CHAPTER VII

THE MONAZITE SANDS

It has been stated that monazite is a not uncommon accessory constituent of many rocks, particularly of granites, gneisses, diorites, etc. The crystalline material, of which an account has been given, is found sometimes in veins in these rocks, more often in tiny crystals disseminated throughout the mass. Most of these monazite-bearing rocks are extremely old, belonging to the Archæan or pre-Cambrian age, and probably none are of secondary (Mesozoic) or later age. It follows, then, that they have been subjected to erosion during practically the whole immense period of which geology can give us any detailed knowledge. Heat, frost, wind, the action of vegetation and of percolating water, the innumerable weathering agents known to the geologist, have been at work on them during countless ages, breaking, crushing, dissolving; rains, brooks, rivers, even ocean-waves have dissolved or washed away the fragments, sorted them out unerringly according to density, and re-deposited them, now in a river-bed, now at the base of some sea cliff, now in a wide alluvial plain from which the water has long since retired. It is in deposits of this nature that the monazite has been concentrated. Its relatively high specific gravity (about 5·0) has secured its separation from the lighter mica, quartz, and felspar of the parent-rock; but the heavier vein or accessory minerals have, of course, been concentrated with it. Zircon is an invariable constituent of these ‘monazite sands,’ as such deposits are called; and others almost as frequently found are rutile, ilmenite, sphene (titanite), and apatite. Common, too, are the characteristic minerals of the metamorphic rocks, garnet, epidote, sillimanite, tourmaline, etc. Rare earth minerals found in the monazite sands include xenotime, fergusonite, samarskite, gadolinite, and allanite. The remaining minerals are oxides of iron and tin, with, of course, a considerable amount of quartz.

It is apparent, from what has been said above, that monazite will be concentrated with the heaviest constituents of the rocks from which it is derived. Very often, indeed usually, these rocks are precisely those in which gold occurs, disseminated sometimes in tiny particles, sometimes collected into nuggets in veins of quartz and pegmatitic minerals. The erosion of these rocks concentrates the gold with the heaviest minerals; and hence it happens that monazite is an almost universal constituent of the gold- and gem-bearing sands and gravels. In the Carolinas and in Brazil, monazite is found in the gold washings; and though in the past the two have always been extracted separately, the gold first and the monazite from the washings or tailings, there appears to be no reason why a system calculated to extract both--where, of course, the content is high enough--should not be put into operation in the future.

A chemical test affords the only reliable method of detecting monazite in a sand. A little of the sand is washed with water to remove the lighter minerals and warmed with concentrated sulphuric acid. A few drops of the liquid are poured off, evaporated to small bulk, and one drop placed on a glass plate. This is placed under a microscope and one drop of a concentrated solution of sodium acetate is added. If monazite is present in the sand, tiny pointed oval crystals of sodium cerium sulphate will separate.

On the commercial scale, monazite is extracted from the sands only, in the manner described below. An effort was made in North Carolina in 1906 by the British Monazite Company, representing the South Metropolitan Gas Light Company of London, to extract monazite from the rock in which it occurs disseminated. The rock was crushed and powdered, and the monazite separated by washing off the lighter particles on concentration tables (see below). In the same year, however, the price of thorium nitrate was suddenly lowered 50 per cent. by the German Thorium Syndicate, which largely controls the Brazilian output of monazite, and the British company stopped operations in 1907. At present it may be said that only the sands are available for profitable extraction.

Up to 1895, the Carolina deposits, which were worked chiefly by the Welsbach Light Company of New York, either directly or indirectly, supplied all the demand, but in that year the Brazilian sands were first worked, and a keen struggle commenced for the market. The American companies, after keeping up a considerable output for some years, were forced to suspend operations in May 1910. The Brazil deposits, worked by the German Thorium Syndicate and the Austrian Welsbach Company, which have an agreement, now meet practically the whole demand. The Brazilian sand occurs chiefly along the shores of the southern provinces, having been concentrated by the action of the tides from the products of erosion of the cliffs; it is very uniform and considerably richer than the Carolina sand, and owing to its occurrence on the sea-shore, the cost of transporting it is very low. It is exported chiefly to Germany, recently also to the United States, and to a small extent lately to England. The method of working it is similar to that employed in Carolina--namely, concentration by washing and magnetic separation.

THE NORTH AMERICAN DEPOSITS[119]

[119] See Nitze, _Bull._ No. 9, _N. Carolina Geol. Survey_, 1895; also Test, _Colorado School of Mines Bull._ Vol. IV. No. 2, p. 125, Jan. 1908.

There are two important regions in North America within which monazite sands occur; one extends over the Carolinas, and the north-western part of Georgia, the other over the Idaho basin and neighbouring counties of the Pacific Slope. It will be best to treat these separately, as the deposits are somewhat different in character.

