ORE-DEPOSITS 45 



pln-noim-nally pure ferric oxides by ground-water. There are 

 vast quantities of lean ores in the same region not thus purified and 

 enriched. 1 



3. Solution and re-precipitation. Ore material may be leached 

 out of the surface-rock by water circulating slowly through it, and 

 carried on until it reaches some substance which causes a reaction 

 that precipitates the metallic matter. This substance may be a 

 constituent of some rock which the circulating water encounters; 

 but more commonly, the precipitation seems to be due to the 

 mingling of waters charged with different mineral substances, the 

 mingling inducing reactions which result in the precipitation of 

 the ore. Precipitation does not necessarily follow such commin- 

 gling; it takes place only when the mingling waters reduce the 

 solubility of the ore material sufficiently. Changes of pressure and 

 temperature also may enter into the process. 



Otherwise stated, the general process of underground ore forma- 

 tion appears to be this: The permeating waters dissolve the ore 

 material disseminated through the rock, and carry it thence into 

 the main channels of circulation, usually the fissures, porous parts, 

 or cavernous spaces. If precipitating conditions are found there, 

 deposition takes place. The precipitating conditions may be 

 merely changes of physical state, such as cooling or relief of pres- 

 sure; but probably much more generally they are found in the 

 commingling and mutual reaction of waters that have pursued 

 different courses, and are differently mineralized. 



Location of greatest solution. Water circulation is probably 

 very slight below the depth of a mile or two, and above that depth 

 there is little reason for supposing that the rocks of one horizon are 

 more metalliferous than others of their kind. Thus there is no 

 assignable reason why the igneous or sedimentary rocks at the sur- 

 face are not as rich in ore material as the igneous rocks two or three 

 miles below. For a given amount of water, solvent action is prob- 

 ably greatest where the temperature and pressure are highest, that 

 is, in the deeper reaches of water circulation; but the amount of 

 water passing in and out of the deeper zone is small compared with 

 that of higher levels, and the total solvent action is quite certainly 

 much greater in the upper zone than in the lower. At the same time, 

 the solutions in the upper zone are quite certainly more dilute than 

 those below. The horizon of greatest solution doubtless lies be- 



1 Van Hise & Lcith, various monographs of the U. S. Geol. Surv. 



