NO. 2005. MOVVT LYELL COPPER DISTRICT— GILBERT d POGVE. 613 



THE MINES. 



Two mines only need be considered in detail, as these represent the 

 major development of the field and embrace the features of interest 

 shown by the minor deposits. 



Mount LyeU mine. — This mine has opened up a pyritic mass of re- 

 markable size, carrying a low copper content and values in gold and 

 silver. The ore is fine-grained, homogeneous pyrite, bearing chalco- 

 pyrite and including evenly distributed gangue of quartz and barite 

 in extremely subordinate amount. The ore body lies entirely within 

 deeply dipping schists, adjacent to and approaching on the foot-wall 

 side within a few inches of the conglomerate fault contact. Its con- 

 figuration is that of a horn-shaped body, tapering downward, with 

 maximum dimensions of 270 by 660 feet at the 400-foot level. It has 

 been mined en masse by the open-cut system, and in 1906 had been 

 developed to a depth of 730 feet. At present the mining is carried 

 on underground, the open cut nearing completion. 



The body as a whole is low grade, the copper averaging from 0.6 

 to 0.75 per cent, with silver running from 1.10 to 1.75 ounces and 

 gold from 0.06 to 0.08 ounce. The mass is singularly free from 

 deleterious elements, containing only about 0.25 per cent arsenic, less 

 than 0.17 per cent antimony, no bismuth, and traces only of selenium 

 and tellurium. Along the contact and about the periphery the ore 

 body is locally richer from deposition of higher percentage of chalco- 

 pyrite and addition of tetrahedrite and enargite bearing areas. 



North Mount LyeU mine. — This mine is the richest property and 

 largest producer in the district. It differs from the Mount LyeQ in 

 that the predominant ore is bornite, which, together with subordinate 

 chalcocite and some tetrahedrite, pyrite, and chalcopyrite, forms 

 lenticular masses in sericitic and chloritic schists. These minerals, 

 with considerable sihca and some barite, have been deposited as 

 stringers and lenses following the lamination of the schistose rocks, 

 forming impregnated zones or "fahlbands" of ill-defined limits, 

 representing combined replacement and interlaminal deposition. 

 The ore is worked by underground mining and the operations have 

 penetrated to a depth of 1,100 feet. The values in copper run much 

 higher than in the Mount Lyell mine, averaging from 5 to 7 per cent, 

 though the precious-metal content is less. Owing to the siliceous 

 character of the ore it forms an efficient flux for the pyritic ore of 

 Mount Lyell, permitting the lowest grades of the latter to be profitably 

 worked. 



