COMPLEXITY OF OPENINGS. 1203 



ground systems of water circulation have been, if not independent, at least 

 partly so. This is well illustrated by the ore deposits of Butte, Mont. 

 Apparently the metallic contents of the individual feeding streams and even 

 the trunk channels were very different within short distances. There are 

 two main zones of mineralization. The more important product of one of 

 these mineral zones is silver sulphide, which is associated with sulphides of 

 lead, zinc, and iron, and with silicate of manganese. The chief product 

 of the other mineral zone is copper, which carries silver in important 

 amounts. 8 



PREEXISTING CHANNELS AND REPLACEMENTS. 



When it is understood that ore deposits from aqueous solutions ordi- 

 narily form in trunk channels the question as to whether ores are deposited 

 in preexisting openings or are replacements is easily answered, as a general 

 proposition. It has been shown that such channels are not originally 

 formed by aqueous solutions. (See pp. 1007-1008.) Original channels for 

 underground circulation are primarily due to the structures of rocks or to the 

 effects of deformation. It therefore follows that ores are, to some extent at 

 least, deposited in preexisting openings. However, the conditions favoring 

 vigorous circulation are also those causing reactions on the wall rocks. It 

 has been fully explained that solution and deposition are common^ simul- 

 taneous processes. Wherever there is a trunk channel it is certain that the 

 walls of the openings are dissolved to some extent, and at the same time, or 

 subsequently, metalliferous minerals are precipitated. Indeed, either enlarge- 

 ment by solution and subsequent precipitation of ore, or synchronous solu- 

 tion and precipitation by which the wall rocks are replaced in various 

 degrees, molecule by molecule, by the ore, or both together, are almost 

 universal phenomena. 



I therefore believe that the large majority of ore deposits, if not all, 

 are partly deposited in preexisting openings and are partly replacements of 

 the wall rocks. However, in some cases the filling of the preexisting cavi- 

 ties is more important, and in other cases substitution for the wall rocks is 

 the dominant process. 



Other things being equal, ore deposits which are largely in preexisting- 

 cavities are more likely to occur in strong refractory rocks, such as quartzite, 



"Emmons, S. F., Notes on the geology of Butte, Montana: Trans. Am. Inst. Min. Eng., vol. 16, 

 188S, p. 54. 



