THE MINERALS OF IDAHO 371 



with the softness of the vein matter, renders mining somewhat 

 difficult. 



The vein matter is composed largely of a soft white gouge which 

 incloses the sulphides. The gouge is sericite, a secondary mica 

 derived from the alteration of feldspars. Pyrite in well-developed 

 crystal aggregates or individuals, is the dominant sulphide of the 

 gouge, but here and there are fragments of older vein material in 

 which quartz, calcite, pyrite, galena, sphalerite, and tetrahedrite were 

 recognized. These sulphides, however, have largely been ground up 

 and incorporated in the gouge. It is evident that there have been 

 two general periods of sulphide deposition and that extensive altera- 

 tion and movement have occurred in the vein subsequent to the first 

 period. Pyrite is the latest mineral in the sericite gouge as proved 

 by its well developed crystal form. According to E. F. Blain, super- 

 intendent of the property, the vein assays from $5 to $15 a ton in 

 gold and $2 to S3 a ton in silver. No assays were made to determine 

 whether the pyrite of the lower tunnel contains any gold and silver, 

 but it is thought more probable that these metals are contained in 

 the older sulphides. 



The samples examined were typical specimens of the sericite gouge 

 from the shear zone and consist in the main of the fine scaly sericite 

 containing disseminated crystals of pyrite up to 1 cm. in diameter. 

 One of the larger specimens contained a mass of quartz and sulphides 

 of the first mineralization and this was examined after the adhering 

 sericite had been removed by scrubbing with a brush and water- 

 The mass is a nodule of white quartz, rounded as though by corrosion, 

 containing pyrite, galena, tetrahedrite, and a colorless to light resin 

 or greenish-yellow sphalerite in well-formed crystals up to 1 cm. in 

 diameter. There is also a little of a mineral of grayish purple color 

 and well-defined cleavage which agrees in optical properties and 

 blowpipe reactions with barite. 



In the hand specimen the sericite is visibly micaceous with a very 

 finely foliated structure and distinctly silky luster. The color varies 

 from cream or dirty white to pale greenish. When moist it is plastic 

 with a greasy feel. The optical properties were carefully determined 

 by Clarence S. Ross, of the United States Geological Survey, and are 

 given in column 1 below, while in column 2 are given, for comparison, 

 the properties of normal muscovite quoted by Larsen. 



