ORE SHOOTS. 1225 



It has been shown, other things being- equal, that the underground cir- 

 culation follows the larger openings. Thus the abundant circulation is 

 converged wherever there are rooms, especially where the} r are connected 

 with passages of considerable width. Moreover, the solutions of such a 

 circulation are likely to be derived from various sources. Hence, upon the 

 average, in the large openings the ore is in greater quantity and very 

 frequently richer than in the narrower ones, where the solutions are both less 

 abundant and less complex. 



It is evident from the above that there may be every variation in the 

 width of an ore deposit due to uneven walls, from zero to many feet. 



Ore shoots are frequent where the fractures, instead of being simple, are 

 complex; that is, where there is a crushed zone, or zone of brecciation and 

 mashing. It has been pointed out (pp. 1086-1088) that some ore deposits 

 are largely due to reactions between the solutions and the rocks through 

 which they pass. Such an ore deposit is most likely to be rich at a crushed 

 zone, where the interaction between the solutions and the rocks through 

 which they circulate is much greater than at a point where there is a single 

 fracture, even if the latter furnishes a larger space than the multitude of 

 smaller openings. (See pp. 1213-1215.) 



The Cripple Creek district is an excellent illustration of a region in 

 which the fracturing is of an extremely complex character. The ores and 

 gangue are largely impregnation and replacement products of the rocks in 

 which they occur, including granite, andesite, phonolite, basalt, etc. The 

 solutions in this district evidently followed the innumerable minute 

 fractures of all kinds and had a very large surface of contact." A condition 

 for the deposition of the ore and gangue was the previous enlargement of 

 the openings by solution. 



Very frequently the rich shoots of ore are located by flexures, the ore 

 being either at the crests of anticlines or at the bottoms of synclines. As 

 pointed out (pp. 1206-1212), this is especially likely to be the case where, 

 in connection with the folds, there are impervious strata. Under such 

 conditions, as has already been fully explained, ore is likely to be 

 converged from ascending solutions in the arches of pervious strata below 

 impervious strata, and from descending waters in troughs of pervious 

 strata above impervious strata. In the cases cited, such as those of 



«■ Rickard, T. A., The lodes of Cripple Creek: Trans. Am. Inst. Min. Eng., vol. 33, 1903, pp. 613-618. 



