84 Transactio7is. 



(4.) Genesis of the Veins. 



In the light of present views in regard to the genesis of ore-deposits, we 

 must conceive that the vein- fissures when formed gave access to thermal 

 waters charged with precious metals, which rose from a magma beneath, 

 wherein rock-differentiation and ore-segregation had taken place. 



The universal occurrence of gold and pyrites, and the very frequent 

 presence of antimony-ore and scheelite. indicate a pretty uniform degree 

 of segregation beneath. The isolated occurrences of copper-ore and cinnabar 

 are no doubt due to some local segregations at points beneath. 



Fault-fractures. — These were formed at a much later date, probably 

 when thermal magmatic waters had ceased their activity. Moreover, being 

 gravity faults, they would not extend to such a depth as compression fractures, 

 even allowing for a reduced thickness of the schists due to denudation. 



Buck Reefs. — The explanation of these occurrences is difficult. Their 

 presence in so many gold-bearing districts would lead us to infer that they 

 have been formed by similar processes to the gold-veins, and probably at 

 the same time. The quartz appears to have been deposited in a wide fissure,. 

 or at least in a fissure which widened as it was being filled, while the solutions 

 were either of meteoric origin or had been robbed of their metallic content 

 some distance below the present surface. It is even possible that the solu- 

 tions were barren of metals, and that these buck reefs illustrate the extreme 

 siliceous product of magmatic differentiation in a portion of the magma 

 where the conditions were not favourable to the segregation of ores. 



(5.) Future Prospects of the Goldfield. 



The history and development of quartz-mining in Otago have been 

 sketched by Mr. Robert Mackintosh, who has also some observations on the 

 future prospects of the district.* The following matters may be emphasized 

 from the standpoint of economic geology : — 



1. Timber is scarce all over the field, while water and fuel are also often 

 at a premium. These factors are obstacles in the way of extensive working. 



2. The small size of the veins, and the fact that the ore is frequently 

 refractory and low-grade, are also objectionable features. 



3. The uncertain occurrence of quartz blocks in the larger veins, and 

 their uncertain quality when found, entail extensive development, and the 

 returns from such workings are not likely to be highly remunerative. 



4. Finally, in consideration of the fact that the veins are practically 

 roots, and that the ore is almost certain to be low grade after a depth of about 

 300 ft., deep sinking is less likely to be successful than development along 

 the strike. There is no doubt that a great deal of payable ore still exists 

 in undeveloped portions of the veins, and that the judicious exploitation 

 of the upper levels would maintain the field for some years to come. 



EXPLANATION OF PLATE II. 



Fig. L Comby quartz [a], and brecciated quartz (b), from the Cromwell vein, Bendigo. 

 Fig. 2. Vein-qviartz, Bendigo. The photo shows the association of coarse and fine 



granules. Magnified 27 diameters. 

 Fig. 3. A siliceous concreticn from the No. 2 level, Barewood, formed evidently during 



secondary enrichment by meteoric circulation. 

 Fig. 4. ChalcopjTite, from Moke Creek. The photo shows some ore (opaque) in a gangue 



of quartz and twinned calcite. Magnified 27 diameters. 



* R. Mackintosh, " Mineral Resources of New Zealand," N.Z. Mines Record, 16th 

 Jan., 1907. 



