ORIGIN OF ORE DEPOSI 41 



acting upon pyrite below the permanent water level, in the pres- 

 ence of a deficiency of oxygen caused a partial oxidation of the 

 pyrite and a consequent metasomatic deposition of the gold. 



Precipitation. Precipitation is the process by which certain 

 constituents of a solution are rendered insoluble in that solution. 

 This is effected in the laboratory by the lowering of the tempera- 

 tnre. by the evaporation of the solvent, or by double decomposi- 

 tion. In nature the fall of temperature occurs with rising ther- 

 mal waters, but ores thus precipitated would be rare. The 

 evaporation of the solvent is common and metallic ores may thus 

 be thrown out of solution. The most common cause of precipi- 

 tation is the mingling of different solutions in the trunk channels. 



Siderite and rhodochrosite are held in solution in waters charged 

 with carbon dioxide. If the carbon dioxide be extracted by 

 relief of pressure, or by any other cause, these metals are repre- 

 cipitated as carbonates and remain as such in the absence of 

 oxidizing agents. The sulphides of many metals are precipitated 

 from their sulphate solution by the action of organic matter, or 

 by the action of hydrogen sulphide upon their acid solution, 

 while others are precipitated as sulphides from their alkaline 

 solutions. Precipitated bedded ores may occur as sheets be- 

 tween sedimentaries, or as crystals, grains, or nodules in the 

 sedimentaries. 



These deposits may be recognized: (1) By parallelism with 

 the enclosing sedimentaries; (2) by their occurrence in different 

 horizons; (3) their margins are clearly defined; and (4) they do 

 not shade into the barren country rock. 



Iron and Manganese as Oxides. These occur as bog-ores. 

 Iron and manganese are thrown out of solution as a mixture of 

 hydrated oxides by the action of algae and bacteria. Siderite 

 may be brought into solution by percolating waters and when 

 oxidized thrown out of solution as ferric hydroxide, according to 

 the equation: 



The primary source of the iron and manganese is the ferro- 

 magnesian silicates of the igneous rocks. The pyrite of the sed- 

 imentaries also is oxidized or decomposed by humic acid and 

 rendered available for precipitation. These minerals of iron and 

 manganese had their primary source in the igneous rocks which 

 furnished the detritus from which the sedimentaries were formed. 



