THE IRON ORES OF ANTRIM.
By T. D. KENDALL, C.F., F.G.S.
From Transactions of the North of England Institute of Mining and Mechanical Engineers, vol. XXX, 1880
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THF north-east corner of Ireland-that occupied by the county of Antrim is almost entirely covered by a sheet of basalt, which varies in thickness from a few feet to several hundred feet. This basalt rests upon chalk, and is supposed, from the nature of the plants yielded by some inter-bedded layers of Lignite, to be of Miocene age. A generalised section of the rocks in this part of Ireland is given in Fig. 1, Plate XXII1.
Generally, the basalt may be divided into two classes, amorphous and columnar, the latter of which must be well-known to all who have visited the famous Giant's Causeway, on the north coast; and the former may also there be seen between and below the two tiers of columns which form such a conspicuous feature of the cliffs.
Although the basalt is not a sedimentary rock, but volcanic, yet, as is often the case with rocks of the latter class, it shows distinct traces of bedding, as may be seen in the cliffs at Pleaskin, near the Giant's Causeway, a sketch of which is given in Fig. 2.
Parallel to the bed-planes of the basalt and inter-stratified with that rock are a number of ferruginous bands, which, of late years, have attracted considerable attention. The precise number of these bands is not known, but they occur one above another like seams of coal, as shown in Fig. 1, Plate XXIV., which is a section of the cliff's near Downhill.
In the above sketch only two seams are shown; in other places, however, there are more. Usually they consist of a ferruginous clay called bole, with an underlying layer of lithomarge; but one seam, the most important of all, and perhaps the only seam that has yet been worked to a commercial success, consists of three beds. The position of this seam, as seen at Portmoon, is immediately under the lower tier of basaltic columns. It appears to be the highest of the series of ferruginous bands, at least the writer is not aware that any have been found above it. The seam which occurs in a similar position, that is, below the lower tier of basaltic columns in the cliffs near the Giant's Causeway, is the same, and it may be found in many other parts of the county. A general section of the seam is given in Fig 2. and described below.
Lower tier.
b.-CLAY.-Slate-coloured, passing gradually into the overlying basalt. The thickness of the clay is very irregular, and it peels off the overlying basalt in laminve, parallel to the sinuosities of the under surface thereof.
c.-PISOLITIC ORE.-This bed consists of a soft brown or reddish ochre, in which are thickly embedded small irregular pieces of harder ore about the size of peas, which are strongly attracted by the magnet, being partly hematite and partly magnetite. Sometimes this bed has an amorphous character and appears as limonite when it is not magnetic. The junction between it and the overlying clay is very distinct, and they separate quite easily. A quantity of fossil wood has been found in the bed, the vegetable tissue of which was replaced by limonite. The pieces which have been seen by the writer belong to a coniferous species allied to the yew. Average thickness of bed about two feet.
d.-BOLE.-A yellowish-red ochre, containing numerous concretionary nodules of basalt. It is moderately hard, and breaks into irregular cuboidal pieces. The junction between this and the overlying pisolitic ore is not very distinct. Average thickness about six feet.
e.-LITHOMARGE.-A variegated soft rock of a prevailing blue slate colour and greasy feel. Like the bole, it contains concretionary nodules of basalt, but they are more numerous in this bed than in the bole. The line separating it from the bole is somewhat indistinct. Average thickness about 25 feet.
.f.-CONCRETIONARY BASALT.-Passes gradually into the overlying lithomarge.
The nodules of basalt included in the bole and lithomarge are very curious. Fig. 1, Plate XXV., is a sketch of three of these nodules occurring in the bole at Ballylagan.
Fig. ,2 shows two similar nodules enclosed in the lithomarge, at Pleaskin. The centres of the nodules consist of compact basalt, which gradually and by concretionary layers passes into either bole or lithomarge.
The extent of the ferruginous bed containing pisolitic ore is not known ; but it must cover many thousands of acres, although it does not occur everywhere within the basaltic area. In many places it has been removed by denudation, being found in the hills but absent in the intervening valleys. The breach of continuity thus brought about is further increased by the numerous faults that traverse the county.
It is possible that there may be more than one band containing pisolitic ore, and that the seams which have been worked by the different Mining Companies operating in the county may not be portions of one original seam as is generally supposed, but so far as is at present known, they appear to be.
The quality of the pisolitic ore is good, as shown by the following analyses:
The bole yields only about half as much iron as the pisolitic ore, and contains a much larger quantity of alumina. The following are analyses of samples taken chiefly from the bole, but having a slight admixture of the pisolitic ore:--
The yield of iron by the lithomarge is too small to render it of any value for iron-malting, as is shown by the following analysis :
ORIGIN OF THE DEPOSITS.
