Van Horn — Occurrence of Proustite and Argentite. 501 



Art. LII. — A New Occurrence of Proustite and Argentite /* 

 by Frank R. Yan Horn. 



In 1902, the Mine Developing Company of Cleveland, Ohio, 

 was operating a property known as the California or Bell Mine 

 on Glacier Mountain about 3 miles from Montezuma, Summit 

 County, Colorado. The ore occurs in a fissure vein in granitic 

 gneiss and consists chiefly of argentiferous galena, sometimes 

 with considerable sphalerite mixed irregularly through it. 

 The vein sometimes shows a simple structure made by the 

 irregular mixture of galena and sphalerite ; at other times it is 

 banded symmetrically, while in a few other instances a brecci- 

 ated appearance was observed. 



The Proustite. 



In September, 1902, the paying portion of the vein widened 

 to about 21 inches, and assumed a distinctly banded structure, 

 with galena and sphalerite in coarsely crystalline masses irreg- 

 ularly mixed on each wall ; this was followed by siclerite, also 

 symmetrical, while the center consisted of massive proustite 

 mixed with finally divided quartz which at times was more or 

 less drusy. The proustite-quartz aggregate was generally 

 about 2 inches wide, but in one case became 14 inches in thick- 

 ness. This aggregate had an average of specific gravity of 4*17 

 as compared with 5'60 for pure proustite. This ore was fol- 

 lowed along the strike for a distance of 30 feet with an upward 

 stope of about 20 feet, when it disappeared. Mr. J. C. Sharp, 

 Case '03, made two analyses of carefully chosen proustite, which 

 gave the following average : 



Found Theoretical 



Ag 3 AsS 3 



Ag 67-60 65-5 



As 13-85 15-1 



Sb -93 



S 17-40 19-4 



99-78 100-0 



The analysis shows a slight' admixture of the pyrargyrite 

 molecule (Ag 3 SbS 3 ), also that the sulphur is too low, which is 

 probably due to analytical errors. The silver is probably too 

 high for the same reason, but might be due to native silver 

 although none was observed on the specimen analyzed. 



*A full description of this occurrence with assays of ores was contained 

 in a paper read before the Geological Society of America at the Albuquerque 

 meeting, December 30, 1907. This article will be published in volume six 

 of the Bulletin of the Society. 



