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University of California Publications in Geology [Vol. 10 



varying amounts of wollastonite, and microscopic diopside. This is 

 gradually replaced by garnet rock with veins and bunches of marble, 

 and the latter becomes more abundant until we find marble with lenses 

 and stringers of garnet and associated minerals, and these become 

 less numerous until a pure marble remains." 5 "Parts of an ore-body 

 may be one hundred feet or more from an intrusive rock; but some 

 portion of every ore mass is close to the contact with one of the two 

 above-named igneous rocks. Usually there is a five to ten-foot band 

 of garnet rock between ore and porphyry." 6 "The ore-minerals are 

 definitely later than most of the gangue-minerals, and occur com- 

 monly as veinlets in the garnet-quartz-calcite mass, or disseminated 

 through it," 7 



Seven Devils, Idaho. — The rocks of the Seven Devils district 

 comprise slate, quartzite, limestone and a vast amount of associated 

 greenstone invaded and metamorphosed by a quartz diorite phase of 

 the Idaho granite and capped locally by Columbia basalt. 8 The ore 

 deposits are in typical contact zones situated in part along the external 

 contact of the quartz diorite but principally adjacent to engulfed 

 blocks of limestone. Garnet, epidote and quartz are abundant gangue 

 minerals and bornite and chalcopyrite are the principal sulphides. In 

 the Queen, Blue Jacket and Arkansas mines the ore occurs on the lime- 

 stone side of garnet zones across which veinlets of chalcopyrite and 

 bornite in a quartz gangue connect with the igneous mass on a dip of 

 about 15 degrees. Mining development along these veinlets has ceased 

 in most places at the igneous contact, but they are known to extend 

 locally twenty-five or thirty feet into the quartz diorite without notice- 

 able diminution in size or change in character. One locality where 

 the veinlets are particularly well shown and are closely spaced is in 

 the Queen No. 1. Intermediate stope. See figure 1. 



In the Peacock Mine the ore occurs in the central part of a large 

 garnet-epidote area bordered on the east and west by diorite and on 

 the south by a green porphyritic rock believed to be an engulfed 

 portion of the greenstone series. The main sedimentary contact lies 

 a few hundred feet to the north across an area of normal diorite. 

 Thus it is suggested that an engulfed block of limestone 'determined 



s Stewart, C. A., op. cit., p. 272. 

 o Ibid., pp. 280-281. 

 - Ibid., p. 282. 



s Lindgren, W., The gold and silver veins of Silver City, De Lamar and 

 other mining districts in Idaho, Twentieth Ann. Kept., U. S. Geol. Survey, Pt. 

 3, pp. 249-253, 1900. Umpleby, J. B., Reconnaissance examination, 1915. Re- 

 port in preparation. 



