Park. — Metasomatism in Formation of Ore-deposits. 33 



At White Island, in the Bay of Plenty, the deposits of 

 sulphur occur in and around the crater-lake, mixed with 

 gypsum. The crater-water is hot, and highly charged with 

 free hydrochloric and sulphuric acids. The gypsum is de- 

 posited in crystalline incrustations on the sides and floor of the 

 crater-lake. The source of the lime has not yet been de- 

 termined ; but the supply must be constant, as gypsum is 

 being deposited continuously. The sulphur is deposited in 

 the water from gas-springs vvhich are seen bubbling every- 

 where in the floor of the lake ; and also from fumaroles 

 around the margin of the crater. 



Art. V. — On the Bole of Metasomatism in the Formation of 



certain Ore -deposits. 



By Professor James Park, M.A.Inst.M.E., M.Inst. M. and 

 M. (Lond.), F.G.S., Director of the Otago University 

 School of Mines. 



[Read before the Otago Institute, 13ili September, 1904.] 



Until lately it was the common belief that ore -deposits 

 merely filled pre-existing fissures and cavities in the country 

 rock. In recent years, writers on ore-formation have become 

 convinced, as the result of microscopic examination, that 

 many ore-bodies were merely metasomatic replacements of 

 country rock that followed certain well-defined crush-zones or 

 zones of metamorphism. According to this, it is surmised 

 that in many cases no previous cavities existed, but that 

 the waters altered and removed certain tracks or zones of 

 rock which they partially or completely replaced with ore- 

 matter and gangue. 



This process of replacement is known to petrologists to 

 have taken place among the constituents of many rock-masses, 

 no matter how dense, including all metamorphic rocks, and 

 all older igneous and eruptive masses. It is known as 

 "metasomatism" (meaning, change of body), and is due to 

 internal chemical reactions which seem to take place as 

 readily in rocks as do the equally obscure metabolic changes 

 in living organisms. 



In many cases minerals are replaced molecule by molecule, 

 giving rise to what is termed "mineral pseudomorphism." 

 But in the processes which affect changes in rock-masses, 

 reactions may be set up between the different constituent 

 minerals, thereby forming new minerals capable of segregating 

 themselves into large masses ; or the rock may be altered, and 

 some or all of the constituents removed and replaced by new 



2— Trans. 



