The Development of Field Spectroscopy for Minerals Exploration: Utah, Fieldwork, and Family
by Stephanie E. Mills
Spectroscopy has been an important analytical method in the geosciences over the past century, and in more recent decades has become an essential tool in minerals exploration across the globe. Spectroscopy can help identify key mineral assemblages that point exploration geologists towards different types of mineral deposits. However, to use the method effectively, an exploration geologist must first understand mineral formation, distribution, and characteristics of different alteration and deposit types. One of the mineral industry’s leading researchers in alteration mineralogy and an early adopter of short-wave spectroscopy is geologist Anne Thompson. Using Utah’s unique geological landscape, the mineral alunite (hydroxylated aluminum potassium sulfate, KAl3[SO4]2[OH]6), and with a nanny and baby in tow, Anne spearheaded using spectroscopy in the field for minerals exploration.
What is spectroscopy?
Fine-grained bladed alunite crystals from Blawn Wash, Beaver County. Image courtesy of Ken Krahulec.
Spectral identification of minerals utilizes a non-destructive analytical technique known as reflectance spectroscopy, which has been used by mineralogists since the early 1900s. Reflectance spectroscopy measures the unique absorption spectra (how electromagnetic radiation is absorbed) of different minerals and focuses on the visible and near infrared (VNIR), shortwave infrared (SWIR), and longwave infrared (LWIR) parts of the electromagnetic spectrum. Early uses of spectroscopy were mainly for mineral identification and defining new mineral species, whereas modern uses include everything from paleoecology to asteroid surface mineralogy. The technology to support the application of spectroscopy continues to develop, moving from early handheld field-based instruments to stationary scanners that integrate traditional photography with spectral readings to satellite-based spectral mapping of entire landscapes.
How is spectroscopy used in modern geosciences and minerals exploration?
Spectroscopy is an ideal method for identifying hydrated minerals due to their strong and unique spectral response, so the use in identification and characterization of clays and micas caught the attention of geologists in the minerals industry in the late 1980s and early 1990s. Clays and micas are of particular interest in minerals exploration because they occur in the alteration zones around mineral deposits, which means they often have a systematic spatial distribution around economic areas of an ore deposit. Characterization of spectrally responsive minerals in alteration zones has become a major field of innovation and a standard method in the minerals explorationist’s toolbox.
In the 1980s the development of models for precious metal deposits that occur in the shallow subsurface, known as epithermal deposits, meant that the mineral alunite, which occurs with these deposits, also became an important indicator of mineralization. At the time there were no formal studies on the formation of alunite, even as exploration geologists needed guidelines to help understand how the alunite they identified might be related to potential ore.
How does Utah fit in?
Adaptation of overview map in Thompson (1991) showing the extent of alunite occurrences in the Marysvale-Pioche belt.
Southern Utah hosts a unique range of alunite occurrences, some of which were exploited as a source of aluminum and potash throughout the early to mid-1900s. In fact, Blawn Mountain, the largest known alunite deposit in the United States, is located in the Wah Wah Mountains of Beaver County. The presence of such diverse species of alunite made a perfect natural laboratory for geologist Anne Thompson, who is today a global leader on alteration mineralogy and the application of spectroscopy in minerals exploration. Back in the 1980s, as industry geologists were scrambling to understand how alunite could help them find epithermal deposits, Thompson was a geological researcher in Utah. At the time, the Utah Geological Survey (UGS) gave small research grants funded by Mineral Lease revenue to non-UGS researchers, and Thompson was one such recipient. Her research in the Marysvale-Pioche mineral belt focused on characterizing alunite associated with argillic alteration, the type of clay- and alunite-rich alteration commonly found around epithermal deposits.
Thompson’s alunite research culminated in UGS Miscellaneous Publication (MP) 91-2 and focused on parsing field relationships, mineralogy, and geochemistry of alunite in argillic alteration with respect to gold mineralization. Thompson recognized that at least three different environments of formation for alunite were present in the Marysvale-Pioche mineral belt. Building on the knowledge she had developed in Utah, Thompson began more detailed comparisons of alunite compositions, including comparisons to known epithermal deposits in Chile. Soon after Thompson completed the Utah research, she met Phoebe Hauff at a conference in Denver. Hauff had one of the earliest handheld spectrometers (a PIMA: portable infrared mineral analyzer) and was developing a mineral database that would become the backbone of spectroscopy in mineral exploration for the next three decades.
(A) Overview of the electromagnetic spectrum and (B) example of the spectral response of different minerals (after Ni and others, 2020).
The pair met in a hotel room during the conference to run Thompson’s alunite samples, and Hauff immediately recognized that the differences Thompson had found in the composition of alunite could be recognized using the much simpler and faster spectrometer. They went on to collaborate for many years, continuing to develop a variety of case studies and applications for field spectroscopy in minerals exploration. Not only did Thompson’s work continue through her career and lead to field-defining publications such as the Atlas of Alteration, it propagated through the Utah geology community. Erich Petersen, Professor of Economic Geology at the University of Utah, worked with Thompson during her time in Utah and noted, “Anne introduced us to VNIR and we took off with it…. [her] legacy continues.”
Thompson’s work on argillic alteration in the Marysvale-Pioche belt is impressive on its own. What many people do not realize is that Thompson carried out this extensive field-based research with her infant daughter in tow. Between her husband working full-time and no family support in the area there were few options for childcare– not that Thompson let this be an obstacle. Thompson started fieldwork when her daughter was three months old and continued to do a week or two at a time until her daughter was 18 months old. She had help from her parents who traveled out west from Florida for one field session and then met a young woman at an outdoors store who became a part-time field nanny. Her daughter and the nanny often spent their days nearby in the field while Thompson mapped and sampled. They patched together a variety of field vehicles, from a Subaru Outback family wagon to rental trucks and then finally Thompson bought a second-hand SUV to get them all around. A mountain bike allowed Thompson to expedite some field visits, while the others stayed closer to the vehicle.
Thompson’s work not only highlights the start of a career-spanning step change in the understanding and detection of alteration mineralogy, but also highlights the importance of flexible work arrangements that allow geoscientists in different family and life phases to continue growing and developing their expertise, especially in the early career phase. To celebrate Thompson’s early work through the UGS, a modern format version of her publication MP 91-2 is planned for release later this year.
