ON CERTAIN MODES OF OCCURRENCE OE GOLD IN AUSTRALIA. 431 



26. Note on certain Modes of Occurrence of Gold in Australia. 

 By Eichard Daintree, Esq., E.G.S. (Read February 20, 1878.) 



[Plates XVII., XVIII.] 



In a paper read before the Geological Society in April 1872, on 

 " The Geology of Queensland," it was pointed out that a large area 

 of Devonian rocks existed in that colony, and that numbers of gold- 

 fields had been opened on such areas. 



Attention was, however, drawn to the fact that the auriferous 

 tracts were entirely confined to such of those Devonian districts as 

 were found to be penetrated by certain plutonic rocks, principally 

 pyritous diorites. 



In these diorites, and at and near their intersection with the 

 Devonian strata, auriferous quartz, calc-spar, and pyritous reefs had 

 been examined, and were proved to be rich in gold, whilst the 

 extension of such veins at any considerable distance from the 

 intrusive rocks was found to be barren. 



Instances were also adduced to show that the pyrites sporadically 

 distributed through the diorites were occasionally decidedly auriferous, 

 and by their decomposition and degradation had yielded alluvial drifts 

 containing gold in paying quantities to work. 



Since this was written I learn from Mr. C. Wilkinson, the 

 Government Geologist of New South Wales, that the same facts 

 hold good for those New-South-Wales gold-fields which lie in 

 Devonian or Upper Silurian areas ; and Mr. G. Ulrich, the talented 

 Curator of the Technological Museum in Melbourne, in his Catalogue 

 of the rocks in that Museum, gives details which prove beyond 

 doubt that the T T pper Silurians of Yictoria owe their auriferous 

 character to the same cause. 



He describes the diorites as occurring in Victoria, mostly as 

 dykes varying in thickness from a few to several hundred feet, 

 traversing Upper Silurian rocks and presenting nearly all the 

 ordinary varieties of texture and mineral composition. They are 

 nearly always impregnated with auriferous pyrites, and traversed 

 by, or associated on one or both walls with, auriferous quartz veins ; 

 and by far the greater quantity of the quartz -gold furnished by the 

 gold-fields occupied by Upper Silurian rocks is derived from such 

 diorite dykes. In support of these statements, Mr. Ulrich mentions 

 several important workings connected with these dykes, and 

 especially notices the dyke of Cohen's Reef (which, he says, is 

 perhaps the richest one in the colony, and some specimens from 

 which represent the variety " diorite aphanite "), and the dyke of 

 the Albion Mining Company, at Crossover Creek in North Gippsland, 

 which is interesting on account of its highly micaceous character, 

 and its influence upon the gold-bearing character of the reefs which 

 traverse it at right angles to its strike, and which, poor in the 

 Q.J.G.S. No. 135. 2g 



