J.D. Kendall on Eskdale iron ore

J.D. KENDALL ON ESKDALE'S IRON ORE

"The Iron Ores of Great Britain and Ireland", by J.D. Kendall F.G.S. (published by Crosby, Lockwood & Son, London, 1893) contains some interesting paragraphs on Eskdale's iron ore, slightly revised from a paper he wrote in 'The Haematite Deposits of West Cumberland' in Proceedings of the North of England Institute of Mining and Mechanical Engineers, vol. xxviii, 1879, pp. 10-12 and Plate xxii [from which the illustrations here have been taken- Fig. 3 from 1879 has been renumbered as Fig. 1, to match the 1893 text; other information from the 1879 paper is in square brackets]

p76: "Deposits in the Eskdale Granite and Ennerdale Syenite. These are somewhat numerous, but so far as proved not of any great importance commercially. The principal deposits hitherto worked are in the granite. One of these is near Boot, in Eskdale. Another, but much smaller, deposit was worked [ditto 1879] near the 'King of Prussia,' in the same valley. Several simlar veins have also been worked in a small way in the syenite of Ennerdale, notably at Crag Fell. They are all very much alike, both in form and inner nature, so that Nab Ghyll vein at Boot, which has been most worked, may be taken as typical. The form of the deposit is that of a vein. Its direction is nearly north and south, and it 'hades' to the east at angles varying from 65° to 80° with the horizon.

In the part of it which, so far as is known, is most fully developed, its existence is indicated on the surface by a small ravine, known as Nab Ghyll, which runs down the southern side of the mountain that lies between Eskdale and the upper part of Miterdale. The ore is worked by levels driven into the hill at various heights [as shown in Fig. 2, a longitudinal section of the vein]. It is not continuous throughout the vein, but, as in most mineral veins, occurs in lenticular forms called 'bunches' or 'bellies.' These bunches are usually thickest near their centre, whence they thin off, or as the miners express it, 'nip out,' gradually towards the edges. Between the bunches there is invariably a 'leader,' marking the vein, but it contains very little ore; sometimes, in fact, none at all, being merely a strong joint. The length of vein that has been worked up to the present time is about 200 fathoms, and the depth to which the ore has been found to extend below the surface is about 50 fathoms. The greatest width of the vein has been about 20 feet, including 11 feet of 'horse' or 'rider.' This was near the surface. Further down it is much narrower, and appears to die out altogether at a depth of 50 fathoms. It is, in fact, a 'gash' vein. A section of it at the surface, as seen in No. 1 Quarry [Fig. 2 top right], is shown in Fig. 1.
Section of Nab Ghyll vein

It will be observed that the vein, at this point, consists of four distinct lots of ore, separated from one another by three ribs of 'horse' or 'rider.' The 'hade' of the ore, as well as its 'direction' horizontally, agrees exactly with that of the large joints (d) intersecting the granite. These joints are very strong and persistent, and give the granite at a distance the appearance of bedded rocks standing on end. The beautiful ravine, on the opposite side of the valley, called Stanley Ghyll, seems to have been cut out along these joints. Frequently, from the main line of ore, small strings of it are found branching out into the granite along some of the minor joints by which this, in common with other granites, is also intersected. If two of these strings cross one another a small bunch is formed.

Near the surface the 'cheeks' of the vein are very much decomposed, being most so adjoining the ore, whence the granite becomes harder and harder, until at a distance of a foot or two from the vein it assumes its normal condition. The same thing occurs in the lower parts of the vein, only the amount of decomposition and the distance to which it extends into the cheek become less and less in depth. It is also observable that the cheeks are more decomposed where the vein contains ore than where it is barren.

Those parts of the vein marked c in Fig. 1 are not solid masses of haematite, but a mixture of red earthy-looking ore, hard fibrous kidney of purplish tint, black oxide of manganese, reddish clay, grains of quartz, similar to those in the granite, and pieces of more or less decomposed granite. Generally these various materials are arranged in broken lines parallel to the sides of the lode, as shown at c.'' Some of the more earthy-looking ore contains pieces of decomposed felspar, and it is sometimes possible to obtain a piece from the vein showing, on the one hand, hard pure kidney, and on the other granite very little latered- that is to say, there seems to be, in such a case, a gradual transition from granite to pure haematite. Some pieces of ore have been obtained from this mine having the appearance of 'ring-ore.' They are usually less than an inch in diameter, and in cross section present a number of concentric rings somewhat like the annual layers of an exogenous tree. Down the centre there is frequently a hole, as if the substance around which they seem to have been formed had disappeared.

Sometimes the ore is stalactitic, occurring then in cavities, and occasionally a piece of ore is met with united to dolomite in such a waY that the two seem 'grown together.' The occurrence of dolomite in some of the veins in Eskdale is very common, and the author is of opinion that it has a most important bearing on the origin of these deposits.

The chemical composition of the ore from these veins is partly represented by the following analyses of ore from Nab Ghyll vein:-

	No. 1. Dried at 212° Fahr.	No. 2.
Ferric oxide	27.43	92.57
Manganous oxide	.03	.02
Silica	2.15	2.05
Alumina	6.10	.88
Lime	23.18	.50
Magnesia	9.04	.08
Phosphoric acid	.04	.03
Sulphuric acid	.02	.01
Carbonic acid and water	32.00	3.70
	99.99	99.84
Metallic iron	19.20	64.80

No. 1 is much worse, and No. 2 better, than the average. No. 1 clearly contains a considerable proportion of dolomite."