AN ASSOCIATION OF KAOLINITE WITH 

 MIAROLITIC STRUCTURE^ 



A. F. BUDDINGTON 



Princeton University, Princeton, New Jersey. 



While engaged in geologic mapping of the Wrangell district in 

 southeastern Alaska, the writer collected a suite of specimens 

 which seem to illustrate very well one mode of origin of kaolinite, 

 though not the most common one. 



The general geology of the occurrence is as follows: A long, 

 narrow stock of granite — ^possibly 25 miles long and 2 miles wide— 

 extends across Etolin Island in a southeast-northwest direction, and 

 is intrusive into a much larger stock of diorite. On the southeast 

 side of Zarembo Island, farther to the northwest, along the same 

 general line of strike, small stocks and intrusive masses of granite 

 porphyry and rhyolite porphyry are found in slates, and are pre- 

 sumably outlying masses of the main stock. Many small masses of 

 conspicuously miarolitic granite porphyry, intrusive into schists, 

 are also found on the islands of Ernest Sound to the northeast of 

 the main stock. The kaoHnite in question occurs as fillings in 

 druses in the rhyolite porphyry, and as an alteration product of 

 the plagioclase feldspars in the granite porphyry of the Etolin 

 Island stock. The rocks examined are in a glaciated region and 

 are fresh and unweathered. 



At the southeastern end of the granite stock the rock is a 

 medium-grained pink granite, with a compact granitoid structure. 

 The major component minerals are microcline-microperthite and 

 quartz, with about 10 per cent or more of albite-oligoclase, a little 

 hornblende and biotite, and accessory zircons and magnetite. About 

 12 miles northwest, the rock has changed to a pink granite porphyry 

 with granophyric ground-mass and a prominent miarolitic structure. 

 The druses average about one-half centimeter in diameter, and are 



^ Published by permission of the Director, United States Geological Survey. 



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