154 PROCEEDINGS OF SECTION C. 



The gangue minerals in the lodes are also found to transgress 

 the limits of the main groups referred to above. 



There are no known instances in which a lode characteristic of 

 one zone or group is actually intersected by a lode belonging to 

 another zone. This evidence, negative though it may be, is cer- 

 tainly most suggestive, since a very large number of lodes have been 

 worked from time to time and thus exposed for examination. It 

 appears that the members of the different groups belong all to a 

 series (or at most, two series^), of which the different components 

 were formed during a single stage m one metallogenetic epoch. 



All these phenomena strongly support the view that the ores 

 which have been mentioned are derived from a common source, 

 and that the differences between them are only quantitative 

 variations in the proportions of the constituent elements.. 



In seeking the cause of the variations it will be necessary to 

 investigate the relation of the distribution of the ores to the geology 

 of the region. 



V, — The Geological Relations of the Ores. 



Geological investigation has shown that all the rocks in the 

 area which are of greater age than the Permo- Carboniferous have 

 been traversed by the vein fissures. The sedimentary and igneous 

 rocks have all been affected by stresses sufficient to produce rupture, 

 and certain of the resulting fractures have served as loci for ore 

 deposition. Thus we conclude that the period of ore deposition was 

 later than that of the main invasion of the acidic igneous magma. 



When the several groups of ores, for which a genetic relation- 

 ship is claimed, are considered separately it is seen that there is a 

 most marked connection between their distribution and the exposed 

 granitic massif of Mt. Heemskirk. There is a zonal arrangement of 

 the groups of ores about the granite such that a very cursory exami- 

 nation of the district suggests some causal interdependence. 



Within the granite borders the lodes are almost exclusively 

 cassiterite-bearing. These lodes do extend beyond the igneous 

 boundaries, but not to any considerable distance. 



The magnetite-bearing ore-bodies are restricted to the contact- 

 metamorphic aureole surrounding the granite, and beyond these lie 

 the pyritic and sideritic lodes as the distance from the granite 

 increases. 



From these facts of occurrence the conclusion to be drawn is 

 that the character of the ore is in some way controlled by its degree 

 of prox.;mity to the granite. 



Moreover, since certain types of ore deposits (for all of which 

 genetic relationship and contemporaneity of deposition are here 

 claimed) are found only within the granite boundaries or for a short 

 distance outside of these, i t follows that the source of these ores is 

 located within the heart of the granite, not at its periphery. 



1 Vide Geol. Surv. Tas., Bulletin No. 8, 1910, p. 68. 



