AKT. 16. ANDORITE-BEARING SILVER ORE SHANNON. 5 



lead has greatly increased. The fact that the alteration has resulted 

 in a great decrease in volume shows that the alteration has been by 

 the removal of silver and antimony rather than by the addition of 

 lead. There is nothing to indicate that this change is the result of 

 surlicial weathering or any attendant process. Apparently all of the 

 constituents of this ore are primary and the ore has not been affected 

 hy secondar}'' downward sulphide enrichment. It would seem that 

 the andorite, stable under the conditions under which it was formed, 

 became unstable under conditions obtaining later in the history of the 

 ■^'ein and broke down, a part of the silver and antimony being re- 

 moved as sulphides. The loss of silver and antimonial sulphides has 

 been in the same ratio to each other in which they are present in the 

 original mineral, namely, AggS.SSbgSs- No simple silver sulphanti- 

 monite of these ratios is known. 



THE GANGUE CARBONATE. 



The carbonate, which forms the earliest mineral of the ore, is pale- 

 pinkish buff (Ridgway) in color and is finely granular, except where 

 it lined open cavities, where it formed crusts of small crystals. The 

 material is brecciated, and some fragments which are isolated in the 

 quartz are largely replaced by finely-divided metallic sulphides. 

 These are too small for their identity to be definitely determined 

 without microscopic study of polished surfaces in reflected light, but 

 it is probable that the grains are mostly andorite and sphalerite. 

 Small fragments of the carbonate were selected free from admixed 

 minerals and analyzed with the following results: 



Analysis of rhodochrosite from Nevada. 



MnO 49. 49 



FeO 7.68 



CaO 3.13 



MgO . 93 



CO2 (calculated) 38. 87 



Total 100.10 



Stated as carbonates of the respective bases this analysis gives : 



MuCO, SO. 15 



FeCO, 12.39 



CaCO, 5.59 



MgCO. 1. 97 



Total 100.10 



This is, as is to be expected, a relatively pure rhodochrosite. Such 

 highly manganiferous carbonates are very frequent as gangue min- 

 erals in hydrothermal silver veins. The significance of this associa- 

 tion will be further discussed in a paper which is in preparation. 



