REPLACEMENT ORE DEPOSITS IN THE SIERRAS 39 I 



successive layers of quartz in an open space, are urged by Fair 

 banks as difficulties in the way of calling these veins filled-in 

 fissures. The gradual replacement of the constituents of dike 

 rocks by the vein material would perhaps account for the great 

 size of the veins as well as their lack of banded structure. How- 

 ever, quite recently Lindgren 1 has brought forward a point as to 

 the different character of the silica deposited as replacement 

 material by a metasomatic process and that deposited in open 

 spaces, which may serve as a criterion to determine, with the aid 

 of the microscope, the two classes of deposits. Lindgren, after 

 referring to the strong solvent nature of carbon dioxide and of 

 alkaline carbonates, and the inert character of silica as a solvent, 

 writes : 



Silicification by the cementation of shattered rock masses by silica is, of 

 course, a common occurrence in and near quartz veins. But silicification by 

 replacement is a less common process, and is observed chiefly in the case of 

 easily soluble rocks, such as limestone or calcareous shales, when it results in 

 fine-grained or cryptocrystalline aggregates of silica. In the metasomatism 

 of bodies of massive rocks penetrated by chemically active solutions silica is 

 formed in many ways, as by the carbonatization of silicates and sericitization 

 of the feldspars, and if no open spaces are available much of this free silica 

 will be deposited within the rock, usually as fine-grained aggregates more or 

 less mixed with opal and chalcedonite. If no material were added the final 

 result of this would not, however, be a silicification, but merely an increase in 

 the total free quartz of the rock. But in case the rock mass is cut by fissures 

 it appears that most of the resulting free silica is not deposited in the rock, 

 but finds its way out in the open ducts, where, if the solutionis supersaturated, 

 it will be deposited 



As for the other possible process of silicification, or a dissolving of the 

 original mineral and a deposition of silica pari passu, it occurs chiefly in 

 easily soluble minerals, such as calcite. In case of the ordinary rock-form- 

 ing silicates it is apparently not common. The resulting silica is generally 

 in the form of fine, cryptocrystalline aggregates. Rocks silicified by either 

 of these metasomatic processes, or by a combination of both, may occur, but, 

 so far as the writer's experience goes, are not often encountered as wall rocks 

 of auriferous quartz veins. But neither of these processes can have produced 

 the massive, white, coarse-grained quartz of gold veins belonging to the 

 normal type. This quartz, which contains native gold and sulphides, shows, 

 1 The Mining Districts of the Idaho Basin and the Boise Ridge, Idaho, 

 Eighteenth Rep. U. S. Geol. Surv., Part III, p. 645. 



