Abstract
Infrared spectroscopy (IRS) is a method used to identify minerals based on their spectral response to infrared light. Spot-based IRS (also known as ‘reflectance spectroscopy’) has frequently been used as an analytical method by the geothermal industry, while infrared imaging spectroscopy (IRIS) has only recently been introduced. Research applying IRIS to the geothermal industry is still limited to academic trials, but sufficient progress has been made in algorithm development and application cases to make this technology ready for uptake by the geothermal industry. In contrast, the mineral exploration industry has embraced IRIS for a number of years and is driving its development forward. In this paper, we review the work that has been done in the geothermal industry with spot-based and imaging IRS, as well as review relevant examples from the mineral exploration realm to look for pathfinders for future uses of the method within the geothermal industry. The review focuses on the application of visible to near infrared (VNIR) and short-wave infrared (SWIR) spectroscopy, as the application of long-wave infrared (LWIR) spectroscopy in the geothermal industry is still very limited. To cater for a wide range of audiences, we will explain the background of infrared spectroscopy as well as the commonly used geothermal index minerals and analytical techniques typically employed by the geothermal industry. Our review shows that IRS has a higher sensitivity in identifying kaolinite (along with its degree of crystallinity), ammonium-bearing minerals, and chemical variations of (spectrally-active) minerals compared to other methods. The ability of IRIS to obtain spectra with high spatial resolution enables the method to identify: (1) less common minerals, and (2) the potential to distinguish smectite and illite formed as interlayered minerals from those formed as two different grains. The latter identification typically cannot be done using spot-based IRS. Other parameters that can only be seen in IRIS include spatial relationships amongst minerals and more robust mineral abundance estimates. The mineral exploration industry has demonstrated that applying IRIS can go beyond just identifying minerals. It can also successfully identify host lithologies on intensely altered rocks, as well as identify and extract the position of veins and veinlets. The latter information is important for geothermal exploration, particularly to indicate permeability and cross-cutting relationships amongst alteration minerals. The IRIS applications that have been demonstrated by the mineral exploration community potentially represent the future trend in the geothermal industry. Spot-based IRS has already proven its added value in assisting geothermal exploration and exploitation. With the latest instruments and algorithm developments in place, imaging IRS is now on the brink of demonstrating its value to the geothermal industry.
Highlights
Hydrothermal alteration (HTA) minerals are an important source of information for characterising geothermal systems
We review the potential of infrared spectroscopy as a geothermal exploration tool by comparing and contrasting it to other commonly used mineralogical methods
We review works that have applied infrared imaging spectroscopy (IRIS) to geothermal exploration (Section 5.1). As this technique is still actively being developed for the geothermal context, we include an example of an ongoing study (Section 5.2) and borrow recent experiences from the mineral exploration industry to get insights into where the developments of IRIS within the geothermal industry could go (Section 5.3)
Summary
Hydrothermal alteration (HTA) minerals are an important source of information for characterising geothermal systems. While the laboratory-scale IRIS studies were research-orientated trials for adapting the technology to the geothermal context, sufficient progress has been made in algorithm development and application cases to make this technology ready for uptake by the geothermal industry This is relevant for volcanic-hosted geothermal systems, as they often underwent similar HTA processes to mineralised systems that are related to the volcanic heat source (i.e., porphyry and epithermal). Guisseau et al, 2007; Liu et al, 2019; Preston et al, 2008) As these analyses used only a small portion of the samples, the research was intended to focus only on particular substances and not the integral characterisation (i.e., mineral identification) of, for example, geothermal drill samples.
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