Reading the tree leaves

As much of the easily accessible gold has been found, prospectors and mining companies have had to dig deeper and venture farther into more extreme and remote environments to find economically viable deposits.

Mineral exploration companies spent nearly US$10 billion during 2019 in the search for nonferrous metals, with over 40% of global mineral exploration budgets spent on gold, according to figures from S&P Global Market Intelligence.

Some industry experts believe that “peak gold” will happen soon — or may already have been reached — and so the cost of finding new gold deposits will only increase.

With exploration becoming more challenging, companies need to look for alternative methods in the search for new deposits.

“Most of the low-lying fruit, so to speak, has already been discovered,” says Colin Rose, executive chairman of Marmota (ASX: MEU). “In today’s world, the most prospective ground is where deposits lie beneath the surface, sometimes deep below. But the problem is: how to get to this gold without the expense of having to drill everywhere?”

Marmota, a mining exploration company in southern Australia, turned to biogeochemical prospecting techniques to find gold.

In 2018, the company collected over 300 leaf samples from acacia and senna trees growing on the Goshawk Zone at Marmota’s Aurora Tank tenement and tested them for over 66 elements, including gold.

The Aurora Tank site lies on the Gawler Craton, a highly prospective but largely underexplored area, about 50 km northeast of the now depleted Challenger gold mine. Discovered in 1995, the Challenger mine, 740 km northwest of Adelaide, produced over 1.2 million oz. gold, worth nearly US$2 billion at today’s prices.

“We first found that trees growing around a known deposit contained elevated levels of gold,” explained Aaron Brown, a senior geologist at Marmota who led the tree sampling program. “This ‘proof of concept’ gave us the confidence to sample trees growing in areas with no known mineralization.”

Buoyed by their initial success, Brown then sampled leaves from Senna artemisioides (a species prevalent throughout central Australia) outside the known mineralized zone at Aurora Tank.

Although calcrete sampling — a traditional geochemical surface sampling technique — had returned no detectable levels of mineralization in the area, a sample taken from a senna tree showed anomalous concentrations of gold. After drilling a hole adjacent to the tree, Marmota discovered a previously undetected zone of gold mineralization.

“I find it remarkable and very exciting to have discovered a new vein based entirely on leaves from one senna tree,” says Rose. “Particularly so, since calcrete testing was telling us not to drill there, but our tree sampling was telling us that there was gold.”

The new high-grade zone occurs over a gold-in-calcrete surface low and has returned 5 metres at 27 grams gold per tonne from 38 metres below surface. New extensional drilling around 100 metres to the northeast of the same hole, conducted last December, yielded another 4 metres of 24 grams gold per tonne and remains open.

“Without the tree sampling, I don’t think we would have found the new deposit,” says Brown. “I believe this is the first time that biogeochemical exploration has been successfully used in prospecting for gold in the southern hemisphere.”

Natural prospecting tools

Gold was first detected in hardwood trees as early as 1900. Since then, concentrations of gold have been reported in a wide variety of tree and plant species collected from mineralized areas in Australia, Canada, Russia, and many other countries and regions around the world.

But it wasn’t until the 1940s with the work of Harry Warren, that biogeochemical exploration became a viable prospecting tool.

A University of British Columbia professor and pioneer of biogeochemistry, Warren used Phacelia sericea, a small purple flower, to locate three gold-bearing areas in British Columbia.

“Warren is considered the ‘father of biogeochemistry’,” says prospector and consultant Colin Dunn. “His work changed attitudes about biogeochemical prospecting from skepticism to the belief that it could be a valuable tool for mineral exploration.”

Dunn has spent the last 40 years developing surface biogeochemical sampling protocols and analytical methods that have been used to support exploration projects in the identification of buried minerals.

Although the specific approach adopted differs with geography, terrain, accessibility, and many other factors, there are several commonalities to all biogeochemical surveys.

For example, while tree sampling surveys, Dunn says, “samples should be collected from the same height around the circumference of trees. Trees should be of the same species and age, and samples should be collected at the same time of year.”

The samples are then sent to the laboratory, where they are dried, milled, and dissolved in strong acids before being subjected to chemical analysis.

The emergence of commercially available inductively coupled-mass spectrometry (ICP-MS) in the late 1990s led to a breakthrough in the field, allowing multi-elemental analysis with detection limits as low as hundreds of parts per trillion.

