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August, 1940

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Haematiteore-bodies have generally been regarded as non-magnetic. It is true that some haematite crystals contain FeO and are noticeably magnetic (Sosman and Hostetter 1917 a and b), but the Cumberland-Furness ore, which is probably a hydrated oxide (Smith 1919) contains very little ferrous oxide. The value k_v=22 x 10^-5 recorded by Wilson (1919, P. 86) may relate to haematite of this kind: rough laboratory tests made for the present purpose upon good Hodbarrow ore (South Cumberland) gave somewhat lower values. By way of comparison it may be recalled that for basalt k_v may be 150 x 10^-5 or more.

In 1934 one of the present authors (J. T. W.) observed that a large piece of Hodbarrow haematite exerted a noticeable force when brought close under a vertical magnetometer. A little later he made a field test along three traverses at Hodbarrow mine, south of Millom; they were actually very near the three lines A, B. C, south of Red Hill Farm (Fig. 1), and the resulting profiles indicated decisively that the ore-body gave rise to a measurable, though small, anomaly. These original curves were very closely similar to profiles A, B, and C (Figs. 2 to 4), which were obtained (A. F. H.) in traverses made independently on the same site four years later in the course of the preliminary trials, a description of which forms Section II of this report.

Magnetic surveying with one instrument is slow and is liable to interruption by disturbances of the earth's field. Further work was therefore undertaken jointly by the present authors, using two magnetometers; and an area about 1,000 ft. square, adjacent to that of the original tests at Hodbarrow, was surveyed in considerable detail. Section III of this paper gives an account of the results of this extended trial and of the methods employed, with special reference to the precautions needed in observing and interpreting small anomalies of the kind encountered.

The authors desire to express their best thanks to Professor G. Poole for his cordial assistance, especially in arranging for co-operation between the several members of the staff of the Mines Department at King's College, Newcastle, and the Geological Survey ; to Messrs. W. Staton, H. M. Nicholson and W. Bullerwell, graduates of King's College, who acted as research assistants in the field work; and to Mr. W. C. C. Rose and Dr. K. C. Dunham of H.M. Geological Survey, who for the past field season have been engaged on a re-survey of Furness and South Cumberland with special reference to the distribution of iron ore. In the course of this work they formed the opinion that use should be made of the magnetic method, so far as it proved suitable, and the areas upon which the present tests were made were recommended by them as providing suitable geological conditions. They also gave considerable assistance in the preliminary trials described below, and advised throughout upon the geology of the areas concerned. Every facility was given by the Hodbarrow Mining Company, Ltd., and by Newton Mines Limited (near Dalton-in-Furness), on whose ground the tests were carried out.

It may be added that in March, 1938, the Geological Survey had access to an unpublished report by Professor A. H. Cox, which had been compiled at the request of the Commission for the South Wales Special Area. This report advocated a magnetometer survey for prospecting in the haematite field of South Wales, and contained the following significant sentence "A short and rather hurried test over the Llanharry deposits shows that, with the help of a Watts magnetometer, it is possible to determine the positions of the larger ore-pockets even under the 200 to 300 ft. of cover present."

II. Preliminary tests
(with one magnetometer)

In order to ascertain the magnetic properties of the local haematite ore, it was decided to make trial observations, in the form of traverses over ore-bodies, of which the existence had been fully proved by mining though the ore had not yet been extracted. The five profiles observed are shown in Figs. 2 to 5.

Two areas were selected, one at Hodbarrow mine and the other at Newton mine, at both of which Carboniferous limestone containing iron-ore veins at a depth of about 60 to 160 ft. is concealed beneath about 60 ft. of boulder clay.

