From Martha to Favona
Moving from Martha to Favona ore means much more than changing where the ore comes from. The rock in each mine has distinct physical, geochemical and geological features that affect the way the ore is processed.
This means that the mill at Baxter Road, which was originally designed for the life of Martha Mine, was shut down at the end of 2006 to carry out major refurbishments. Over the last two months new sizing screens have been fitted, a new conveyor and magnet have been added, the SAG mill grates have been replaced to produce a finer ore grind and two new cyclones have been installed. These modifications are nearing completion, and the mill is expected to be operating on Favona ore soon.
Concurrent milling trials on Favona ore showed that concentrations of some elements, quite different to levels previously encountered with Martha ore, alter the types of treatment required for water prior to discharge to the river.
In order to better understand the effects of working with Favona ore and to make modifications before the mill starts back in production, the company has undertaken ore sampling and geochemical modelling, is trialling a range of water treatment processes and is assessing water discharge consenting issues. A decision on when to restart the mill is expected soon.
The aerial surveys will take place over the areas as marked on the map. Weather permitting, the flights will be completed by the end of March.
Exploration
The aim of the company’s exploration programme is to discover high grade quartz vein gold deposits suitable for underground mining methods like those used in the Favona operation. In underground mining the deposits are able to be mined with little surface effect. Ore would be transported to the existing mill in Waihi for processing.
Airborne Geophysics Surveys
Two aerial survey methods are being employed by Newmont Waihi Gold during March 2007 – magnetics and electromagnetics (EM).
The surveys are flown at low altitude by helicopter at approximately 80 - 90 metres elevation with a towed load at approximately 50 - 60 metres elevation. The complete survey will cover an area close to 2,000 square kilometres. Coverage will be undertaken along parallel lines spaced 100 - 400 metres apart.
The magnetic and electromagnetic survey systems will alternate depending on logistical issues. The two surveys overlap and in some areas the helicopter will need to fly more than once along the same lines. It will be in each area for up to two days.
While the survey will not take place over built-up areas, it is likely to be noticed since the helicopter will be operating at low altitude as well as towing a load. However, the helicopter being used has a turbine engine and is not expected to be noisy, in contrast to more common piston-engined top-dressing planes that can be intrusive when performing their tasks. The company will make every effort to avoid flying over cowsheds at milking times. Deer farmers in the survey areas have been notified about the activity as a precaution to help minimise effects on stock.
In the magnetics survey a magnetometer 'bird' is towed on a cable measuring 30 metres in length.
Magnetic Survey
Rocks of the earth’s crust contain varying amounts of the magnetic mineral, magnetite. Epithermal gold/silver deposits such as those in the Hauraki-Coromandel area are products of past geothermal activity similar to that seen today around the Rotorua and Taupo areas. Heat from geothermal activity destroys magnetite in the host rocks, therefore subduing their magnetic signature.
This produces subtle variations in the earth’s magnetic field that can be measured with an instrument called a magnetometer.
This very sensitive instrument does not transmit any signal but simply records magnetic data and is portable so able to be used on the ground for detailed surveys of small areas. Alternatively it may be used in the air from helicopters or fixed wing aircraft to cover larger areas and assist in locating epithermal systems. Newmont Waihi Gold is using a magnetometer slung 30 metres beneath a helicopter to carry out the aeromagnetic survey between Waihi, Whangamata and Kopu.
In the EM survey a loop structure resembling a rotary clothesline is towed by a helicopter.
Electromagnetic (EM) Survey
Electromagnetic surveys aim to measure and map the conductivity of rocks within the earth’s crust. Rocks of different types tend to exhibit varied resistance to electromagnetic fields due to their primary composition, permeability and porosity.
Quartz is a very poor conductor of electricity (resistive), while in comparison, clay and sulphides can be good conductors (show low resistivity). In Waihi quartz ore has a distinctive resistivity character within the volcanic host rock. Airborne EM surveys are able to identify other areas that show a similar resistivity character.
The targeted material may be masked by covers such as barren volcanic rocks or sedimentary material and could be as deep as 200 metres below the surface.
The Newmont Waihi Gold freephone is attended 24 hours a day, seven days a week.
0800 NEWMONT
(0800 639 6668)
Factors such as terrain and groundwater conditions can have an effect on the accuracy of the information collected in the EM survey and a great deal of skill must be applied in interpreting the results.
Newmont Waihi Gold’s EM survey involves towing a large transmitting (Tx) and receiving (Rx) loop structure about 30 metres below a helicopter. The loop structure is similar in appearance to the frame of a rotary clothesline. The Tx loop is energised for a period of five milliseconds (msecs) every 20 msecs. During the ‘off time’ the signals generated interact with the earth and a return signal is generated, which is recorded by a receiving coil located within the centre of the transmitter loop. The longevity of this signal is used to discern the resistivity character of the earth below the loop structure.
The signals generated from the system operate at such high frequencies and speeds (of light) that the methods pose no health risks on the ground or for the helicopter pilot. Similarly, the system is safe in relation to potential detonation of electronic charges in blasting environments or to sensitive electronics.