Integrated remote sensing and geophysical techniques for shallow base metal deposits (Zn, Pb, Cu) below the gossan zone at Kalabar, Western Aravalli Belt, India

Highlights

• ASTER spectral data analysis has great potential in characterizing gossan zone.

• HERT tomograms show narrow vertical zone of shallow base metal deposit upto 127.9 m.

• GIS analysis of resistivity data used for estimation of the geometry of the ore body.

• Validation of resistivity tomograms with drilling data and geochemical analysis.

Abstract

Occurrences of small base metal deposits are reported along the western margin of Aravalli Belt in India. Surface manifestation of one such deposit is found to occur as gossan zone within the host rock of amphibolite in Kalabar area of Rajasthan State. Assessment of this occurrence through drilling technology is a time-consuming exercise, more expensive and requires huge manpower. In the present study, an attempt has been made to rediscover the mineral deposit in Kalabar area using remote sensing and High-resolution Electrical Resistivity Tomography (HERT) techniques. Spectral data analysis with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor shows the characteristic mineral constituents of the gossan zone. 2D electrical resistivity tomograms helped in identifying a narrow vertical zone with low resistivity but high chargeability value up to a depth of 127.9 m. Geospatial analysis of the resistivity data provides the size of the sub-surface deposits. Several exploratory drillings confirm the existence of such deposit up to a depth of 180 m due to presence of highly conductive metallic ore body (Zn, Pb, Cu). Thus, a unique approach is adopted in mineral exploration technology through integrated analysis of surface to sub-surface information. HERT technique is found to be a cost-effective geophysical survey for identifying the geometry and type of shallow base metal deposits.

Introduction

Rajasthan is the richest state in India and the sole producer of base metal deposits which accounts for 95% of zinc (Zn), 85% of lead (Pb) and 20% of copper (Cu) resources (GSI, 2011). Due to high growth of steel and galvanizing industry, the demand of base metals in India is increasing day by day. In order to reduce the dependency on imports and achieve self-sufficiency in the production of Zn, Pb and Cu metals, it is essential to search for new deposits of economic significance (MECL, 2005; IBM, 2018; GSI, 2018). A large number of deep seated (200–500 m) base metals deposits have been found in Eastern Aravalli Belt of India viz. Rampura-Agucha Pb-Zn deposit (Gandhi et al., 1984; Mishra, 2004; Vishwanath et al., 2006; Morgan, 2012), Zawar Pb-Zn deposit (Mookherjee, 1965; Chauhan, 1984; Roy, 1995; Paradis et al., 2015; Evrard et al., 2018), Rajpura-Dariba Pb-Zn deposit (Nair and Agarwal, 1976; Mishra et al., 2006; Mishra and Deb, 2012), Sindesar-Khurd and Kayad Pb-Zn deposit (HZL, 2018) area. But the occurrence of base metal deposits of smaller dimension also has been reported along the western margin of Aravalli Belt e.g. Kalabar in Pali-Ajmer district (MECL, 2005) and Pipela in Sirohi district of Rajasthan. Signature of base metal deposits (Pb, Zn, Cu) in deep seated condition is generally depicted by the outcrops of gossan zone (red cap rock). Gossans are dark brown to reddish-yellow colored Fe-oxides and hydroxides which can be captured by satellite images. Limonite, hematite, goethite, jarosite are the major minerals found in the gossan zone (Kelly, 1981; Skarpelis and Argyraki, 2009; Ghrefat, 2017). The depth of gossan zone may extend from a few centimeters to hundreds of meters. Satellite based multispectral and hyperspectral data analysis has been done extensively for mineral exploration (Rowan et al., 2005; Azizi et al., 2010; Bhadra et al., 2013; Alimohammadi et al., 2015; Salem et al., 2016; Pour et al., 2017; Bhadra et al., 2019). Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data is generally used for detecting iron oxides in VNIR band (0.4–1.0 μm), hydroxide-bearing clay and carbonate minerals in SWIR band (0.9–2.5 μm) and silica in TIR band (8–14 μm) in different parts of the continents (Crosta et al., 2003; Crowley et al., 1989; Rowan and Mars, 2003; Zhang and Pazner, 2007; Liu et al., 2011; Bhadra et al., 2013). So far, no attempt has been made in Kalabar study area to characterize the gossan zones using multispectral or hyperspectral satellite images.

Integrated geological and geophysical surveys like electrical, electromagnetic, gravity, aeromagnetic, magneto-telluric etc. have been carried out for exploration of sulphide mineral deposits (Alilou et al., 2014; Chen et al., 2010; Pierwoła, 2015; Willocks and Moore, 1999; Gurin et al., 2015; Biswas, 2017). Electrical resistivity and Induced Polarization (IP) are the two important geophysical techniques used for locating massive and disseminated sulphide deposits in different parts of the world (Gole and Woodhouse, 2000; Saein et al., 2012; Alilou et al., 2014; Pierwoła, 2015; Evrard et al., 2018; Heritiana et al., 2019). In Rajpura-Dariba Belt of Rajasthan, geological and airborne geophysical data have been studied for prognosticating new areas for the exploration of lead and zinc (Chauhan, 1977; Ametha and Sharma, 2008; Yadav and Avadich, 2010; Yadav, 2015). Different geophysical investigations (SP, magnetic, IP, Resistivity) have been applied successfully in other base metal deposits in Rajasthan (Naskar et al., 2018; Rani et al., 2019; Rao et al., 2019).

In the past, Geological Survey of India (GSI), Mineral Exploration Corporation Ltd. (MECL) and Hindustan Zinc Ltd. (HZL) made several attempts for reserve estimation of base metal deposits in Kalabar area in Pali-Ajmer districts of Rajasthan (MECL, 2005). However, no mining activity could have been established due to lack of proper assessment of the sub-surface investigations. Traditional field mapping and core drilling are cumbersome and laborious exercise for targeting sub-surface deposit. To augment the exploration method, spaceborne satellite data analysis coupled with High-resolution Electrical Resistivity Tomography (HERT) survey techniques have been applied in the study area. Detailed study of gossan zone characterization using ASTER spectra, 2D resistivity tomogram generation across the gossan zone, GIS analysis for area estimation of mineralized zone and the correlation of resistivity and induced polarization (IP) layers with the existing drilling datasets are the major highlights of this research work.