On Rainbow Mountain

About 20 kilometres south of Rotorua, heading towards Taupo, Rainbow Mountain suddenly looms in front of you, guarding the intersection of State Highway 5 and State Higway 38; its northern slopes scarred with sheer walls and escarpments of pink tinged andesite that contrast against the dense green of the native bush cover.

Reflecting its volcanic origins, Rainbow Mountain (or Maungakakaramea, meaning ‘mountain of coloured earth’) still smoulders in places, but the quarry site is a quiet and elevated dormant ridge with extensive ‘smoko’ views north to Mt Tarawera and part of its lake, and south all the way to the mountains at the southern end of lake Taupo.

“It’s a bloody beautiful spot when the machinery is turned off and you are taking a breather,” says quarry operator Clark McRobbie who has worked numerous stone sites as a contractor.

In contrast, the opening ceremony couldn’t have been held on a more miserable winter day on the Volcanic Plateau. A freezing rain lashed easterly across the central North Island with wind gusts so angry and strong that they threatened to pick up the marquees erected for the function and carry them off towards Taupo. Elevated at some 700 metres the winter air up on the quarry site is always a few degrees below that down on the plateau, but on that day the wind-borne chill cut through the guests like a butcher’s knife.

Just before Clark McRobbie (pictured right) officially welcomed us, a weak sun broke through the low ceiling of bruised clouds and beamed down – hanging in there until he had finished his speech and we had wolfed down our morning tea and toured the quarry. Then the rain came back with a vengeance and it was slow trip home.

The local iwi, who now own the land through a treaty land deal with the Government that went through in July of this year, had insisted on the site being blessed in their own manner some weeks before our tea and biscuit ceremony, so maybe were looked after by powers more significant than a benign break in the local weather?

Andesite is a volcanic rock (named after the Andes) that is common in all the mountain-building zones around the Pacific Ocean rim. Quarryable resources of andesite on the Volcanic Plateau are few and far between because of the pumice, volcanic ash and mudflows that make up the sandy soil in this region, the result of numerous historic eruptions from the Taupo volcano now lying below the lake.

This rare resource of hard rock, made up of Andesite and Dacite, was originally found by Stancorp Exploration while the contractor was constructing roads through the surrounding Kaingaroa pine forest, owned by Timberlands at the time. Stancorp did the resource test drilling and set up the initial stage of the quarry before selling it to the Central Group in 2006 along with its Okareka quarry. Central Quarries operates six quarries in the North Island, including Stonyridge on Waiheke Island

Central had the quarry stripped of its thin layer of overburden and then approached McRobbie Dowling to take on the quarry sub-licence. Central’s chief executive Noel Horan says the company did not have the funds or the equipment to maximise the development of the quarry with its very hard stone.

“We had already enjoyed a good relationship with McRobbie Dowling through their blasting expertise. We also knew they were very good at getting mobile operations running efficiently and products up to specification very quickly. We are now getting great results from rock tests and the resource is improving all the time.”

Clark McRobbie honed his quarry skills after taking over a company originally set up by his father and uncle in the Pokeno region. He set up a new company with Hamish Owen and Bob Dowling, two old friends with strong family connections. In 2002 their company McRobbie Dowling won a Carter Holt forest roading programme contract from Welsford to the Far North. With a staff of 10, McRobbie Dowling had the task of scratching around the region for a stone resource close to each forest to make the gravel access roads for log and heavy equipment haulage. 

“We operated 20 quarries over this period, producing 80,000 to 100,000 cubic metres of different material a year for the Northland forests,” says McRobbie.

“We got to understand a wide range of blasting techniques and got a good grounding in quarrying that has lead us to Rainbow Mountain.”

Since 2006, McRobbie Dowling has concentrated on the Pokeno Quarry on the southern slopes of the Bombay Hills (featured in Q&M December/Januray 2009 issue), and worked on blasting contracts with Central Quarries. The company now has the contract on Rainbow Mountain Quarry, 25 kilometres south of Rotorua and a new contract with Central to operate its Waipu quarry. Sub-licence operations at Rainbow started in February this year and by the time of the opening ceremony in June, McRobbie Dowling had already extracted over 60,000 tonnes of high quality sub-base and basecourse products as well as Gabion rock and large rocks.

To keep a number of quarries going at maximum capacity, the company uses mobile equipment that moves from one quarry to another. The nature of the resource at Rainbow Mountain means blasting results in some sizable and very hard rocks that have to be broken up with a hammer. A large Komatsu BR 380 jaw crusher is then used to build up a surge pile of between 10,000 and 15,000 tonnes and is moved on to the next quarry. A Terex Pegson cone rusher and a South African-made vertical shaft twister is used in combination with a Finlay 883 reclaimer screen to produce the variety of product from the stockpile.

A 79C Cat 40 tonne dumptruck is on site to cart stone to the stockpile or move overburden.

The quarry uses Blast Metrics 3G software, developed in Austria, to plan out and design its rock blasting. McRobbie says the computer system performs much better than eye-ball mark-ups and plotting by hand. “It is very precise, optimises the blast pattern and maximises rock for the explosives used. It’s a great little tool.”

The quarry is producing a range of product from M4 to GAP 65 and 40, and AP 100.

“Rainbow Mountain has plenty of high quality grade rock for use in the region, which has a scarcity of this sort of product,” says McRobbie.

Demand is very promising.

“It is certainly meeting our expectation. The nearest quarry which produces M4 is up near Matamata and Cambridge, or Fulton Hogan’s Tepuke quarry.”

A resource closer to the Rotorua district reduces heavy traffic on the roads, he adds. “Every body is talking of carbon credits and if we can cut the cartage distances down by 100 kilometres then that a huge improvement. The cost saving also means local councils have more to spend on products for their roads.”

Rainbow Mountain is also one of the ‘options’ for supplying Fulton Hogan and the Taupo Link Road project being built over the next five years.

McRobbie doesn’t know how big the total quarry resource is at Rainbow Mountain.

“The area has been drilled and tested and we have set aside some 400 acres for the quarry. Potentially, it is far bigger than that. Nobody really knows. The initial test holes were so far apart.”