Park. — Formation of Zones of Secondary Enrichment. 93 



Art. V. — Notes on the Formation of Zones of Secondary Enrich- 

 ment in certain Metalliferous Lodes. 



By Professor James Park, M.A.Iust.M.E., M.Inst.M. and M. 

 (Lond.), F.G.S., Director Otago University School of Mines. 



[Read before the Otago Institute, Sth August, 100.5.] 



It has been noted in many parts of the globe that masses of ore 

 of exceptional richness often occur in the oxidized portion of 

 the ore-body — commonly in that portion lying at the boundary 

 of the oxidized and unaltered sulphides. 



Microscopic investigation has proved that these rich masses 

 are not of primary but of secondary origin. Their genesis is 

 supposed to be due to the migration of the valuable contents of 

 the upper part of the vein to, and their concentration in, the 

 lower part of the oxidized zone. 



In some cases the processes of dissolution, migration, and 

 redisposition may have taken place over and over again, each 

 cycle resulting in an increasing degree of concentration. 



The veins in which secondary enrichment are most often seen 

 are those of gold, silver, copper, lead, and zinc. 



Gold-ores, in the zone of weathering, are often augmented in 

 value by the long-continued disintegration of the vein and the 

 enclosing rock, whereby the gold set free from its matrix is per- 

 mitted to concentrate at the outcrop. 



Extensive areas of the Australian Continent have been sub- 

 ject to subaerial denudation since the close of the Palaeozoic 

 period ; and it is doubtless due to this cause that so many notable 

 •examples of mechanical enrichment of gold-bearing veins have 

 been found in Victoria and Western Australia. 



The migration of gold, copper, silver, lead, and zinc from the 

 upper to the lower parts of the veins is effected by descending 

 surface waters in the zone of vadose circulation. The processes 

 involved in the migration are chemical dissolution and electro- 

 chemical deposition. 



Chemical processes may operate in various ways to cause 

 secondary enrichment, as follows : — 



(a.) By the removal of worthless metals, thereby leaving the 

 valuable contents in a purer form. 



(b.) By removal of worthless metals, and their replacement 

 by valuable metals removed from a higher part of the 

 vein. 



