TECHINICAL FILE
By Grain Wyson Phillip Malunga – FIMMM Mining and Environmental Management Expert
An Economic Window Through Malawi’s Countrywide Airborne Geophysical Survey
Abstract
Airborne geophysical survey provides an effective and cost efficient way of undertaking reconnaissance surveys over large areas. Detailed data is thereafter collected over localised areas. The data improves discovery of underground structures such as archaeological artefacts, extended geological boundaries and is used to target the discovery of mineral deposits. Recent airborne geophysical survey (2013 – 2014) will help improve understanding of the geology of Malawi and speed up discovery of mineral deposits including oil and gas. The paper describes common geophysical methods that are used and how interpretation of hidden treasures is made.
Introduction
Geophysical surveying is a subsurface characterisation of geological features and archaeological artefacts detected by geophysical instruments. Geophysical instruments can detect buried features when their physical properties contrast measurably with their surroundings. Many of these technologies are traditionally used for exploration of economic materials such as metals and hydrocarbons, and for the search of ground water through understanding of geology and geological structures. Geophysical technics are non-destructive, efficient, combine various detections for comprehensive analysis and they are cost effective.
Methods used in geophysics include Magnetics, Gravity, radiometrics, Electromagnetics, Ground penetrating radar (GPR), Resistivity, Seismic refraction (and / or near surface seismic reflection), Spontaneous potential (or “SP”)and Induced polarization (or “IP”).Recent airborne geophysical survey (which was termed “kauniuni”) used magnetics, Gravity and Radiometrics.
Magnetic Method
The magnetic method detects small variations in magnetic mineralogy of minerals such as magnetic iron, pyrrhotite and titanomagnetite. In most cases, total-magnetic field data are acquired. Magnetic anomalies may be related to primary igneous or sedimentary processes that establish the magnetic mineralogy, or they may be related to secondary alteration that either introduces or removes magnetic minerals (B. Hoover et. al, 2004). The method may help to detect Banded Iron Formation. Figure 1 shows magnetic anomaly on Mlindi Structure which is composed of Metapyroxenite, vermiculite and biotitite.
Recent airborne surveys have confirmed the existence of a North-South trending dyke (Mchinji Dyke) crossing Malawi from Mchinji to Chitipa. A portion of it is shown cutting across metagabbro dykes near Mkanda.
Gravity Methods
Gravity measurements define anomalous density within the Earth. Gravity anomalies are computed by subtracting a regional field from the measured field, which result in gravitational anomalies that correlate with source body density variations (B. Hoover et. al, 2004). Deposits of high-density, such as chromite and hematite yield gravity highs while deposits of low-density, such as weathered kimberlite or deep sedimentary formation yield gravity lows.
Radiometric Methods
Scintillometry or gamma-rays are used to identify the presence of the natural radioelements of potassium, uranium, and thorium. Total count and individual radioelements can be measured to define their abundance. Gamma-ray methods are commonly used in uranium exploration because they provide direct detection. Thorium content, like uranium content, tends to increase in felsic rocks, such as in granite centres or alkali rocks. Figure 4 shows radiometric anomaly over Chikangawa nepheline syenite. This is dominated by high Uranium anomaly.
Social Economic Issues
Interesting geological features will be discovered and reaffirmed. These features will reveal new geology and lead to possibility of discovering economic minerals. Further work will be required to carry out ground follow up in order to undertake ground geophysics, geological mapping and exploration drilling to confirm mineralisation. There will be a long lead time to mineral extraction and employment opportunities will be growing to support these activities.
A national consensus and strong legislation on how to undertake the above activities are required. Efficient communication mechanism is required to disseminate information relating to how mineral resource development is undertaken right from ownership of mineral resources, mineral resource rent, distribution and sharing of wealth generated from the mining sector.
Proper communication and management of people’s expectation, including political and civil leadership, will lead to creation of positive signals to investors. Mining investors are not rich but are a special group of investors who take risks mobilising financial and technical resources through share capital and loans.
Until an economic mineral deposit is discovered and developed will this special group be in a position to meaningfully get involved in negotiating community development agreements that bring sustainable development.
References
- Malunga G. W. P. 2014. An Analysis of Mineral Resources of Malawi, ISBN 978 99908 95001, Design Printers Ltd
- Hoover B., Douglas P. Klein and Campbell D. C., 1995, Geophysical methods in exploration and mineral environmental investigations: U.S. Geological Survey Open-file Report, CHAP3, pp 19 – 27.
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This piece was initially published in Malawi’s Mining & Trade Review Issue Number 52 (August 2017).
The full edition is available for download here. This monthly publication is edited by Marcel Chimwala.
