NOT WORKED OUT

Waipa Post, Volume 47, Issue 3359, 26 August 1933, Page 2

NOT WORKED OUT

• ■.——• OTAGO'S GOLD. GEOLOGIST'S OPINION. The gold-bearing deposits in Central Otago were discussed at the last meeting of the geological section ,•? the Wellington Philosophical Society.. when Mr. E. O. Macpherson read a paper on "The Terrestrial Sediments of the 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 the present study had been undertaken with the object of finding out whether these goldfields were actually exhausted, or if geological complication*, or the economic influences existing at the time that mining ceased caused their wane. "The sequence of formations commencing with the lowest consists of schist and argillites (the 'main bottom' of the miners) and overlying these are quartz conglomerates which range in thickne'ss from 10 to 450 feet; these deposits are gold-bearing. Overlying these are gypsiferous 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 greywacke fragments and quartz pebbles. "This group of sediments has been folded and faulted, and the main feature 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 have been dragged up and overturned, and erosion of their upturned edges liberated gold which enriched the placers of the early days of Otago. "The main gold-bearing is in the lowest formation, the quartz conglomerates; these are richer right on the schist bottom, but bands of conglomerates carrying gold occur 50 to 100 feet up from the base. From the distribution of known gold occurrences in the Manuherikia Valley and their relation to the geological structure it appears that the gold is concentrated as 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 have 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 on the future of these goldfields, for we are justified in assuming," said Mr. Macpherson, "that pay streaks in the quartz conglomerates, although having limits, "will be continuous over considerable areas. This will simplify prospecting and mining methods. "The Otago goldfields are not worked out; in fact, if the main deductions set forth in this discussion are correct they are about to enter anew phase. Problems of prospecting and mining must be overcome, and the latest tool at the disposal of the miner, geophysical prospecting, 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 bottom is close below the main gold-bearing horizon, valuable 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 graduations between. "There are several hundred square miles of country in Otago that are 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."