Uraniferous calcrete mapping using hyperspectral remote sensing

Abstract

Calcrete, the calcium carbonate duricrust generally occurs in arid/ semiarid region. Globally, it has been used as a non-traditional constructional material. The recent discovery of calcrete hosted secondary uranium deposits in Australia and Namibia has kindled interest in exploration of calcretes in many parts of the world. In this study, palaeochannel related calcretes of the Thar desert, India is explored as a possible source for uranium mineralization. The adopted exploration strategy includes mapping of calcretes using hyperspectral satellite data, in-situ radiometric survey, analyses of field collected samples for mineralogy and geochemistry. Calcretes along palaeochannels were mapped using Hyperion data, field spectra and Spectral Angle Mapper (SAM) technique. It was found that the adopted technique could also differentiate high-, and low-Mg calcretes with 72% accuracy. Mineralogy of calcrete includes calcite (40-60%), quartz (30-45%) and feldspar (10-15%) with minor amount of clay minerals (montmorillonite, kaolinite, illite, goethite). In in-situ radiometric survey, the range of uranium, thorium and potassium in the calcretes varies from 2.4-11ppm, 5-28.2ppm, and 0.8-4.2% respectively.

Spectral pathways for exploration of secondary uranium: An investigation in the desertic tracts of Rajasthan and Gujarat, India

“…The host rocks in the present investigation, the calcretes, have characteristic absorption bands between 2.50-2.55 lm and 2.30-2.35 lm wavelength regions in addition to three other weak absorptions between 2.12-2.16 lm, 1.97-2 lm, and 1.85-1.87 lm (Hunt and Salisbury, 1970;Gaffey et al, 1993;Clark, 1999;Christensen et al, 2000;Gupta, 2003;Van-der-meer, 2004;Lagacherie et al, 2007;Bharti and Ramakrishnan, 2014). The important absorption features for uranium are located in 0.84 lm, 0.86 lm, 1.11 lm, 1.35-1.54 lm, 1.62 lm, 1.73 lm and 2.26 lm wavelength regions (Bates, 1965;Lévesque et al, 2001;Zhang, 2008).…”

“…These characteristic absorption features can be easily identified using image derived endmembers/lab measured spectra and airborne/spaceborne HRS data. Bharti and Ramakrishnan (2014) reported that the uranium concentration in calcretes correlate well with the high Mg-calcite content. In the investigated area, the high Mg-calcites are commonly observed with palaeochannels and playas having sluggish groundwater flow regime.…”

“…Due to molecular vibration, water has overtone absorption features at 1.4 lm and 1.9 lm (Clark, 1999). In this study, the spectral noises caused by water vapor absorptions were corrected from the measured spectra using the procedures of Kusuma et al (2010) and resampled them to Hyperion bandwidth (Bharti and Ramakrishnan, 2014). Field samples were also analyzed for the mineralogy (semi-quantitative) and major oxide chemistry using XRD (PANAlytical-X'Pert PRO) and XRF (PANAlytical-PW2404) respectively.…”

“…Since absorption wavelengths are the key for identification and quantification of a mineral, an accurate wavelength calibration is critical and most important in analyzing the hyperspectral data. This warrants specialized data processing and analysis techniques such as atmospheric correction, dimensionality reduction, identification of pure endmembers and spectral analyses (Hubbard and Crowley, 2005;Kruse, 1996;Kusuma et al, 2012;Bharti and Ramakrishnan, 2014).…”

“…Conventionally, image processing techniques such as band ratio (Crosta and Mc.Moore, 1989;Tangestani and Moore, 2000;Ranjbar et al, 2004), supervised classification (Martínez-Montoya et al, 2010), linear mixture modeling (Bryant, 1996), data fusion with decorrelation stretching (Kavak, 2005) and spectral analysis (Crowley, 1993;Ramakrishnan et al, 2013;Bharti and Ramakrishnan, 2014) are extensively used to target such deposits based on characteristic spectral features. Spectral absorption features (particularly their position and shape) are unique to mineral species and are caused by electronic and vibration processes which reveal their chemistry (Burns, 1970;Hunt and Salisbury, 1970;Goetz et al, 1985).…”