ORE BODIES AND ASSOCIATED IGNEOUS ROCK. 157 



(b) The Mode of Derivation of the Ores from the Magmatic 

 Hearth. — Passing mention has been made above of the presence in 

 the acidic magma at the time of its iriiiption of certain volatile 

 ingredients. These ingredients, called " mineralizers," combining 

 with the metallic contents of the magma, formed volatile compounds 

 of the several metals which were incapable of solidification while 

 they were retained within the highly heated magmatic hearth. 



From the study of the metallic ores it is found that the minera- 

 lizers which were contained within the particular magma under con- 

 sideration were principally compounds of boron, sulphur, and carbon. 

 Fluorine was present, but in relatively small amount, and arsenic 

 was present in traces. 



The metals present in the magma were iron, zinc, lead, tin, 

 copper, antimony and silver, with very much smaller amounts of 

 tungsten, molybdenum and bismuth. 



Over and above these mineralizers and metals there has been 

 present a considerable amount of aqueous vapour, which has 

 undoubtedly served in no small degree as the transporting agent 

 of the metallic compounds. 



Upon the development of fissures, which penetrated to the 

 heart of the cooling magma, these gaseous compounds were 

 enabled to pass outwards through the already solidilied portions 

 of the igneous magma into the surrounding rock masses. In 

 such fissures the metallic burden was deposited. 



As has been already indicated, the zonal arrangement of the 

 metallic ores about the granite is well- defined, and it remains to 

 investigate the probable nature of the control exerted by the 

 granite over the precipitation of the ores. 



(c) The Precipitation of the Metallic Ores. — ^The principal facts 

 of occurrence, to which reference has been made above, are the 

 gradual alterations in the mineralogical characters of the ore- 

 bodies in passing from Heemskirk on the west to Zeehan on the 

 east, and the general restriction of the broader groups of mineral 

 associations (vein-types) to the granitic, contact metamorphic and 

 transmetamorphic terrains. 



In the attempt to determine the causes which have contributed 

 to establish these geological relationships, it is necessary to consider 

 briefly both the causes which produce primary variations in ore- 

 bodies in general, and the special circumstances applicable to this 

 area in particular. 



(a) General Principles. — In the case of the gradual consolida- 

 tion of such a magma as that which has here been described, the 

 segregation of the metalliferous vapours appears to take place at 

 a number of different centres, especially when the magmatic 

 reservoir is a large one. Within these different centres of segrega- 

 tion the proportions of the different metals are variable, sometimes 

 so much so that from one centre ores of one metal result, and from 

 another centre ores of another metal. These differences will vmder 

 such circumstances be more apparent to the mining community 

 than to geologists who possess some knowledge of the geology of 



