THE MINERALS OP IDAHO 223 



BOISE COUNTY 



Curved saddle-shaped crystals of opaque pale gray dolomite were 

 seen with crystals of quartz and arsenopyrite in gold ore from the 

 I. X. L. mine in the Pearl district. The ore consisted principally 

 of quartz containing finely disseminated arsenopyrite. 



BONNER COUNTY 



The Cambrian limestone at Lakeview on the southern shore of 

 Lake Pend d'Orielle is principally dolomite. At Bayview, where 

 this rock has been metamorphosed, it is a pure dolomite marble, 

 except in certain beds which are quarried and burned to quicklime. 



CUSTER COUNTY 



Dolomite is present in most of the limestone of the Alder Creek 

 (Mackay) district, but it does not occur in the garnet contact rock 

 with the ores, the magnesium here having been used up to form other 

 minerals, principally diopside, during the earlier stages of the meta- 

 morphism. 32a In northwestern Custer County dolomite occurs rarely 

 as a gangue mineral in the lead-silver deposits. 33 



SHOSHONE COUNTY 



Dolomite is mentioned by Ransome 34 as a gangue mineral in the 

 Silver Cliff copper prospect east of Mullan. Recent examinations by 

 the writer indicate that many of what have heretofore been called 

 siderites in the area of copper prospects in the Wallace and St. Regis 

 formations to the east and south of Mullan and also in the numerous 

 prospects in the Prichard formation around Kellogg and on Pine 

 Creek are mixed carbonates intermediate between dolomite and 

 ankerite. These are commonly associated with quartz as the gangue 

 of pyrite, pyrrhotite, or chalcopyrite. These carbonates usually 

 show the characteristic rhombohedral cleavage of the group and in 

 color, on fresh fracture, are white to pale buff or, more frequently, 

 bluish or gray. When weathered for a year or more on the dumps 

 they assume a pearly lustered ocher-brown color. There is repre- 

 sented a gradation from dolomite containing a small amount of iron, 

 to ankerite containing relatively little magnesia, most of the specimens 

 consisting of an intermediate mixture containing both iron and 

 magnesia. Several gangue carbonates from the Mullan area have 

 been analyzed and those from other parts of the region will be exam- 

 ined as opportunity permits. Those which have a molecular pre- 

 ponderance of magnesia over iron are here described as ferriferous 

 dolomites. Those in which the reverse relation obtains are mentioned 

 below under ankerite as are those which have not yet been analyzed. 



}U J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 52-53, 1917. 



33 J. B. Umpleby. U. S. Geol. Survey, Bull. 539, p. 52, 1913. 



'* F. L. Ransome. U. S. Oeol. Survey, Prof. Paper 62, p. 95, 1908. 



