THE MINERALS OF IDAHO 305 



Garnet occurs in the Starlight mine in Elkhorn Canyon near 

 Ketchum as massive fine-granular brown garnet rock containing 

 galena and also as small brown crystals associated with sahlite, etc. 

 Another specimen from somewhere in this district consists of a large 

 mass of greasy brown garnet, which, against included calcite, occurs 

 as well-formed crystals. 



CUSTER COUNTY 



Lime garnet is known from numerous localities in Custer County. 

 It is common in the Phi Kappa mine in Phi Kappa Canyon near the 

 head of Big Lost River in granular massive grayish form containing 

 galena, sphalerite, and chalcopyrite, probably developed in calcareous 

 beds in Ordovician shale. Another specimen from a prospect in the 

 same vicinity (" Prospect at 8560' and N. 30 E. of high point on ridge 

 across canyon") shows massive garnet rock and light-brown dodeca- 

 hedral crystals up to 2 cm. in diameter with chalcopyrite, galena, 

 sphalerite, and pyrite. Another specimen shows brownish granular 

 massive garnet with galena. Specimens from the Basin group 

 prospect which is a small tunnel at an elevation of 8,050 feet N. 24° 

 E. of the top of the round knob 8,700 feet high just west of where the 

 road from Ketchum to Mackay crosses the Trail Creek summit and 

 across Park Creek contain abundant garnet. This is apparently a 

 metamorphosed calcareous lens of shale which contains garnet in 

 association with vesuvianite, epidote, prehnite, etc., and some 

 sulphides. The garnet varies from gray to brown and some speci- 

 mens show good brown crystals up to 7 mm. in diameter in calcite. 



In the Copper Basin district at the head of east fork of Big Lost 

 River garnet occurs abundantly associated with other lime silicates 

 and magnetite as the gangue of copper ores in quartzite and calcareous 

 shale or slate. 25 Specimens from the Reed and Davidson claims 

 in this district consist of resinous brown granular masses of garnet 

 rock made up of small crystals. A part of the garnet rock is porous 

 and the cavities are lined with greenish-brown crystals which are 

 dodecahedral in form, modified by faces of the trapezohedron. 



In the Alder Creek district garnet is by far the most abundant 

 mineral in the contact copper deposits about Mackay. The com- 

 position ranges from grossularite to andradite, while computation of 

 analyses indicates the presence of very minor amounts of the alman- 

 dite, spessartite, and pyrope molecules. The garnet occurs as 

 euhedral crystals where embedded in calcite or along joints but else- 

 where the individuals are anhedral owing to mutual interference. It 

 locally includes cores of mixed magnetite calcite and quartz and con- 

 centrically distributed grains of chalcopyrite. Much of the garnet 

 shows zones of anomalous birefringence under the microscope in 



2 « J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 104, 1917. 



