THE MINERALS OP IDAHO 3 



The most prominent geologic feature is the great central mountain 

 mass bordering Montana and continued on the north by the Clear- 

 water and Coeur d'Alene Mountains. The eastern part of this 

 mountain mass consists of pre- Cambrian and Paleozoic strata, the 

 extreme western part of Triassic sediments. Between these two lies 

 a great intrusive mass of granitic rocks, principally quartz monzonite, 

 continued on the north and east by smaller intrusive areas. The 

 northern or panhandle part of the State is mainly built up of the 

 steeply inclined sedimentary strata of a thick pre-Cambrian complex 

 known as the Belt series. The southern and southwestern parts of 

 the State are covered by thick volcanic flows of Tertiary age, and this 

 area connects on the east with the lavas of the Yellowstone Park 

 region and Utah. Toward the Nevada line rise a number of short 

 ranges of the type of the Basin Ranges of that State, and the extreme 

 southeastern part of Idaho is occupied by north-south ranges built 

 up of folded and faulted Paleozoic and Mesozoic rocks which may be 

 considered as a northward extension of the Wasatch Mountain 

 system. 



Pre-Cambrian deposits are not present in Idaho, except possibly 

 in some of the less important districts in the northern part of the 

 State, where mineralization seems to have accompanied intrusion of 

 diabase sills in the Belt series. Here as elsewhere two important 

 kinds of deposits may be recognized — those which were formed 

 shortly after the intrusion of the great central batholith of Jurassic 

 rocks, probably in late Cretaceous time, and those of late Tertiary 

 age which developed after the outburst of the Tertiary lavas in the 

 southern and central parts of the State. The latter are confined to 

 the gold and silver veins of Owyhee County and to a belt of lavas 

 in the central part of the State, including such deposits as the Custer, 

 in Custer County; the Singiser veins in Lemhi County; and the 

 Thunder Mountain deposits in Idaho County. 



A few unimportant deposits of copper and lead ores of uncertain 

 age occur in the Paleozoic limestones of the ranges in the extreme 

 southeast corner of the State. 



The upper parts of the late Cretaceous ore deposits have to a 

 considerable extent been removed by erosion and the parts now 

 exposed contain ores formed at considerable depths. They are 

 mainly fissure veins, but in places, as in southern Lemhi County and 

 at the Wood River (Hailey) district, these merge into replacement 

 deposits of galena in limestone. Contact-metamorphic deposits 

 containing copper ores are developed on a rather extensive scale at 

 Seven Devils, near the Oregon boundary line, and at White Knob, 

 in the southeastern part of Custer County, but neither place has 

 proved a large or steady producer. 



