ORE BODIES AND ASSOCIATED IGNEOUS ROCK. 153 



IV. — ^The Metallic Ores and the Relationship Betaveen 



Them. 



The metallic ores of the area, considered apart, show con- 

 siderable diversity of character, and yet are characterised by an 

 equally noteworthy blending of types. The diversity is expressed 

 by the fact that common reference is made to the western part 

 (Heemskirk) as a tinfield. while the eastern portion (Zeehan) has 

 hitherto been regarded as essentially a silver-lead field. But this 

 passage from a tinfield on the west into a silver-lead field on the east 

 is not characterised by any sharp line of demarcation in the matter 

 of distribution nor in the mineralogical composition of the ores. 



A more detailed discussion of the mineralogical constitution of 

 the ore-bodies will be found in an official publication ^ For the 

 purposes of this paper it will suffice to give the following sum- 

 mary : — 



The tin-bearing lodes of the Heemskirk district exhibit some 

 variety of composition^ They are marked in almost every case 

 by the presence of pyrites, and less often by the existence of small 

 amounts of bismuthinite, molybdenite and wolframite with the 

 cassiterite. Of the gangue minerals, tourmaline is the most 

 characteristic and most widely distributed. Fluorite is not 

 abundant. 



Lying to the eastward (and southward) of this area, in which 

 cassiterite is the most constant metallic mineral, are the magnetic 

 masses of the Comstock district. The magnetite is found in very 

 large bodies which are seldom free from admixture with other 

 metallic minerals. Although often nearly perfectly pure, the 

 magnetite is found associated with galena, blende, chalcopyrite and 

 pyrite, and one instance is known in which cassiterite accompanies it. 



Still further to the eastward lie the large pyritic lodes of the 

 western portion of the Zeehan field. Pyrites js the most abundant 

 mineral, and blende and galena are associated with it. At one 

 point there is a notable development of stannite, with which are 

 associated pyrite, chalcopyrite, galena, bismuthinite, wolframite, 

 tetrahedrite, siderite, and sporadic traces of fluorite. 



The eastern limit of the area under consideration — the Zeehan 

 field proper — is characterised by lodes of which a considerable 

 portion consists of siderite, with which are found galena, blende, 

 and smaller quantities of tetrahedrite and chalcopyrite. 



Between these broad groups lie many associations which clearly 

 mark transitional stages. Certain metals have been proved to 

 exist in almost every type, and as the work of developing the mines 

 proceeds fresh discoveries are continually being made of the 

 presence of small proportions of certain metals in lodes which were 

 formerly not known to contain them. 



1 Geol. Surv. Tas. Bulletin No. 8, 1910, pp. 45-54. 



2 G. A. Waller: " Report on the Tin Ore Deposits of Mt. Heemskirk," 1902, pp. 8-10. The 

 author acknowledges his great indebtedness to this and other reports of Mr. Waller, whose researches 

 in the region considered have so largely contributed to the knowledge of the ore-bodies and of their 

 genetic relationships. 



