50 C. W. Cook — New Occurrence of Ilsemannite. 



Aet. VI. — A New Occurrence of Ilsemannite; by Chas. 



W. Cook. 



The molybdenum-bearing mineral ilsemannite seems 

 to be of sufficiently rare occurrence to make such occur- 

 rence worthy of note. It has been reported from Blei- 

 berg, Carinthia ; x Cripple Creek, Colorado ; 2 Natal, S. 

 Africa f Ouray, Utah; 4 and as possibly occurring in the 

 mine waters at Idaho Springs, Colorado. 5 During an 

 extensive investigation of molybdenum deposits, in which 

 a large number of localities in the United States and 

 Canada were visited and hundreds of specimens from 

 localities not visited were examined, the writer observed 

 the occurrence of ilsemannite at but two points ; one at 

 Ouray, Utah, mentioned above, and the other in Shasta 

 Co., California, four miles west of Gibson, a station on 

 the Southern Pacific R. R. 



In the Gibson locality molybdenite, associated with 

 pyrite, occurs disseminated in what appear to be bowlders 

 of aplite, no definite connection between the molybdenite- 

 bearing masses and other rocks having been established. 

 Dikes of similar lithological character were found in the 

 district but in no instance, where it was certain that the 

 rocks were in place, was any molybdenite found. Not 

 infrequently the molybdenite has been completely altered 

 to molybdite and this mineral may be seen entirely filling 

 the cavities formerly occupied by the molybdenite. In 

 other instances, where the molybdenite has not been com- 

 pletely altered, a bluish-colored zone shows around the 

 molybdenite nucleus. This zone is plainly a staining of 

 the quartz and feldspar by ilsemannite or some closely 

 allied substance. 



At the time of the examination of the property, some- 

 thing over one hundred tons of ore had been removed, 

 according to reports, and some of this ore was still on 

 the ground at the railroad awaiting shipment. This 

 broken rock had been exposed to the action of the atmos- 

 phere for several months and many pieces had been com- 

 pletely discolored blue on the surface, giving them an 

 entirely different appearance from that of the freshly 



1 Hof er, N. : Jahrb. Min., 1871, p. 566. 



2 Lindgren and Eansome : U. S. G. S. Prof. Paper No. 54, p. 154, 1906. 

 3 duToit: So. African Jour. Sci., vol. 13, p. 153, 1917. 



4 Hess: U. S. G. S., Min. Kes., 1917, p. 913, 1920. 

 B Horton: U. S. Bur. Min., Bull. Ill, p. 15, 1916. 



