Q&A: Snow, soil and trees – the keys to identifying mineral deposits?

Materials World magazine
,
1 Feb 2018

Bruce Madu, VP of Minerals and Mining at Geoscience BC, talks to Gary Peters about developing techniques that use snow, soil and trees to identify buried mineral deposits.

Can you tell me about your background and career to date?

Most of my career has been spent in mineral exploration and land management, with recent positions as regional geologist with the British Columbia Government and Director of the BC Geological Survey in Vancouver, British Columbia, Canada.

What about Geoscience BC and its work?

Geoscience BC is an independent, non-profit organisation in British Columbia that generates earth science in collaboration with Indigenous groups, communities, governments, academia and the resource sector. Projects are primarily focused toward the mineral and energy sector.

Ideas come from a variety of sources – some are generated internally by volunteer advisory committees and/or staff, while there are open calls for proposals that address specific challenges. We work with communities that have questions that we may be able to answer.

Your research project has developed techniques that use snow, soil and trees to identify buried mineral deposits – how did this come about?

It was received in response to a Geoscience BC call for proposals to generate an exploration method or technique that enhances exploration. We chose to provide funding alongside a large amount of in-kind funding from others attracted to the proposal. 

This project involved Geoscience BC as the funder and our project leaders – Dave Heberlein, a consulting exploration geochemist based in Vancouver, Dr Colin Dunn, previously Head of Geochemistry Research Section at the Geological Survey of Canada, and Sarah Rice, Senior Geochemist for the Geochemistry Division of ALS.

How does it work?

Mineral explorers commonly test for more abundant pathfinder elements in the natural environment to gain clues as to where concentrations of economic elements might be. Often, this follows established relationships, for example a relatively rare concentration of gold is often associated with a broadly dispersed, elevated level of a more commonly found element like mercury or arsenic.

Halogen elements, such as fluorine, chlorine, bromine and iodine, are also candidate pathfinder elements since they are common constituents of most rock types. Where they are concentrated, and derive gases, they play an important role in the mobilisation and transport of metals in ore-forming systems. 

When ore-forming systems are subjected to near-surface decomposition, the halogens can be released as volatile vapours – bromine and iodine – and/or more stable compounds or water-soluble ions (fluorine and chlorine) that disperse to form detectable anomalies in the surficial environment.  

This project evaluated several different media and techniques to detect halogen elements above two known mineral deposits. One was to capture fluids given off from trees by trapping tree sweat in plastic bags. Another was to collect halogen elements emanating from soils using buried collectors left in the ground over several months that passively absorb ions. Snow was also sampled because it can intercept and hold halogen elements. Soils and tree foliage were sampled directly.

Each sample type was prepared and analysed – no small undertaking when dealing with multiple sources such as water, soils, bark, carbon/resin collectors and snow. 

What are the benefits?

The minerals sector is continuously innovating to make exploration more efficient, while facing the reality that discoveries are lower grade and more deeply buried than ever before. Research projects that better detect buried deposits using simplified techniques should make the sector’s challenge of finding new deposits easier.  

The techniques we used can be conducted with a negligible effect on the environment, supporting ultra-low impact exploration. In addition, the protocols for collecting the various analytical summaries for each type are well described, meaning that explorers may conduct their own investigations on the more promising techniques.

What were the challenges involved in this?

Since much of this research evaluated original concepts, many challenges needed to be overcome, including designing passive ion collectors suitable for several months’ burial, fixing plastic bags to tree branches to collect fluids and determining the best depth at which snow should be sampled.  

In the laboratory, halogen elements are relatively difficult to analyse, as there is a lack of commercially available halogen analytical packages for use, few case histories exist and costs are currently high. 

How did you overcome them?

We developed robust sampling protocols that prevented contamination, preserved sample integrity, and led to accurate and precise analysis. ALS, whose research developed new procedures and techniques needed to measure low-level halogen element concentrations in our samples, provided significant support. 

Samples were collected on Vancouver Island at the Mount Washington prospect and from the Lara zinc-copper-lead-silver-gold showing – why did you choose these areas?

The sites are established field laboratories and they have been subjects of previous research programmes for both traditional and new geochemical exploration techniques. 

In addition, they produced data for direct comparison to our programme results. Each site represents a unique deposit type and since the Lara deposit has never been exploited and remains uncovered, it was an ideal test site for detecting buried mineralisation.

How effective was your testing?

Like most research, the effectiveness of testing for halogens and volatile compounds over these two deposits produced successes and inconclusive results. 

We found that fluids transpired from mountain hemlock needles produced a convincing dataset for reflecting mineralisation at Mount Washington, while at Lara traditional soil horizons outlined the surface expression of the buried deposit.  

For many of the different sample types, the results of this form precedent – if soil samples show no increase over the mineralised body, that medium might not be an effective choice for future programmes.

What are the current limitations of this technique?

The overall intent is to define the limitations for sampling this group of elements and compounds. Field sampling techniques used materials that are generally available to most explorers such as plastic, paper and polypropylene bags and snow sampling tubes, so that they can conduct their own research. Some materials and timeframes, including those required in the buried passive ion collectors, need more preparation and commitment compared to traditional geochemical sampling techniques. 

Limitations still exist in the laboratory, as some halogen elements are more difficult to analyse, commercially available analytical packages for halogen are not widely accessible, and costs are consequently still high. 

Another limitation is the small number of case histories available, from which comparative conclusions can be made.

What’s the next step for the research?

Further advancement of the analytical methods is required if halogen elements are to become widely used by the mineral exploration sector. The sites in this study are located in a particular bio-geoclimatic zone and more work is needed to test the techniques in other parts of British Columbia that have different characteristics, such as colder and dryer conditions. 

Is this research the first of its kind?

There are other published case histories showing that halogen elements and volatile compounds provide effective responses and delineation of zones of concealed mineralisation. There has been a limited amount of this type of research in British Columbia, however, where particular challenges are associated with extensive surficial materials obscuring the underlying mineral potential.