362 F. P. Mennell—The Rhodesian Banket Beds. 



IV. Economic Importance. 



The geological correspondence of the Rhodesian conglomerates 

 with those of the Rand naturally raises the question of their proving 

 of similar economic importance. As a matter of fact, though it has 

 not been generally recognised, ' banket ' has been worked for some 

 time on the Wanderer Mine at Selukwe, a big low-grade deposit at 

 the junction of the conglomerate and the banded ironstone, the latter, 

 however, forming the bulk of the ore. The conglomerate has been 

 tested in numerous other localities, with poor results, and so far it is 

 only at the Eldorado Mine in the Lomagundi district that gold has 

 been proved to exist in payable amount. Recent developments at 

 this locality have shown that not only is the ore in general of very 

 high grade, but that the ore shoot contains occasional patches of 

 phenomenal richness. The conglomerate is here altered and sheared 

 to a greater extent than usual. The matrix has passed into the 

 condition of hornblende schist, and consists of deep-green ragged 

 hornblendes, irregular felspar granules, and some possibly secondary 

 quartz ; epidote, biotite, and chlorite being also present in addition 

 to magnetite and pyrites. The pebbles, which are occasionally large 

 enough to be termed boulders, are chiefly of granite or granophyre, 

 considerably altered, but not so much so as to leave any doubt as to 

 their nature. Nearly black quartz is common, and what may 

 possibly be banded ironstone also occurs. Pebbles of white quarta 

 are rare, but many little veins of secondary quartz are seen, some- 

 times traversing faulted pebbles in a most interesting manner, while 

 there is often a good deal of pyrites. The rich specimens enable the 

 distribution of the gold to be studied with unusual ease. The 

 precious metal is practically confined to the matrix of the con- 

 glomerate, and is principally seen in cracks formed after it had 

 assumed its present characters, apart of course from those due to 

 changes resulting from the introduction of the gold itself and that of 

 the other secondary minerals by which it is accompanied. Even 

 where very rich the gold is fine and not nuggety ; it is rarely seen 

 in the pebbles. When they do show it, it is always along cracks 

 formed during the extension of the pebbles at right angles to the 

 pressure, or along slipping planes, sometimes with and sometimes 

 without secondary quartz and pyrites. Round the pebbles, and 

 especially at their ends, where slips would tend most to loosen the 

 rock, the gold is often very abundant, forming a regular coating ta 

 the pebbles in extreme instances. It is clear that here, as on the 

 Rand, the gold is of secondary origin, and was not present during 

 the original deposition of the rock. 



I may conclude by expressing my indebtedness to Dr. H. Sauer 

 for specimens, etc., and for permission to describe the Eldorado 

 rock ; to Messrs. J. L. Popham and A. J. C. Molyneux, F.G.S., for 

 specimens and slides ; and to Mr. H. E. Jones, A.R.S.M., for some 

 valuable notes and suggestions. 



