82 T ransactio7is . 



It is noticeable that the vein-bearing district spreads out in a fan froni^ 

 the head of Lake Wakatipu to Macrae's in the north and Lawrence in the- 

 south. This fan encloses all the known auriferous veins of Otago, although 

 it must be remembered that veins exist at Preservation Lilet, in the far 

 south-west, and also probably in the Longwood Range, in Southland. Never- 

 theless, the fact remains that the gold-veins of Otago proper are contained 

 within the area indicated. 



Origin. — The fissures are evidently due to compression-forces, as they 

 have all the typical features of compression veins, described by Emmons^ 

 For the cause of this compression there are two alternatives : (1.) It may 

 have been caused by a tectonic force acting from west to east and south- 

 east, with its greatest intensity at the head of Lake Wakatipu, in the near 

 vicinity of the belt of igneous rocks which were intruded during the Jurassic 

 mountain-formation. Such a compression movement would conceivably ex- 

 tend its area of effect as suggested by the above " fan," and would gradually 

 diminish in intensity as it passed towards the coast. This would be favour- 

 able to the formation of bedded shear-zones, as seen at Macrae's. (2.) It 

 may have been caused, as Professor Park claims,* by local intrusions of 

 igneous rocks which did not reach the surface. As against this view, it 

 is strange that none of the intrusives are to be seen exposed. The evidence 

 is not yet sufficient to decide which view is the correct one, but from a con- 

 sideration of the gold-bearing veins throughout the South Island there is 

 strong evidence for concluding that the fissures were formed during adjust- 

 ment of the strains set up by the alpine folding at the end of the Jurassic. 



Age. — On this view the Assuring is Post-Jurassic, and probably occurred 

 during subordinate earth-movements following on the main extensive folding. 

 They may be regarded as of Cretaceous or early Tertiary age. 



The fault-fissures are much younger, being, from geological consider- 

 ations, of late Pliocene or even younger date. 



(2.) The Vein- for motion. 



The vein-formation varies, as has been seen, according to the nature 

 of the fissures, and all gradations may be traced from quartz-filled fissures 

 to replacement veins. The typical quartz vein is seen at Barewood ; 

 Bendigo is also a good instance of fissure-filling. At Skipper's, Waipori, 

 and Macrae's, replacement has been the chief process. 



The characteristic metasomatic alteration, as revealed by analysis, is 

 sericitization of the wall-rock, accompanied by a reduction of specific gravity 

 and a decrease in volume. A calculation of the constituents of the altered 

 rock at Barewood shows that about 33 per cent, of sericite is present. As 

 regards changes in constituents, the altered rock shows generally a con- 

 siderable loss in silica, greatest in the case of veins which carry segregated 

 quartz (Macrae's), in iron and titanium, in the earths and in the alkalies. 

 The added constituents are generally sulphur and carbon-dioxide. The 

 nature of the metasomation is somewhat similar to that of the Freiberg 

 gneiss, examined by Scheerer and Stelzner.f 



Ore-shoots. — Nearly all those veins which have been reported to contain 

 ore-shoots are now unworked, and I was unable to investigate the subject. 

 It is therefore inadvisable to speculate or make statements based on rumour 

 and on records of doubtful value. 



* Park, Bull. No. 5, N.Z.G.8., 1908, p. 63. 



t Beckweed, " Nature of Ore-deposits," 1905, p. 397. 