Method used in preliminary tests. — Small anomalies of the present kind may be completely obscured by variations in the earth's field during the observations. For work requiring less accuracy it is usual to correct readings taken at successive stations by means of simultaneous values recorded at a neighbouring magnetic observatory, in this case Eskdalemuir. At a distance of 50 miles or more it is, however, by no means certain that the variations at two places are strictly parallel ; in the present tests, therefore, a local base was selected, at which base readings were taken between the occupation of each successive pair of stations along a traverse. The observed anomaly was thus obtained by direct difference from a strictly local base value. The interval between readings was about five minutes and it was found that, under the quiet magnetic conditions prevailing at the time, the earth's force remained sufficiently constant to allow differences to be taken with an accuracy of about ±2 gamma.

The instrument used was an Askania vertical variometer (G.S. 229), adjusted to a sensitivity of 20 gamma per division. Temperature correction was eliminated by the use of a local base as described above.

Provision is made on the instrument for reversing the vertical field-balance to obtain a result that is the mean of two sets of readings with the beam pointing E.-W. and then W.-E. This reversal eliminates errors in levelling, and is of value in extended surveys, where re-adjustment of the levels may be required during working ; but for short traverses of the present type it was found preferable to take only three readings, with the beam in the first position, using special care in levelling. The position of the level directly affects the reading and can only be taken as constant over a limited period, but the accuracy obtainable within such limits is at least as good for the present purpose as with the method of reversal, and there is a great gain in speed and convenience. The tripod should be kept in the same orientation throughout, especially if the auxiliary magnet is in use, for any eccentricity in the position of the latter may introduce an error if the tripod is in a different orientation at different stations.

Results of test traverses on known ore-bodies. — As stated above, two localities were investigated. The first was the field south of Red Hill Farm, Hodbarrow mine, near Millom (Fig. 1), where a long narrow ore-body extends N.W.-S.E. in the position shown in plan in the sketch-map. This ore-body is a vein dipping S.W. at from 60 to 45 deg. In it the ore has replaced limestone along a fault (the more north-easterly of two shown in Fig. 1), especially in the hanging wall. The top of the limestone is covered by about 60 ft. of boulder clay. Typical cross-sections, under traverses A, B, and C, are shown in Figs. 2, 3, and 4. A total thickness of over 100 ft. of limestone is underlain by the Basement Beds of the Carboniferous System, and the ore preferentially replaces the upper and middle part of the limestone, sometimes extending into a flat-like shape as in Fig. 3. The bottom 50 ft. of limestone is more compact, of calcite-mudstone type, and is relatively barren. Further details regarding the geology are given later in the paper.

The ore-body had been proved by several levels and crosscuts, but only the lowest part under traverse C had been removed. Cross-sections were supplied by Hodbarrow Mining Company and are shown (omitting levels) beneath the respective profiles in Figs. 2 to 4.

At the second locality, Newton mine, near Dalton-in-Furness, two traverses (P and Q, Fig. 5) were made over a north-west vein lying about midway between the Dendron and Stainton roads, near their intersection at Stainton Tithe Barn about half a mile W.S.W. of the Crown limestone quarry. The traverses were parallel with the aerial ropeway from the mine and about 150 ft. distant from it on either side, that with the larger anomaly being on the north-west side.

Rather irregular ore-bodies occur in about 100 ft. of limestone overlying a shale bed. They appear to follow north-west fissures or faults, and they dip towards the southwest at an angle varying between vertical and 45 deg. Typical cross-sections, supplied by Newton Mines Limited. are shown in Fig. 5. Some ore has been removed, beginning from the top.

In general form these ore-bodies closely resemble those at Hodbarrow, and the magnetic profile P is quite comparable. The profile Q is weaker, but corresponds in general form with that calculated for a 30-ft. vein with about 60-ft. of cover (Fig. 6); the vein shown in the section is about 20 ft. thick (horizontally) and nearly 80 ft. below the surface. Closer comparison must not be attempted in view of the variable section along these veins.

Interpretation. — No exact correspondence is to be expected between the effects produced by an irregular ore-body and by one of simple geometric form; but a comparison may be of value so long as there is an approximate similarity of shape and size.

