242 Watson — -Association of Native Gold with Sillimanite. 



the appearance is that of gradation from pegmatite into schist, 

 while in other portions the two rock phases are rather sharply 

 differentiated from each other. 



In the hand specimen abundant large and small flecks of 

 native gold are noted in intimate association with biotite, the 

 light-colored minerals, and garnet, suggesting in the latter 

 mineral partial rimming and filling of minute fractures or rifts. 



Under the microscope the thin section shows intergrowths 

 of biotite and muscovite, quartz, sillimanite, orthoclase and 

 plagioclase feldspar, garnet, occasional pyrite, and several 

 unimportant microscopic accessory minerals. Biotite is partly 

 altered to chlorite, is dark brown in color, has strong absorp- 

 tion, and contains numerous inclusions of zircon surrounded 

 by characteristic dark borders. Muscovite, in large and small 

 shreds possessing good cleavage and partly intergrown with 

 biotite, is plentiful. Orthoclase is probably in slight excess 

 over plagioclase and both feldspars show intergrowths with 

 quartz in micrographic structure. A considerable portion of 

 the thin section is occupied by an aggregate of sillimanite 

 fibers having partial radiating arrangement, intergrown with 

 biotite, and closely associated with muscovite, the longer axis 

 of the sillimanite fibers crossing at all angles the cleavage 

 direction of muscovite. Much of the sillimanite is colored 

 brown and is pleochroic from fine scales of biotite lying 

 between the sillimanite fibers. The relations of the two min- 

 erals suggest possible derivation of sillimanite, in part at least, 

 from biotite, a change which according to Yan Hise* may take 

 place under conditions of elevated temperature and pressure. 

 Quartz shows optical disturbance and contains liquid and solid 

 inclusions. Garnet is without crystal boundaries and exhibits 

 rifts or minute fractures. Pyrite is only sparingly present but 

 appears entirely fresh. 



That portion of the specimen from which the thin section 

 was cut shows unusual richness in native gold, the particles of 

 which range from tiny granules up to irregular grains 2 mm or 

 more in diameter. The gold occurs associated with all the 

 principal minerals including sillimanite and garnet, and is in 

 juxtaposition with several grains of pyrite, partly enclosed by 

 biotite; but in each case the boundaries between the two min- 

 erals are sharply defined and the pyrite shows no indication of 

 alteration. It occurs as interstitial grains between the quartz 

 and feldspar and the other minerals, and in some cases is 

 embedded in the substance of the quartz and feldspar, and to a 



* Van Hise, C. R., A Treatise on Metamorphism, Mon. No. XL VIE, U. S. 

 Geol. Survey, pp. 342-343, 1904. Dr. Van Hise states that biotite rarely 

 alters into hypersthene and sillimanite, and the formation of these from 

 biotite "usually occurs in connection with contact reactions of igneous 

 rocks :...." 



