BLACK HILLS GEOLOGY. 295 



The siliceous gold ores occur in the Cambro-Silurian and Car- 

 boniferous formations, and are the ones with which this paper is 

 chiefly concerned. The others will be considered only so far 

 as they are geologically related to them. 



A. Ores in the Algonkian. 



The ores in the Algonkian form impregnated zones in the 

 slates and schists. The gold is associated with pyrite, and the 

 ores are free-milling in the upper parts of the deposit, but pass 

 into more refractory sulphurets as workings advance to greater 

 depths. The most important development is the great Home- 

 stake vein of Lead City, but many other minor impregnated 

 zones occur throughout the Algonkian exposures. The ores 

 in this formation average $T).?>7 to $4.00 per ton, and are essen- 

 tially low grade. Concerning the Homestake vein Dr. Carpen- 

 ter says •} " The part of the ' belt ' belonging to the Homestake 

 combination is gold bearing for a distance of 6,000 feet. The 

 ' ore ' is not continuous throughout this distance, but occurs in 

 shoots or vast 'pipes,' lenticular in cross-section. The beds of 

 argillite, phyllite and amphibole schists, in which these shoots oc- 

 cur, strike north T)7^ degrees west ; which is also accurately the 

 strike of the plane or ' ore channel ' in which the shoots occur. 

 The dip of the beds is as a whole to the east. The shoots dip 

 east also, but athwart this plane at an angle of about 45 degrees. 

 The ore and enclosing rocks have indifferently the same general 

 cleavage structure." 



Many dikes of porphyry cut the Homestake deposits, and 

 sheets of the same rock overlie it. The porphyries have, in the 

 opinion of the above authority : ' 



1st, Made the ores more free-milling. 



2d. Produced in their neighborhood either an enrichment of 

 the deposit or a further concentration of the gold which origin- 

 ally existed in it. 



As these deposits have not yet attained, in the area mapped, 



1 Trans. Am. Institute Mining Eng., XVII, 574, Feb., 1889. 

 Op. cit., page 575. 



