Geophysical & Geochemical Methods for Pegmatite Exploration
Geophysical Exploration: X-Ray Diffractometry
To overcome the challenges surrounding the detection of lithium in-situ, one method that has been tested for detection of lithium in rocks at the surface of these cap zones is x-ray diffractometry (XRD). This well-developed science has been hard to implement in the field of mineral exploration (in the field especially, owing to the size of the machines used to run x-ray diffractometry and spectroscopy testing (Trueman & Cerny, 1982)), but with the downsizing of many of these implements in recent years the portability of this science is becoming more widely exploited. The new developments in this technology are marketed (Olympus Corporation, 2020) and studied under the acronym pXRD, or portable x-ray diffraction, and while the currently commercially available detecting equipment cannot identify lithium itself, it is adept at location and identifying a suite of other Alkali metals and high field strength elements (among which are the Lanthanide series elements cesium and lanthanum (Olympus Corporation, 2020).
Geophysical Exploration: Remote Sensing
Remote sensing has also been problematic in detecting these types of deposits, however, due to the proficiency of this method in detecting silicon and oxygen-high zones within rocks (due to hydrothermal interaction or alteration in most cases), I would opt for its use in determining the overall deposit extent as well as the size of the alteration zones in the pegmatite itself. This information would be a valuable supplement to any developer or mineral explorer, as it would give insight into the ease at which the deposit could be extracted, and perhaps even processed. Furthermore, it has been shown recently in several case studies in Northwest China and exploration ventures across the globe that by utilizing a sampling program assaying mineralized rock samples for reflectance spectra one can view the deposit area as a much more defined body. In the aforementioned geophysical and geochemical studies in exploration of LCT pegmatites in Dahongliutan the exploration strategy developed for locating spodumene-bearing pegmatites was anchored by the identification of pegmatite veins on high-resolution hyperspectral remote sensing images and anomalies related to pegmatite-type Li deposits using multispectral remote sensing images. The meter-wide Li-bearing pegmatites in the study area are associated with rare metal geochemical anomalies, as determined from the geochemistry of rock samples from trenches and diamond-drillhole core (Yongbao, et al., 2020). Using spodumene’s reflectance criteria, the remote sensing images can then be enhanced to clearly show said mineralized regions, making it easier to identify the presence of pegmatite clusters through different band combinations and principal component transformations when analyzing images returned from satellites (which are principally used in the collection of remote sensing data). The additional clarity and ease in both determining a pegmatite’s rare earth metal content as well as its ability to identify ‘fertile’ granite (those granites with elevated levels of Rb, Cs, Sn & Ta).
Geochemical Exploration
To further constrain the deposit and the areas of alteration around a possible pegmatitic orebody, a geochemical sampling program would be necessary to develop deposits of this like. The general characteristics of lithium and its common hardrock mineral parent spodumene (and within the LiO2, lithia, that is benefacted for market sale) are such that the detection of lithium elementally as a means of gaining insight into a deposit is almost impossible. However, one way that a geochemistry program could be implemented is by looking at the associated light rare earth minerals such as cesium and tantalum that sometimes occur with pegmatitic lithium deposits (LCTs). To do this, a simple, yet cost efficient method would be surface sampling both of the hanging wall and footwall areas adjacent to a pegmatite of interest. The area to be sampled could be catered to fit the strike and dip of the prevailing pegmatite arm/dyke and would need to capture a certain correspondent area of data for further analysis via inductively couple mass spectrometry (ICMS) (Aylmore, 2018). This methodology has been proven both inexpensive (relative to other well known geochemical testing methods) and easily completed by mineral exploration professionals and geochemists in Western Australia’s pegmatite fields to determine major elements (Ca, Fe, K, Al, Mg, Mn, P, S, Si, Ti, and Ba), and then analytically reworking the results to derive possible lithia content within a sample of spodumene or lepidolite, etc. (Aylmore, 2018). While the overall benefit of this geochemical exploration program would be minimal in progressing a potential orebody to economically viable status, it would further enable geologists and geochemists (in conjunction with metallurgical insight from professionals of that field) to determine the ‘mine-ability’ of such a prospect. This information would indeed be a valuable insight for any potential developer of such a pegmatite.
Sources Cited:
Aylmore, M. (2018). Assessment of Lithium Pegmatite Ore Bodies to Determine Their Amenability to Processing for the Extraction of Lithium. Extraction.
Bradley, D., & McCauley, A. (2013). A Preliminary Deposit Model for Lithium-Cesium-Tantalum (LCT) Pegmatities. Washington: USGS.
Johnson, A. (1979). Pegmatite Deposits In The Black Hills Of South Dakota. Littleton: SME.
Munson, G., & Clarke, F. (1955). Mining and Concentrating Spodumene in the Black Hills of South Dakota. Littleton: SME.
Olympus Corporation. (2020). Olympus Portable X-ray Fluorescence (pXRF) and X-ray Diffraction (pXRD) for Lithium Exploration in Lithium-Caesium-Tantalum (LCT) Pegmatite Deposits. Tokyo.
Trueman, D., & Cerny, P. (1982). Exploration for Rare – Element Granitic Pegmatites. Mineralogical Association of Canada, 463-494.
Yongbao, G., Leon, B., Kan, L., Moushun, J., Yuegao, L., & Jiaxin, T. (2020, December 10). Newly Discovered Triassic Lithium Deposits in the Dahongliutan Area, NorthWest China: A Case Study for the Detection of Lithium-Bearing Pegmatite Deposits in Rugged Terrains Using Remote-Sensing Data and Images. Frontiers in Earth Science, Vol. VIII.
