1916] Umpleby: Ore on the Limestone Side of Garnet Zones 33 



of the occurrence of ore on the limestone side of garnet zones. The 

 relations shown in figure 6 do not support the general idea, but it is 

 introduced because it represents a part of the same chimney of ore 

 shown in the other two figures and has value as a check to sweeping 

 generalizations. Within the area represented by it the writer would 

 expect to find clear evidence of replacement of the silicates by the 

 sulphides. 



Mackay, Idaho. — The contact metamorphie copper deposits at 

 Mackay, Idaho, occur well within a mass of granite porphyry which 

 has invaded folded strata of Carboniferous age. 10 Several engulfed 

 blocks of limestone occur in the vicinity of the ore deposits and have 

 been variously metamorphosed so that some are bordered by only a 

 narrow margin of marble, others are more than half changed to lime- 

 silicate rock, and still others are completely transformed to garnet- 

 diopside rock and ore. Marmorization accompanied intrusion but 

 the intense metasomatism followed the fracturing of a thick magma 

 shell. In many places the granite porphyry bordering included lime- 

 stone masses is extensively changed to garnet-diopside rock. 



The ore in the Copper Bullion Mine occurs along the margins of 

 a block of limestone several hundred feet long and is separated from 

 the granite porphyry by a zone of garnet-diopside rock from three 

 or four to more than twenty-five feet wide. Marble forms the other 

 wall, but this gives way within a few feet to normal limestone. The 

 relations are illustrated in figure 9. 



The Alberta ore bodies, situated along a pronounced fault which 

 traverses the igneous mass, comprise two groups of branching 

 chimneys which occur in the central parts of large garnet areas (fig. 

 10). Here no central limestone body remains, and the occurrence 

 is interpreted as representing the limiting stage of ore deposition on 

 the limestone side of a garnet zone which surrounds an engulfed block. 

 A study of the ores shows that the sulphides locally are contempo- 

 raneous with the garnet crystals but that most of them are of distinctly 

 later development. 



Whitehorse Copper Belt, Yukon. — The rocks of the Whitehorse 

 district comprise Carboniferous limestone largely destroyed by three 

 distinct igneous invasions, the principal of which, though of variable 

 composition, is predominantly hornblende granite. 17 The larger 



is Umpleby, J. B., The genesis of the Mackay Copper Deposits, Idaho, Eeon. 

 Geol., vol. 9, pp. 307-358, 1914. The ore deposits of the Mackay Region, 

 Idaho, Prof. Paper, U. S. Geol. Survey (in preparation). 



1 7 McConnell, R. G., The Whitehorse Copper Belt, Yukon Territory, Can. 

 Geol. Survey, No. 1050, 63 pp., 2 pis., 2 figs., 8 maps, 1909. 



