616 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 45. 



one section the bornite grains, when examined at 200 diameters, are 

 seen to be bordered by chalcopyrite, or by tetrahedrite, or both. (See 

 fig. 3, PI. 51.) The relation between bornite and chalcocite is often 

 that indicative of contemporaneity; the boundaries are ordinarily 

 intricate and clear-cut, and good examples of graphic intergrowths 

 were observed. (Figs. 3 and 4, PI. 50.) 



Chalcopyrite. — In addition to the associations with bornite just 

 described, and minor interstitial development in pyrite, chalcopyrite 

 at North Mount Lyell occurs alone inclosed in sericitic or chloritic 

 schists. In polished sections under the microscope the chalcopyrite 

 shows in every proportion from the merest development (fig. 1, PI. 49) 

 to a solid opaque body with only a scattering of gangue (fig. 2, PI. 49). 



Chalcocite. — So far as studied chalcocite is a rather minor constit- 

 uent of the ore and is always in close association with bornite. In 

 sections across such specimens the bornite occupies relatively large 

 areas, with the chalcocite occurring here and there in smaller patches 

 either as sharply defined individuals but with marginal lines variously 

 embaying, and embayed in, bornite in the most completely intimate 

 fashion, or rarely as graphic intergrowths. (See fig. 4, PL 50.)* 

 Such chalcocite is clearly of contemporaneous development with 

 bornite. Sections of this chalcocite, when etched by immersing a 

 few minutes in dilute nitric acid, develop characteristic cleavage lines, 

 as shown in figure 4, Plate 49. In two of the sections studied rela- 

 tively large areas of bornite were found which assumed a granular 

 character toward their margins, and were encased in pure chalcocite. 

 This structure is suggestive of secondary chalcocite, but no further 

 examples were found and even the ones in question were destroyed 

 when the sections were repolished preliminary to more detailed study. 

 It may be safely said that chalcocite deposited by descendmg surface 

 waters is an ununportant constituent of the North Mount Lyell ore. 



Tetrahedrite. — This mineral is somewhat analogous to enargite at 

 Mount Lyel] in that its development is highly localized. Like enar- 

 gite, too, it is intimately associated with extremely fine chalcopyrite 

 discernible only under high magnification. On the section tetra 

 hedrite appears both as irregular patches up to 10 mm. in diameter, 

 and as sharply defined, exceedingly narrow, marginal zones si.r 

 rounding bornite grains. In many instances the bounding zone will 

 consist in part of chalcopyrite, the two alternating and together form 

 ing a beautifully sharp, irregular zone completely encasing the bornite 

 area. (See fig. 3, PI. 51.) This mode of occurrence for chalcopyrite 

 is confined absolutely to tetrahedrite-rich areas, and its relationship to 

 bornite is totally different from that in the conventional tetrahedrite- 

 free ores of the North Mount Lyell mine. 



1 These figures closely resemble crystallographic intergrowths of bornite and chalcocite in the copper 

 ores of Virgilina, Virginia. See Laney, Proc. U. S. Nat. Mus., vol. 40, 1911, pi. 68. 



