﻿Vol. 66.'] IN THE LEAD AND ZINC VEINS OF GEEAT BRITAIN. 



323 







Extraction of metal by : — 







fc'olution. 



1. 



Kaulin. 



2. 3. 4. 



Caleanous ' C'lierty ' Shell 

 grit. i limestone. | limestone. 



I 

 5. 6. 7. 8. 



9. 



Burnt 

 ■hale. 



10. : 11. 



Cke. Paraffin 



Load (Nitrate) .... 24-50% 

 Zinc (Sulphate).' 9-25 



70-85 

 52-46 



91-55 ! 92-64 



! 

 85-65 82-57 



70-21 

 5U-89 



71-34 1 90-88 42-50 



1 

 54-31 ! 83-29 22-37 



44-45 

 21-49 



40-16 | 39-38 

 24-53! 25-26 



It will be seen, first, that lead is deposited more freely, in 

 every case, than zinc. The chemical influence of the rock is 

 evidently more important in the case of lead, a conclusion which 

 is supported by microscopic and field-observations. 



With regard to the influence of the different substances, it 

 appears that the silicate, kaolin, is the least active of all, while the 

 carbonates are the most active. In the case of all the carbonates 

 (Nos. 2 to 7), thick deposits of lead and zinc carbonates were 

 formed in the bottom of the jars, binding the powders into a firm 

 cement, while much lime had gone into solution. In these cases 

 the deposition was effected by replacement of the rock-material by 

 the metal. At the same time, it is to be observed that lime is 

 much more effective than magnesia in these laboratory tests, an 

 effect probably due to its greater solubility during replacement. 

 In the veins, on the other hand, dolomitic limestones are the more 

 favourable beds as a rule, especially for the deposition of secondary 

 calamine. This has been ascribed to the greater porosity of the 

 dolomitic rock. 1 The discordance between the experimental results 

 and the facts of observation in the field is probably due to the 

 elimination, in the former case, of the physical element of porosity 

 by fine grinding. Further, the much longer time-factor and the 

 more dilute solutions in nature would greatly modify the results. 



Turning to the carbonaceous substances (Nos. 8 to 11), it is seen 

 that the extraction of metals by these is very much lower than by 

 the carbonates. In considering this fact, it must be borne in mind 

 that time, concentration, temperature, and composition of solutions 

 were not comparable with the conditions which existed during 

 ore-deposition. The variation of these factors makes it very 

 difficult to apply the experimental results in explanation of natural 

 phenomena. In particular the solutions in nature were alkaline 

 carbonates and sulphides, whereas those here used Avere acid 

 sulphates and carbonates. The lower influence of the organic 

 matter as here shown is probably due to its more refractory nature, 

 which rendered it less amenable to replacement. The replacement 

 of calcareous substances, in short, is more effective under the 

 conditions of these experiments than the reducing influence of 

 organic matter on the oxidized salts of the metals. In the case 

 of vein-solutions containing metallic sulphides dissolved in alkaline 

 sulphides, however, the influence of organic matter was probably 

 more important. At the same time, it is probable that organic 



1 H. F. Bain, ' Lead & Zinc Deposits of the Ozark Region ' 22nd Ann. Eep. 

 U.S. Geol. Surv. (1901) pt. ii, p. 128. 



