62 ALASKA GEOLOGY 



the region, and later charged with gold-bearing pyrite by 

 mineralizing solutions. 



The sodium-syenite consists essentially of almost pure 

 albite, in granular or partly idiomorphic aggregates, with 

 which is more or less orthoclase and quartz, the latter 

 generally in micropegmatitic intergrowths filling the in- 

 terspaces of the albite. Ferromagnesian constituents were 

 probably present in the unaltered rock, but their nature 

 can not now be determined, beyond the fact that there 

 was a small amount of biotite, now changed wholly to a 

 greenish mica or to chlorite. Other original accessory 

 constituents are apatite and titanite, fairly abundant, and 

 sparing zircon. The secondary products are, first and 

 most important, pyrite, which is abundant in all slides in 

 sharp crystals, often surrounding small feldspar crystals 

 and often associated with calcite. Calcite and sericite 

 are very abundant, being developed especially in the cen- 

 ters of albite crystals, which they sometimes completely 

 honeycomb, leaving only a narrow rim of unaltered feld- 

 spar. Grains of epidote and zoisite are sometimes abun- 

 dant in the feldspar, and rutile in sagenitic groups is seen 

 in the chloritized biotite. Quartz in evidently secondary 

 forms is surprisingly small in amount, although this is ac- 

 counted for partly by the choice of material for slicing, 

 which was the freshest obtainable. It should, however, 

 be said that sections from rocks which were pronounced 

 by those most familiar with the ores to be undoubtedly 

 gold-bearing, and sections made from portions of the 

 syenite shown by assay to be barren, were practically 

 identical in character, there being but little observable 

 difference in decomposition or in content of quartz or 

 pyrite. Hand specimens of the barren rock usually 

 showed a darker color, due to the greater amount of 

 chlorite, but this distinction was not constant. In fact it 

 was continually a matter of surprise that the syenite was so 



