446 Transactions. — Geology. ^ 



several classes, and in an alluvial form, it being from the alluvial workings 

 that by far the greater quantity of gold has been obtained. The goldfields 

 have been divided on the Geological Map into Northern, Central, Western, 

 and Southern, and the gold from the different locahties varies considerably in 

 purity. That from Otago or the Southern fields and from Westland is pure 

 or nearly so, being alloyed with less than 6- per cent. Ag., and a little copper. 

 In Nelson it is alloyed with 10 to 14 per cent, of silver, while at the Thames 

 or Northern goldfield it is generally alloyed with over 30 per cent, of silver, 

 thus corresponding in composition to theElectrum* of Klaproth, which con- 

 tains gold 64, and silver 36 per cent. There is also a marked difference in 

 the associated minerals north and south, which, of course, is dependent upon 

 the rocks from which the gold is derived. Thus in the Southern goldfield 

 it is associated Avith Platinum, Zircons, and Garnets, as well as Black Hema- 

 tite and Scheelite ; in Nelson it occurs with the rare minerals Osmiridium 

 and Platiniridium, and in the Northern goldfields Native Arsenic, Copper 

 Pyrites, Galena, Zinc Blende, and Stibuite occur in the reefs, these being 

 also more or less represented at Collingwood and at parts of the "West 

 Coast. The different characters of the reefs and mode of deposition are, 

 however, the most marked and interesting in different localities. 



In the Southern goldfield at Macetown and Cromwell these reefs are of 

 a truly brecciated character (Hector, Geol. Eep. 1878-9, p. 24), being com- 

 posed of angular fragments of slate and schist cemented by quartz, with 

 which gold has been infiltrated in a very pure form. The gold in these 

 reefs must have been introduced by mechanical means, but some has prob- 

 ably also been deposited from solution ; but these reefs which occur in 

 metamorphic schists have probably been formed subsequently to those in 

 the foliated schistose rocks and slates, which belong to the next group, 

 and from which they have derived tliek gold to a large extent. The reefs 

 of the Eeefton district, on the other hand, are true fissure reefs, and owe 

 their origin to fractures produced by contortion, those which occupy the 

 synclines widening as they descend, while those on the anticlines die out 

 in depth or come to be nothing more than strings. Outliers of the same 

 formation of reef occur at Cardrona. These fissures having been formed by 

 the same action which induces cleavage elsewhere, during the plication of 

 the strata on a large scale, have given rise to subterranean channels, in 

 which the quartz and gold have been deposited fi-om solution. The reefs 

 thus formed consist of solid, compact quartz, and correspond more to the 

 Australian reefs than any others. It is hard to assign any reason for the 

 precipitation of gold in cases of this sort, and a field of research yet remains 

 open for anyone who feels disposed to take it up. 



* Hector " Trans. N.Z. Inst.," vol. ii., p. 366. 



