232 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Prichard formation on Pine Creek contain a little ankerite and no 

 siderite has been seen from any of these. In this respect they 

 resemble the zinciferous ores in the Prichard formation of the Nine- 

 mile group. All of the siderites contain an appreciable amount of 

 manganese carbonate and this seems highest in ores which are high 

 in silver ratio and contain tetrahedrite. Some of the most trans- 

 lucent, brownest and most coarsely crystalline siderite of the district 

 occurs in the narrow silver-rich veins in the Wallace formation be- 

 tween Kellogg and Wallace, where the siderite forms the gangue of 

 argentiferous tetrahedrite. Typical of these veins are the Yankee 

 Boy on Big Creek and the Polaris at the head of Polaris gulch. 

 The siderites from these veins also seem highest in manganese carbon- 

 ate although the} 7 have not yet been analyzed. 



Although especially abundant near certain areas of intense min- 

 eralization, a carbonate supposed to be siderite is widely distributed 

 through the quartzitic rocks of the district. Although the dis- 

 seminated carbonate has a much wider distribution than the ores it 

 is most abundant where the rocks have been most folded and fissured. 

 It apparently forms more readily in some rocks than in others, 

 being abundant in the Burke and in parts of the Prichard forma- 

 tion but rare in the calcareous rocks of the Wallace. 



The siderite is visible in few weathered exposures, owing to its 

 ready decomposition to limonite, consequently the presence of the 

 carbonate disseminated in weathered rocks is indicated by rusty 

 streaks and specks. In thin sections of the country rock the car- 

 bonate occurs in individuals, a majority of which are larger than the 

 clastic grains of the rock, the mineral being crystallographically con- 

 tinuous for a diameter of several millimeters. Their form varies 

 and while many of the larger grains appear ragged, close examina- 

 tion shows them to be determined in large part by rhombohedral 

 planes. In other grains the rhombohedral form is more distinct, in 

 some cases perfect, and in any specimen the rhombohedral crystal 

 boundaries cut sharply across quartz grains establishing the fact 

 that the carbonate developed by molecular replacement of the 

 quartz rather than by filling cavities. The larger grains frequently 

 inclose quartz grains and, much less commonly, scales of sericite. 

 The carbonate therefore replaces the sericitic matrix of the rock 

 more readily than the larger grains of quartz. 4 ' Disseminated iron- 

 bearing carbonates which leave rusty pits when weathered occur in 

 quartzitic rocks of the Belt series in wide distribution in the area 

 surrounding the Coeur d'Alene district and remote from known 

 mineralization. It seems probable that the same confusion obtains 

 with regard to the disseminated carbonates as in the gangue car- 

 bonates of the veins and probably both ankerite and siderite occur 



<» F. C. Calkins. Prof. Paper U. S. Geol. Survey, No. 62, p. 97, 1908. 



