proceedings: geological society 359 



with metamorphosed limestone. This ore body is elliptical in plan; 

 it is 100 feet long, 20 feet wide, and has been stoped upward to a height of 

 10 or 12 feet. From it were extracted 2000 tons of ore carrying $4 in 

 gold, 35 per cent excess iron, and 2 per cent in copper. 



The ore consists of a mixture of pyrrhotite and chalcopyrite in a 

 gangue of augite. Surrounding the sulfide mass and grading into it is a 

 body of dark heavy rock of fresh appearance and of even-grained granitic 

 texture, which proves upon microscopic examination to be a pyrrhotite 

 augite diorite. This rock in turn grades laterally outward into quartz 

 monzonite composed of plagioclase, orthoclase, quartz, biotite, and horn- 

 blende, which is the normal country rock. The facts pointing to the 

 primary igneous origin of the sulfides are the following: (1) The lack of 

 evidences of hydrothermal alteration in the ore and wall rocks; (2) the 

 textural relation of the sulfides to the augite as shown by the tendency 

 of the pyroxene to develop idiomorphic boundaries against the sulfides; 

 and (3) the marked differentiation that has taken place in the magma 

 concurrently with the segregation of the sulfides, expressed mineralogi- 

 cally by the decrease of the othoclase, quartz, and biotite, and the con- 

 current increase of the ferromagnesian minerals, as the ore body is 

 approached. 



The mineral composition of the primary ore as a factor determining the 

 vertical extent of the secondary sulfide zones: W. H. Emmons. It is 

 generally supposed that waters descending from the surface in sulfide 

 ores rapidly lose acidity. Two samples taken from the same column of 

 water in a shaft in a body of pyrrhotite ore at Ducktown, Tennessee, 

 indicate a decrease of acidity of more than one-half at a depth only 37 

 feet below the top. of the body of standing water. 



At Ducktown, Tennessee, and in many other copper deposits contain- 

 ing abundant pyrrhotite, the chalcocite zones are restricted to a few 

 feet in vertical extent, whereas they may be hundreds of feet in vertical 

 extent in pyritic ores containing little or no pyrrhotite. With a view to 

 ascertaining the effect of dilute acid sulfate waters on several sulfides 

 Dr. R. C. Wells carried out the following experiments. Equal volumes 

 of ground sulfides similarly sized were treated separately with dilute 

 ferric sulfate and acid sulfate solutions, and also with acid sulfate solu- 

 tions about 1/20 normal. The rate of attack with acid sulfate was in 

 the following order: Pyrrhotite, sphalerite, galena, chalcopyrite and 

 pyrite. The action with pyrrhotite was about 25 times as rapid as with 

 sphalerite and with sphalerite at least 4 times as rapid as with pyrite or 

 chalcopyrite. The generation of hydrogen sulfide was proved in the 

 action on pyrrhotite, sphalerite and galena. 



Since hydrogen sulfide precipitates copper, gold, silver and some other 

 metals from acid solutions, it would not be supposed that these metals 

 would be carried far below the zone where hydrogen sulfide was being 

 generated by the action of acid on sulfides. They could probably be 

 carried to greater depths in ore composed of pyrite and chalcopyrite 

 without pyrrhotite or sphalerite than in sphalerite ores, and to greater 

 depths in sphaleritic ores containing no pyrrhotite than in ores contain- 



