PRE-CAMBRIAN STRUCTURE OF THE NORTHERN BLACK HILLS 705 



Discussion 



Prof. J. D. Irving: The remarks just offered by Mr. Sidney Paige on the 

 Homestake ore-body have been of considerable interest to me, as it was my 

 good fortune to make an examination and publish a report 2 on the Homestake 

 ore-body some 15 years ago. I am not very familiar with the broader struc- 

 ture and lithology of the rocks which inclose the ore-body, in so far as they 

 were observed underground during the very brief study which I was able to 

 make, permission to enter the mine having been withdrawn by the manage- 

 ment shortly after the beginning of my field work. The surface geology was 

 done by Professor Jaggar and his assistants, so that I am unable to comment 

 on the structural features mentioned by Mr. Paige. 



A very large collection of specimens of the ore, however, was made on all 

 levels of the Homestake mine, extending to a depth of approximately 900 feet. 

 These represented all varieties of- ore found in the Homestake mine, and I was 

 able to make from them a rather complete petographic study, the results of 

 which, to a large extent at least, seem to me to make this explanation im- 

 probable. 



The most notable feature about the Homestake ores is that they exhibit no 

 lithologic or textural features which seem to distinguish them from the barren 

 country rock; so well has this feature been recognized that the company has 

 never been able to distinguish the ore without either panning tests or assays. 

 When we first took up the study of the Black Hills ores, we noted that through 

 some of the rock which carried the gold pyrite, pyrrhotite and subordinate 

 arsenopyrite occurred in considerable abundance. In other varieties this was 

 accompanied by quartz, and in some stopes the ore was largely made up of the 

 silicate Cummingtonite, which was, I believe, first recognized by Mr. W. Lind- 

 gren. In still other varieties dolomite was observed in considerable quantities 

 and after red garnet, presumably of the variety pyrope. In all of these va- 

 rieties of ore the appearance of the rock was apparently characteristic. Such 

 ore, however, represented only a small part of the ore mined. The larger part 

 of it was simply silicon schist containing no carbonates and in no respect dif- 

 ferent from the non-auriferous schist. Such ores were simply mica schist, 

 schist, quartz schist, chlorite schist, and other varieties of non-calcareous 

 schists and metamorphic rocks, whose only distinction from barren country 

 rock lay in their containing small quantities of gold. Careful microscopic study 

 yielded no indication of extensive alteration of any kind and showed no tectural 

 or mineralogical features which were not also observable in the unmineralized 

 rocks. 



In addition to this, no evidence of replacement was discernible and the ores 

 passed insensibly into barren rock. While, therefore, Mr. Paige's hypothesis 

 as to the origin of these ores by replacement of calcareous members in the 

 Algonkian group may be of limited application in a few of the ore-masses of 

 the mine, it certainly is not applicable to the major portion of those ores ex 

 amined by myself. Since those constituted a very complete and representative 

 series of all of the ores there exploited, I fool that Mr. Paige's hypothesis of 

 origin by replacement of calcareous rocks is, to say the least, highly improbable. 



'Professional Paper No. 2C, U. S. Geological Survey. 



