Eirated Institution of Miring Enguan

Trans xptions 1890-91.

VOL II, PLATE VI.

To illustrate M. David Burns' paper "On the bending of Beds near Veins."

SOME BANKET DEPOSITS OF THE GOLD COAST, WEST AFRICA.

BY EDWARD HALSE, A.R.S.M.

The auriferous conglomerate or banket deposits, discovered in South Africa only a few years ago, are by this time fairly well known,* while concerning those of West Africa, which have most probably been worked by the natives for about two centuries, little has been written,† and in this paper an account will be given of several banket deposits which the writer has lately had an opportunity of examining on the Gold Coast.

The word banket (in Dutch=sweetmeats, hence almond toffy) can, strictly speaking, be applied to any conglomerate, whether auriferous or not, and is practically equivalent to "pudding-stone," "cement" (Australia), and "blue lead" (California). Mr. Brough Smyth describes cement as “a stratum of very ferruginous conglomerate, composed of rounded pebbles of quartz, angular fragments of the same rock, and small pieces of schist,”‡ which, in some districts, is highly auriferous, and worked at a profit. But the gisement of the banket reefs is entirely different from that of the so-called cement or blue lead, for whereas the former, usually of primary age,§ are found interstratified with other contemporaneous rocks of a quartzitic character the latter belongs to "deep lead" or ancient Tertiary alluvial deposits, filling valleys and hollows or "gutters," preserved from destruction by layers of basalt, and usually lying unconformably on the much older bed-rock. The cement is found below the wash-dirt, "overlying the bed-rock-in some places touching it, and in others a few inches or a few feet above it" (Brough Smyth). Cement has also been found to occur in recent alluvial deposits.

The term banket-reef is misleading, as it is suggestive of a true fissure-vein; it would be better to call that class of deposit a banket bed or seam. Knochenhauer says " Die de Kaap Riffs sind Lagergänge,"|| and, indeed, the equivalent beddedvein is more applicable to the quartzite reefs of De Kaap, and bankets of Witwatersrand than to the conglomerates of the Gold Coast; for, as we shall see the gold of the former has been deposited subsequently to, and of the latter contemporaneously with, the bed in which it occurs.

* See C. L. Alford, F.G.S., in the Mining Journal, Oct. 5, 1889, republished in pamphlet form; A. R. Sawyer, Trans. N. Staff. Inst. Min. and Mechl. Engrs., Vol. X.; the Engineering and Mining Journal, E. B. Dorsey, July 20, 1889, J. S. Curtis, Feb. 5, 1890; B. Knochenhauer, Die Goldfelder im Transvaal, Berlin, 1890. For the conglomerates of the De Kaap districts, see C. L. Alford, Mining Journal, April 27, 1889; W. H. Furlonge, Trans. Am. Inst. Min Engrs., 1890 (Abstract in Engineering and Mining Journal, March 8, 1890).

+ See Skertchly; Journ. Roy. Geogr. Soc., Vol. 48, pp. 274-283; Cameron, "The Gold Fields of West Africa," and Burton, "Gold on the Gold Coast," in Journ. Soc. Arts, Vol. 30, 1882, also op. cit. below; E. McCarthy, Mining Journal, July 1, 1882, and P. Dahse in Deutsche Geographische Blätter, 1882.

The Gold Fields and Mineral Resources of Victoria, 1869, pp. 352-354; see also J. Arthur Phillips, F.R.S., A Treatise on Ore Deposits, 1884, pp. 458-9, and Lock's two volumes on Gold. Compare C. Waldeyer in R. W. Raymond's Mining Industry West of the Rocky Mountains, 1874, chap. xi.

Auriferous conglomerate of Cambrian age is found in the Black Hills, Dakota, of Siluro-Devonian age in New South Wales, of Devonian age at Witwatersrand, of Carboniferous age at Bessèges, in Languedoc, in New South Wales, Tasmania, and New Zealand, and of Permian age in New South Wales, while "gold has been recently discovered in the marine conglomerate and shale beds, which are believed to be of Cretaceous age, and which rest upon the flanks of the Silurian, Devonian, and granite formations in the Mount Brown or Albert Gold Fields." Catalogue of Exhibition, New South Wales Court, Mining and Metallurgical Exhibition, Crystal Palace, 1890.

Op. cit.

The chief mines of the Gold Coast are situated in the Wassaw country, between 4° 50′ to 5° 30′ N. lat. and 1° 50′ to 2° 25′ W. long., comparatively near the banks of the Ancobra River, which, running in a general north and south direction, falls into the sea a little west of Axim. Leaving recent alluvial deposits out of account, the mineral deposits of the district may be divided into two classes :—

(1) Gold quartz fissure-veins traversing schist, and (2) Bedded alluvial deposits enclosed in sandstone.

The latter are said to lie unconformably on the former (E. McCarthy), and are, therefore, of more recent date, and while (1), as a rule, occur west of the Ancobra, (2) are found to the east of that river. Among the exceptions are the auriferous quartz vein, marked by M. Bonnat,* on the east bank, a little north of Tumento, and those existing on the west bank of Prince's River, and which, according to a French map, published in 1750, was formerly called "Gold River" by the Portuguese. In point of fact, the whole country is so densely covered with bush that hitherto comparatively few mines have been discovered, and for the same reason the geology of the country has still to be worked out.

