NOT WORKED OUT

Evening Post, Volume CXVI, Issue 34, 9 August 1933, Page 5

NOT WORKED OUT

OTAGO'S GOLD

GEOLOGIST'S OPINION

The gold-bearing deposits in Central Otago were discussed at the. last meeting of tho geological section of tht Wellington Philosophical Society, when Mr. E. O. Macpherson read a paper on "The Terrestrial Sediments of th« Manuherikia Depression.?' "The gold found "in Central Otago," said the speaker, "was an important factor in the economic life of New Zealand of the past generation, and tha present study had been undertaken with, the_xjbject of finding out whether these goldfielda were actually exhausted, or, if geological complications or the econo- ' mic influences existing at the time that mining ceased caused their wane. '' The sequence of formations commencing with the lowest consists o{ schist and argillites (the 'main bottom' of the miners) and overlying thejsa are quartz conglomerates which range in thickness from 10 to 450 feet; these deposits are gold-bearing.. Overlying these aro gypsiferpus clays, approximately 300 feet thick;' these are lake beds. The clays are in turn followed by 300 to 800 feet of rusty bedded gravels consisting of schist and grey* wacke fragments and quartz pebbles. "This group of sediments, has been folded and faulted, and the main fea* ture of the geologic structure of the Manuherikia Valley is the existence of a powerful thrust fault along the western flank of the valley. Along this fault the sediments described .hava been dragged up and overturned, and erosion of their upturned edges liber' ated gold which enriched the placers of the early days of Otago. ."The' main gold-bearing bed is in the lowest formation, the quarts con* glomerates; these are richer right on the schist bottom, but bands of conglomerates carrying gold occur 50 to 100 feet up from the base. From th€ distribution of known gold occurrence! in the Manuherikia Valley and their relation to the geological structure it appears that the gold is concentrated- aa a sheet or bedded deposit, and that these deposits were laid down by rivers which flowed over a reduced land surface and distributed the quartz gravels and concentrated the gold. The flood plains of these ancient miners hay« since been buried under later deposits and deformed by land movements.; "The knowledge' that these deposits are sheet or bedded deposits should have an important economic bearing oa the future of these goldflelds, for wa are justified in assuming," said Mr. MacphersoMj "that pay streaks in tha quartz conglomerates, although having limits, will be continuous over considerable areas. This will simplify prospect* ing and mining methods.. .■•".'■' "The Otago goldfields are npt worked out; in fact, if the main deductions set forth in this discussion are correct they, are about to enter a new phase. Problems of prospecting and mining must be overcome, and the latest tool at the disposal of the miner, geophysical pro* specting, will play an important part. The seismic method in particular will be extremely valuable, for by this method the schist bottom .can be contoured, and as this bototin is cloje below the main gold-bearing horizon, ValuaWo deductions can be drawn, which will aid in solving prospecting, drainage, and mining problems.., "Little is known about the distribution of values through the quartz conglomerates; very poor and extremely rich areas have been discovered, but these two classes of values cannot alone be present; there, must _be .gradations between. \ '■'"' ' "'■'. ' *' There are several hundred, squaro miles of in Otago that . aro underlain by these gold-bearing beds; the pioneer miners prospected .and worked along the faulted edges; the next phase should be systematic prospecting and mining."