“ICP-MS offered a faster, cheaper, and more sensitive and precise method for conducting multi-elements analysis,” says Dunn, “and has become a powerful tool for defining the surface chemical signatures of mineralization buried beneath the surface.”

The technique is now routinely used to look for the minute traces of gold and other metals present in trees and plant samples at concentrations — even for vegetation growing over mineralized areas — often no higher than a few parts billion.

Industry takes notice

In 2018, Canadian senior companies spent $1.3 billion on mineral exploration, up 19% from the previous year, according to Natural Resources Canada. Around one-third of this expenditure was on deposit appraisal.

Market conditions and commodity prices are key factors that dictate how much companies spend on mineral exploration. However, long-term projections indicate that the cost of finding new deposits will increase.

“A biogeochemical survey of a prospective deposit is a fraction of the cost compared with conventional surveying,” says Brady Clift, a geologist and minerals manager at Geoscience B.C. “And there’s no better place to look than around existing deposits.”

Geoscience BC, a not-for-profit organization based in Vancouver, funds independent research that, among other goals, aims to provide a “clearer understanding of the geology, mineral potential, and geothermal reserves buried in central B.C.”

The $4 million Targeting Resources for Exploration and Knowledge (TREK) project, launched in 2013 by Geoscience BC, aims to assist in the identification of undiscovered deposits around the Blackwater deposit in B.C.’s Northern Interior Plateau. The deposit, discovered in 2012 by New Gold (TSX: NGD; NYSE-AM: NGD), was found to contain mineral reserves of 8.2 million oz. gold and 61 million oz. silver.

Funded by Geoscience BC, Dunn provided advice on a tree-top sampling program over the plateau in 2015 conducted by Wayne Jackaman. Characterized by a thick layer of glacial sediments and lava flows, the area has potential untapped mineral wealth and geothermal resources buried below.

A subsequent project on the same tree-top samples “builds on research that showed possible links between the level of halogens — fluorine, chlorine, bromine, iodine — in spruce needles and the underlying mineralization,” explains Dunn.

Halogens are commonly instrumental in the emplacement of metal deposits and sometimes generate broader haloes of enrichment than the metals themselves.

“All four halogens are particularly enriched in differentiated magmas, and the hydrothermal fluids and volatile compounds derived from them play an important role in the mobilization and transport of metals in ore-forming systems,” says Dunn.

Weathering at the earth’s surface causes the host minerals to breakdown either mechanically or chemically and release halogens into the environment. As they are highly soluble in water, they can be taken-up by trees and other plants growing in the area.

The halogen project, conducted by Dunn and another consultant geologist, Dave Heberlein, could provide mineral exploration companies with additional geochemical data to help identify zones of metal enrichment from deep-seated or covered mineralization.

“What’s really interesting is that they’ve surveyed an area where the bedrock is covered by a thick layer of vegetation,” explains Clift, “so they are trying to ‘see’ through this cover to the underlying rocks where we’d expect to find mineral deposits. The work could provide proof-of-concept for the technique, reducing the risk for companies wishing to use it.”

Although the technique is relatively untested, PJX Resources (TSXV: PJX), an innovative mineral exploration company with properties in the area of Cranbrook and Kimberley, B.C., has embraced the work.

While attending the Minerals South 2019 conference in Cranbrook last November, John Keating, chief executive officer, president and a director at PJX Resources, listened with increasing interest to Dunn as he presented his work to the audience.

“I was fascinated to hear about Colin’s work with halogens, and so, with his guidance, the next month we started a tree sampling program around an area of known surface and buried mineralization,” says Keating.

Keating and his team collected branches from Douglas fir trees within a 1 sq. km grid at the company’s Vine property.

The tree samples are now being tested at the laboratory for over 50 elements and four halogens. The results from the multi-element analysis will then be analyzed for correlations between the halogens and the other elements to see if these correlate with the surface mineralization zone.”

“We should have the results of the analyses within a month or so,” says Keating. “And if the method works, we will use it to look for other zones of mineralization heading west across the property.”

As the demand for metals increases, and the search for new mineral deposits becomes ever more challenging and costly, biogeochemical prospecting could prove to be a valuable addition to the prospector’s toolbox.