The Journal of Geology, May-June 1893 / A Semi-Quarterly Magazone of Geology and Related Sciences

by Various

THE TRANSPORTATION OF IRON AND MANGANESE IN NATURE.

The process that goes on in this interchange of iron and manganese from older to younger rocks is as follows:

(1) The conversion, by surface agencies, of the minerals containing iron and manganese into forms that can be taken into solution by surface waters.

(2) The solution of the iron and manganese in surface waters, acidulated with organic and sometimes inorganic acids, and their transportation in this form from the areas of older rocks to areas over which younger rocks are being deposited.

(3) Finally, the precipitation in one or more of several ways of the iron and manganese contained in solution.

The iron and manganese thus chemically precipitated may be deposited either with mechanical sediments, such as sand, clay etc., or without them. If the deposition of mechanical sediments is largely in excess of the precipitation of iron and manganese, the final products will be beds of ferruginous shale, sandstone, etc., common in many geologic horizons. If the precipitation of iron and manganese is in excess of the deposition of mechanical sediments, the resulting products are deposits of more or less pure iron and manganese ore. Between these two extremes there are all gradations in the admixture of the iron and manganese with mechanical sediments.

Frequently the iron and manganese which were originally finely disseminated through shale, sandstone, etc., are subsequently concentrated into bodies of comparatively pure ore, and very commonly this concentration takes place by a process of re-solution of the iron and manganese and re-deposition by replacement with limestone, or, more rarely, with some other material. The limestone or other material which thus acts as a precipitant is often in the same series of strata from which the iron and manganese were removed, and thus these two substances, which were once in a finely disseminated condition, may be converted into deposits of comparatively pure ore and yet remain in the same general series of strata in which they were originally deposited. A remarkable case of this is seen in the iron deposits of the Penokee series in Michigan and Wisconsin,[16] to be mentioned again on page 370. It has also been suggested by H. D. Rogers[17] that certain siderite deposits in the Coal Measures were formed by the conversion of finely disseminated sesquioxide of iron into carbonate of iron by organic matter, and the subsequent segregation of the carbonate as now found in layers and nodules.

The surface waters that carry the iron and manganese to the strata being deposited at a given time are sometimes derived from areas in which iron predominates, sometimes from areas in which iron and manganese are both abundant, and sometimes, though rarely, on account of the scarcity of such regions, from areas in which manganese largely predominates over iron. If iron and manganese were always precipitated from these waters in similar chemical forms and under the same conditions, it would be expected that the strata deriving their iron and manganese from surface waters would contain those substances in the same relative proportions as they had existed in the rocks from which they were derived, and that they would be in an intimately mixed condition. Such is doubtless often the case, or at least approximately so; but it is also often the case that iron and manganese occur in separate deposits, yet in close proximity to each other and often alternating along the same horizon. Besides this, the two substances frequently form parts of the same deposit and yet are distinctly separate from each other. In such cases the question arises as to why the iron and manganese are not intimately mixed in the form of a manganiferous iron ore, as would be expected if they had been precipitated together. Moreover, deposits sometimes occur which are composed largely of manganese ore, with little or almost no iron, and when the source of the manganese is looked for, we often find that the rocks which probably supplied it contained both manganese and iron, and that the iron was present in a much larger proportion as regards the manganese than in the new deposit. Here again the question arises as to why the iron and manganese are not in the same relative proportions in the new deposit as they were in the rocks from which they were derived.

Four principal causes suggest themselves in explanation of this separation:

(1) It might be supposed that the deposits containing mostly iron and those containing mostly manganese received these constituents from waters derived from different sources, and carrying iron and manganese only in the proportions in which they deposited them. Under some conditions this explanation might suffice, but in many cases, such as when iron and manganese alternate along the same geologic horizon, and yet in close proximity with each other, the explanation is entirely inadequate, for the deposits are too close to each other to have been formed from different supplies of surface waters.

(2) It might be supposed that the iron or the manganese had been leached out of a deposit of the mixed ores, leaving one free from the other and depositing the dissolved ore somewhere else. This explanation, except in special cases, also appears inadequate, because the reagents in surface waters, which dissolve iron and manganese, seem to affect both about equally, so that if one were dissolved, the other should be taken up in the same way. Doubtless small differences could be found in the behavior of the organic and inorganic compounds in surface waters towards iron and manganese minerals, but they would be small as compared with the more active reactions which go on.

(3) It might be supposed that a separation could be produced by secondary concentration such as segregation, replacement, etc. This has doubtless sometimes been the case, but where the concentrating action is not assisted by a difference in the chemical behavior of the two substances, the separation would only be on a small scale. Even in the case of concentration by replacement of limestone, if iron and manganese both acted in the same way during the replacement, it would be expected to find them deposited in an intimate mixture. Though this secondary concentration, therefore, unassisted by other agencies, would not produce all the results found in nature, yet, when it is thus assisted, it often plays an important part.

(4) The fourth, and what seems the most important, factor in the separation of iron and manganese, is that, though very often they are precipitated in the same form from the same solution, yet sometimes they are precipitated in different forms; and even when precipitated in the same form, the precipitation of one sometimes requires different conditions from the precipitation of the other. This fact will explain the alternate association and separation of iron and manganese, not only when no secondary concentration has gone on, but also in cases where such concentration has taken place, such as in the replacement of limestone, etc.

It will now be attempted to show how the various degrees of association and separation of iron and manganese found in nature may be produced by different conditions during deposition.