Commercial mineral exploration during the past two decades have increased manifold due to relaxed mining policies and the growing economies of third world countries. Mineral prospectors are constantly on the lookout for newer technologies which can help them identify mineral reserves in a quicker timeframe and at reduced costs. One such technology that is being widely used during reconnaissance and preliminary prospecting is integrated geospatial technology and remote sensing.

Commercial mineral exploration during the past two decades have increased manifold due to relaxed mining policies and the growing economies of third world countries. Since the demand of prospecting and mining has increased, mineral prospectors are constantly on the lookout for newer technologies that can help them identify mineral reserves in a quicker timeframe and at reduced costs. One such technology which is being widely used currently during reconnaissance and preliminary prospecting is integrated geospatial technology and  remote sensing is a significant tool of this technology.

The authors explored the usage of remote sensing technology to identify surface anomalies such as hydrothermal alterations  for mapping  mineral exploration with two main objectives: 1. Mapping and updation of geological attributes (lithology and structure) at regional and local level  2. Recognition of hydrothermal alteration zones that may be associated with mineral deposits.

Remote Sensing and Mineral Anomaly Mapping

Prior to the use of satellite remote sensing, altered rocks were recognized by their appearance in the visible spectral bands or using ground surveys (geological mapping, geochemical and geophysical prospecting). Most mineral deposits are accompanied by hydrothermal alteration of adjacent country rocks. Not all alteration is associated with ore bodies, and not all ore bodies are accompanied by alteration, but the presence of altered rocks is a valuable indicator of possible deposits. Prospectors have long been aware of the association between hydrothermally altered rocks and ore deposits.

Today, advanced technologies like remote sensing and digital image processing have enabled researchers to use additional spectral bands viz., Short Wave Infra Red (SWIR) and Thermal Infra Red (TIR) for mineral exploration studies. In the early 70’s Landsat TM or ETM satellite images were used to identify potential mineral zones. But with the passage of time this was replaced by Advance Spaceborn Thermal Emission and Reflection Radiometer (ASTER), which gave better results in recognizing assemblages of alteration minerals that occur in hydrothermally altered rocks. The authors utilized ASTER satellite image for the geological mapping and identification of hydrothermally altered zones.

ASTER sensors measure reflected and emitted electromagnetic radiation from Earth’s surface and atmosphere in 14 channels (or bands). There are three groups of channels: three recording visible and near infrared radiation (VNIR), at a spatial resolution of 15m; six recording shortwave infrared radiation (SWIR) at a spatial resolution of 30m; and five recording thermal infrared radiation (TIR) at a resolution of 90m. The higher spectral resolution of ASTER especially in the shortwave infrared region of the electromagnetic spectrum makes it possible to identify minerals and mineral groups.

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RMSI Technological Advancement

RMSI has consistently been trying to innovate technologies to reduce field work and evolve techniques that directly give field inputs through advance satellite image studies to save both time and costs. Prior to this, RMSI had worked on many metallic and non metallic minerals to identify surface mineral anomalies,via, gossans, oil microseepages, hydrothermal alteration zones (Sricitisation, chloritisation etc.,) and geomorphic anomalies, which in turn became direct evidences for detailed Geophysical and Geochemcial prospecting.

RMSI conducted this study in the Ramagiri area (15 04 47 North and 77 10 50 East and 14 42 05 North and 77 39 09 East) of Andhra Pradesh, a state in Southern India. ASTER L1B satellite image of leaf off season (Dated – 3rd January 2006) was used to carry out the image based mapping and exploration/targeting. Secondary data used included available geology maps (1: 250,000 Scale) and Geological research records and research papers. The study focused on application of ASTER satellite images for structural mapping and exploration of hydrothermally altered areas for identification of probable mineralized zones.

Various image processing technologies were utilized by RMSI to locate the potential mineral zones. This included satellite image normalization (conversion of reflectance to radiance image) georeferencing, and image enhancement.

The stepwise key processes are explained below:

• Mapping and interpretation of geological features: Lineament, fault fracture, mapping and structural analysis
• Spectral Analysis: Image interpretation using various band combination algorithm and study of its spectral behavior. These spectral curves were matched with the USGS and JPL spectral library available with the ENVI software, diagnostic of each mineral
• Band Ratio Method: The involved applying band ratio and indexing of characteristic signature
• Principal Component Analysis (PCA): Various PCA combinations were used to identify spectral anomaly that could be related to mineralization.
• Integrated Geospatial modeling: The derived outputs are further integrated in GIS environment and analyses were made to find mineral potential zones. Finally by doing weighted based GIS models, Mineral Potential zones were derived.

This diagram illustrates the step-by-step methodology followed in the mineral exploration study.

 

Results

The results of the different image processing technologies are outlined below. These results showed the different types of alteration zones identified that were further validated using several other methods. For instance, presence of Gossan was tested using band ratio, spectral band combination and PCA methods. All these methods resulted in similar results. The structural mapping using image enhancement techniques and visual interpretation are also shown below which further validated the presence of hydrothermal zones.

Figure 1 - Gossan identification

Figure 2 - Alteration zones identification using satellite images.

Figure 3 - Structural mapping.

The integration of these data using GIS models led to the identification of mineralized zones. An area rich with Gossan signature coupled with strong sulphidation signatures, indicated prospects of base metal mineralization. This led to the identification of further area specific results:

• Sulphidation areas identified through this study correlated with shear zones which could be potential zones for base metal and gold mineral in the study area.
• Various hydrothermal alteration zones identified also correlated with mineral association as well as various structural and geomorphic anomalies are potential for base metal and gold mineral bearing zones in this area.

 

Conclusion

It can be concluded that remote sensing and GIS-based mineral prospecting studies not only save exploration time and cost, but also provide information about similar areas. This helps in identifying and planning alternate prospect areas. Also, multi spectral images with different channels in SWIR and TIR like ASTER can be effectively and economically used to identify the fracture pattern and hydrothermal alteration zones. This concept can form the basis of remote sensing based mineral exploration method in vegetative terrain also. Although this method has been tested by various scientists and RMSI, confirmed mineral target areas are further enhanced using hyper spectral image based analyses.

The authors all work in the Agriculture and Natural Resource vertical at RMSI, SubhashBabu Ainampudi is manager, Raushan Kumar is an assistant project manager, and Dr. K.S. Siva Subramanian heads the group.

References
Aleks Kalinowski and Simon Oliver (2004), ASTER Mineral Index Processing Manual, Published by Remote Sensing Application, Geosciences Australia.

Floyd F. Sabins (1999), Remote sensing for mineral exploration, Ore Geology Reviews 14, page – 157–183.

Junek P (2004), Geological Mapping in the Cheleken Peninsula, Turkmenistan Area Using ASTER data, ISPRS.

Nouha Mezned, Saadi Abdeljaoued and Mohamed Rached Boussema, ASTER Multispectral Imagery for Spectral Unmixing based Mine Tailing Cartography in the North of Tunisia.

Rajesh H M (2004), Application of Remote Sensing and GIS in mineral resource mapping – an overview, Journal of Minerogical and Petrological Science, volume – 99, page – 83 – 103.

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