400 H. IV. TURNER 



In such ore the pyrite is often grouped in bunches and at other times is des- 

 seminated through the quartz. The distribution of the pyrite seems to be 

 without effect upon the tenor of this variety of ore, which is usually rich in 

 proportion to the quantity of pyrite. The other variety of ore consists of 

 fragments of the syenite which have been, as it were, soaked in the auriferous 

 liquid. They are impregnated chiefly with carbonates and pyrite, only a little 

 silica penetrating where there were no open fissures. The pyrite in this 

 variety is also either bunched or disseminated, and all the mine foremen 

 assert that where this pyrite is scattered, the ore is nearly or quite worthless. 

 It appeared to me that the disseminated pyrite represents ferromagnesian 

 silicates attacked by sulphydric acid or soluble sulphides, and study of the 

 ore under the microscope lends strength to this hypothesis, though without 

 absolutely proving it. 



Wherever the ore is strongly mineralized the ferromagnesian silicates have 

 totally disappeared from the syenite, and the pyrite is scattered in it in about 

 the same manner as the iron-bearing silicates in the fresher material. On 

 the other hand, as the bunches of pyrite are accompanied by much calcite 

 they could not have been produced from any ordinary accumulation of ferro- 

 magnesian silicates, and I think such pyrite must have entered the rock in a 

 state of solution. 



The alteration of the soda-feldspar dikes seldom or never 

 goes so far as to constitute a complete replacement of the origi- 

 nal material, and the ores are, so far as I know, uniformly of low 

 grade. Such ore deposits may perhaps be called partial replace- 

 ments. According to Lindgren 1 the term substitution is sometimes 

 used for deposits of this character. In a paper on the aurifer- 

 ous veins of Meadow Valley 2 Lindgren describes the alteration 

 of granodiorite along fractures by solutions containing heavy 

 metals and boron. The deposits consist of epidote, zoisite, 

 pyroxene, tourmaline, quartz, mica, titanite, ilmenite, calcite, and 

 auriferous sulphides. These lodes seem to be of the nature of 

 replacements. H.W.Turner. 



PLATE. 



A. Photomicrograph of thin section of the red silicified shale of the Dia- 

 dem lode, showing the outline of a foraminiferattest, without the analyzer, X 29. 



B. Photomicograph of same view of thin section of red silicified shale as 

 last, but taken with the analyzer, X 29. 



The two photographs by J. V. Lewis. 



'Stanford University Engineering Journal, February 1898, p. 10. 

 3 Am. Jour. Sci., Vol. XLVI, 1893, p. 201. 



