THE TELLURIDE ORES OF THE RED CLOUD AND COLD SPRING 



MINES, GOLD HILL. 



By B. SiLLlMAN.* 



In May, 1873, 1 briefly announced the discovery of tellurium gold-ores 

 at the Eed Cloud mine in Colorado, and stated that Prof. JST. P. Hill, of 

 Black Hawk, had proposed to send me specimens of these ores.t The 

 specimens sent by Professor Hill were long in reaching me, and it is only 

 recently that I have examined them. The observations made in the 

 summer of 1873 by the officers of Dr. Haydeu's expedition have sup- 

 plied the data needful to understand the mode of occurrence of these 

 ores, for the details of which reference is made to Mr. Marvin e's notes 

 and map, which form part of this paper. 



It appears from them, in general, that near the mining-hamlet of Gold 

 Hill, about twenty -five miles northwest of Denver City, and at an ele- 

 vation of almost 8,000 feet above tide, is a wide dike of porphyry cut- 

 ting the metamorphic rocks, probably of Archaean age, about six miles 

 west of where the Triassic rocks die out at the base of the mountains. 



A section of this dike, A, furnished 

 by Mr. Marvine, is annexed, showing 

 the tellurium-bearing veins B and 

 on its sides. The porphyry of which 

 it is composed has distinct crystals of 

 feldspar imiDlanted in a purplish-gray 

 paste. These crystals have a green- 

 A, porphyry-dike ; B, c, veins with gold ish-white color, and are evidently 

 and teiiurium-ores. partly decomposed. As seen in a mi- 



croscopic section, it shows the usual obscurely crystalline ground-mass 

 of felsite, with crystals of quartz, and sections of feldspar crystals show- 

 ing the parallel bands of a triclinic species. A glance at the map shows 

 the position and course of this dike, and also the existence of other dikes 

 of porphyry in the same region. The porphyry from the " 7.30" and 

 " Central" mines closely resembles that from the " Eed Cloud," while 

 that from a dike (No. 136) between the "7.30" and the "Americus" is 

 distinctly trachytic, and that from the " Niwot" mine, at the west margin 

 of the map, (No. 181,) is a quartz-porphyry, 'with distinct crystals of 

 biaxial mica. Those from the dikes at Jim Town, (specimens No. 147,) 

 on the north border of the district, are distinct sanadin-trachyte. 



The tellurium-ores have been explored so far only in connection with 

 the dike near Gold Hill, shown in the section, although they exist with 

 the dike at " 7.30" and the " Central." These ores are found along the 



* Tlie substance of the following remarks was originally communicated at the April 

 session (1874) of the National Academy of Sciences at Washington, and afterward 

 appeared in the form of an article entitled " Mineralogical Notes ; Tellurium Ores in 

 Colorado ; by B. Sillimau," in the American Journal of Science and Arts for July, 1874, 

 and from which they are now here reproduced by the permission of the author. It was 

 in connection with this article that the preceding notes on the general geology of the 

 region about Gold Hill were prepared and approved. 



t American Journal of Science and ArtSj III, vol. V, 286. 



