Park. — Forvmtwii of Zones of Secondary Enrichment. 95 



The property possessed by silica, clay, and porous substances 

 ■of absorbing metals from dilute aqueous solutions may be an 

 impoilant factor in the formation of zones of secondary enrich- 

 ment in the oxidized portions of metalliferous lodes. In this 

 we may have the explanation of the rich kaolin ores of silver 

 at Broken Hill, of the concentration of gold in the talcose clays 

 of the lode-formations of Kalgoorlie, and of the copper-bearing 

 shales of Europe and America. 



The lesearches of Emmons,* Weed,t and others have thrown 

 much light on the secondary enrichment of vein deposits, and 

 much still remains to be done before safe generalisations can be 

 formed. 



Impoverishment of Veins in Depth. 



T. A. Rickard, when discussing Professor Posepny's paper 

 on " The Genesis of Ore-deposits," states that the general non- 

 persistence of ore in depth is a fact capable of proof. { He 

 contends that since heat and pressure are the two factors which 

 increase the solubility of mineral substances, the deep region 

 will favour solution the most, while the shallow zone will favour 

 precipitation owing to the decrease of heat and pressure. 



There is much in favour of this contention, and many ex- 

 amples could be adduced in its support in all parts of the globe. 



Moreover, progressive poverty in depth below a certain point 

 must be the natural corollary of the general law governing the 

 orderly distribution of ores in horizontal zones in vertical dis- 

 tance through the agency of ascending waters. 



In some cases impoverishment in depth is determined by 

 the prevailing geological conditions. Ore-veins which are con- 

 fined to a single overlying formation often die out or become 

 exhausted on reaching the underlying rock. 



A notable example of this is afforded by the hydrothermal 

 veins of the Thames, Tapu, Coromandel, and Kuaotunu mining 

 districts in the Hauraki mining region of New Zealand, where 

 the gold-bearing veins are contained in altered andesites which 

 rest on a highly eroded surface of Lower Mesozoic slaty shales 

 and sandstones. 



Mining operations have in all cases shown that when the 

 veins which occur near the borders of the andesite-flows reach 

 the basement rock they die out completely, or end in small 

 strings which soon disappear in depth. 



* S. J. Emmons, Trans. Am. Inst. M.E., Vol. xx. 1900. 

 • t W. H. Weed, Bull. Geol. Soc. Am., Vol. xi, 1900, ]>. 179 : and Trans. 

 Am. Inst. M.E., Vol. xs, 1900, p. . 



X T. A. Rickard, " Genesis of Ore-deposits " : Disciissioii, p. 190. 



