position and form of the ore body 299 



Position and Form of the Ore Body 



The form and position of the surface croppings of the Homestake ore 

 body may be gathered from a study of the open cuts. It is observed that 

 there are two parts : a wide part, that to the south (the great open cut at 

 Lead), and a narrow part, that to the north, terminating in the De Smet 

 cut on the southern rim of Deadwood Creek. 



The narrow portion begins to spread and merge into the wide portion 

 at a point where stratigraphic work indicates the presence of the main 

 body of the calcareous series. The narrow portion appears to be a single 

 calcareous bed thrown into a sharp fold well exposed as a narrow syn- 

 clinal trough in the northern end of the De Smet cut. 



The western boundary of these surface exposures is a relatively straight 

 line and is believed to be, for reasons already set forth, a fault-plane. 



The outcrop of the Caledonia ore body has been shown to coincide very 

 nearly with the outcrop of a part of the calcareous series. The strike is 

 directly toward the main Homestake ore body; in fact, a portion of the 

 two open cuts are very close together. 



Beneath the surface the form and position of the ore body is even more 

 significant. The wider portion, on a given level, is delimited in certain 

 definite ways. First, on the western or footwall side by a relatively 

 straight line — the fault-line ; second, on its southern end by a number of 

 narrow fingers, which examinations show to be actual folds — that is, the 

 ore is sharply cut off by wall rock which curves around it ; and, third, on 

 the north-northeastern side — that is, in the direction of strike of the 

 rock series — by indefinite fading away of ore. The inference is that here 

 a definite series of beds striking north- northeast have been folded, cut otf 

 by a fault, and mineralized. 



The ore body plunges in the direction of the intersection of the fault- 

 plane with the steeply dipping beds, and at each succeeding level is 

 capped by a greater thickness of barren rock. The Caledonia ore body 

 pitches with the dip — that is, the replaced beds are cut by no fault-piano. 

 Its northern extension is taken to be limited by the distance from the 

 fault-plane that solutions were able to replace limestone, its southern ex- 

 tension by late igneous intrusion, which separated the ore body from the 

 main mass along the fault. 



Character of the replaced Rocks 



It has been implied in the preceding paragraphs that the ore bod} Is 

 related to a definite set of beds, 7 and in describing surface relations it 



1 The main ore body is here referred to. There are probably other Ledfefl occupying 

 mineralized slate bands. 



XXI— Bull. Geol. Soc. Am., Vol. 24, 1912 



