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The Pamphlet Collection of Sir Robert Stout: Volume 2

No. 2

No. 2.

Almost every known variety of iron ore has been discovered in the Colony, but none have yet been successfully worked, chiefly owing to the want of enterprise and practical acquaintance with the subject.

Until lately, the amount of iron required in the Colony has also been limited, and imported iron has been cheap; but the recent rise in price, and the demand for rails and other iron-work required in connection with the public works now in progress, are causing attention to be directed to this branch of our natural resources.

For the purpose of classification, the iron ore may be divided into Granular and Massive, the former group including all the varieties of iron sand (which have always had great and, I think, undue prominence given to them in New Zealand), and the latter including all the ordinary ironstones, occurring either as stratified masses or as vein-stones.

I. Granular Ores or Iron Sand.—Iron sand, or black sand, as it is usually termed, is found in every part of New Zealand, there being few soils or stream gravels that will not yield a considerable quantity when washed in a pan in the same manner that the gold-diggers prospect for gold. The chief deposits are, however, to be found on the seashore of the west coast of both islands, the best known deposit being that at Taranaki, where the shore between tide marks is, for many miles, almost wholly formed of this black iron sand, to the depth of several feet.

Several companies have been formed, both in England and in the Colony, to manufacture steel from this iron sand, and very considerable sums have been spent, but as yet without success. Lately, a large extent (about five miles) of the beach has been leased to a new company, and they are now erecting furnaces. At the Manukau Heads, near Auckland, a similar deposit of iron sand has also been worked, but the company failed.

Before describing the processes which have been employed, and those which are adapted to this class of ore, a few words may be said page 16 on the variety of mineral compositions it presents, and the geographical distribution of those varieties, which is somewhat singular.

Samples from twenty-six different localities have been analyzed, as shown in the appended Table I. The iron they contain is present as magnetite (Fe2 O3 Fe. O.), hematite (Fe2 O3), or as titaniferous oxide (Fe. O. Fe2 O3). The acidic rocks, such as the granite of the South Island and trachytes of the North, have been the chief source of the magnetite. The metamorphic schists afford the drifts abounding in hematites or specular ores, while the titanic oxides are derived from the diorites and basaltic rocks.

These observations give a fair indication of the formations where, in future, the different ores may be looked for in the massive form.

The various methods which have been proposed for working these iron sands were described in my evidence before the Colonial Industries Committee last year, as follows:—

"Iron sand was first worked in 1842, by Mr. Home, a steelmaker and cutler in London, who extracted 60 per cent, of malleable iron from iron sand obtained in America, and which he converted into steel. The Japanese and natives of India have also long used iron sand as ore for the production of a fine quality of malleable iron for conversion into steel. Patents were granted for producing cast steel direct from these sands in crucibles, being, in fact, an extension of the ordinary method used in the laboratory; but it was not found possible to produce uniform results on a large scale. In 1845, Heath proposed to reduce ores such as iron sand by the addition of a small proportion of charcoal, and thus produce a spongy mass of malleable iron, which was then plunged in a bath of molten cast iron, in a proper proportion to make steel of the compound. A modification of this process is now in use in Sweden, but requires the use of crucibles. In 1868, Leckie, of Montreal, proposed to mix the iron sand up into a lump with charcoal, and place it in a hearth at the back of a bath of molten cast iron in a reverberatory furnace. After the reduction, the lump was to be tipped into this bath. This was an attempt to work with a single furnace without crucibles, and by a continuous process. It does not yet, however, appear to have been a success. In 1868, Ellerschausen proposed to decarbonize pig iron by the addition of oxides, such as iron sand, while the metal was flowing from the furnace. This process, which requires very peculiar machinery, is now in use at Pittsburg, in the United States. In 1851, Stenson obtained a patent for working the iron sands of New Zealand by means of a blast furnace, the sand being worked up with clay containing a small proportion of lime, ground in a pug-mill and formed into bricks. These bricks were then treated as common earthy iron ores, the results being a pure cast iron, which might be converted into steel by a second process. Many patents have been taken out relative to the New Zealand iron sand, which have all relation to some supposed virtue which they possess from their containing titanium, and which would give them such extra value for the production of steel as to warrant the employment of expensive processes of manufacture. None of these have been a practical success. We page 17 thus have four processes for the conversion of these iron-sand ores:—(1.) By cementation with charcoal, the result being malleable iron. (2.) Being mixed with clay, they are reduced by a flux in an open blast, the result being cast iron. (3.) The cement sponge, obtained by a process like No. 1, is plunged in a bath of cast iron, the result being steel. (4.) The decarbonization of cast iron by the addition of the iron sand, the result being malleable iron of a fine quality. There is no necessity for experimental research being undertaken, as the iron sand is the same in New Zealand as that which has been and is being tried on a large scale in other countries."

