NO. 2005. MOUNT LYELL COPPER DISTRICT— GILBERT d- POGVE. 619 



the mineralization. The succession of deposition, as evolved, is in 

 exact harmony with the order of increasing copper content and with 

 that of decreasing ii-on content. Noting this apparent agreement as 

 the work with the metallographic microscope progressed, a sample of 

 pyrite free from included chalcopyrite, so far as could be detected with 

 even the highest power objective, was tested qualitatively and found 

 to give a copper reaction.^ While it is by no means certain from this 

 that the copper present in the pyrite is not due to chalcopyrite of sub- 

 microscopic order, it is of importance in coordinating the true pyrite 

 with the chalcopyrite; for a dissemination of chalcopyrite so fine as 

 not to be distinguishable under the highest magnification, must have 

 been present during solidification of the pyrite, and from this sub- 

 microscopic chalcojjyrite there is every gradation up to the mega- 

 scopically prominent chalcopyrite of the pyrite ore, referred to a po- 

 sition consequent to joyrite in the order of crystallization. Accord- 

 ingly, starting with what may reservedly be termed cupriferous pyrite, 

 which is at once the most extensively developed and the earliest of 

 the ore group, and passing successively through chalcopyrite, bornite, 

 and chalcocite, the tendency is so marked as to make the different 

 mineral species seem indicative of successive points in a steadily di- 

 minishing iron content and increasing copper content in solutions of 

 constantly diminishing quantity. 



SECONDARY ENRICHMENT. 



This subject can be discussed only in a general way, because few 

 only of the specimens available showed characteristics referable to 

 this process. Also the writings of Sticht and Gregory, while making 

 frequent reference to enrichments in the ore bodies, do not in every 

 instance present criteria suitable for discriminating whether such 

 enrichment is due to descending surface waters, or is merely a special 

 phase of primary deposition; indeed, such criteria are difficult to 

 obtain and a problem of this kind could be successfully attacked only 

 through a metallographic study of specimens collected with this end 

 in view. As the enriched portions of the ore are the ones naturally 

 most completely worked out, such an attainment is obviously 

 impossible. 



In general, it may be said that enrichment is more prominent in 

 the Mount Lyell body than in the North Mount Lyell deposit. The 

 latter is marked by a uniformity of its mineral associations excepting 

 a slight increase in proportion of chalcocite to bornite in the lower 



1 This result is the reverse of that obtained by Laney (Bull. 21, North Carolina Geol. and Eeon. Survey, 

 1910, p. 92), Simpson (Econ. Geol., vol. 3, 1908, pp. 628-«35), and Finlayson (Econ. Geol., vol. 5, 1910, p. 

 420), from metallographic study of "cupriferous pyrite" from Gold Hill (North Carolina), Butte (Mon- 

 tana), and Huelva (Spain), respectively; all of whom found that in these ores the copper content is due to 

 definite copper minerals recognizable under the microscope, and where such are not visible the ore is copper- 

 free. 