(_a_) ~The Carolina Deposits~, including the unimportant Georgia deposits, which belong to the same field, occur over an area approaching 4000 square miles. The area is occupied chiefly by the Piedmont plateau, which is drained by a number of streams rising in the South Mountains, an eastern outlier of the Blue Ridge; it is in the basins and valleys of these streams, particularly at the head-waters, that the monazite is chiefly found. The geology of the district is very complicated,[120] the rocks being very highly altered granites. The chief bed is known as the Carolina gneiss, and includes several types of gneiss, usually very much weathered. The sands, which average about 1 per cent. of monazite, are worked in and near the stream beds; they occur in the beds, and in layers 1 to 2 feet in thickness a few feet below the surface of the surrounding soil.

[120] See Sterret, _U.S. Geol. Survey_ (_Minerals_), 1906, p. 1195.

Concentration was formerly effected chiefly by a crude process of washing. In this process the sand is thrown on to a sort of sieve, fixed over the upper end of a long wooden trough, by one workman; a jet of water is directed on to the sieve, washing the sand through it. The heavier particles fall to the bottom of the trough, whilst the lighter are washed right through. A second workman continually turns over the sand left in the box and on the sieve; at the end of a day’s work the ‘concentrate’ is collected. This averages from 15 to 70 per cent. of monazite, according to the nature and amount of the heavy minerals accompanying it in the sand. The concentrate is dried either on rubber or oiled cloths in the sun, or on an iron plate covering a trough in which a fire is lighted. The iron minerals are then picked out by means of a magnet, and the sand filled into sacks for transport.

Before treatment for thorium nitrate, the sand is at the present day further concentrated by powerful magnetic separators. In a few cases the older method of concentration by hand-washing has been abandoned for machine concentration, the Wilfley table being sometimes employed. The principle here is exactly the same, the sand being fed into a hopper by means of a moving belt and thence on to a machine-shaken table from which running water constantly removes the particles, sorting them according to their specific gravity.

Further separation of the dried concentrate has been effected by three kinds of separators.[121] The first was of the Edison, or fall-and-deflection type; in this the sand is allowed to flow in a thin vertical stream past a horizontal magnet, which deflects the minerals containing iron; these fall on one side of a partition, the part richer in monazite on the other. The second was an electrostatic machine; the heated sand is borne on a moving belt underneath a rotating vulcanite cylinder, excited by felt-covered rubbers; the lighter particles are attracted to the cylinder, and dropped on one side, the heavier passing on. Neither of these machines is of much value in effecting concentration, and neither is in general use.

[121] See Pratt and Sterrett, _Trans. Amer. Inst. Min. Eng._ 1909, ~40~, 313.

The third, and by far the most efficient and most widely-used machine, is known as the Wetherill electro-magnetic separator. It depends on the principle, first applied by the American engineer Wetherill, that not only the iron minerals, but a large number of other minerals may be attracted if the magnetic field be sufficiently strong. In all types of this machine used in cleaning monazite concentrates, four magnetic fields of increasing intensity are traversed by the sand; the first removes magnetite, ilmenite, and the larger fragments of garnet; the second removes all the remaining garnet and ilmenite; the third removes the coarser, and the fourth the finer monazite, tailings of zircon, rutile, and silica passing on. Careful adjustment of the magnetic fields will readily give a 97-99 per cent. monazite.

Two types of this machine are in common use. In the first the magnetic fields are obtained by four successive electro-magnets, arranged so that a broad horizontally-moving belt passes between the poles of each in succession. The upper poles are ground down to a fine edge perpendicular to the direction of the belt, to secure a more powerful field. Just beneath these edges, and just above the broad belt are four rapidly driven horizontal belts moving at right angles to the first or main belt; these carry off and deposit in separate bins the minerals attracted by their respective magnets. This type is known as the Rowand separator.

In the second type four horizontal belts are arranged in the form of descending steps, as shown in the diagram (Fig. 2). The magnets are placed at the end of each belt, and within it. The attracted mineral is held to its own belt, whilst the remainder drops on to the next; the attracted mineral falls into a bin as soon as its belt carries it out of the magnetic field. The sand to be cleaned is fed on to the first belt by means of a hopper.

The almost pure monazite so obtained is now treated chemically for its thorium. The processes proposed and in use are described in Chapter XVIII.

As already stated, the extraction of monazite in the United States has practically ceased; but the processes outlined above, which were first brought into use in the Carolinas, have been adopted for the treatment of the Brazilian sands.