The method employed by the company in Auckland was to mix the iron sand with scoria or volcanic slag, which abounds in the neighbourhood. A small quantity of iron was produced, but, as might be expected, was most uncertain in quality.

At Taranaki, the method proposed is to mix the iron sand with calcareous clay and some carbonaceous matter, and to form bricks, which are afterwards to be treated as iron ore. The introduction of impurities into a naturally pure ore, and the uncertainty of the product, whether iron or steel, is the defect of this system.

The only practical trial which has been made on this plan resulted in the formation of a large ball of cement-stone and a small quantity of impure cast steel.

In working these granular ores, the chief difficulties to be met with are—
1.In the collection of the ore in the pure form.
2.In getting pure fuel (coke or charcoal) sufficiently cheap.
3.In getting the labour required to carry out the annealing process.
4.Or, if any of the newer processes are attempted, to get the requisite apparatus and skilled labour.
5.The cost of export from the exposed coasts where the granular ores most abound.

II. Massive Ores.—The massive iron ores have not yet been applied to the manufacture of iron in New Zealand on a large scale; but in one case pig or cast iron of excellent quality has been manufactured on a small scale, as an experiment, from a brown ore, at Parapara, near Collingwood. The same ore is also used in Nelson for the manufacture of iron minium, or ochre pigment.

A selection of the principal iron ores which have been examined is given in Table II.

No. 1, though placed with the massive ores, is really the common black sand, which in certain localities becomes cemented by oxidation, so as to form a very compact stratum, which can be broken up and worked like an ordinary ironstone. This form of ore is tolerably abundant south of the Manukau Heads, and might be obtained for admixture with other varieties.

No. 2 is from a vein of loadstone that occurs in chlorite schist in the interior of Otago. The rock at this place is a very compact foliated diorite, having crystals of magnetite dispersed in the same way that garnets occur in mica schist. No attempt has been made page 18 to ascertain the thickness of this lode, as its position is at present too inaccessible to permit of its being worked.

No. 3 is from a lode of hematite, or specular iron, 6 feet in thickness, that occurs in mica schist in the vicinity of the Wakatipu Lake. Other veins of a similar nature, and also lodes yielding 20 per cent, of copper, have been found in this district. Limestone also occurs in the vicinity, and, as charcoal can be obtained in abundance from the extensive forests, there is a fair prospect of metallurgical operations being commenced when the railway now in course of construction, to connect the Lake with Bluff Harbour, is completed.

No. 4 is from a vein from the serpentine series of the Dun Mountain, or what is known as the mineral belt of Nelson. This series forms a range along the east side of Blind Bay, which is singular in having a barren appearance, due to the large amount of magnesia mixed with the soil. The sections of these hills show them to consist of slates, limestones, and indurated sandstones, containing triassic fossils, that pass in ascending series to diabase breccias, associated with diorite, and a peculiar form of compact olivine rock known as dunite. This rock appears at the surface as large masses, several miles in extent, and is speckled with grains of chromic iron; but the principal deposits of ore are in a band of serpentine lying between the dunite and a limestone formation. The serpentine is traversed by dykes of felstone, diallage, and a variety of other minerals; the district being one of the most interesting in New Zealand to a mineralogist. It is here that the hematite vein was found in 1853, along with red, grey, and blue oxides of copper. The thickness was not ascertained at the time, and the works have now fallen in.

This sample of hematite is interesting on account of its containing an appreciable amount of gold.

A company was established in London for working the mines at this place, and after expending large sums of money, without, however, any systematic mining exploration being performed, they last year finally wound up their affairs, abandoned the ground, and sold off all their appliances Only the chrome ore was worked, 5,000 tons having been raised. The mine, which is at an altitude of 2,500 feet above the sea, was connected with the port of Nelson by a railway 12½ miles in length, and having a gradient for a great part of its course of 1 in 18; but the rails have now been removed and sold.