A method of profile calculation for simple geological models has been described (Hallimond 1930 b). In the present case the ore-body approximates to a parallel-sided inclined mass, extending indefinitely in the horizontal direction and terminated above and below by horizontal plane surfaces. Such a mass is shown in cross-section in Fig. 6. The profiles given above were calculated (using graphical methods) by means of the formulae already published in the discussion of the Lornty Dyke (Hallimond and Herroun, 1933, pp. 310, 311). Two thicknesses were considered, 30 ft. and 60 ft. measured horizontally, with depth of cover 60 ft. and vertical depth of ore-body 100 ft.

If the earth's field-strength is 0.53 Oersted inclined at 70 deg., the intensity of magnetisation, i, of the top and bottom of an ore-body with susceptibility k will be 0.45 k. The normal to the sides is inclined at about 35 deg. to the direction of (lip of the earth's field and they will have a magnetisation of 0.53 k cos 35 deg., in the absence of permanent magnetisation. The formulae used are (details will be found in the paper mentioned above)

Top or bottom ... v=2i (Θ₂—Θ₁)

Side ... ... v=2i{log r₂/r₁ . sin ø-(Θ₂—Θ₁) cos ø}

In these formulae k appears only as a factor in i, while the remainder of the expression is geometrical. Thus if the susceptibility of the ore is changed the only effect is to change the vertical scale of the anomaly without any change in form.

Now if the value of k be put as low as 10 x 10^-5, the height of the calculated curves is lower than that of the observed profiles. In order to obtain a sufficient agreement, it is necessary to put k=50 x 10^-5. When this has been done there is a very good coincidence between the observed profile and the superimposed theoretical curve, as will be seen from Fig. 7. Geometrically the interpretation seems to be fairly clear, so much so, that for veins of similar form and orientation the magnetic profile might be used by comparison with Figs. 2 to 6 to forecast the approximate size and position of an ore-body.

As mentioned above, the value 50 x 10^-5 assumed for susceptibility in drawing the profiles of Figs. 6 and 7 seems definitely greater than that observed in the laboratory in a stronger field. This difference might be due to a difference in susceptibility in the weaker field of the earth ; to a magnetic "creep" in the haematite under prolonged exposure to the earth's field; or to permanent magnetisation. If the latter is present, it is in a direction substantially parallel with the earth's present field. Whatever the cause, the net result is that the magnetic anomaly actually observed is of the right order of size but is numerically greater than that which would be forecast from the susceptibility measured in the laboratory. If the ore-body is a vein-like deposit with reasonable persistence in depth, the preliminary tests have shown that it will be easily possible to detect a vein about 20 ft. thick under 60 ft. of cover, or a proportionately wider vein under thicker cover; the shape of the profile will depend, however, upon the orientation of the vein in relation to the earth's field.

Effect of concealed pipes in mine workings. — In considering the trial profiles over known ore-bodies, great care must be taken to avoid a false conclusion due to the iron-work present in the mine.

The Hodbarrow Mining Company have kindly supplied a plan of the workings beneath traverses A, B and C, with details of iron-work as follows :—

The depths are reckoned as below the magnetometer. Rails are 3 in. high.

The pipes and rails are mainly in levels more or less at right angles to the traverses and are in the lower part of the ore-bodies or in the limestone below. It will be seen in the traverses H and L that the effect of a pipe at surface was only apparent up to about 30 ft. away; most of the near-by effect was probably due to permanent magnetisation in certain sections of the pipe. It seems unlikely, therefore, that the profiles A, B and C can be due to the iron-work: at most, they may be slightly modified in form. But a final proof that the iron-ore is the real cause of the anomaly is found in the detailed survey north of the farm, described in Section III. In this case an anomaly occurs in typical form and size in traverses H, I and J, over ground where no mining at all has been done.

* From "Bulletin of the Geological Survey of Great Britain — No. 2, " by permission of the Controller of H.M. Stationery Office.

Drawings and Photographs accompanying the article