GOLD ORES. 1171 



small vertical distance (from a few meters to 100 meters or more) a large 

 part of the gold which, as a first concentration, may have had a vertical 

 extent of 1,000 to several thousand meters. 



It has been seen that gold is soluble in ferric chloride, cupric chloride, 

 sodic carbonate, alkaline sulphides, and in other compounds. There 

 are likely to be present in descending waters ferric chloride and ferric 

 sulphate, and, in case the lode is a copper-bearing one, also cupric 

 chloride. It is therefore believed that in the upper part of the belt of 

 weathering these reagents or others take the gold in solution. As the 

 solutions pass below the belt of weathering and into the belt of cemen- 

 tation they come into contact with sulphides of the base metals, tellurides, 

 or organic materials, or are mingled with reducing solutions. It has 

 already been seen that any of these conditions results in the precipitation 

 of gold from its solutions, and therefore the gold is thrown down by 

 these compounds in the upper part of the belt of cementation. Probably 

 in the majority of instances the sulphides are the chief reducing com- 

 pounds, although the others are not unimportant. The reduction of gold 

 by sulphides is somewhat different from the reduction of any of the metals 

 previously considered. Zinc, lead, copper, and silver are thrown down 

 from their salts as sulphides by the baser sulphides. The gold is thrown 

 down from its salts by those sulphides not as a sulphide but as metallic 

 gold, because gold and sulphur have such weak affinity and gold is so 

 easily reduced to the metallic form. 



As denudation goes on the enriched upper portion rises into the belt 

 of weathering. The sulphides and tellurides are there again oxidized, 

 the gold is again partly dissolved and transported downward to be again 

 precipitated, and thus an horizon of steadily increasing richness and breadth 

 is formed below the belt of weathering in the belt of cementation, the gold 

 commonly being larg-el}^ free, but associated with sulphides. 



During the process not all the gold which rises into the belt of weath- 

 ering is dissolved, and the richer the sulphide zone which rises into the 

 belt of weathering the greater the amount of gold left behind. This 

 process of chemical concentration in the belt of weathering is supplemented 

 to an important degree in many districts by residual concentration. As 

 denudation continues the rocks are disintegrated, dissolved, and transported 

 to the streams to a greater extent than is the gold, and thus the gold in the 



