136 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



Examination of quartz-porphyry from the Bullwhacker mine. This porphyry OCCUrS ill the 



form of a dike in the above-mentioned mine. It contains numerous cubes 

 of pyrite distributed throughout its mass, which sometimes measure as much 

 as one-eighth of an inch. The pyrite is bright, and shows no signs of weath- 

 ering except where it has been exposed for some time to the action of the 

 air. Clean crystals of this pyrite were picked from the matrix and were 

 assayed for gold and silver, 377.09 grains being pulverized and roasted 

 sweet in the muffle. The roasted mass was mixed with 770 grains litharge, 

 580 grains bicarbonate of soda, 270 grains bitartrate of potash, and 700 

 grains borax. The whole mass was melted two and one-half hours and the 

 resulting lead button cupelled. 



Per cent. 



Value in silver, 65 cents 0.0017236 



Value in gold, 80 cents 0.0001327 



The amount of silver and gold contained in this porphyry when it was 

 assayed without separating the pyrite was: 



Per cent. 



Silver, 6 cents 0.0001591 



Gold, 12 cents 0.0000199 



From this it will be seen that the ratio of the gold to the silver was 

 greater in the porphyry than it was in the pyrite itself, and that either the 

 coarse crystals of pyrite (those that were selected for assay) were purer 

 than the fine crystals, which is highly improbable, or that the porphyry 

 carried gold and silver independently of the pyrite. Usually pyrite is 

 found to be the matrix of gold and not of silver, but in this instance these 

 relations seem to have been reversed. 



A sample of porphyry from the surface, in which the pyrite had been 

 completely decomposed by the continued action of the atmosphere, gave 

 very nearly the same amounts of gold and silver as that which contained 

 undecomposed pyrite. The results were: 



Per cent. 



Silver, 7 cents 0.0001856 



Gold, 13 cents 0.0000215 



The amount of pyrite contained in the porphyry was 1.89 per cent. 

 This was determined by Dr. Melville, assistant chemist of the Geolog- 

 ical Survey, by calculation from the amount of sulphur in the rock. 



