ON THE KOCKS OF THE HAUKAKI GOLD-FIELDS. 269 



joining the reef on the hanging wall probably increase the yield of 

 gold for a time, but leaders from the foot-wall seldom."* On this 

 I must remark that I have often seen small veins carrying rich 

 gold witliout any pyrites at all, but in the larger reefs pyrites 

 generally occurs. 



Theories. — Sir James Hector, who has liad excellent opportu- 

 nities for studying the subject, finds the source of the gold in the 

 pyrites of the country-rock In his printed instructions to me in 

 1867 he says, "The composition of the several rocks in the 

 vicinity of the lodes at Coromandel shews their singular character, 

 arising, as I suspect, from all the soluble matters of what was 

 once a basic rock having been removed and replaced by silica and 

 partly by iron pyi'ites containing gold. That this mineral is the 

 main source of the gold is shewn by a section of the lode ground 

 I made 1864, when 1 found that the so-called quartz reefs were 

 contained between two varieties of pyritous rocks, the sulphurets 

 having been removed from the overlying rock, but still remaining 

 in the lower, the reef itself being a band of mullock containing 

 kernels and geodes of quartz and carbonate of lime, and evidently 

 formed by infiltration." The attached analyses shew thnt the 

 hanging wall contained neither gold nor pyrites, while the 

 foot-wall contained about 11.68 per cent, of pyrites : but it is not 

 stated that this pyrites contained gold. In 1869 Dr. Hector 

 says " whatever may be the age of the impregnation of these 

 rocks with sulphidrs, the gold they contain seems first to have 

 appeared in them at the same time.f He then says that the 

 quartz was brought up from below, as I have already mentioned, 

 and he adopts the generally accepted opinion that thermal waters 

 and acid vapours were the agents that produced the changes. 



Mr. Davis, I suppose, agreed with Sir James Hector as their 

 conclusions are identical. I also held the same opinion in 1869. 

 In 1882 Mr. Cox pointed out that the pyrites in the decomposed 

 rocks is not itself decomposed, and could not therefore be the 

 source of the gold. The pyrites, he thought, was formed con- 

 temporaneously with the gold in the veins, the mineral waters 

 which deposited the gold and the quartz in the reefs having 

 found their way through numerous small joints in the rocks, 

 decomposed their felspathic constituents and deposited from 

 solution tlie crystals of "crystalline grains of pyrites.; Mr. Cox 

 thus accounts for the presence of pyrites in the surrounding rock 

 being a favourable indication of gold, although it is not de- 

 composed ; he looks upon these pyrites as the overspill from the 

 reef of matei'ials brought up in fissures. 



Undoubtedly, under ordinary circumstances, much of the 

 pyrites remains in the rock unaltered, and can be washed out of it, 



* Reports Geological Explorations, 1870-1, p G8. 

 + Reports Geological Explorations, 1868-9, p, 39. 

 I Reports Geological Exjilorations, 1882, p. 44. 