No. 5 is a sample of mixed hematite and magnetite, containing a marked percentage of manganese, and is found in brecciated sandstones and slates, probably of triassic age, on the west side of the Gulf of Thames. Pure manganese ores are very common in this formation, and have been worked on the Island of Pakihi, and also near the Bay of Islands. The occurrence of copper lodes at Kawau and the Great Barrier, in connection with the same formation, affords an indication of its being worthy of a more thorough mineral exploration than it has yet received.

The above are only a few of the many anhydrous massive iron page 19 ores which exist in New Zealand; and the next group consists of those ores which contain a large proportion of combined water, and are generally known as brown or hydrous ores.

No. 6 may be taken as a type of the bog iron ore which forms in swampy localities, generally near the outcrop of the brown coal formation. From the quantity of sulphur generally contained in this ore, it is not adapted for making good iron; but it could be obtained in considerable abundance, and under favourable circumstances for working, being generally close to supplies of coal and limestone.

No. 7, from Raglan, may be taken as an average sample of the brown iron ores from the Tertiary coal formations, which occur generally as ballstones or concretionary masses dispersed through the shale, and can generally be obtained in large quantities wherever that formation prevails. As this ironstone is of great practical importance, a detailed analysis of its composition has been made, as follows:—
(No. 1,395)/L Sesquioxide of iron 72.69
Oxide of manganese .56
Alumina 1.16
Lime .27
Magnesia .69
Phosphoric acid .70
Sulphide of iron traces
Siliceous matter 6.30
Water hygroscopic 4.61
Water constitutional 13.02

No. 8 is a hydrous hematite, formed by the decomposition of the hematite ore associated with the manganese veins alluded to under No. 5. This particular sample is from the island of Kawau, but this ore is generally to be found in thin, irregular veins wherever the sandstone formation has undergone decomposition into a form of laterite, which takes place to a very great depth in some places. As an iron ore, however, it would be very difficult and expensive to collect, unless to supplement a supply derived from better-defined deposits of ore.

No. 9 is from a deposit of hydrous hematite that occurs in the upper tertiary drifts at Parapara, in the province of Nelson, but is also not infrequent in many other localities.

The ore occurs as the matrix of a quartz conglomerate, but often containing large masses of nearly pure ore, of several hundred pounds weight. On breaking these there is frequently a kernel of undecomposed sulphide of iron, showing the origin of the ore to be probably from the denudation of a mineral vein. The deposit at this place covers about fifty acres in extent, and is 100 feet thick, and, as previously mentioned, is worked on a small scale for the preparation of pigment.

Taking the foregoing as typical samples of the different varieties of page 20 hydrous iron ore, it now remains to notice the occurrence, in the upper secondary coal formation, of iron ores containing a sufficient per centage of carbonate of iron to entitle them to be classed with the black bands or spathic iron ores, which are generally considered the most favourable for the purposes of a blast-furnace. Nos. 10 and 11 are samples of this ore, taken from two veins that occur along with coal seams at the Collingwood Coalmine, and on account of their importance, I also give the following detailed analyses of their composition:—
No. 1296. No. 1361.
Protoxide of iron 35.23 40.38
Sesquioxide of iron 25.77 5.26
Oxide of manganese 1.10 traces
Alumina 2.11 .40
Magnesia 1.94 .63
Lime .71 .66
Silica .90 .55
Phosphoric acid not determined not determined
Sulphuric acid traces traces
Carbonic acid 21.12 21.97
Sulphide of iron .41 .09
Water 1.96 .39
Organic matter 5.72 12.98
Silicates undecomposed by acid 3.03 16.69
100.00 100.00

These ores occur at Collingwood under circumstances very favourable for their being worked, as they can be mined along with coal of the very finest quality that occurs in New Zealand, the only drawback being that the coal seams are not more than 3 feet thick, while the ore is from 10 to 20 inches. The coal-measures are now being explored by a tunnel 700 feet in length, and, so far as it has gone, they show an improvement from what was anticipated from the examination of the outcrop. Close to the coal mine there is abundance of fine crystalline limestone, belonging to an older formation; and deposits of graphite and also micaceous hematite, that might be useful in working puddling furnaces, are found in the same district.

The area of country near Collingwood occupied by a similar formation is about ten or twelve square miles; and as Golden Bay offers very favourable conditions for the erection of deep-sea wharfs, I think it likely that this locality will be the first in New Zealand where the mineral resources will lead to the establishment of a manufacturing industry.