The mines about to be described are from 41 to 56 statute miles in an air-line from Axim, and, travelling by river and bush, 60 miles must be traversed to reach the centre of the group. Proceeding in a hammock to the mouth of the Ancobra, one ascends the river in a steam launch, canoe, or surf-boat as far as Tumento on the left bank, and thence by hammock along a bush track to the mines, the chief of which are about 22 miles from the river side. The writer left Axim in a surf-boat, manned by 14 Kroomen using paddles, and, crossing the bar, arrived at Bonsa station on the Bonsa River, in a little over two days' time, travelling mostly by moonlight, and being often lulled to sleep by the peculiar and monotonous chanting of the Kroomen. Soon after ascending the river, and having passed the dense mangrove swamps, one is struck by the great height of some of the trees, and by the marvellous wealth of tropical vegetation lining the banks of the river. At night, when the highest tree-tops are obscured in white mist, and the moon casts but a pale and sickly light over all, the scene is weird in the extreme, and is not likely to fade soon from the memory of the beholder.

TOPOGRAPHY,

Commander Cameron's map of this region shows a number of hills from about 1,000 yards to 6 miles in length, and running, most of them, a little east of true north. The hills are crossed at right angles by small valleys, the bottoms of which are of a little higher elevation than the main north and south valleys-the gradient of the latter towards the sea-coast is only 24 feet to the mile, and yet hydraulicking has been recommended for this district! The whole country is covered by forest and

* Carte des Concessions de "The African Gold Coast Company," 1879.

To the Gold Coast for Gold, by R. F. Burton and V. L. Cameron, 2 vols., 1883. The map of Messrs. Walker and Bonson having the claims of the African Consolidated Mines, Ld., marked on it, seems to be mainly taken from Cameron's map in the above book. Dahse's map is taken from the above and other

sources.

From Chama to Taquah is only 28 miles in an air-line; the Government is now making a good road by that route.

dense bush. Both the auriferous veins and the banket seams, as a rule, follow the direction of the hills. Plate VII. is a general map of the district, after Dahse; Plate VIII., after Messrs. Walker and Bonson, shows the relative positions of the banket mines, and the direction of the hills on which they are situated. The chief seams are marked down on both maps, from data taken from various sources.

TAQUAH MINE.

The present workings, from surveys made by the writer, are shown in plan on Plate IX., and in section on Plate X. The deposit strikes N. 47° E. (magn., 1890), or N. 32° 20′ E. (true). It has a distinct bend, however, the southern portion striking N. 13° E. (magn.), or N. 1° 40′ W. (true). The deposit appears to form an acute angle with the average run of the hill, which is 260 feet above sea-level. The main bed appears to form a distinct saddle above the new tunnel (Plate X.); and, although a quartz vein is seen in places along the huckle, which might be taken for a saddle joint, the seam in the two levels would really appear to form one main seam, divided by a "horse" of sandstone rock, the general dip being north-westerly. Unfortunately, the older lower workings in the Verillon tunnel cannot now be surveyed, owing to falls of ground, etc. One wing of the saddle-bend dips N.W. 31° and the other S.E. 49°, gradually lessening towards the lower portion until it becomes horizontal.

The seam has been worked by shafts driven by the natives for a long distance N.E. and S.W. of the present workings. Some of the shafts are covered by bush, some have fallen in, and the remainder are so small, and the workings so narrow, that it is very arduous work to examine them, and for these reasons an accurate plan of the whole mine does not exist. The Taquah Seam, at first sight, does not look like a banket deposit; however, a minute examination seems to show that it is really a very fine conglomerate, arranged in layers, merging occasionally into coarse conglomerate of the ordinary type on the one hand, and into moderately fine sandstone on the other. For the most part it consists of more or less rounded grains of quartz, from coarse to fine, of titaniferous iron ore in layers and in scattered grains, and of extremely thin plates of silvery-white talc. The bed is thickly strewn with the latter and has little cohesion, crumbling to pieces with very slight pressure, but in other respects resembles Brazilian itacolumite.* The fragments are very gritty to the touchcarbonate of lime and clay, if present at all, are there in small quantities only. The seam may roughly be said to consist of layers of titaniferous iron ore, carrying free gold alternating with layers of practically non-auriferous talcose sandstone. The width of the layers varies, as well as that of the whole seam; in the portion now being worked, close to the new tunnel, the width has been proved to be 18 feet. Sometimes the titaniferous iron ore occurs in solid layers, several inches thick, and sometimes well crystallized in quartz, filling cross joints.

"Itacolumite is a friable sandstone, consisting mainly of quartz sand, but containing talc or mica, and sometimes also possessing a certain amount of flexibility when in thin lamina." J. Arthur Phillips, op. cit., p. 613; also W. J. Henwood, Trans. R. Geol. Soc. Cornw., vol. viii., p. 172. When specular iron ore replaces the talc or mica, it is termed itabirite.

VOL. II.-1890-91.

J