1006.] on Ore Deposits and their Distribution in Depth. 817 



be steadily dissolving materials, l)ut al)ove it the water will l)egin to 

 deposit the material, which it has l)ronght with it from greater depths. 

 The water has left the zone of accinnulation, and has reached that of 

 deposition. 



According to the mode of deposition, there are three main types 

 of ores : — 



1. The steady ascent of the solution carries it continuously to 

 regions of less and less pressure and of lower temperatures. Both 

 changes favour deposition, and as the changes both in pressure and 

 temperature are gradual, the material will be deposited continuously, 

 and fairly uniformly over a great vertical range. 



2. A more rapid and patchy deposition occurs where the solution 

 happens to come into contact with some rock, such as limestone, 

 which decomposes the dissolved salts, and leads to the immediate 

 precipitation of the metals, probably as sulphides. 



3. "When the solution approaches the surface, the rate of cooling 

 is accelerated, and it may be mixed with surface waters ; and thus 

 the balance of the metals is quickly precipitated, and a high-grade 

 ore results owing to its deposition in a narrow range. 



Hence most lodes have their richest ore bodies on the surface, for 

 the agencies that lead to ore formation tend to rapid precipitation 

 there, as the conditions are more complex and the variations in 

 conditions more sudden. 



Surface Enrichment — Primary and Secondary. 



This primary richness of the outcrop is re-enforced by a secondary 

 enrichment, to which the richest bonanzas and prizes of the mining 

 industry are due. The secondary enrichment is, from a mining point 

 of view, and especially in the valuation of a mining field, the more 

 important process. Its nature may be illustrated by reference to a 

 gold quartz vein, formed by an ascending solution, which had 

 deposited gold uniformly in a vertical lode ; if the ground containing 

 the lode is slowly lowered by denudation, the gold in the uppermost 

 part of the lode is dissolved by a solution formed by the action of 

 surface waters on the iron pyrites in the lode. The gold is carried a 

 stage lower, where the ferrous sulphate is reduced and gold and 

 pyrites re-deposited. So the cap of the lode now has concentrated 

 within it the metals originally distributed through twice the length. 

 The process is repeated again and again until a rich gold patch, such 

 as the famous Londonderry pocket, may cap an otherwise worthless 

 lode. 



It is as an index of the extent to which ores have ])een thus 

 secondarily enriched that the mineralogical study of lodes is of high 

 economic importance. 



All ore formation is essentially a process of concentration ; the 

 specific gravity of ordinary ores may be taken as 3*4; the rocks of 



