ORE BODIES AND ASSOCIATED IGNEOUS ROCK. 155 



VI, — ^The Relationship of the Ore-Bodies to the Igneous 



Rocks. 



A. — The Source of the Metallic Ores. — In seeking to establish 

 the relationship of the ore-bodies to the igneous rocks it will be 

 apparent that the attention must be directed towards certain 

 particular phases in the genetic evolution of both. 



It has been already stated that there are good reasons to 

 believe that the metallic ores of Heemskirk, Comstock and Zeehan 

 constitute a genetically related series. In common with almost 

 all the ore-bodies of Tasmania they are associated with the complex 

 intrusion which occurred in Devonian time. The several ore-bodies 

 of different types are arranged zonally about a visible outcrop of 

 granite, and the ores of the innermost zone extend downwards into 

 the heart of the granite. 



If, then, our hypothesis of the inter-relationship of these ores 

 is correct, the\^ have all proceeded from some source within the 

 heart of the granite. 



On the other hand, in discussing the igneous rocks of the 

 acidic group, it has been pointed out that there has been a pro- 

 gressive alteration in the nature of the magma during consolidation. 

 The granite is the earliest product of consolidation, and the peg- 

 matite, aplite and quartz-tourmaline dykes are the latest products 

 to which the name " igneous rock " would be accorded without 

 hesitation. 



These dykes cut through the granite and have clearly pro- 

 ceeded from a source situated within its innermost core. 



We find, then, that there are on the one hand metallic ore veins 

 (cassiterite-bearing and tourmaline-bearing), and on the other 

 hand, quartz-tourmaline, aplite, or pegmatite dykes proceeding 

 from the same source. 



Upon these tabular deposits of mineral material — the dykes 

 and the veins — we may therefore centre our attention. 



At once we are confronted with the difficulty of establishing 

 a definition which will ser^^e to satisfactorily distinguish between 

 a " dyke " and a "vein." We may only conclude from the field 

 evidence that certain of the tabular bodies, composed of quartz 

 and tourmaline, solidified from liquid material less highly charged 

 with aqueous vapour than others. Yet all gradations exist and 

 may be visible in a single dyke or vein.^ 



So the structural features — especially that of crustification — 

 have no great value as distinguishing criteria betweenveins and dykes. 



Turning to other phenomena in the search for such criteria, 

 very little satisfaction is given if we examine the wall rocks 

 traversed by these dykes and veins. It is apparent that a very 

 much greater alteration of the wall rock takes place in some cases 

 than in others, but this alteration is by no means proportionate to 

 the mass of the vein or dyke. For, in many cases, there is a con- 

 siderable width of altered wall rock on either side of a mere film, 

 occupying the central fissure. 



1 Entirelvsimilar phenomena recur in the neighbouring tin field of North Dundas. Vide Geol. 

 Surv. Tas., Bulletin No. 6, 1909, pp. 53-54. 



