202 W. IT. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



scribes the occurrence of the famous ore bodies of the Smuggler and 

 Mollie Gibson mines at Aspen. The ore consists of barite and poly- 

 basite, with tennantite. Although Mr Spurr gives no definite statements 

 as to the possible secondary origin of these ores, yet the sketch which 

 he gives on page 183 and the descriptions all indicate that the original 

 ore was a silver-bearing lead sulphide, with more or less zinc sulphide, 

 formed along inclined faults, and that subsequent to the formation of 

 these ore bodies nearly vertical faults displaced the ore and formed the 

 two bodies now worked at the mines mentioned. Although in these 

 vertical or nearly vertical fissures rich polybasite ore is now found, it 

 does not extend far in either direction from this fault, and the descrip- 

 tion of the ores given by Mr Spurr indicates that it is derived by second- 

 ary alteration processes from the lead and zinc ore bodies. This also is 

 indicated by the fact that the polybasite is in part altered to native silver 

 at the extreme lower end of the ore body. 



In conversation Mr Spurr has admitted the possible secondary origin 

 of these polybasite bodies, but he has no new evidence upon the subject. 

 He says : 



" This ore was of a rich character, having large amounts of polybasite and native 

 silver. This polybasite body appears to lie in a sort of subordinate shoot, trending 

 south of east and lying at the Gibson fault plane. This shoot is marked by excep- 

 tionally large and rich bodies of a nature not found elsewhere in the mine. It is 

 noteworthy that this rich shoot is practically the lower termination of the ore of 

 the Gibson fault. Most of the ore below this is native silver, which, from the 

 nature of its occurrence, is manifestly a secondary deposit leached from the rich 

 ore above. Some of these secondary deposits are, however, of considerable size, 

 and empty vugs are often found beautifully and elaborately festooned with deli- 

 cate wires of silver. Above the polybasite ore, however, the ore appears to be 

 pretty continuous, but the amount of silver becomes less." 



It will be noticed that he recognizes the secondary nature of the silver, 

 and that the polybasite lies between the native silver and the lead surface, 



In his chapter on the chemical geology of the region, where he dis- 

 cusses the alteration of the ore deposits and of the limestones, he does 

 not adduce any new facts concerning the formation of the polybasite ore, 

 but he does state that iron pyrites carrying small amounts of arsenic, 

 lead, copper, zinc, cadmium, cobalt, and nickel are found, and that tetra- 

 hedrite is also very common. The polybasite is said to be later than the 

 barite. 



A very interesting and remarkable case of secondary enrichment 

 occurs at the Broken Hill Consolidated mine, Australia, described by Mr 

 George Smith.* Mr Smith shows by his description and sketches that the 



* Trans. Am. Inst. Mining Engineers, 1890, p. 69. 



