Gcoioyy of the Diamond Creek Area. 339 



replacement of pyrites by limonite. An occasional six-sided crystal 

 of quartz, partially replaced by sericite, is present. The original rock. 

 is thus seen to have been a quartz felspar porphyry. A similar 

 conclusion was reached as to the original character of the Diamond 

 (^reek dyke. 



Section A 21, from near the edge of the Dry Creek dyke, contains 

 numeroTis xcuoliths of sandstone and slate. The plagioclase is 

 zoned, and the quartz is frequently eaten into and replaced by 

 .sericite. 



•5. Diamond Creek dyke. — This dyke was sectioned and examined 

 before the writer had seen the Dry Creek dyke, and the main con- 

 clusions regarding the nature of the alteration it has suffered were 

 thus deduced previous to the examination of the latter dyke. 

 Jn hand specimen the Diamond Creek dyke is yellow green in 

 •colour, and contains abundant minute cubes of pyrite. Micro- 

 scopically it is a hypocrystalline, very even grained aphanitic 

 I'ock, with skeleton outlines of original porphyritic constituents. 

 Microcrystalline quartz grains and flakes of sericite constitute con- 

 jsiderably over 90 per cent, of the rock, the other minerals present 

 being pyrite, stibnite, bleached biotite, rutile, carbonate occasion- 

 ally, and possibly arsenopyrite and zircon. Idiomorphic outlines 

 of the original felspar crystals are distinguished by the mox'e com- 

 pact nature of the sericite in such areas. The metasomatic replace- 

 nient of the felspar is generally complete, and so the stages in the 

 alteration are not determinable. In only one section of the Dia- 

 mond Creek dyke did the wa-iter see original orthoclase remaining. 

 Karely a residual phenocryst of quartz occurs. Sericite usually 

 occurs in cleai', colourless microscopic flakes, show'ing the usual 

 delicate polarisation colours. Where replacing felspar, a linear 

 arrangement of the sericite is sometimes seen. The original femic 

 constituent of the rock was apparently biotite. As a result of 

 the alteration, the l)iotite was leached, and simultaneously i-utile 

 separated out in the biotite areas. This separation was usually 

 in the form of " segenite "^ webs, but occasionally rutile occurs 

 in prisms roughly parallel to the length of the biotite. Carbonate 

 often occurs associated with quartz in veins through the dyke, but 

 otherwise it is rare. Some of this vein carbonate was examined and 

 found to be dolomite. Pyrite is common in cubic crystals, and 

 stibnite is always present, but generally in very small amount when 

 not near the quartz veins. 



1 Rosenbusch, IddinjfS, p. 146. 



