SECONDARY DEPOSITS OF ZINC MINERALS 205 



of thin-sections of the Drum Lummon ore, Marysville, Montana, seem 

 to indicate a secondary origin for the extremely rich silver-gold ores of 

 that lode. 



In the first few years following the discovery of the enormously rich 

 placer deposits of Montana many discoveries of rich silver-bearing lodes 

 were made. These held enormously rich deposits of silver sulphides 

 and sulphantimonides beneath the shallow layer of oxidized ore; but 

 these enrichment deposits, which led to most extravagant ideas of the 

 future of this region, were soon exhausted, and when the leaner, baser, 

 primary ores were encountered work was usually suspended. 



ZINC 



The secondary origin of bodies of zinc blende at Leadville, Colorado, 

 lias been maintained by Blow,* who, in describing their occurrence 

 there, writes : 



The zinc sulphides are the most widely disseminated and show plainly the re- 

 sult of their more ready solubility than the other sulphides and the redeposition 

 of a large portion of the zinc which has thus been removed from the carbonate 

 ores. This fact is clearly shown in many ways, but most satisfactorily just at the 

 line of transition. The sulphides first encountered are invariably heavy sulphides 

 of zinc, carrying a little iron and very little lead. They have a close crystalline 

 structure and lie in a laminated form, the lines of fracture being nearly vertical. 

 Upon these cleavage planes crystals of cerussite are found, and often a small in- 

 crustation of native silver. Such deposits, where first encountered in passing from 

 oxidized to unoxidized ores, are always lowest in silver. In their further exten- 

 sion the zinc gradually grows less and the laminated structure disappears. Beyond 

 this, again, the zinc sulphides appear to predominate along cleavage and contact 

 planes with the gray porphyry or along the lines of minor faults and cracks in the 

 limestone. Such characteristics are also universally observed in other instances 

 besides those of Iron Hill. 



It seems probable that a large proportion of the zinc, which was totally removed 

 from the carbonate ores, has been redeposited as a sulphide, and principally just 

 below the line of complete oxidation, by surface waters, and such redeposition has 

 advanced and increased pari passu with the limit and extent of such oxidizing 

 action. 



As a corollary of the above, it is believed that at the present stage of develop- 

 ment in Leadville the sulphide of zinc forms a larger part of the unoxidized ores 

 than will be found in future and deeper exploration. 



There is also evidence going to show that the action of alteration and second- 

 ary deposition has extended for a considerable distance within the sulphide bodies. 

 In ores of this class the silver values are found concentrated, as it were, with pre- 

 ponderance of either the zinc, lead, or iron occurring in lenticular masses or 

 patches, surrounded by low grade ore, and forming bonanzas of great value. These 

 bonanzas are rarely found near the flanks or sides of the chute, but generally in 

 its center and with no connection one with another. 



* A. A. Blow, in Trans. Am. Inst. Mining Engineers, June, 1889. 



