4 ECONOMIC GEOLOGY 



come from a long distance and derive the dissolved minerals 

 from a large amount of rock, but the precipitation occurs near 

 the surface. Each of the above theories seems incomplete in 

 itself. They supplement each other in general harmony with 

 underground circulation. The largest ore deposits like the 

 copper deposits of Michigan have sometimes been formed by a 

 single ascending solution and subsequently enriched near the 

 surface by descending solutions; and also both near the surface 

 and at greater depths by lateral secreting solutions. 



The Causes of Precipitation. Some authors have attributed 

 precipitation to diminishing temperature and pressure of rising 

 solutions alone. This is insufficient. The precipitation is 

 generally due to the mingling of different solutions in the trunk 

 channels whereby new chemical compounds are produced. 

 Whenever a neutral or slightly acid solution of a lead salt comes 

 in contact with hydrogen sulphide the lead is precipitated as 

 lead sulphide. The resulting mineral is galenite. Precipita- 

 tion is also influenced by the character of the wall rock, by the 

 infalling of fragments from the fissure walls and the presence 

 of minerals already formed. Replacement is also not an infre- 

 quent method of forming an ore body. 



There is a time-honored belief that ore bodies increase in 

 richness with depth. This theory is absolutely untenable. 

 Whether the ore body will increase with depth depends upon 

 many factors; as the breadth of the fissure at the lower altitudes 

 prior to the filling; the amount of concentration of the ore that 

 has taken place at the point of the filling, and the amount of 

 enrichment that may have occuired through faulting or the 

 introduction of intrusive bodies. 



If the upper unit of measurement is large and there is great 

 irregularity in richness, the general trend is toward impover- 

 ishment with depth. This may arise from the combined efforts 

 of descending and lateral secreting solutions upon deposits 

 already formed by ascending currents in conjunction with the 

 erosion of large masses of overlying strata. Occasionally where 

 the surface unit is large, there is little decrease in width with 

 descent. This is due to the fact that the ore resulted from a 

 single enrichment by ascending waters. 



Gossan. The contents of the upper part of the original vein 

 become oxidized in the zone of weathering, as shown in Fig. 1, 

 and are either washed away in alluvial deposits or are carried 



