130 ^V it*. Jmincr: 



greatly changed as a result of the alteration. It appears very 

 likely that the niineralisers had their origin in the cooling niaguia, 

 and that their escape was made possible Ijy the relief of pressure 

 during the inti-usion of the diorite. Tlie gases thus expelled from 

 the magma would permeate the solidifying rock, and cause such 

 alteration as has been described. 



II. Hydrothermal alteration adjoining vein fissures. Narrow 

 veins of auriferous cjuartz, generally only an inch or two in width, 

 but as much as twelve inches wide in places, traverse the diorite. 

 On either side of these veins, the coimtry rock has been bleache(l 

 for varying distances, generally less than one foot, and rendered 

 nuich finer grained. Considerable development of pyritt- and 

 arsenopyrite together with some stil)nite, has taken place in close 

 pi-oximity to the quartz veins. These sulphides are most abundant 

 along the vein wall, and decrease in amount outwards. That the 

 bleaching is a result of the alteration of the diorite by the vein 

 .solutions, is certain. 



Specimen No. 140, Caledonia Mine, Yow Yow, shows an inter- 

 mediate stage in the alteration. Macroscopically, it is a fairly even 

 grained rock, greenish in colour, and showing fairly abundant 

 pyrites scattered through it. A section of the rock examined micro- 

 .scopically shows no hornblende or felspar. The hornblende remain- 

 ing from the propylitic alteration has not been first changed to 

 chlorite, and then to carbonates and muscovite, but apparently it 

 has in part passed directly to carbonate, and in part to a whitish 

 or yellowish, cloudy, nondescript material. Idiomorphic outlines 

 of hornblende are occasionally preserved in this material. The 

 carbonate is dolomitic or ankeritic in character, as dilute hydro- 

 chloric acid has very little action on the powdered rock in the cold. 

 Chemical analyses of similar carbonates developed in the diorites 

 near Wood's Point have shown them to be varying compounds of 

 the carbonates of lime, magnesia, ferrous oxide, and manganous 

 oxide. Chlorite still remains in considerable amount, Init epidote 

 has disappeared, having apparently succumbed to the action of the 

 carbonate solutions. Local patches of carbonate in the chlorite are 

 closely allied in shape to the epidote grains in the propylitic rock, 

 and undoubtedly replace the latter mineral. Some of the chlorite 

 is beginning to })ass to colourless mica (muscovite), and carbonates 

 have been intruded along the cleavage planes, and replace the 

 chlorite in part. The felspars have been almost completely re- 

 placed by closely packed sericite flakes. Under high powers, 