LITHOMARGE AND BOLE.-
The manner in which these beds have been produced seems to the writer to be rendered perfectly clear, by the presence in them of the concretionary nodules already noticed. The graduation from bole to basalt and from lithomarge to basalt, as seen in the nodules of the respective beds, leaves no doubt whatever in his mind that both bole and lithomarge are the result of metamorphic action on basalt. The appearance presented by these nodules cannot possibly be explained on any other assumption, but whether the rock from which the two beds were produced was exactly alike in chemical constitution, as well as like that of the basalt now above and below them, it is impossible at present to say. The probability is, in the writer's opinion, that the beds of rock from which the bole and lithomarge were produced differed in their chemical constituents. If not, and it is assumed that they were both subjected to the same metamorphic action, then the two beds of lithomarge and bole must represent two different stages of the metamorphic process. But if bole were metamorphosed lithomarge, that is, if it had undergone a higher degree of metamorphism than lithomarge, there would be found in the concretionary nodules of the bole, a transition from basalt to lithomarge in the first place, and then from lithomarge to bole; in other words, between the bole and basalt forming the centre of the nodules, there would be found a layer of lithomarge, which is not the case. Then again, if lithomarge were altered bole, there would be found in the concretions of the lithomarge bed a layer of bole immediately round the basaltic kernels, which did not appear. These facts incline the writer to the idea that the bole has been produced from a bed of basalt chemically different from that which, by alteration, has resulted in lithomarge. Below are two analyses of basalt from Antrim.
They show how variable that rock is, and that its composition differs quite as much in the same locality as lithomarge does from bole. A fact which may throw some light on the mode of metamorphism may be observed, in connection with the bed of clay lying between the pisolitic bed and the overlying basalt. This clay, the writer believes, is also metamorphosed basalt, for there is a most gradual laminated passage from hard basalt to soft clay, as shown in Fig. 1, Plate XXVI. Wherever this clay bed crops out to the day it appears to thicken, as shown at a, Fig. 2, which is a section of the pisolitic ore and accompanying beds, as seen at Ballylagan. This thickening of the clay-bed at the outcrop would seem to indicate that whatever was the metamorphic action which produced the clay, it acted most powerfully near the surface. The metamorphism cannot, however, have been recent, or the upper part (c) of the bed A might be expected to be also converted into clay. This may have been so at one time and have been subsequently removed by glacial or pre-glacial denudation.
PISOLITIC ORE.-
This bed has been accounted for in various ways. Some have supposed it to be of igneous origin, others consider it to be metamorphosed bole, whilst not a few are of opinion that it had an aqueous origin. The fact, previously referred to, that fossil wood has been found in the bed, seems to favour the last idea. It clearly precludes the possibility of the first and second. Besides, if this ore is the result of metamorphic action on bole, why is the same sort of pisolitic ore not found accompanying the other bole beds. They were all alike, overlaid by a bed of basalt, which, according to the holders of this view, was instrumental in producing the pisolitic ore.
The precise mode of deposition usually advocated by those who believe in the aqueous origin of this bed, is the sedimentary. But that view, it seems to the writer, is scarcely reconcilable with the facts. The freedom of the ore from the mechanical admixture of other rocks, alone seems a sufficient argument against its deposition in that way. The most likely lode of origin appears to be that of precipitation from a chemical solution, possibly by organic agency, but not necessarily so. On that supposition, the absence of foreign matter, such as would almost assuredly have been present had the bed been of sedimentary origin, offers no difficulty, whilst the occurrence of wood in the ore is as easily explained by assuming the ore to be a sediment. Suppose the bed of basalt from which the bole was produced to have been in a comparatively soft and decomposed state at the time the pisolitic ore was deposited, that would also afford a probable explanation of the fact already mentioned, that the junction of the pisolitic bed and bole is somewhat indistinct. Some of the first precipitated ore would almost certainly find its way into the decomposed bed below, the effect of which would be to produce the appearance of a regular transition from iron ore to bole. The magnetic character of the ore is probably due to the influence of the overlying basalt whilst it was in the condition of molten lava.
It may be thought somewhat improbable that three beds, such as the pisolitic ore, the bole, and the lithomarge, should be found lying together in such intimate relations and yet two of them to have originated in one way, and the other in a manner entirely different. It may be asked why, if the pisolitic ore was precipitated from a chemical solution, should it be underlaid by bole and lithomarge. It might as easily have been thrown down, it may be said, on unaltered basalt. But it should not be forgotten that one of the very conditions which is necessary to produce a chemical precipitate, that is the presence of water, is also one that is likely to afford the agent necessary to effect the metamorphic change which resulted in the formation of bole and lithomarge, so that these beds might have been formed at the same time or nearly so, and yet in very different ways. Besides, by adopting that view, the difficulty of explaining the absence of pisolitic ore from any of the underlying bands yielding only bole and lithomarge is avoided. On the precipitation hypothesis it need only be supposed that if the lower beds, like the upper one, were submerged in water, iron was present only during the deposition of the pisolitic ore accompanying the upper bed, or it may be that then only were the necessary organisms present to effect the precipitation.
AGE OF THE DEPOSITS.
Being inter-bedded with rocks of miocene age, it is certain that these ores are not older than middle Tertiary; nor can the pisolitic ore be younger, as it must have been formed contemporaneously with the rocks in which it occurs. The bole and lithomarge may be much younger if they are metamorphosed basalt, but the evidence afforded by the blended junction of the pisolitic ore and bole suggests that these two beds, at any rate, originated at the same time. It is also probable that the lithomarge was produced then as well, so that the whole three beds may be dated as middle Miocene.
The CHAIRMAN said, that this paper was one of a character which deserved their best consideration. The commercial importance of the subject was evidenced by the fact that in 1860 the produce from the Antrim fields was only 20,000 tons; at the end of the nest five years it had increased to nearly 100,000 tons ; in 1875 it was 122,000 tons ; and in 1879 it was 155,000 tons of iron ore. As there was still some other business to get through, he thought the discussion had better be postponed till the paper was printed and in the hands of the members.