MINING FOR GOLD

New Zealand Herald, Volume LXXI, Issue 21763, 31 March 1934, Page 13

MINING FOR GOLD

LOW LEVELS AT THAMES AN UNEXPLORED REGION "RICHEST PART OF FIELD" TESTING BY DEEP BORING No. VI. It is a striking fact , that while vast sums of money have been spent in the effort to locate gold in the Thames and Coromandel area there is very little exact information available concerning the nature of the country in the deeper levels, particularly at the southern end of the Thames flat.

There is every prospect that capital will soon be available to work large deposits of ore in those parts of the Thames area which have been worked over in the past. Nevertheless, it is evident that the field here and at Coromandel will not be fully exploited for its potential wealth until tests havs been made, preferably by boring, to indicate whether the undoubtedly heavy cost of penetrating the lower levels with workings would be justified. . The possibilities of this low-lying strata were dealt with at some length by Mr. Colin Fraser in the No. 10 Bulletin which he prepared in 1910 tor the New Zealand Geological Surrey. In speculating then on the future of the Thames field, he said that perhaps in no other quam-reefing centre in the world with a record of upper-level mining paralleling that of Thames had less attempt been made to prove the deeper levels. It seemed certain that only by a well-directed effort in this direction was there a hope of re-establishing the industry on the Thames gold field upon a satisfactory basis. Former Effort to Explore In the early years of the present century an effort was made to explore these depths. The Queen of Beauty shaft at the base of the foothills was sunk to a depth of 1000 ft. and a level was driven • northward in response to the urging of companies located in that direction and against the expressed opinion of the Geological Survey Bulletin No. 10.

Had this exploratory work been pupsued in a southerly direction, it is possible that- Thames would not- hare experienced the slump it is suffering today. Huge sums of money were expended to littla or no effect. Much of this expenditure went in the installation of the Cornish plunger pump, better known as the Thames Big Pump. This pump had considerable capacity, but was expensive to instal and operate. Eventually the 1000 ft.. workings released large quantities of underground water and the venture had to be abandoned, the complete removal of the pump following some years later. Realising the effect that the discovery of gold-bearing country at depth would have on the fortunes qf the district, the Thames Prospecting Association, on the advice of Mr. E. F. Adams, mSn* ing engineer, has secured the area known as Block 27, containing nearly 92 acres of ground, with the object of handing it over to any parties having finance to undertake .its deep level prospecting. As Adams has pointed out, the ground now held really form* the nucleus of an area forming the southerly part of the central .block of the field which has prored beyond the powers of the local investor, whose plant- and methods were not- applied in a manner likely to mine it successfully* Depth of Productive Rock

The trouble lies in the fact that the ground is deeper ihan the mining companies were accustomed to, the covering of unproductive country overlying the country rock encasing i&e pay-ore reefs not having' suffered erosion as elsewhere.

Where the productive rock was exposed at 100 ft. above sea level at the extreme north of the field, says Mr. Adams, mining was immediately on ore. At the southerly end of the field the productive rock is from 600 to 700 ft. below sea level. The lowest mining on Block 27 is 450 ft. below sea level.

The recommendation made to .the Prospecting Association by Mr. Adams is that the area should be prospected by boring. His view is that it belongs to the richest part of the field. A considerable amount of capital would bo required to develop it it depth, but he has advised that it could be explored relatively cheaply by tioring.

There are three main facts to be ascertained. The first is the depth at which the change-over from the over-, lying barren country to gold-bearing rock takes place. Then it has to be determined whether there is a prevalence of hard rock which is detrimental to mining. The depth at which the basement rock is situated is the third point to be cleared up. The IOOOFt. Level This basement rock has always been looked upon as the limit, ot mining at Thames, but may not prove to be so. . Mr. Adams thinks it would probably bo found at the northern end. at the Moanataiari tunnel at 1800 ft. and at the southern . tend very much deeper, following the dip of the strata. He is of opinion that the 1000 ft. level would prove suitable for purposes of development. Following successful piospecting Mr. Adams sees no difficulty in development to 1500 ft. below sea level and probably more. The essentials are a pumping outfit capable of from 600 to 700 galloha a minute working capacity and an efficient ventilation plant, lu the earlier venture the ventilation scheme used was of the crudest description, so that there was a constant menace from carbonic acid gas. The expensive plunger pump would be replaced by highly-efficient electric pumps involving a fraction ot the capital outlay of the Big Pump. One difficulty with the Cornish pump was that its capacity .was well beyond the normal flow from the workings, the idea being that it would be able to cope with a sudden inrush of water when underground streams were released. -

Giant Doing Pigmy's Work It was a veritable giant doing ft pigmv's work. Modern pumps can be installed to deal with die normal flow, while spare units can be available to take up unexpected" rushes of water. It is certain that some financial assistance would be required from the Gov- | eminent to meet the cost of boring, bufc., it is felt that the vast amount of defin- ,i;J ito information that would be acquired j would fully justify the outlay. In spite of the years of activity on the field, J| the only considerable bore ever put down was that sunk by tho Kuranui Caledonian Company near the entrance ; to the Moanataiari tunnel. Of this, it'-• ifi was said by Mr. Colin Kraser'that it gave more real information as to the > nature of the country at various levels, ; * than coulji be -acquired in any other- | feasible ■ manner.