Park. — Thermal Activity and Metalliferous Veins. 23 



level, which is the greatest depth reached by mining opera- 

 tions up to the present time. The propylitisation of the 

 andesites is not widespread, but confined to small areas 

 grouped around the old volcanic vents. Away from the 

 eruptive centres the andesites have suffered surface-decom- 

 position, but are not propylitised. The propylitisation was 

 apparently effected by the fissures, which are now veins, 

 having served as channels for the circulation of the hot 

 mineral waters. From these fissures the waters acted on the 

 rock on each wall, and where the fissures were near each 

 other the metasomatic processes operating from one fissure 

 met those coming from the other. Where the processes of 

 alteration did not meet, narrow irregular sheet-like masses of 

 unaltered rock — the " bars " of the miners— were left between 

 the vein fissures. 



At Waihi and surrounding districts the veins are chiefly 

 composed of chalcedonic or micro-crystalline quartz, possess- 

 ing all the characteristics of solfataric origin. Some of the 

 larger lodes can be traced on the surface for a distance of 

 16,000 ft., but the length of the majority is under 5,000 ft. 

 Besides veins having linear extension, there are many huge 

 mushroom-shaped masses of chalcedonic quartz, closely re- 

 sembling in form the siliceous deposits now forming in the 

 volcanic regions around Eotorua and Lake Taupo. 



At Kuaotunu and Great Barrier Island there are many 

 mushroom-shaped deposits of chalcedonic quartz of great size, 

 in some cases covering hundreds, in others thousands, of 

 acres. At Kuaotunu they are more or less circular in shape, 

 and seldom exceed 20 ft. in thickness. 



At Great Barrier Island the largest deposit is of an unusual 

 character.* It is nearly two miles long, half a mile wide, 

 and from 50 ft. to 700 ft. thick. The pipe is completely filled 

 with mineral matter. It has been intersected in four mines 

 in a distance of a mile, and opened up by levels for many 

 hundreds of yards. Ic- varies from 12 ft. to 40 ft. in width, 

 and is filled with very dense banded chalcedonic quartz, in 

 which iron and silver sulphides are sparingly distributed. 

 The evidence furnished by the mine-workings implies that the 

 overlying mushroom or umbrella of quartz was deposited on 

 the surface from thermal water issuing from a long fissure or 

 rent in the andesite. 



The molybdenite deposits at Jeff's Camp, in the Hod^- 

 kinson Goidfield, in Queensland, are described by W. E. 

 Cameront as roughly circular or oval-shaped outcrops of 



*J. Park, "The Geology and Veins of Hauraki Goldfields," Trans. 

 N.Z. Inst. Min. Eng., vol. i, 1897, p. 137. 



+ Walter E. Cameron, " Wolfram and Molybdenite Mining in Queens- 

 land," Geol. Survey Eeport No. 188, Brisbane, 1904, p. 7. 



