Petrology and Geology of Queenstown. 



131 



tliese ureas of sericite are resolved into a mixture of quartz and 

 sericite. Carbonates are notably present in only very small amounts 

 in the altered felspar. i The extent to which sericitisation has taken 

 place in the wall rock, is shown by an analysis made for the alkalies. 

 Thus :— Na^O 0.21%. 

 K^b 3.45%. 

 It will be lemembered that in the original rock, soda was in marked 

 excess of potash, and hence it must have been leached out during 

 the alteration whilst tlie potash was fixed and probably more intro- 

 duced. It is often assumed that the waters causing such sericitisa- 

 tion were relatively rich in salts of potassum and contained little 

 sodium salts. I see no reason, however, for this assumption,- and 

 since the vein solutions have passed through the diorite on their 

 way to the surface they should presumably become richer in soda, 

 and correspondingly weaker in potash, owing to sericitisation, as 

 they approaclied the surface. Hence, even if the original uncon- 

 taminated waters were richer in potash salts than soda salts, tlie 

 relative proportions of these constituents should soon become re- 

 versed. Probably the correct explanation is that under the conditions 

 of temperature, pressure and concentration, existing at the time of 

 the alteration, no stable sodium mineral could form. Paragonite 

 and albite, for instance, are usually high temperature minerals, 

 found in the crystalline schists and contact metamorphic deposits, 

 and are very rarely formed under conditions of hydrothermal vein 

 alteration. The potash mica corresponding to paragonite — namely, 

 sericite, however, has a wide range of existence, occurring exten- 

 sively in the crystalline schists, and almost invariably in rocks 

 affected by vein solutions, and in the presence of the latte)-. it 

 appears to be particularly stable. A very little pyrrhotite remains, 

 but iron pyrites has increased considerably in amount. It is scat- 

 tered through the rock, and shoAvs no preference for regions where 

 chlorite and ilmenite are most common. Quartz has suffered very 

 little change as a result of these alterations. Its peculiar parallel 

 cracks simulating a cleavage, still remain. On close examination 

 under the high power, these cracks are seen to be not regular and 

 straight, but to be occasionally curved and branching. Apatite, in 

 general, is little changed. Occasionally it is fractured and replaced 

 by carbonates. Ilmenite is "almost entirely replaced by leucoxene. 



1 Rosenbasch, " Eleniente der Gesteinlehre," 1898, pp. 70-71, states that ealcite, sericite, and 

 •quartz, are the products of the attack of carbonated waters on plagioclase felspars. 



I It might be susf'ested since sericitisation is accompanied by partial dehydration of the rock, 

 that the ^•ein solutions contained very little water ; yet presumably this is not so. 



10a 



