182 W. 11. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



It is, perhaps, unnecessary to add that while sulphide enrichment is 

 believed to be a common phenomenon in veins, it is not an invariable 

 accompaniment, but is very often wanting. 



Enkichmknts described by previous Writers 



The importance of this secondary sulphide enrichment seems to have 

 escaped the attention of most geological writers, though such deposits 

 are familiar to many practicing mining engineers. The most prominent 

 of French writers on ore deposits, De Launay, has, it is true, recognized 

 this phenomenon, describing it as part of the phenomena of superficial 

 alteration of ore deposits in an essay entitled " Contribution a L'Etude 

 des Gites Metal li feres." The second part of his paper is devoted to the 

 phenomena of superficial alteration and of the renewal of migration in 

 the constitution of ore deposits.* The recognition of the "renewal of 

 migration " is the essential element of secondary sulphide enrichment. 

 De Launay regards many vein fillings as formed by the direct leaching 

 of preexisting ore deposits through a concentration of material. In the 

 latter part of his paper De Launay gives a summary of what is known 

 regarding the derivation and deposition of the various metals, and brings 

 out the main facts upon which he bases his views. 



Other writers have alluded in a casual way to this feature of ore de- 

 posits, Posepny, for example, says : 



" The chemical effects proceeding from the present surface . . . involve not 

 only the phenomena on the surface itself, but extend beneath it to ground water 

 level and even below that level as far as the vadose circulation is traceable. t • • • 

 The solutions formed by surface waters, like those of mine waters, mostly find 

 their way to the point where the water level reaches the surface, yet as a part of 

 the ground water penetrates to greater depths, such solutions may very likely 

 produce in the deep region itself impregnations which must, however, differ in 

 character from those produced by the deep circulation.^ Meteoric waters carrying 

 oxygen, some carbonic acid, and minute amounts of chlorides will first oxidize 

 whatever is oxidizable, especially metallic sulphides. The ferric sulphate formed 

 by the decomposition of the easily attacked FeS 2 will immediately attack the latter 

 minerals of the series." \ 



Penrose refers very briefly to this feature and says : " Hence the richest 

 bodies of ore in a deposit often occur between the overlying altered part 

 and the underlying unaltered part.' 1 | 



*Sur le r61e des phenomeues d' alteration superficielle et de remise en mouvement dans la con- 

 stitution de ees gisements. Ann. des Mines, xii, 1897, pp. 119-228. 

 f Genesis of ore deposits, pp. 135, 136. 

 X Loe. eit., p. i;$G. 

 I Loe. cit., p. 137. 

 || The superficial alteration of ore deposits. Jour, of Geology, vol. ii, p. 294. 



