﻿Vol. 66,'] METALLOGENY OF THE BRITISH ISLES. 295» 



Magmatic differentiation and ore-deposition. — The 

 local distribution of certain ores, such as the gold ores of Merioneth- 

 shire, the pyritic deposits of Wicklow and Anglesey, the silver- 

 cobalt ores of Alva and Hilderston, and other similar occurrences, 

 is very striking when considering the geographical distribution of 

 the metals. While some of such peculiarities may result from 

 selective chemical affinity and from influences due to the country- 

 rock, it is clear that there is a much more important and more 

 general cause at work. The heterogeneous distribution of ores in 

 the rocks of the earth's crust cannot be accounted for by assuming 

 an originally heterogeneous distribution of metals beneath. It is 

 agreed that the ultimate source of all heavy metals is in igneous 

 magmas, and further, the differentiation of certain metals with 

 certain types of plutonic rocks has long been recognized. It follows 

 that the distribution of ores, as now found, is an index to the 

 former distribution of metals in the magmatic sources beneath, a 

 distribution brought about by progressive differentiation of these 

 magmas, with correspondiug differentiation and segregation of the 

 heavy metals. 



We are here brought into touch with a feature emphasized by 

 Mr. J. E. Spurr, namely, the relations observed between petro- 

 graphic provinces and provinces of ore-deposits. 1 This feature,, 

 which is illustrated to some extent in the British area, clearly 

 indicates that the differentiation of igneous rocks and that of ores 

 have been closely involved. That petrographic provinces with a. 

 certain individuality are accompanied by metallogenetic provinces 

 with a corresponding individuality is illustrated in many regions.. 

 I may instance the pyritic deposits of Huelva, Norway, and. 

 Wicklow ; the nickeliferous pyrrhotites of Norway and of Sud- 

 bury ; the Tertiary goldfields of Hungary and of the Pacific belt ;. 

 and the ancient goldfields of Gondwanaland (Erythraean Province 

 of Dr. Maclaren). 2 Each of these types has its characteristic 

 assemblage of igneous rocks, and generally also its characteristic 

 tectonic structure due to certain definite sub-crustal processes. 



It is clearly evident that the differentiation and concentration of 

 ores, as reflected in such cases, has been intimately bound up with 

 the differentiation of magmas, and hence has been dependent, 

 throughout geological time, on epochs of intense crust-movement. 

 To these circumstances is due the occurrence and recurrence of 

 defined metallogenetic epochs, corresponding with widespread epochs 

 of crust-movement and igneous activity. The facts of magmatic 

 differentiation in relation to ore-deposition on the one hand, and to 

 tectonic processes on the other, are clearly indicated. The full and 

 complete explanation of these facts is a labour for the future, and 

 must wait upon further knowledge of the processes involved, and of 

 their relation to the earth's history. 



1 J. E. Spurr, loc. supra clt. 



2 Trans. Inst, Min. & Met. vol. xvi (190G-1907) p. 15. 



