(icoluijij of the i) id mo ml Creek A reel. 341 



From tlif fair amuiiut nf hintitr and lime soda felspars pro- 

 bably orij^'iiially present, it may \'.v inferred that the percentages 

 of lime and maj^'nesia were vei-y mueh higher than in the altered 

 roek. The iron has probably not changed very much in amount, 

 but sulphur has been introduced either as sulphuretted hydrogen 

 or as alkaline sulphides, and this has united with the iron forming 

 pyrites. It is not possible to say from the evidence of a single 

 analysis what migration has taken place in the alkalies, although 

 it seems 2>i'<>bal)le that a reduction in amount of Ixith potash and 

 soda has occurred. Silica and watci- have both apparently increased 

 in amount. Sericite is essentially a potash mica, and in the 

 analysis over one per cent, of soda is represented. 'J'liis may pos- 

 sibly be due to the presence of the soda mica paragonite, although 

 microscopically no distinction could be made out. Kegai-ding the 

 temperature of tlie altering solutions, Kirkl states "that where 

 sericite can be certainly identified, it becomes a useful corroboiative 

 criterion in the interpretation of previous hydrothermal liigh pres- 

 sure coiulit ions. " Kutile is also mentioned as forming under eim- 

 siderable pressure and modei-ate temperatures. We may, therefore. 

 conclude that the alteration which the Diamond Creek dyke has 

 suffered was prol)al)ly of the nature of a solfatarie after-effect 

 undei- moderately high pressuie and temperature operating on the 

 quartz felspar porphyry. This type of alteration agrees with 

 " pi'opylitisation '"' as defined by Vogt.2 Kirk^ has summed up in 

 tabular form the various alterations during propylitisation of 

 the Butte granite, and the table below is a partial extraction. 



Chemical Alterations. 



Iron gained, sulphui- added to form pyrite. Losses in lime, 

 magnesia and soda; transfornuition of iion o.xides to sulphides; 

 decrease in all bases except potash, (xains in iron sulijliide, silica 

 and water, alunnna. i)0tash, etc. 



M inei'a! .\lteiat ions. 



Development of sericite, (piari/. })yiite, chlorite, epidote, rutile. 

 etc. Femie minerals are first altered and felspars are more- 

 resistant. 



1 Kirk. Op. cit.. p. lu. 



2 Vosjt. Cienesis of ore liejiosite. Tran>. .^niiT. Inst. Miii. Emu'., liinl, |>. "KiS. 



3 Op. cit.. p. (i7. 



