FiNLAYSON.— Geo/o(/?/ of Quartz Veins of Otago Goldfields. 67 



For a goldfield, the district is poor in igneous intrusions. An outcrop 

 of actinolite-schist near Ophir is probably a metamorphosed dyke.* There 

 are, at Gibbston and Moke Creek, outcrops of serpentine-talc rock, repre- 

 senting magnesian intrusives of doubtful age.j Finally, there is the im- 

 portant middle Tertiary series of volcanic rocks developed especially round 

 Dunedin, and along the east coast as far north as Oamaru and inland to 

 Macrae's.J The rugged and little-known district between the lakes and 

 sounds doubtless contains extensive igneous intrusions, as judged from 

 pebbles picked up in the Clutha and Kawarau Eivers. . 



The next formation, chiefly developed along the coast, is the Tertiary 

 limestone series of New^ Zealand, seen as a small patch at Bob's Cone, near 

 Queenstown, where it has been preserved from denudation by being involved 

 in a fault-line in the schists. 



Then follow the Pliocene auriferous gravels and cements, shales, and 

 sands, which fill the valleys of the goldfield (formerly old lake-basins), and 

 finally the Pleistocene river and lake terraces, for the most part auriferous. 



(b.) Geological History. 



As far as we know at present, sedimentation was continuous, with minor 

 breaks, from the middle Palpeozoic, and perhaps earlier, till near the end of 

 the Jurassic period. During this extended time a vast thickness of sedi- 

 ments was deposited. Then followed, throughout the South Island, the 

 main upheaval of all these older beds, accompanied by the intrusion of an 

 important belt of granitic rocks, which can be traced from Nelson, through 

 Westland, and probably through western Otago, down to Preservation 

 Inlet and Stewart Island. At the same time the metamorphism and alpine 

 folding of the older rocks was mainly effected. 



With elevation, denudation became active, and the younger Mesozoic 

 beds were removed from the central portions of Otago, where the uplift 

 was greatest. 



Subsidence followed in the early Tertiary, and a series of coals and lime- 

 stones was deposited, the sea extending far into the interior. Then followed 

 elevation, accompanied by faulting and volcanic activity. Denudation at 

 the same time almost entirely removed the Tertiary deposits, and the pre- 

 sent drainage-system of Otago was inaugurated. Subsequent movements 

 are not quite clear, but there probably occurred a Phocene depression, 

 during which the deposits of the Central Otago lake-basins were laid down, 

 followed by a late Phocene and early Pleistocene elevation, accompanied 

 by glacial extension, and then a subsidence, with retreat of the glaciers. 



The Veins. 



With some exceptions, the veins are small, rarely exceeding 2 ft. or 3 ft. 

 in width. The filling is largely crushed and altered rock, accompanied 

 in the smaller ones by veins and stringers of quartz, and in the larger by 

 lenses or blocks of quartz, varying in size. 



The predominant ore is gold, with very little silver (fineness, 960), ac- 

 companied by auriferous pyrite. Other minerals are scheeUte and stibnite, 

 which are common, and sometimes constitute the dominant ore ; also galena, 

 bournonite, and zincblende, which are rarfe. 



* Park, Bull. No. 2, N.Z.G.S., 1906, p. 41. 



t Finlayson. " Notes on the Otago Schists," Trans. N.Z. Inst., 1907, p. 76. 



j Marshall, " Geology of Dunedin," Quart. .Journ. Geol. Soc, vol. Ixii, p. 422. 



