510 THE ORE-DEPOSITS OP BORAH CREEKj 



There occur in the Tingha District — 



A.(l) Tin-veins proper — with no metallic sulphides. 



(2) Tin-molybdenite veins. 



(3) Tinarsenopyrite veins. 

 B.(4) Tin-copper-arsenopyrite veins. 



(5) Tin-copper-galena veins. 

 C.(6) Tia-silver-galena-zincblende-copper-arseuopyrite veins. 

 These six types fall into three groups — 



A.Tin as oxide in association with oxides. 



B.Tin as oxide in association with sulphides. 



C.Tin as sulphide (stannite) in association with sulphides. 



Thus a complete gradation can be traced from tin-veins proper 

 to lead-veins in this district. This, then, indicates a common 

 origin for both types of veins. In this connection it is interest- 

 ing to compare the Conrad Mine with other silver-lead mines in 

 different jtarts of the world. 



Professor Vogt* has written an interesting summary on this 

 point which reads as follows: — "For the older as well as the 

 younger ones [mineral veins], we may declare that a clear genetic 

 connection with eruptive rocks can be established. In some 

 eruptive districts the latest eruptives of the series exposed are 

 even later than the ore-veins; hence the formation of the latter 

 must have occurred during the eruptive epoch." 



" Partly for this reason, and partly because of the fact that, 

 on the whole, the veins are generally independent of the petro- 

 graphic nature of the country-rock, I think we are warranted, 

 in this department also, in assuming, as a working-hypothesis, 

 that the ore-material was extracted from a magma. With regard 

 to the younger veins especially, we must keep in mind a possible 

 extraction from a laccolitic magma in depth." 



" In support of this hypothesis, we may cite the transitional, 

 or intermediate occurrences between the cassiterite- and the 

 silver-lead-veins. Thus, in Cornwall, the tin-, tin-copper- and 



* Vogt, J. H. L., "Problems in the Geology of Ore-Deposits." Trans, 

 ^Amer. Inst. Min. Eng., Posepney Vol.1901, pp. 656-7. 



