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



335 



ore bearing country. It has been shown also that this irregular 

 surface is far above the ground water. It thus appears clear 

 that the oxidized rock was before oxidation simply the bluish 

 white decomposed porphyry sporadically impregnated with py- 

 rite and chaleocite. This ore then, which by oxidation has con- 

 tributed secondary sulphides to the lower ground, must, itself 

 have been mineralized by a similar secondary process. In other 

 words, the copper ore which yields by oxidation the materials 

 for the enrichment of the lower ground, is not a primary de- 

 posit, but has been derived from some primary source not yet 

 apparent; and our so-called secondary enrichment is in reality 

 at least a tertiary enrichment. This conclusion immediately 

 raises the question as to the original unenriched copper deposit, 

 or the product of the primary concentration. The answer to 

 that question cannot be given on the basis of direct evidence, 

 but only as an inference from certain suggestive facts observed 

 throughout the district. 



The only primary deposits of copper that have been observed 

 in the district are limited bodies of chalcopyrite intimately asso- 

 ciated with garnet rock, or with garnet and quartz. These pri- 

 mary ores occur in two distinct situations. One of these is at 

 the contact of intrusive rocks, particularly the monzonite and 

 monzonite porphyry with the Carboniferous limestones. Here 

 they are clearly referable to a process of contact metamorphism. 

 By this is meant not merely thermal metamorphism, but reac- 

 tionary metamorphism, whereby solutions bearing silica and iron 

 from the intrusive mass react upon the lime carbonate and give 

 rise to the lime-iron silicate, garnet. These same solutions ap- 

 pear to have brought with them the materials necessary for the 

 deposition of the chalcopyrite, and sometimes there was an excess - 

 of silica, which was precipitated as quartz. 



The other situation in which primary copper ores have been 

 detected is in the quartz blout and here again it is intimately 

 associated with garnet. From considerations set forth in pre- 

 vious pages, it appears very probable indeed, if not a demon- 

 strated fact, that the quartz blout is largely the result of a silici- 

 fi cation of the limestone, chiefly on the periphery of the copper 

 bearing porphyry, but also partly along fissures in the limestone 



