NO. 2005. 3I0VNT LYELL COPPER DISTRICT— GILBERT & POGVE. 61 Y 



PARAGENESIS OF ORE MINERALS. 



The one mineral whose genetic relationships are everywhere sharply 

 defined is pyrite. Its bearing toward the other sulphides is clearly 

 that of a mineral of prior development. In most sections, however, 

 there are certain examples of interassociation which suggest that the 

 sulphide development from pyrite onward was one of sequential stages 

 rather than of distmct isolated periods. 



Among the copper minerals proper there is nowhere any sharply 

 defined order of sequence such as exists between the group as a whole 

 and the pyrite. Indications pohit strongly, however, to chalcopyrite 

 as having been the first to follow the lead of the iron sulphide. Its 

 wide diffusion, the intimacy of its occurrence everywhere with the 

 manifestly earlier pyrite, and the frequency with which it is to be 

 found inclosing and enmeshing pyrite clusters with the whole engulfed 

 in bornite, by themselves would be conclusive. Elsewhere, however, 

 chalcopyrite and bornite occur intimately intergrown (see fig. 2, 

 PL 51) as if of contemporaneous development. From these two 

 types of • relationships the inference would be that a period of clialco- 

 pyritization passed into one productive of chalcopyrite and bornite 

 together. 



Of the rich copper minerals bornite shows itself not only the one 

 most extensively developed, but the one most intimately associated 

 in order of continuity with chalcopyrite. In some instances it is 

 intergrown with chalcopyrite (fig. 2, PL 51); in other instances it is 

 intergrown with chalcocite (fig. 4, PL 50) ; there are also numerous 

 intermediate examples of its occurrence independently of chalcopyrite 

 or chalcocite. These associations point strongly to a period of bornite 

 development that was inaugurated while chalcopyrite was still form- 

 ing, continued through a period of its own, and closed with simulta- 

 neous precipitation of bornite and chalcocite. Certain relations of 

 the latter mineral tend further to indicate that it continued to form 

 for a while after bornite ceased depositing. 



Tetrahedrite, occurring as replacement rims to bornite grains, is 

 distinctly later than that mineral. Though never associated with 

 chalcocite so as to indicate relationship, tetrahedrite is judged to be 

 subsequent to it also, since chalcocite is m part contemporaneous 

 with bornite. With tetrahedrite occurs chalcopyrite, in minute pro- 

 portions, having analogous bearing toward bornite; this chalcopyrite 

 is of course also later than bornite and represents a second generation 

 of chalcopyrite. This chalcopyrite and the tetrahedrite show every 

 indication of synchronous deposition. Their formation is due either 

 to descending waters or is the result of further changes in the primary 

 ore-bearing solutions, dependent upon some specialized condition. 



Tetrahedrite at North Mount Lyell is paralleled by enargite at 

 Mount Lyell. Both minerals are of localized occurrence and are 



