SECONDARY DEPOSITS OF SILVER MINERALS 201 



from the removal of the sulphides which it formerly held. Frequently, too, the 

 red and gray silver ores are associated there. These ores are the pacos, chlorides, 

 etcetera, of the Spanish-American miners. 



Deeper down, at 80 to 150 meters, the bonanza zone is encountered, where the 

 silver is in the form of glance (Ag 2 S), the copper as chalcocite, gray copper (often 

 argentiferous), and bornite. Iron is wanting or is present as oxide. Lead, if pres- 

 ent, is in small amount and mostly in the form of the carbonate. 



Still lower beneath the ground water level, which varies from 400 to 500 meters, 

 one finds a complex assemblage of primitive metalliferous sulphides, galena, which 

 is more or less argentiferous, copper and iron pyrites, arsenopyrite, blende, and 

 rarely silver minerals. 



Iii the secondary ores of copper, gold and silver, which are commonly 

 present in small amounts in copper ores, are concentrated in gray coppers 

 or as native gold or silver, which we see commonly in bornite.* 



Professor Vogt also describes f the recent formation of a gold and 

 silver bearing zone beneath the iron hat. He says that in the Rio Tinto 

 region the " iron hat " is from 35 to 50 meters deep, and consists of iron 

 oxide or hydrated oxide, with from 35 to 50 per cent of iron, some 

 silver in part as basic sulphate, and a few ten-thousandths per cent of 

 arsenic, while on the other hand the copper contents are, as already re- 

 marked, entirely oxidized and dissolved out. In one mine, North vein 

 number 2, at Rio Tinto, there occurred between the " iron hat " and the 

 underlying comparatively fresh pyrite a layer of earthy, porous material 

 bearing gold and silver. This earthy ore, though a few decimeters in 

 thickness, may be followed continuously over the entire ore body. This 

 very marked layer follows closely the irregular plane between the " iron 

 hat '' and the underlying pyrite. It everywhere contains an average gold 

 and silver contents of from 15 to 30 grammes gold and 1.025 silver, with 

 a value of about 150 marks per ton. In stripping off the "iron hat " 

 this earthy mass is carefully laid to one side, and has thus yielded fully 

 a thousand tons of ore. It is clear that the formation of this gold and 

 silver bearing zone is connected with the oxidizing process that formed 

 the "iron hat," and that the gold and silver comes from the very small 

 percentage of such metals in the primary ores. 



Another interesting structural feature of silver bonanzas is their occur- 

 rence in connection with faults or later fractures. This is also true in the 

 copper mines of Butte, as pointed out by Emmons. It is probably true 

 in the Broken Hill Consolidated mines, Australia, and in the Aspen 

 district, Colorado. 



In his monograph on the Aspen district, Colorado, Mr Spurr J de- 



*De Launay, loc. cit., p. 195. 



fZeitschrift fur Praktische Geologie, July, 1899. 



J J. E. Spurr : Monograph no. xxxi, Aspen mining district, U. S. Geological Survey. 



