FiNLAYSON. — Geology of Quartz Veins of Otago Goldfielda. 83 



Ckanges of Primary Ore-content in Depth. — The deeper workings appear 

 to show that iu most cases the gold in depth becomes largely involved with 

 sulphides, the free gold diminishing in proportion. The metal is still largely 

 free at Barewood, but this appears due to secondary enrichment, and primary 

 or unleached ores in Otago are all probably refractory. 



In regard to variations in the sulphides at different depths, the data 

 are either unreliable or too meagre to indicate anything. 



Paragenesis. — The following are the known cases of paragenesis in 

 Otago : (1) Gold, iron-pyrites ; (2) gold, iron-pyrites, stibnite ; (3) gold, 

 iron-pyrites, stibnite, galena ; (4) gold, iron-pyrites, galena, zincblende ; 

 (5) gold, iron - pyrites, bournonite, zincblende; (6) gold, iron - pyrites, 

 scheelite ; (7) gold, iron-pyrites, scheelite, stibnite ; (8) gold, iron-pyrites, 

 scheelite, stibnite, galena; (9) gold, iron - pyrites, cinnabar: (10) gold, 

 iron-pyrites, copper-pyrites. 



(3.) Subsequent Changes in the Veins. 



1. Movement along the Fissure-walls. — Subsequent movement of the 

 fissures is shown by the frequently brecciated ore, but there is nothing to 

 indicate enrichment by uprising thermal waters during the movement. 



2. Secondary Sulphide Enrichment. — Gossan enrichment is best seen 

 at Bendigo and the Carrick, and illustrated by the manner in which workings 

 were largely confined to the oxidized zone. The phenomenally rich patches 

 of ore recovered near the surface in the early days are further evidence. 



The occurrence of film or paint gold on clay selvages, as at Skipper's, 

 is another proof of enrichment, and the yellow kaolinic wall-rock seen here 

 and at Barewood is no doubt due to the same cause. 



A zone of enriched sulphides occurs, as discussed above, at Barewood, 

 and probably in other veins as well. 



3. Denudation. — During the frequently recurring periods of elevation 

 since the vein-formation denudation would be active, and the upper enriched 

 portions of the veins would be continually shorn off, the gold going to supply 

 the alluvial wealth of Otago, for which the district is more justly famed. 

 A consideration of the relative amounts of vein and alluvial gold obtained 

 in Otago shows that the value of the veins has been largely indirect. De- 

 nudation has, indeed, gone on to such an extent that the Otago veins must 

 be considered as practically vein-roots, and when it is possible to calculate 

 the amount of denudation that has taken place we will have a legitimate 

 field for speculating on the depth to which the veins will be payable. 



4. The Source of Alluvial Gold. — The early diggers in Otago, as in all 

 other goldfields, maintained that the alluvial gold was derived from quartz 

 veins. Messrs. McKay* and Eickard,t however, claim that the quartz 

 veins are insufficient in quality and quantity to have supplied the gold, 

 and they maintain that the gold has been derived mainly from the schist. 

 In another paper I have pointed out that the occurrence of gold in the schist 

 is an unproven hypothesis.J Further, after a careful study of the great 

 number of veins which exist, and of the prolonged enrichment and enormous 

 denudation which have taken place since the veins were formed, I have 

 come to the conclusion that the early diggers held the correct view^ — that 

 the quartz veins were a sufficient source of supply, and that no other source 

 supplied any appreciable quantity. 



* :McKay, " Gold-deposits of New Zealand " (Wellington, 190.3), p. G7. 



f Rickard, " Goldfields of Otago," Trans. Amer. Inst. Min. Eng., vol. xxi, [i. 442. 



j Finlayson, " Notes on the Otago Schists," Trans. N.Z. Inst., 1907, p. 77. 



