NOTES ON SOME OXIDISED COPPER ORES. 477 



since they are the only indications to guide the selector, — on the 

 one hand not to throw away as worthless stuff containing more 

 than 4 percent., and on the other hand not to retain as valuable 

 stuff which runs below that and which would tend to reduce the 

 smelting average below from 6 to 8 per cent. 



At San Francisco and La Prieta, about half-a-mile north- 

 ward from El Promontorio, the ores occur in a highly crystalline 

 limestone breccia, which fills irregular cavities occasioned by, or 

 at least intimately associated with, the garnet-rock and felsitic 

 intrusions. In other places, as at the part known as San Carlos, 

 there appears to be a distinct vein fissure occupied by a similar 

 crystalline brecciated filling — the whole cemented together by 

 compounds of copper and by siliceous and calcareous infiltrations 

 of a tufaceous nature. The ores from these portions of the 

 property, when properly selected, yield about 12 per cent, of 

 copper, and perhaps 16 ounces of sHver to the ton, while 

 occasional masses are met with which are more than twice as 

 rich. When properly selected and properly mixed from the 

 different ore-deposits so as to give an ore-mixture running about 

 8 per cent, of copper, no flux is needed, the iron and lime present 

 being sufiicient to flux the whole mass. The smelting of this 

 mixture yields at one operation argentiferous black-copper 

 ("base bullion") of about 97 per cent, of copper, and from 100 

 to 200 ounces of silver per ton, and f of an ounce of gold ; also 

 a small quantity of argentiferous copper matte running over 60 

 per cent, of copper, and a black glassy slag containing I J per 

 cent, of copper, and 45 per cent, of silica. 



Probable origin of Deposits. — As already stated the ores con- 

 sist mainly of black and red oxides, blue and green carbonates ; 

 variously tinted silicates, and yellow and gray suljDhides of 

 copper, mingled with oxide and carbonate of iron, and associated 

 with calcite, barytes, and garnet. As appears from many partially 

 altered specimens, and from a careful study of the dej)osits in 

 situ, the original ores seem to have been sulphides, which have 

 become oxidized by aeration and by the percolation of oxygen- 

 ated and carbonated waters. Throughout the mines there is 

 abundant evidence of the former action of metalliferous springs 

 and of hydrothermal action, both carbonated and siliceous. As 



