Near Infrared Measurement Options for Today’s Exploration Geologist
Near infrared spectroscopy (NIR) is a proven analytical technique that measures light reflectance in the visible and near infrared region of the spectrum (350 to 2500 nanometers) to identify and differentiate various minerals. While not all minerals are discernable with NIR, many of these minerals that are identifiable with NIR are often difficult to identify visually or distinguish with other analytical methods. These are often important deposit vector minerals and are critical to identify for the exploration process.
NIR mineral analysis has been a valuable tool for exploration geologists in recent decades, particularly for those geologists investigating hydrothermal systems. NIR’s ability to quickly and easily delineate hard-to-identify mineral species like clays, in particular, as well as chlorites, carbonates and iron oxides can greatly assist the exploration geologist by providing quick and accurate mineral results so the geologist can vector faster to the ore body. This, of course, speeds up the investigation, lowers exploration costs and helps build a better overall deposit model. Additionally, since the power source of this analytical technique is a halogen light, NIR requires no safety protocols and the instruments are safe for the layman to operate.
Twenty years ago, miners that wanted to perform NIR mineral analysis had to purchase an instrument and perform the spectral interpretation themselves. Today, modern geologists have a number of options that can make this process quicker and much more efficient. The rest of this blog will discuss NIR options for today’s geologists.
Purchasing a NIR spectrometer for an exploration project or mine site fully takes advantage of the strengths of this analytical technique since these instruments are safe, easy to use and can provide real-time or same day results so decisions can be made on the fly.
- out-of-the-box ready for geologists,
- inexperienced employees can be easily trained to operate the spectrometer,
- collect quality spectral results and download results with a half-day of training.
Every analytical technique has its quirks, so the author would recommend more detailed NIR training to take full advantage of the instrument and the data it provides.
While there are many companies that manufacture NIR spectrometers, only two companies market their spectrometers specifically to mineral industries by providing full range (350-2500 nanometers), field-worthy instruments designed specifically for mining: Analytical Spectral Devices Inc - now MalvernPanalytical (ASD) and Spectral Evolution. Both companies manufacture and ship their instruments from the USA. A popular first generation field spectrometer, the PIMA, is no longer being manufactured.
- ASD has been manufacturing the TerraSpec ® brand of field spectrometers since 2004. the most widely used spectrometer in the mineral industry.
- Two types of TerraSpec:
- a portable benchtop spectrometer, the TerraSpec 4
- a handheld spectrometer, the TerraSpec Halo. Since it’s release in 2013, the TerraSpec Halo has become the most popular ASD mineral spectrometer due to its portability and ease of use.
- Two types of TerraSpec:
- Spectral Evolution manufactures the oreXpressTM field spectrometer. In form factor, the oreXpress is comparable to the TerraSpec4 with better accessories to make it easier to manage in the field. While Spectral Evolution does not have a handheld spectrometer, Spectral Evolution did release an “Ultra High Resolution” spectrometer, the SR-6500, in late 2016 that may provide added mineral analysis capabilities and “identify minerals in trace amounts.”
If capital is not available to purchase an instrument or not appropriate for the size of project, a number of laboratories offer NIR mineral interpretation services. Usually, this involves sending core or chip samples to the laboratory for analysis. Major laboratories like ALS and Bureau Veritas offer a range of NIR services, as well as some smaller laboratories. If a miner is already sending samples to the laboratory for XRD or other analysis and doesn’t need real-time results, it may make economic sense to have the lab scan and interpret the mineral spectra for the miner as part of a larger analytical project.
If a miner has historic or recent NIR mineral spectra but has no way to interpret the spectra to get a mineral result, there are now more cost effective options than there were in the past. Two software programs that can automatically interpret NIR spectra:
- The Spectral Geologist (TSG): Historically, the most widely used mineral interpretation software, TSG now has daily, weekly and monthly pricing options to make spectral interpretation more cost effective. (http://www.thespectralgeologist.com)
- aiSIRIS: A new program that is rapidly gaining in popularity, aiSIRIS, also offers similar pricing on a per spectrum basis. (http://www.ausspec.com).
Core imaging involves shipping core boxes (or chips) to a facility for spectral analysis of the entire core unless an onsite imager can be arranged. Generally, other analytical techniques, such as thermal or long wave infrared, can be added to the NIR scanning to give the customer a full mineralogy image of the core. As the size of these imaging units is quite large, they generate a lot of data and may require a few people to operate. These units are generally located at a regional facility where a price per meter or overall project cost is negotiated. Larger sites can arrange for an onsite unit where a turnkey service is typically provided. For the geologist interested in core imaging, there are a number of options:
- HyLoggingTM System: With initial trials back in 2002, the HyLogger has evolved to an automatic, robotic sampling system that can continuously analyze drill core, chips and pulps. Output includes digital images, surface profiles and dominant mineral interpretations. Originally developed by CSIRO scientists in Australia, it has been adopted by private laboratories, all Australian state and territory geologic surveys as well as the Mexican Geologic Survey. In 2017, the HyLoggingTM System was licensed to Australian company CoreScan, who will continue development of the HyLogger with a next generation product scheduled for release in 2018. For more information: http://www.corescan.com.au
- Core Imaging Solutions: More detailed analysis of the core including complete mineralogy and textural variations can be determined by using core imaging solutions. These solutions use hyperspectral cameras to image the entire core providing detailed, complex mineralogy that generates significant amounts of data. With appropriate conditions, quantitative mineralogy can be determined to produce powerful 3D core logs and downhole mapping of mineral assemblage changes. Because of complexities, these highly detailed solutions are best provided as a service to interested miners. Options include:
- CoreScan: In addition to HyLogging, CoreScan offers complete core imaging solutions and provides a wide variety of services and turnkey options. http://www.corescan.com.au
- TerraCore: TerraCore offers a range of imaging options from onsite to regional-based facilities. ALS also offers TerraCore services. http://terracoregeo.com
Near infrared spectroscopy is a powerful tool for the exploration geologist. Ultimately, deciding what NIR technique to use will be determined by use case, exploration process and budget. For a successful project, it’s possible that one or all of the NIR techniques described may be used during the exploration process.
For example, early stage exploration projects may want a spectrometer on site so quick decisions can be made to expedite the exploration process and answer short-term questions. In addition, scanning core or chips with an instrument is a “hands on” technique that forces site geologists to learn more about the core, a good practice particularly early in the exploration process. Once a miner is closer to the ore body, more analyses may be needed in which NIR could be combined with other analytical techniques when sent to a laboratory. As a project becomes more promising, more detailed mineralogy may be required to further define mineralization, at which point selected cores may be sent for imagery. Larger projects may arrange for an onsite imager. Ultimately, which NIR technique a miner selects will be based on the individual needs of the miner at that time. There is a range of options for today’s exploration geologist.