KAOLIN ITE WITH MIAROLITIC STRUCTURE 151 



The kaolinite and calcite were deposited in the druses of the 

 rhyoHte porphyry still later than the quartz-specularite combination, 

 and hence probably at still lower temperatures under hydrothermal 

 conditions. The possible source and origin of the kaolinite is 

 indicated by certain phenomena found in the granite porphyry now 

 exposed at the surface. The plagioclase feldspars of this rock are 

 zoned, and the more basic cores of many of them have been altered 

 to fibrous kaolinite, which has been partially or completely removed 

 so that many of the plagioclases show cavities and pits with an 

 etched solution surface, whereas others are still partially filled with 

 a porous kaolinitized feldspar residue. It was impossible to isolate 

 the crystalline fibers to prove whether they were sericite or kaolinite, 

 but the porous character suggests the latter. It is reasonable to 

 assume that the rhyolite porphyry passes transitionally downward 

 into a granite porphyry similar to that just described, and that the 

 kaolinite and calcite in the druses of the rhyolite porphyry have 

 been derived through the partial alteration and leaching of the 

 plagioclase feldspars of such an underlying granite porphyry by 

 thermal carbonated waters of relatively low temperature. This 

 conclusion is in Line with the latter part of the statement of Lindgren^ 

 "In brief, kaolin is never a high temperature mineral but is either 

 a product of alteration by descending waters containing sulphuric 

 acid of carbon dioxide, or of alteration by ascending weak carbonated 

 waters close to the surface." 



^ W. Lindgren, "The Origin of Kaolin," Econ. Geol., Vol. X (1915), p. 90. 