(_b_) ~The Idaho Deposits.~--Monazite was first observed in placer-gold deposits in the vicinity of Boise city near the Snake river. This deposit was a gold-bearing sand derived from granite. Later the gold-bearing sands of Oregon were also found to contain monazite; these sands are rich in zircon, and contain platinum and allied metals as well as gold. The sands of the Pacific slope are the so-called black sands, derived from hornblende, and augite-granites, usually porphyritic, which are much weathered at the surface. The soil is loose and is largely composed of granite fragments; the rain and streams constantly bring it down to the valleys, and continually renew the deposits. The concentrates obtained by washing are rich in well-crystallised zircon, with titanite and garnet.

In 1906 a company was formed to extract monazite from the black sand residues left after the extraction of gold. By 1909 they had erected plant and commenced operations at Centerville, and proposed to work the poorer auriferous sands for gold during the monazite washing. This, it was expected, could be done by washing the sands in boxes lined with amalgamated copper plates, which would retain the gold. Considerable amounts of monazite had already been extracted from the tailings when a disastrous fire put a stop to the operations in 1910.

Since then the production of monazite in the United States has practically ceased.

THE BRAZILIAN DEPOSITS

Brazil first became a serious competitor in the world’s market with the United States, for the supply of monazite, in 1895. The greater percentage of thorium, the more even quality of the sands, and above all the occurrence on the sea-coast, rendered the Brazilian monazite cheaper from the beginning, so that it soon ousted the Carolina sand, and since 1910 has supplied the whole demand. The deposits at present worked lie along the coasts of Bahia, Minas Geraes and Espirito Santo, and whilst they are very rich in monazite, there is the disadvantage that their position and extent, and so also the possibility of working them, depend very largely on the variations in the tides, etc. The largest of these deposits is on the shores of a bay near the island of Alcobaca, on the southern coast of Bahia.

Monazite also occurs to a considerable extent in the diamond sands and gold-bearing sands of many of the interior provinces. In Minas Geraes it has long been known to occur at the celebrated mining centres of Diamantina and Ouro Preto, where xenotime and other rare earth minerals are also found; it is also known at various localities in the surrounding mining provinces of São Paulo, Goyaz, and Matto Grosso. More recently, extensive inland deposits have been found by Freise, in the province of Espirito Santo.[122] In the plateau-basin of the Muriahé and Pomba rivers he found a sand known locally as ‘catalco’ which carries an average of 2·1 per cent. of monazite and a gold-content of 1·75 grams per ton. In the Aymoré’s mountains he found monazite, both massive and granular, in pegmatite veins in granite; analysis showed a thoria content of 9·23 per cent., which is very high. These deposits would form a very valuable and extensive source of thoria, if the difficulties of transport could be overcome.

[122] _Zeitsch. pr. Geol._ 1909, ~17~, 514; _ibid._, 1910, ~18~, 143.

At present, as stated above, only the beach deposits are worked. The Brazilian Government has laid a very heavy tax on all monazite exported; it is stated[123] that the German Thorium Syndicate pays 50 per cent. of its profits in royalties to the Government. In spite of this, the high quality of the sand and the low cost of transport have enabled this combine to lower the price of thorium nitrate to a point at which the Carolina sands cannot be worked, and it appears probable that the world’s markets will be supplied for some time, at least, entirely from Brazil. The methods employed in working the sand are similar to those already described.

[123] _U.S. Geol. Survey_ (_Minerals_), 1906, p. 1195.

In the last few years monazite deposits have been found in various places, notably in Australia, India, and Ceylon. In the latter locality it occurs sparingly in the gem-gravels, in association with the much more valuable thorianite and thorite, but the supply is uncertain, and the minerals cannot be worked regularly. In Australia it occurs in Victoria and in Queensland. In Victoria the deposits are poor in monazite--about 0·025 per cent.--so that working is not profitable. In Queensland it occurs in beach sands on the southern coast, with gold, platinum, and cassiterite; there seems to be no reason why these deposits should not be profitably worked when sufficient labour is forthcoming. It also occurs in North Queensland, on the Walsh and Tinaroo mineral fields; here it is found massive and granular in veins in granite, associated with wolframite, molybdenite, and cassiterite.

Quite recently, deposits of considerable extent have been found near Travancore, India.[124] These sands contain about 46 per cent. of the mineral, which is itself very rich in thoria, containing about 10 per cent. of the oxide; the unconcentrated sand is therefore as valuable as a source of thorium nitrate as the ordinary Brazilian concentrates, which average 4 per cent. or less of the oxide.

[124] _Bull. Imp. Inst._ 1911, vol. ix., No. 2, p. 103.

Monazite has also been observed in the tin-bearing sands of Embabaan, Swaziland, South Africa, and in the province of Ottawa, Canada.