E/MJ : engineering and mining journal . entre 8 et 92 Mi 5 746,000 entre 9 et 9 i-j 13 378 700 entre 914 102 et 891 500 Plus de 10 Il résulte de ce tableau qu'environ 35,500,000tonnes (ce qui représente environ 40  % de l'ensemble) a une valeur de moins de 6^ o tpl. par tonne, ou d'un actualvalue de 27s. 6 d. par tonne, sans permettre pour miningand pertes métallurgiques. La mise en œuvre de ces droits de douane de 10 p. 100 comme une moyenne approximative, il est évident qu'il y a un petit mois marge coûts dans le quartier de 22s.6d. Monter 1 tpl. dans la classification, il y a 39 000 000 tonnes d'une valeur supérieure à 7i/o tpl. De t

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E/MJ : engineering and mining journal . tween 8 Mi and 9 2 5, 746.000 Between 9 and 9i-j 1 3, 378, 700 Between 914 and 10 2 891, 500 Over 10 It will be seen from the table that about 35, 500, 000tons (representing about 40 per cent of the whole)has a value of less than 6^ o dwt. per ton, or an actualvalue of 27s. 6d. per ton, without allowing for miningand metallurgical losses. Putting these at 10 percent as a rough average, it is evident that there is asmall margin mth costs in the neighborhood of 22s.6d. Going up 1 dwt. in the classification, there are 39, 000, 000 tons valued at over 7i/o dwt. Of this, 27, 500, 000 (as valued at over Si/o dwt.) can rightlybe considered as high gi-ade. Higher Values Caused by Eliminating Low-Gi-ade Ore The shutting out of low-grade tonnage has, auto-matically and mathematically, had the general effectof increasing the value assigned to ore of commercialgrade. But there are exceptions to this, as thevalues at Brakpan and Springs have each di-opped by3/2 dwt. and at the Van Rjti (where, in addition, a. THE GOVERNMENT GOLD MINING AREAS (MODDERFONTEIN)CONSOLIDATED LTD. Areas stoped out shaded. The ore reserves are indicatedthe dotted lines. more stringent safety factor has been employed) bypractically a pennyweight. In one or two instances improved values are inevidence, and, as this applies to the whole tonnagedeveloped, both in the past and currently, it meansthat the latter has been exceptionally favorable. Acase in point is that of the City Deep, the most im-portant mine in the vicinity of Johannesburg, wherethe value of tlie whole reserve has increased from9 to 9.6 dwt. That improvement is in respect of noless than 3, 350, 000 tons. The advance is owing tothe fact that the previous estimate, including oreconservatively valued pending further work, and now 724 Engineering and Mining Journal Vol. 108, No. 18 found to be higher in value than anticipated, andthe years development (8, 500 ft.) has exposed satis-factory values, namely 19 dwt., over

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