Vol. 4] Lawson. — The Robinson Mining District. 



339 



nately the carbonates of copper disseminated as they are rather 

 diffusely through the porphyry do not lend themselves to ordi- 

 nary pi-oeesses of concentration as does the chalcocite. In order 

 to handle this ore, therefore, it may be necessary to resort to some 

 leaching process. 



The porphyry thus charged with copper carbonate has other- 

 wise the character of the zone of oxidation at the Ruth, except 

 perhaps, that kaolinization has not proceeded so far and that 

 the rock is fairly strong and coherent. Between this zone of oxi- 

 dation with its content of copper carbonate and the ground 

 water, there is as in the case of the Ruth a certain depth of the 

 sulphide zone in a dry condition down into which the oxidized 

 rock extends irregularly. This sulphide zone both above and 

 below the level of the ground water resembles closely that at the 

 Ruth. It is a bluish white porphyry impregnated with pyrite 

 and chalcocite. Locally it has in it nests of bronzy or brownish 

 mica in foils of considerable size. The mica appears to be sec- 

 ondary and is said by the miners to occur in those portions of 

 the porphyry where the content of the chalcocite is highest. 



The upper part of the unoxidized zone at Copper Flat carries 

 higher copper values than the deeper ground, and doubtless rep- 

 resents a zone of enrichment by downward leaching at a former 

 higher stage of the water plane. The general average value of 

 the ore in the unoxidized ground differs but little from that at 

 the Ruth. 



It is probable that the copper ore of the oxidized zone ex- 

 isted earlier as chalcocite, itself a product of secondary concen- 

 tration, and that in the oxidation process an excess of carbonic 

 acid determined the carbonatization of the copper ore dissem- 

 inated through the rock. The presence of this carbonic acid 

 may perhaps be ascribed to a later period of eruptive activity 

 which gave rise to flows of rhyolite, remnants of which are to be 

 found in the vicinity of Copper Flat. 



With the exception of the mines at the Ruth and Copper 

 Flat practically all other attempts at copper mining in the dis- 

 trict have been made upon rather unsatisfactory and limited 

 deposits of chalcopyrite either on the contact zones of the intru- 

 sive rocks or in the quartz blout. 