An estimate of the cost of the erection of suitable blast-furnaces has been obtained from a reliable source, which shows that with a capital of £22,000 invested, 18,000 tons of pig iron could be produced annually, at a cost of less than £100,000 for working expenses. (See Pari. Papers, 1873, E. 10, p. 25.)

page 21
I.—Tabular Statement of Iron Sands.
No. Locality, and matrix from vrhich probably derived. Magnetite. Hematite. Titanite. Per cent. of Iron. Accessory Minerals.
1 Upper Buller River, Nelson—Hornblende rocks 87.5 9.4 70.2
2 Lower Buller River—Tertiary gold drift of diorite slate 54.0 42.3 59.0 Auriferous
3 Upper Molyneux River, Otago—Mica schists 82.7 9.7 65.9 Auriferous
4 Lower Molyneux River—Mica schist and tertiary strata 74.4 2.5 58.7 Auriferous, and with 12 percent, of glauconite.
5 Mountain Stream, Canterbury—Palæozoic slates, &c. 62.7 37.2 66.2
6 Mountain Stream, Otago—Palæozoic slates, &c. 86.1 10.5 58.5 Auriferous
7 Tuapeka, Otago—Old gold drift 2.2 92.8 63.8 Auriferous
8 Wakatipu, Otago—Mica schist 80.0 7.6 52.9 Auriferous
9 Mataura River (Upper)—Diorite slato 9.8 70.9 41.2
10 Mataura River on a spur close by—Old gold drift 63.5 16.1 8.0 60.6
11 Stewart Island—Granitic rocks, with greenstone dykes 77.8 20.1 57.3 Auriferous
12 Stewart Island—Hornblende rocks 71.5 20.0 8.2 70.1 Auriferous
13 Anatoke, Nelson—Granite and hornblende 79.8 7.7 3.4 60.2 Auriferous
14 Mahinepoa Lake (old channel of Hokitika)—River drift from diorite rocks 58.0 29.1 Auriferous, with garnets, topaz, disthene, &c.
15 Sea Beach, Hokitika—Sea sand drift 75.0 54.0 Auriferous
16 Motueka River, Nelson—Tertiary strata and granite 33.0 32.5 42.0 Auriferous
17 Wairau River, Marlborough—Tertiary strata and granite schist 21.0 48.4 38.9 Auriferous
18 Wanganui River, Nelson—Granite and tertiary 54.0 13.0 43.2 Auriferous
19 Saddle Hill, Otago—Basaltic 58.3 25.6 52.9
20 Green Island, Otago—Basaltic or sea beach 53.3 29.6 50.3
21 Hooper Inlet—Basaltic 20.0 74.2 53.0
22 W. Bluff, Southland, Foveaux Straits—Diorite, or sea beach 12.2 40.6 28.6 Auriferous and Platiniferous
23 D'Urville Island, Nelson—Diabase and Granite 78.6 57.4 Chrome iron
24 Taranaki Beach—Trachyte 91.9 6.2 70.1
25 Taranaki Beach—Trachyte 71.0 8.0 56.1 Olivine and hornblende
26 Tauranga Beach—Trachyte 87.4 8.6 68.0
page 22
Table II.—Massive Iron Ores, Oxides, and Titanites.
No. Variety. Locality. Centesimal Composition. Percentage of Iron. Remarks.
Magnetite. Hematite. Titanic Iron. Siliceous matters. Water.
1269 Impure magnetite Manukan, Auckland 60.20 37.90 traces 1.90 70.06
426(b) Magnetite Dunstan Gorge, Otago 86.32 traces 13.68 63.60
426 (a) Hematite Dunstan, Otago 96.11 3.89 68.30
Magnetite Dun Mountain, Nelson 90.60 7.60 1.8 63.40
1274 Mixed magnetite and hematite Maramarua, Auckland 2.24 87.10 traces 10.66 62.30
22 Bog iron ore Spring Swamps, Auckland 73.17 13.83 13.20 51.22
1395(b) Brown iron ore Raglan 72.69 9.68 17.63 50.88
1199 Brown iron ore Kawau 67.98 19.65 12.37 47.58 Manganese oxide, 1.38
1230 Hydrous hematite Parapara, Nelson 62.68 24.08 13.24 43.87 Contains a little manganese
Black Bands or Spathic Iron Ores.
No. Variety. Locality. Prot-oxide of Iron. Sesquioxide of Iron. Carbonic acid. Silicates. Percentage of Iron.
1296(1) Black band Collingwood, Nelson 35.23 25.77 21.12 3.93 46.06
1351 Black band Collingwood, Nelson 40.38 5.26 21.97 16.69 35.12