abstracts: geology 259 



dark blue-green color, and to signalize this feature they are termed augite 

 melaphyres. 



Toward the northeast the slates and graywackes pass gradually into 

 phyllites and schists, which become progressively more and more crystal- 

 hne toward the diorite gneiss that forms the northeast boundary of the 

 gold belt. The schists where they adjoin the gneiss are garnetiferous, 

 staurolitic, cyanitic, and coarsely biotitic. 



The gneiss on the northeast, which in local usage is known as the 

 granite hanging wall of the Juneau gold belt, is the crushed and foliated 

 margin of the great intrusive quartz diorite core of the Coast Range. 

 The gneissic structure is best developed near the contact with the schist 

 and fades out northeastward into the ordinary structure of normal 

 granitic rocks. It is essentially a cataclastic effect which was produced 

 by the crushing of the component minerals of the quartz diorite, and 

 which was impressed on the diorite soon after the diorite had consoli- 

 dated from fusion. In fact, a period of dynamic deformation set in dur- 

 ing the pegmatitic stage, for some of the pegmatite and aplite dikes are 

 sheared like the gneiss, but others have escaped the general dynamic 

 metamorphism. The sedimentary rocks adjoining the gneiss have re- 

 crystallized into schists, whose crystaUinity, as already stated, diminishes 

 gradually from the contact. The region thus affords a remarkably 

 fine illustration of a belt of highly crystalline schists formed as an effect 

 of the heat and pressure accompanying a great batholithic intrusion of 

 late Mesozoic age. 



The ore bodies are exclusively gold deposits. The great majority 

 are stringer lodes, but include some mineralized dikes and a few fissure 

 veins. Except for sporadic sheets of rich ore, the stringer lodes are of 

 low grade. They range in width from a few feet to 100 feet, and appar- 

 ently at a few prospects, to 300 to 400 feet. The greatest depth attained 

 by mining anywhere in the district does not exceed 200 feet. 



Some of the mineralized dikes are of economic importance; all are of 

 considerable interest because the profound alteration which they have 

 undergone throws light on the character of the solutions that brought in 

 the gold. A common change consisted in a large introduction of soda 

 and the formation of albite, and this albitization is here shown to be a 

 regional feature of the Juneau gold belt. Other changes consisted in the 

 introduction of apatite into the altered wall rocks and the conversion of 

 amphibole into biotite. From these and other features the vein-forming 

 waters are believed to have been hot, ascending solutions of deep-seated 

 origin, probably connected with the intrusion of the diorite magma. 



A. K. 



