614 PROCEEDINGS OF THE NATIONAL MV8EVM. vol.45. 



THE ORE MINERALS— MOUNT LYELL MINE.» 



The ore minerals at the Mount Lyell mine, in the order of their 

 observed prominence, are pyrite, chalcopyrite, enargite, tetrahedrite, 

 sphalerite, galena, bornite, and chalcocite. 



Pyrite. — The Mount Lyell ore is dominantly pyritic and the average 

 ore specimen appears to consist purely of densely granular pyrite with 

 accessory quartz. Where the development of quartz is sufficient to 

 be conspicuous its distribution is not uniform, but tends toward 

 segregation into bands. In polished sections the banding becomes 

 more apparent and presents a pUcated and distinctly schistose 

 aspect. (See fig. 1, PI. 48.) Under magnification the pyrite appears 

 characteristically granulated and is not intergrown with other sul- 

 phides. Its bearing toward quartz is both that of contemporaneous 

 and of later development. 



Ghalcopyrite. — The copper content of the Mount Lyell ore is 

 largely due to the presence of chalcopyrite, which permeates the 

 pyritic ore in the most intimate fashion. It is imperfectly distin- 

 guishable in hand specimens, but under the microscope is seen to 

 traverse the pyrite in stringers and form a network enmeshing grains 

 and cementing fractured iadividuals. (See fig. 3, PI. 49.) Its devel- 

 opment is especially marked along the quartzose bands, and the 

 mclosing fUaments decrease outward from such areas, in some in- 

 stances leaving the denser pyrite of the section entirely free from 

 observable chalcopyrite. Such pyrite, free from visible chalcopyrite 

 even at a magnification of 200 diameters, however, was found to react 

 for copper. 



Enargite. — ^The occurrence of this mineral is highly localized. 

 Where present it does not permeate the ore after the fashion of chal- 

 copyrite, but occurs as relatively large, irregular, confluent areas 

 uiclosing breccialike pieces of pyrite. (See fig. 2, PI. 48.) Its most 

 interesting microscopic feature is an ever-present impurity in the 

 form of chalcopyrite, which is disseminated throughout the enargite 

 in minute patches, networks, stringers, and disconnected points. (See 



1 The microscopic work was done on polished sections with a metallographic microscope using veritcal 

 illumination from an acetylene light, at magnifications ranging from 30 to 200 diameters. The minerals 

 were identified by noting their characteristics, such as color, hardness, structure, tarnish, and etching effects, 

 upon areas sufficiently large to furnish fragments for blow-pipe tests ; by which means criteria were obtained 

 for the identification of these minerals even where microscopically developed. In making the photographs 

 it was found desirable to increase the color contrasts by developing tarnishes by brief treatment with acid; 

 for this purpose nitric acid was most useful. 



A useful hardness test, applicable to the determination of the relative hardness of adjacent grains In 

 opaque sections, was developed during the course of the study, and may be applied as follows: Having 

 centered the microscope tube on the contact between two mineral grains, place a small metal straightedge 

 on the section and move until its edge intersects the two grains; then remove the section from the microscope 

 stage, holding firmly the straightedge in place, and, by means of a knife point, draw a line across the two 

 grains, being careful to press uniformly throughout. Replace the section under the microscope and note 

 the size of the channel as it passes from one grain to the other. The larger channel, of course, will lie in 

 the softer mineral. This method is sensitive to within a half degree of hardness in the customary scale and 

 is applicable to grains as small as 0.5 mm. in diameter. 



