NO. 2005. MOUNT LYELL COPPER DISTRICT— GILBERT d POOVE. 615 



figs. 1 and 2, PL 50.) A noteworthy feature of this included chal- 

 copyrite is that it is everywhere richer toward the margin of the enar- 

 gite areas and is not infrequently strongly concentrated close to or 

 at the border. It nowhere crosses into the pyrite, nor does it seem to 

 be related to the chalcopyrite already referred to as disseminated 

 through the pyrite ore. 



Bornite, chalcocite, tetraJiedrite. — These minerals, while important as 

 correlating the Mount Lyell ores with those from North Mount Lyell, 

 are exceedingly limited both in quantity and in extent, and since thek 

 mode of occurrence is analogous to that observed in ore from. North 

 Mount Lyell they may be reserved for discussion under that head. 



SpTialerite and galena. — These minerals represent highly localized 

 phases of the ore and are nowhere prominently developed. They are 

 confined to the pyrite and were in no place observed in association 

 with the copper sulphides. Sphalerite is much more prevalent than 

 galena and wherever the latter does occur it is in intimate association 

 with the former. Occasional sections show both together, or sphal- 

 erite alone, as disseminated grains in the pyrite ore; but commonly 

 their occurrence is as veinlets traversing the section. 



THE ORE MINERALS— NORTH MOUNT LYELL MINE. 



In the North Mount Lyell workings the same minerals are to be 

 found as at Mount Lyell, but in relative proportion so different as to 

 produce ores of entirely divergent character. Here the minerals, in 

 order of their importance, are bornite, chalcopyrite, chalcocite, tetra- 

 hedrite, and pyrite; and these form mineralized zones in the schists 

 and not a great sulphide body as at Mount Lyell, where pyrite is 

 dominant. 



Bornite. — The ore mineral of widest development and greatest sig- 

 nificance at North Mount Lyell is bornite. It occurs alone, m associa- 

 tion with pyrite, and admixed with other copper sulphides. (1 ) Where 

 occurring alone it forms lenses within the schists and presents no 

 microscopic features of note. (2) In association with pyrite it is con- 

 fined to quartzose patches and channels of megascopic proportions 

 withm a pyrite-quartz rock similar to the typical ore of Mount Lyell. 

 (3) The third type of occurrence is the dominant one. The bornite 

 is in close association with chalcopyrite, or chalcocite (with or without 

 tetrahedrite), or both, and the ore forms lenticular areas and stringers 

 within the inclosing schists. In polished sections scattering pyrite 

 grains show up in ore and gangue alike, and in places granular pyrite 

 aggregates are visible. Toward chalcopyrite, bornite has a varied 

 bearing. While the two are often developed in intimate association, 

 as if intergrown (fig. 2, PI. 51), there is in other sections a distmctly 

 noticeable tendency for the chalcopyrite to associate itself with granu- 

 lar pyrite aggregates where such occur in the section, and for the 

 bornite in a general way to envelop the association as a whole. In 



