90 The Philippine Journal of Science i»i3 



veins or in the case of leaching the sulphur constituent of sul- 

 phides, in which cases the usual conditions are encountered. 



Enrichment of vein matter has resulted from the leaching 

 out of barren material, from the solution and reprecipitation of 

 valuable minerals, and from mechanical concentration of valuable 

 minerals by water. The leaching of the calcite from a vein 

 reduces the density of the vein, but does not carry with it any 

 appreciable minerals of value. Therefore, the leached portion 

 contains a much greater value per ton than the primary ore. 

 This is very pronounced in the Colorado and Eastern mines, 

 where it can be shown that this leaching alone has increased the 

 value per ton about 100 per cent. The leaching of iron and 

 manganese, while affecting the ore value to some extent, is 

 relatively unimportant. 



In all the veins some solution of gold has taken place. In 

 copper-bearing veins and in manganese-quartz veins this has no 

 doubt been an important factor, but the rapid erosion of the sur- 

 face has washed away most of the leached material and has 

 left exposed on the surface the enriched portion of the vein 

 where precipitation has taken place. In veins of quartz, manga- 

 nese, and calcite, the typical veins of the Philippines, the solution 

 of gold appears to have been very slight and the leaching effect 

 of manganese-bearing veins discussed by Emmons 10 appears 

 to be lacking, but only observations in shallow workings are 

 available. This condition will be discussed in a separate paper. 



The highest values have been due to mechanical concentration 

 of valuable minerals in crevices, streaks, and waterways within 

 the vein or next to one of the walls. These are of common 

 occurrence. In manganese-bearing veins soft streaks of black 

 oxides of manganese are found, frequently extending from the 

 surface to almost a hundred meters in depth, which carry ex- 

 ceptionally high values in gold. It is probable that part of this 

 gold was precipitated from solution, but the greatest part seems 

 to have resulted from mechanical concentration. In manganese- 

 free veins rich streaks, carrying large amounts of iron oxide or 

 iron and copper oxides, are common. The large volumes of water 

 that flow through the veins during each rainy season must of 

 necessity carry with them valuable particles which are filtered 

 out at a lower depth as the fissure or waterway fills with 

 sediment. 



"Bull. Am. Inst. Min. Eng. for 1910 (1910), No. 47, 767. 



