1922] Whitman: Genesis of the Ores of the Cobalt District 



301 



in cases of metasomatism, where precipitation is not hasty but is very 

 deliberate, since under such conditions colloidal particles, even though 

 formed, would have ample opportunity to undergo re-solution and 

 re-deposition as crystalloids, according to the principles of crystal 

 growth set forth by Wm, Ostwald. 24 



Coming now to a consideration of the conditions existing in the 

 vein walls at a point where deposition of ore is about to occur, we 

 must recall that the fissure in question is under longitudinal compres- 

 sive stress and consequent lateral tension. A given wall particle is 

 under differential stress, and is therefore several times more soluble 

 than it would be otherwise, its solubility being proportional to the 

 stress difference. At a point, then, midway between the ends of a 

 split joint, and where it bottoms on a strong shear joint or flat fault, 

 the stress difference is the greatest. That is the point, as has already 

 been indicated, where the richest ore has been deposited. There the 

 wall rock tends most strongly to pass into solution. 



The fissure and the pore spaces of the rock are saturated with ore- 

 bearing solution ; and it may be assumed that all solutes are at the 

 saturation point. It is assumed also that one of these solutes is silver 

 sulphide, the solvent being a solution of sodium thiosulphate. The 

 exact mechanism of replacement which takes place under these con- 

 ditions can not yet be stated. However, were the replacement to be 

 by chemical substitution, volume changes must occur and contraction 

 or expansion would be evidenced by vugs in the vein or strain in the 

 walls; but both these are absent. The replacement clearly is a matter 

 of relative solubilities and of volume-for-volume substitution. It 

 would seem that in the reaction the wall rock on passing into solution, 

 as a result of the differential stress affecting it, must absorb energy, 

 which assists the precipitation of metalliferous substances in which 

 the solution is saturated. At the same time the solution of the wall- 

 rock silicates probably consumes a certain amount of the solvent of 

 the ore material, thus adding another cause for its precipitation ; but 

 the amount leaving the solution is just enough to fill the space vacated 

 by the silicate, as otherwise a back-pressure might be established, so 

 great as to inhibit further reaction. 



The selective precipitation of ore and gangue minerals may have 

 been governed by the fact that precipitating crystalloids exert a 

 pressure due to the force of crystallization, and the substance with 



2 * Wm. Ostwald, The scientific foundations of analytical chemistry, ed. 1900, 

 p. 22. 



