300 S. PAIGE ORIGIN OF THE HOMESTAKE ORE BODY 



was pointed out that the outcrop of the main lode corresponded to the 

 position of a calcareous series as determined by stratigraphic work. 

 Specimens were collected underground and examined microscopically, 

 with a view to determining three points : the nature of the beds where 

 ore graded into rock too poor to extract, the nature of various wall rocks, 

 and the nature of the ore. 



It is not desirable to describe in detail here the results obtained, but 

 the essentials are as follows : A number of specimens collected on the 

 north-northeast side of the ore body (from various levels), where the ore 

 graded into country rock, proved to be distinctly calcareous. They would 

 be termed calcareous or dolomitic slates or schists and carry 50 per cent 

 or more of carbonate. The walls where the ore body is clearly defined 

 are either siliceous slate or quartzite, or, as found in one case 50 zb feet 

 from the footwall 8 of the Caledonia ore body, a decidedly calcareous or 

 dolomitic schist. An examination of several specimens of the ore proper 

 shows an abundance of carbonate replaced by silicate minerals, sulphides, 

 the oxides magnetite and specularite, and quartz. Prominent among the 

 silicates are the iron amphibole 9 cummingtonite, phlogopite, chlorite, 

 and some biotite. Among the sulphides are pyrite and pyrrhotite. The 

 form of the cummingtonite was significant in that it possesses that in- 

 complete crystallization of individuals and poikilitic inclosure of other 

 minerals which has been described as occurring in replaced limestones. 10 

 That the silicate minerals do not show a noteworthy amount of calcium, 

 but are magnesian silicates, suggests that the replaced beds were dolo- 

 mitic 11 and indicates one of many chemical problems awaiting solution. 



Summary 



It is suggested that the main Homestake ore body owes its origin to 

 the presence of a strong fault and the subsequent mineralization of a 

 calcareous series, its form being that which would be taken if a folded 

 sedimentary series were cut by a fault and replaced by solutions arising 

 along the fault. The particular form which observed facts would require 

 of such a calcareous series as is present agrees with the form of the ore 

 body as determined by mining. Microscopic examination of ores and 

 wall rocks, as far as carried on, supports this hypothesis. 



8 A narrow dike at the place. 



9 The amphibole observed agrees in its optical properties with cummingtonite described 

 by W. J. Sharwood : "Analyses of some rocks and minerals from the Homestake mine, 

 Lead, South Dakota." Economic Geology, vol. 6, No. 8, p. 729. 



10 F. L. Ransome : Geological and ore deposits of Bisbee quadrangle, Arizona. Profes- 

 sional Taper No. 21, TJ. S. Geological Survey, p. 130. 



Waldemar Lindgren : The copper deposits of the Clifton-Morenci district, Arizona. 

 Professional Paper No. 43, U. S. Geological Survey, pi. vii, A. C. D., pi. xv. 



11 Analyses have proved this to be the case. 



