Exploring for mineable minerals

Like Sherlock Holmes, exploration geologists are detectives - their business, in this case, to find minerals in concentrations of economic interest. Elementary … literally

The search for mineable gold, silver, copper, iron and other metals depends on clues in the rocks, visible only to the experts. Learning to find these clues, and decipher their meaning takes years of training and experience in the field and the laboratory. That is part of what geology is – the study of the Earth, and the rocks and minerals of which it is formed.

Gold exploration is a prime example of the geologist’s art and science. A quartz vein running through a rock formation may contain a lot of gold, or it may not, and there is no immediate way of telling, if the gold flakes are too small to be seen by the naked eye, as is often the case in New Zealand. The search starts by finding the right geology or rock composition and structure. The roots of old volcanoes, their tops long eroded away, is one good place to start looking for gold. There is plenty of that to be had in New Zealand; in the Coromandel, Northland, and the Central North Island.

The quartz veins found in such volcanic rocks suggest a history of hot, chemically-rich fluids that moved through fissures in the rocks ages ago, escaping a volcanic heat source, cooling and depositing minerals as they went, including metals, over very long periods of time. Gold is one of the first to drop out of solution; other metals such as iron, manganese, lead, zinc and mercury travel much further, before solidifying. Imagine a cloud of quartz veins in rock under the ground, with gold and silver in the core region and successive layers of other mineralisation, further and further out. In this scenario, the presence of mercury sulphides in rocks suggests there may be gold linked to the same geology, somewhere in the vicinity – that could be hundreds of metres in any direction.

The old-timers found gold at the surface, in quartz reefs, or as flakes in river gravels, from gold-bearing rocks washing into rivers and being broken up into smaller pieces, in floods, and landslides. Such hard rock gold, and alluvial gold, respectively, fuelled the gold rushes of the 1850s, in the Coromandel, the West Coast of the South Island, Northwest Nelson, and Otago. Today the easy gold has been largely won; the challenge is to look for gold where it cannot readily be seen or inferred.

This is where the exploration drilling rig comes into its own. Transported by truck or helicopter, this piece of equipment is like that used for sinking a water bore, except that the hole is narrower, usually deeper, and the objective is the recovery of a continuous core of solid rock, sometimes hundreds of metres long. This is laid on the ground in parallel sections, each 2-3 metres long. The experienced geologist can usually spot the host rock of interest, and send it to the laboratory for chemical analysis. A record of the grade is returned, matched to the exact part of the rock core from which it came. In the case of gold, the typical measurement is grams per tonne of surrounding rock. In New Zealand, gold is mined at grades as low as 1 gram per tonne, or less.

A gold-rich intersection in one rock core does not a mine make. More drilling is required, typically in a grid pattern, to allow the geologist to develop a 3D model using computer software of the underground mineralisation. The more drilling, the better the model. In this way, the Correnso ore body, sitting 350 metres below Waihi township was determined, with patience, effort and time. An estimated $2 billion worth of gold, distributed in vein quartz in host volcanic rock, represents an exciting resource. But to improve on the informed guesswork, only mining can reveal the true amount of gold present, and its grade and distribution in 3D space.