132 The Philippine Journal of Science wis 



Enrichment, therefore, is caused by two conditions: (1) By 

 the leaching of the calcite, which causes a removal of a value- 

 less element, leaving a smaller mass of richer ore; and (2) by 

 mechanical concentration of fine gold along channels caused by 

 fracturing and by the removal of the calcite. 



The removal of the calcite reduces the mass of the ore carrying 

 the gold from 30 to 75 per cent. This alone would increase the 

 value of the ore from 50 to 300 per cent. Certain portions of 

 the veins in the Eastern and Colorado mines have been so thor- 

 oughly leached of calcite that the network of quartz remaining 

 has the appearance of a sponge. The removal of the calcite 

 and the oxidation of the pyrite in the quartz and the pyrite 

 originally with the calcite but left behind in the leaching process 

 creates an ideal condition for mechanical concentration of the fine 

 gold in the primary ore. The gold is carried down with the 

 manganese and iron oxides by water, and deposited along the 

 cracks and fissures in the vein. The manganese being difficultly 

 soluble and being so abundant in the ore is concentrated in 

 large amounts simultaneously with the gold, forming soft black 

 bands. This would account for the association of high gold 

 values with the manganese. 



Some of the examples of deposits in the United States cited 

 by Emmons, 7 which seem to bear directly on points under dis- 

 cussion and to confirm these conclusions, are given below. 



II. Black Hills, S. D. — The principal minerals are quartz, dolomite, cal- 

 cite, pyrite, arsenopyrite, and gold * * *. Some of the ores at the 

 surface were below the average tenor, while other surface-ores were two 

 or three times as rich as the average * * *. In general, according 

 to S. F. Emmons, secondary enrichment by surface-leaching has had rela- 

 tively small importance. 



III. Treadwell Mines, Alaska. — The minerals include quartz, albite, rutile, 

 chlorite, * * * epidote, * * * chalcopyrite, and molybdenite. 

 Manganese-minerals are not reported. * * * Nothing in the character 

 of the ore indicates any important concentration of values by oxidizing 

 waters. 



VII. Ophir District, California. — The gangue is mainly quartz with a 

 little calcite. * * * The extensive development of placers, the value 

 of the ore near the surface, and the occurrence of valuable ore-shoots just 

 below the surface are opposed to the notion of extensive migration of gold 

 in these deposits. 



IX. Phillipsburg , Mont. — At the Cable mine the deposits are included 

 in a long, thin block of limestone in contact on either side with quartz- 

 monzonite. The principal minerals are calcite, quartz, pyrrhotite, pyrite, 

 magnetite, and chalcopyrite, with chlorite, muscovite, and other silicates. 

 At one or two places small traces of manganese dioxide have been noted 



f Opus cit., 818 to 836. 



