, and farther on carbonate of manganese. This not only points to the well-known fact that carbonate of iron is more easily oxidized than carbonate of manganese, but it also leads to the belief that the bicarbonate or other salt of iron in the water is more easily oxidized than the manganese salt.
An action somewhat similar to that described by Fresenius readily explains the occurrence of manganese sometimes in entirely separate deposits, sometimes in distinct but closely alternating deposits.[29] Under certain conditions, if the waters from which the precipitation took place were moving, the iron and manganese, owing to the difference in oxidability, as stated above, would be laid down in different places, resulting in the formation of deposits of iron ore free from manganese, and manganese ore free from iron in different positions along the plane of the same geologic horizon. Such occurrences are often seen in the iron regions of the Appalachian Valley, where there are often found, in different places along the same belt, deposits of iron ore and deposits of manganese ore in positions similar with relation to the enclosing rocks.
These conditions of moving water might also cause the occurrence of the two ores in interstratified layers, as is sometimes the case. Such a condition would result if iron were deposited in a certain place at one time, and if, later, on account of some increased facility for oxidation, iron was deposited before it reached that place, and the manganese, being less easily precipitated, were carried on and laid down upon the first deposit of iron.
Suppose the metalliferous solutions to be confined in a shallow basin, or, at least, to pass through it so slowly that they become thoroughly oxidized. Under such conditions the deposition of iron and manganese would go on continuously, and so nearly on the same spot that a comparatively homogeneous manganiferous iron ore would be formed. If the supply of metalliferous solutions were not continuous, but were intermittent, as is sometimes the case in local basins, such as coastal lagoons, which are often dependent for their supply of water on the changes of season and the sudden fluctuations of weather, then interstratified layers of iron and manganese ore might be produced. The iron, becoming oxidized on the surface, sinks to the bottom, possibly in some cases to be converted there to the simple carbonate by organic matter. Further oxidation precipitates hydrous oxide or carbonate of manganese on top of the iron. A renewed supply of surface waters brings more solutions of iron and manganese, or else the evaporation of the water in the closed basin concentrates the materials which have not yet been precipitated. In either case there is a further alternate deposition of the two ores.[30]
Another process of separation of iron and manganese in nature might take place by the formation of sulphide of iron. It has already been shown that iron is sometimes deposited as sulphide and later oxidized in the same manner as the carbonate. Manganese, on the other hand, is rarely found as sulphide, and there is reason to think that the sulphide never represented the original form of any large sedimentary deposits of manganese ore (see pages 364 to 365). It seems probable, therefore, that from a solution of iron and manganese in surface waters the iron might, where the conditions are favorable, be precipitated as sulphide (FeS2) and the manganese might be carried on in solution to be deposited somewhere else as oxide or carbonate. Subsequently the oxidation of the ores would give rise to oxide of iron from the sulphide and oxide of manganese from the carbonate; and the two ores, though occurring at the same horizon, would be separated by a greater or less distance.
After the deposition of the sulphide of iron, the conditions might change and permit the deposition, in the same place, of the carbonates of iron and manganese together. This is an easy case to imagine, and where such a deposit was exposed to surface influences, the resulting product would be oxide of iron from the underlying sulphide and a manganiferous iron oxide from the overlying isomorphous carbonates. Hence another possible cause of the frequent association of pure iron ores and manganiferous iron ores. It is possible also that after the solution of iron and manganese had been freed from the former by precipitation as sulphide, the manganese might be carried on and laid down as carbonate on a previous deposit of iron sulphide, and when such a combination was oxidized, the result would be oxide of iron and oxide of manganese in beds closely associated but yet distinct.
By supposing the iron sometimes to be deposited in sea water as glauconite, a manner in which manganese is not laid down (see page 365), a further means of separation of the two metals would result.
Thus by alternating the conditions of the deposition of iron and manganese in different forms, a great variety of methods of association and separation of the two metals can be produced.
The above discussion refers not only to the deposits of iron and manganese ores of notable size, but also to the iron and manganese frequently found disseminated through shales, sandstones etc. In these rocks they usually form a small but often a very important part, for in many cases the iron and manganese is taken into solution from the rocks and redeposited by a process of replacement with carbonate of lime in neighboring beds of limestone, or more rarely by replacement with other rocks, thus giving rise to important ore deposits. The question of the association and separation of the iron and manganese in these replacement deposits depends on a number of conditions, the principal of which are, just as in the class of deposits that has been discussed, the conditions during deposition and the forms in which the iron and manganese are precipitated. The processes by which association and separation occur in replacement deposits differ somewhat in detail from the processes just discussed, but are based on the same principles.
Many of the iron and manganese deposits of the Appalachian region are supposed by many to be replacement deposits. N. S. Shaler[31] in 1877 suggested that some of the iron deposits of Kentucky and Ohio were formed by the solution of iron from certain rocks, and its deposition in the form of carbonates by replacement with underlying limestone. Subsequently it was changed by oxidation to brown hematite. A notable case of replacement has also been shown by R. D. Irving and C. R. Van Hise[32] in the iron deposits of the Penokee series of Michigan and Wisconsin. Here the ore is supposed to be partly a replacement of chert in a trough between quartzite and igneous rocks. The solution that contained the iron was derived from strata in the same series of rocks in which the iron was re-deposited and contained a certain amount of manganese. It is shown how the iron and manganese were more or less separated in the replacement process and that the separation was due to the difference in the oxidability of the carbonates as explained on page 363.
R. A. F. Penrose, Jr.