OEE SHOOTS. 1229 



tated as a result of reducing - solutions furnished by the adjacent sulphides 

 of the main vein. At the famous Camp Bird mine there are thin black 

 seams deep below the surface, which are extraordinarily rich in gold." The 

 main part of the vein has its gold and silver values in copper sulphides, 

 galena, blende, and other base sulphides and in tellurides, but the fine 

 black material in the small veins apparently contains free gold, as is shown 

 by the fact that 75 per cent of the value of the ores is collected on the 

 plates. It seems perfectly clear that these minute seams of rich gold- 

 bearing material are the results of precipitation in secondary fractures. 



Ore shoots in many cases are explained by the influence of the wall 

 rocks. It is well known that where ore deposits intersect a complex set of 

 rocks the pay shoots are likely to have a decided preference for one rock. 

 For instance, if a fissure passes from granite to diorite, or from either of 

 these to limestone, or from any of these to sandstone, the character and rich- 

 ness of the deposit may vary greatly as the rock changes. For this vari- 

 ability, due to the character of the wall rocks, different explanations apply 

 in different cases. In some instances the restriction of the ore shoots to one 

 rock is largely explained by its more ready solubility. This is particularly 

 applicable to the substitution deposits, the wall rock being dissolved in 

 advance of or pari-passu with the deposition of the ore. By the solution of 

 the soluble rock sufficient room is furnished for a large ore deposit. The 

 above is undoubtedly the partial explanation in many cases of the occurrence 

 of the ores in limestone rather than in the adjacent more insoluble rocks. 

 Gypsum is even more readily soluble than limestone. Of the Enterprise 

 mine of the Rico district, Ransome 6 says the rich blanket of ore is "due 

 essentially to the removal by solution of a massive bed of gypsum which 

 may have been from 15 to 30 feet [5 to 10 meters] in thickness." All 

 stages of the process of replacement are seen. 



In other instances the occurrence of the rich and large bodies with one 

 wall rock rather than with another is due to the fact that the wall rock itself, 

 by reaction upon the , solutions, precipitates the ore material. This also 

 partly explains the preference of certain ore deposits for limestone, for, as 

 already explained (pp. 1116-1117), calcium carbonate probably accelerates 



oRansome, F. L., A report on the economic geology of the Silverton quadrangle, Colorado: Bull. 

 U. S. Geol. Survey, No. 182, 1901, pp. 89-90. 



b Ransome, F. L., The ore deposits of the Rico Mountains, Colorado: Twenty-second Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1901, pp. 278-295. 



