124 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



SECONDARY NATURE OF ORE MINERALS. 



It is probable that all these metallic minerals are nearly always secondary. 

 Ruby silver and argentite are often observed in this camp as secondary minerals 

 coating cracks, as well as horn silver (silver chloride) and the bromides and iodides. 

 The free gold appears probably secondary. In a few microscopic sections studied 

 argentite has formed as an alteration of silver chloride, itself probably secondary. 



REARRANGEMENT OF VALUES DURING OXIDATION. 



00 feet 



All the ores in the mine are oxidized or semioxidized, for the zone of 



oxidation goes down to the 

 600-foot level or below, 

 beneath which the vein is 

 cut off by the flat Siebert 

 fault. 



The facts that the ores 

 in their present form are 



FIG. 25. Horizontal diagrammatic' plan of Mizpah vein as exposed in the Oddie largely altered and that 

 and McMann lease, 20 to 30 feet below the surface. Of the crosscutting frac- ' J 



tures (dotted lines) limiting the ore, as is shown, some are evidently premi- many postmineral frac- 

 neral fractures or cross walls, and some postmineral fractures. In the latter 



case it appears probable that in some cases the postmineral fracture has origi- tUl'CS are present Suggest 

 nated by the continuation of movement along a premineral fracture. 



the inquiry as to how tar 



the values have been rearranged and concentrated during the alteration process. 

 A study of the assay plan of the mine failed to show any decisive change 

 at different depths in the relative proportion of gold and silver, the chief 

 metallic minerals present in the ores. A more accurate statement of this 

 investigation may be of interest. 



Proportion of gold to silver in Mizpah vein. 



"The word "lift" is here used to designate one of the horizontal zones into which the vein and mine have been 

 divided for the purpose of measurement. The use of the word is similar to that in speaking of the different "lifts" of 

 leather on a shoe heel, and the writer is under the Impression that the word in in use by mining engineers with the same 

 significance as given above. 



In value the gold constitutes 25 to 30 per cent of the ore. 



