Gold 



301 



rocks. In 48 tests of igneous rocks the gold content varied 

 between extremely wide limits. Wide variation exists between 

 the amounts of gold in similar rocks of different localities. 

 Moreover, the same rock from the same locality frequently 

 shows equally marked variations. In six granites containing 

 gold thought to be primary the highest contained 67 times 

 more gold than the lowest. The evidence at present available 

 tends to show that the average gold content of igneous rocks 

 is about six parts per one hundred million, equivalent to about 

 3 1 /2 cents per ton. No placer deposits where gold has been 

 derived from the primary content of igneous rocks have proved 

 of economic value. Gold is distributed in the mass of igneous 

 rocks approximately evenly throughout its mass. Important 

 concentration begins with the destruction of the igneous rocks 

 by erosion. Gold tends to gather in sediments, more in coarser 

 and less in finer sediments. If there were any considerable 

 bodies of igneous rocks assaying 3 5 cents per ton some of them 

 certainly would have produced placers of economic importance. 

 No such ores have been found. 



Courtesy U. S. Bureau of Mines 



FIG. 84. Bonanza Ore, from Gold Bug mine, showing free gold in cal- 

 cite (slightly reduced). 



