The serpentine group of minerals found in serpentine rocks is used as a source of magnesium and decorative stone. ALAN TITCHALL takes a closer look at the only ‘serp’ quarry in the North Island.
The serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate minerals.
Their olive green colour and smooth or scaly appearance is the basis of the name from the Latin serpentinus, meaning ‘serpent rock’. Serpentine comes in many varieties but one thing is certain – animals, including humans, have a fascination with it.
Wairere Serpentine at a glance
- Extracting – 120,000 tonnes of serpentine a year.
- Wheeled machines – 30-tonne articulated Volvo and Terex dump trucks, plus 20 tonne Volvo and Hyundai wheeled loaders.
- Tracked machines – 50 tonne 480 Volvo excavator and a D6 Cat Dozer shared between the two Piopio quarry sites.
- Crushing plant – Universal 36 x 24 jaw crusher.
- Plans for expansion – Not for this site as yet, but Rorisons RMD has expanded over to a different site across a boundary river with a new plant extracting Ag lime.
Mankind has made jewellery and ornamental carvings from the harder (antigorite) serpentine for centuries. Maori carved beautiful objects from local serpentine, which they called tangiwai (tears). In 1965 the State of California designated serpentine (the mineral) as its official ‘state rock’.
When I was photographing a stockpile at the Wairere Serpentine (Serp) quarry in the North Island, a large rabbit stared passively back at me. It was in the middle of the day and in bright sunshine.
“They love to burrow into it,” commented the quarry foreman Jason Phillips. Which was unfortunate for them when the loader got its bucket into the pile.
“In the morning, when we open up, the stockpile is also covered with wild goats. We used to think they were just getting out of the rain, but they are there all weathers.”
This quarry is one of two that Rorisons RMD operates along Aria Road, just south of the little Waikato settlement of Piopio. Wairere Serp is the older quarry and has been extracting a rich deposit of serpentine, a source of magnesium for agriculture and farming across the North Island, since the 1940s.
Lime also used to be extracted at the site but this is now spent and the serpentine is blended to different recipes with lime from the other RMD quarry with a Sandvic QE341 scalper.
The Waikato serpentine is soft and crumbles once exposed to the air. When aerial topdressing took off after WW2, it became a practice to add it to superphosphate, as serpentine’s free-running properties, after being crushed, allow it to be sprayed from aircraft.
It has also been added to other fertilisers and applied to magnesium-deficient soils. By 1993, some 4.2 million tonnes of serpentine had been quarried in New Zealand, with the biggest producers at the time being Greenhills near Bluff and Wairere Serpentine at Piopio.
By this century many accessible deposits had been worked out and the development of granulated, free-running superphosphate saw production fall. At some locations, serpentine was found to contain asbestos fibres and quarries were closed for health reasons.
Investing in the wrong mineral…
A dirty word now, asbestos was mined in this country extensively and used to manufacture pipes and corrugated roofing from the late 1930s until 1987, even though the health risk of inhaling asbestos fibres was exposed back in the 1970s.
Despite this news, several mining companies spent considerable time and effort exploring the potential of the asbestos deposits in South Westland’s Red Hills region, where a large deposit with good-quality fibre had been found.
A mining company tried to blaze a trail with a bulldozer from Arawata to this remote claim. To make a path they had to use explosives in the Monkey Puzzle Gorge, and drilled and blasted enormous boulders. Swamps, forests, and rough ground added to the obstacles.
The motley convoy, which included tractors, sheds on sleds, and other gear, never reached its destination and returned from the 200-kilometre trip without doing a single day’s prospecting.