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Ohinemuri Regional History Journal 27, September 1983

by J.B. McAra

Account of a talk given to Paeroa Historical Society members at Waihi Mine Lake at 16/2/83.

(Having lived in the vicinity of the lake and worked in that part of the Mine long before the lake came into existence Mr. McAra was able to supply some first hand information regarding its unique history).

As we stood on the smooth grassy slope beside the Waihi Mine Lake on a fine summer morning and looked on its placid beauty with the water-lilies in bloom and the tree ferns and pines reflected along its further shore, it was hard to realise that this beautiful spot was once the scene of intensive mining activity with many men working deep beneath the surface digging and blasting the stubborn rock, the noise of heavy machinery working on all sides on the surface, with the railway ore trains puffing, rattling and whistling back and forth from numbers 4, 6, 7 and the Grand Junction shafts, to the Battery at Waikino and the noise of the winding engines at these shafts clanking and rattling as they hauled and dumped the hundreds of tons of gold-bearing quartz into the hoppers.

There was no lake here when I knew this place but the shallow depression into which No. 1 Shaft collapsed near the far side of the present lake was kept pumped out and discharged into a deep drain leading to Walmsley's Creek [Eastern Stream (Mill Stream) – E]. Immediately in front of where we stood the old formation of the railway line to No. 6 Shaft hoppers was clearly visible owing to the exceptionally low level of the water in the lake in this very dry year, and it was on this line that the trucks of ore were hauled from the No. 6 Shaft hoppers by horse and formed into rakes of 40, 2½ ton trucks for transport by steam locomotive to Waikino. This railway formation slopes fairly steeply down under the water towards No. 6 Shaft at the northern end of the lake due to the gradual and general subsidence of some 16 acres of land where the lake is now.

About 100 metres short of the western shore and about 16 metres above the level of the lake, No. 1 Shaft headframe once stood, so it can be readily seen that the whole aspect of the surrounding country has been greatly changed due to this Subsidence. At one time there were houses along the lake side of Junction Road but as they tilted more and more towards the subsiding area they eventually became uninhabitable and were moved away. As most of the houses had a passageway floored with polished lino one had to be careful in stepping in the front door that one did not find oneself suddenly going out the backdoor on one's bottom. The fall in Junction Road at the corner beyond the lake is also due to subsidence and there was a large crack across the footpath at the corner near the parking area.

In order to comprehend more fully this subsidence which is a rare phenomenon in reef mining it is necessary to understand a little of the structure of the reefs themselves which constitute the ore of the Martha Mine. The Martha reef (the main one) which is on the northerly side has an approximate width of 15 to 30 metres and also a number of branches. It runs N.E. and S.W. that is roughly on a line from No. 6 to No. 7 Shaft and dips S.E. at about 60 degrees from the horizontal. The other reefs are roughly parallel and lie on the S.E. side dipping towards the Martha so that they tend to form a large wedge shaped structure with the bottom of the wedge pointing downwards.

In the area between numbers 2 and 6 Shafts many reefs intersect on each other in the vicinity of the former No. 1 Shaft where they formed a mass of intersecting veins in fairly weak ground so that when they were being excavated the country became so weak that in 1907 the upper section of the Martha reef in this area, from above No. 5 level collapsed onto the footwall, that is the N.W. wall but as the stopes had been kept well filled with surface filling, little damage resulted.

This was the beginning of what became known as "The milking cow" which for many years produced ore of good grade at the rate of round about 200 tons per day. This ore was usually damp and clayey and was diluted with a considerable amount of mullock. A band of gold could often be seen on some of the Wilfley concentrating tables while it was being processed. The usual method of mining this material was by caving, using crosscuts spaced at close intervals vertically and horizontally, driven from sub-levels, some 40 men being employed in this specialised work over an estimated 25 years and well over a million tons of ore is estimated to have been produced (See Gold Mining in Waihi 1878 to 1952).

Care had to be taken to prevent water accumulating on the surface as the ground subsided because an inrush of water could have had fatal consequences; also the surface was surveyed regularly to measure the subsidence and make sure that it corresponded with the volume of ore removed so that no large "hang ups" occurred. To my knowledge only one inrush occurred after heavy rain when large boulders were pushed about 100 metres along a crosscut and about a million gallons ran down the shaft. However this occurred after midnight when there was no one in the area.

The ore was tipped into shaft hoppers at No. 6 Shaft at No. 8 and 9 levels, loaded by gravity into 3 ton skips and hauled to the surface by a large steam winding engine with cylinders approx. 0.75 metre diameter and 1.5 metre stroke. This engine seemed to give a few quick puffs and the loaded skip from the 1,000 ft. level would be on the surface. The driving shaft which carried the approx. 5 metre diameter winding drums was about 0.45 metre diameter.

A brief description of the probable manner in which the gold-bearing reefs at Waihi were formed may be of interest in helping to understand their structure. With most geological processes, if we look round us we can very often observe examples of their different phases and this is the case with these quartz reefs. They have, in all probability, been formed by hydrothermal processes acting along fault lines such as we now see in action at places like Wairakei where steam boreholes have yielded cores of mineralised quartz. In spite of appearances the hard glassy material which we know as quartz is slightly soluble, particularly in superheated circulating water and steam. Fissures or faults were first necessary to provide channels for the hot water to circulate in and next the surrounding rocks had to contain the appropriate minerals even though in extremely dilute form.

At Waihi the Martha reef lies in the main fault and is surrounded by layers of quartz andesite rock estimated to be about 700 metres thick which contain minute quantities of widely dispersed gold and which have, over periods of millions of years, been leached out along with the quartz and other minerals, concentrated and deposited in the fault channels, gradually building up on the sides of the cavities, alternating bands of mineralisation and quartz. The other reefs which dip into the Martha probably fill subsidiary fissures caused by slipping of the adjacent country towards the main fault.

The Lake was formed when pumping stopped on the closure of the Mine and its level fluctuates with the levels in the shafts showing that it is connected with them. As the lake covers one of the former richer zones of the reef system it would disappear in any mining operation which might be undertaken, but in the case of a large open cast mine being worked, it might eventually be replaced by a larger and equally beautiful one in the future perhaps 25 years hence.