340 N. R. Jn liner: 



(d) /)/seussio/i of the alteration of the (h/l.-a. 



It will l)L' seen that the Diamond Ci-et'k dj'ke consists of a very 

 mucV altered porphyry. The alteration has been complete, and all 

 of tliG original minerals have been replaced, and the ground mass 

 has been recrystallised. Petrologically, it is seen that tiie biotite 

 was the first mineral to be attacked, as in all sections of the Dry 

 Creek dyke examined biotite was always entirely replaced, although 

 plagioclase and orthoclase were sometinics only partially altered. The 

 alteration apjjears to start first in the areas of orthoclase, although 

 orthoclase occasionally remains when all the plagi(jclase has been 

 replaced. Kirk.l in discussing somewliat similar mineral changes 

 to those here described, has shown that the iron magnesium minerals 

 are the first attacked, and that plagioclase commences to be re- 

 placed ])y sericite and ijuartz before the orthoclase is attacked. 

 Replacement of tlie felspar frequently starts along planes of Aveak- 

 ness. such as cracks and cleavage planes. An analysis of the Dia- 

 mond Ci-eek dyke was made for the purpose of studying the chemical 

 migrations during the alteration. Tlie analysis gave the follow- 

 ing results : — 



A. Diamond Creek Dyke, 800 feet level, 

 ;d)out 200 feet from the underlay shaft 

 along the north drive. Diamond Creek 

 Mine. S.G = 2.72. 



— N. R. Junner, Analvst. 



100.17 



1 C. T. Kirk. Conditions of inint'i'alisatiori in the copper veins at lintte Montana. Kc'onoiuie 

 Geology, vol. ii.. No. I., 1012. 



«■ TiOvi incliiflcd Hitli tlie AI-jO;i 



