96 Transactions. 



The principle of secondary enrichment precludes the con- 

 tinuance of the enriched portion of the vein downward in verti- 

 cal distance. 



When the values of secondary enrichment are added to ore 

 already of a payable quality, the result is a rich shoot or 

 bonanza ; but when, as often happens, the secondary values 

 are added to lean ore, then the net result is to render the lean 

 ore just profitable. Hence below the zone of enrichment the 

 ore will be lean and unprofitable. 



Absorption of Metals by Clays in relation to Secondary' 

 Enrichment. 



It has been noted in many mines that the ore in the zone 

 of secondary enrichment is commonly associated with, or con- 

 tained in, a matrix consisting of clay or other finely divided 

 mineral matter. Of this there are no better examples than the 

 kaolin silver-ores of Broken Hill and the talcose gold-ores of 

 Kalgoorlie. 



Clays and clayey matter are the natural products of the alter-^ 

 ation of rocks and ores in the zone of oxidation, hence their 

 presence calls for no comment. But the frequent occurrence of 

 rich ores in a clayey matrix in the zone of oxidation in certain 

 lodes has led to much speculation as to the relation existing be- 

 tween the clay and its metallic contents. _ 



It has been suggested by some writers that this association is 

 not accidental, nor the result of paragenesis, but due to some 

 quality of the clay. That clay and finely divided matter possess 

 the property of absorbing or extracting metals from their aqueous 

 solutions has long been known ; and with this knowledge in 

 mind it has been contended that the clayey matter acting as a 

 porous filter in the lower part of the zone of oxidation has ab- 

 sorbed the metals from the descending solutions, thereby effect- 

 ing a concentration of the valuable contents. 



There is much to be said in favour of this view, but it has 

 still to be determined whether clayey matter is a primary- or 

 merely a contributing cause in the formation of zones of second- 

 ary enrichment. 



Walter Harvey Weed* early in 1905 described some experi- 

 ments made by himself and others in the laboratory of the United 

 States Geologi(-al Survey on the absorptive property of clays, 

 &c. The results obtained confirmed the researches of W. Skey 



* W. H. Weed, " Absor]itioii in Orc-(le]iositiou," Engineering and 

 Mining Jonrnal. Fel). 23. IfiOo. 



