374 University of California Publications in Geology [Vol- 10 



exposed the latter is surrounded by a considerable thickness of gouge 

 composed mostly of kaolin and silica partly replaced by pyrite, an 

 illustration of which is shown in plate 26, figure A. The pyrite com- 

 monly grades out from the solid ore through the gouge into the wall 

 rock. In some of the most recent workings a hard type of pyrite has 

 been encountered. Although a small mass of this was struck on the 

 lower level, yet the main body was encountered in a raise from this 

 level. This ore contains a considerable quantity of chalcopyrite. The 

 wall rock adjacent to the ore, and into which it grades, is highly silici- 

 fied and contains small masses of kaolin. It is more thoroughly crystal- 

 line, contains a larger percentage of silica, and is much harder than 

 the other type of pyrite. An illustration of this is shown in plate 26, 

 figure B. 



Mineralogy. — The minerals derived from the ore according to 

 Schaller 33 are pyrite, chalcopyrite, copper, melanterite, pisanite, chal- 

 canthite, copiapite, epsomite, hematite, limonite, alunogen (?) and 

 boothite. Besides these the writer has observed chalcocite, bornite, 

 and sphalerite in the ore. The pyrite, however, is the only mineral 

 of economic importance and is used to make sulphuric acid. 



Metallographic Study of the Ore. — -A number of polished surfaces 

 of the pyrite and of the country rock partially altered to pyrite were 

 studied for the purpose of determining the nature of the metasomatic 

 replacement of the rock by the pyrite and associated sulphides. The 

 two types of pyrite already described show a rather marked difference 

 under the metallographic microscope. The crystals of pyrite in the 

 massive or softer ore are not so well cemented together as are the 

 crystals in the harder type, but appear to be more dense and do not 

 contain as much silica. The mineral has a very irregular pitted 

 surface even after careful polishing, and the whole mass is made up 

 of materials varying in density and hardness and also in the degree 

 of cementation of their constituent parts; consequently no good pol- 

 ished surfaces were obtained. Quite often the outlines of large cubes 

 can be seen, but the pyrite constituting these is in very small crystals 

 and may often be picked out with a needle point. Between these large 

 cubical or irregular masses are distributed smaller patches of pyrite, 

 which are generally hard and can be polished. The remaining area 

 is mostly occupied by very small crystals of pyrite mixed with dark 

 silica. It is very probable that this portion of the ore is still in the 

 making ; that is, the pyrite has not replaced all of the rock and some 

 of the more resistant materials are still present as a part of the ore. 



is Univ. Calif. Publ., Bull. Dept. Geol., vol. 3, no. 7, 1903. 



