Geophysical surveying used in the search for magnetite and hematite irons ores Research Paper

Summary

This literature review deals with examples of geophysical surveying technique that are used in the exploration of hematite and magnetite iron ores. The examples dealt with in this literature review include gravity methods; which defines the anomalous density in the earth surface.

This method has the capability of predicting total anomalous mass. Another method dealt with is magnetic techniques; it makes use of small changes in magnetic mineralogy in magnetic iron, as well as iron-titanium oxide minerals, such as magnetite.

The technique has the capability of detecting iron ore deposits. With this capability, the method uses useful deducing sub-surface lithology as well as structure. Another method is seismic techniques, which due to high costs; it has not been fully utilized.

Thermal method is also a technique that has been applied in the exploration of hematite and magnetite. Last but not least, this review looks at electric method, which has several techniques. However, for the purpose of these assignments, the paper deals with direct current resistivity method and electromagnetic technique. The review concludes that, the method to be applied depends on the ore physical and chemical properties, along with environmental issues and costs incurred.

Introduction

There are many examples of geophysical surveying techniques used in search for magnetic and hematite irons ores. Such methods have been divided into these that make use of earth’s natural fields, and these that need ground artificially generated energy.

Natural field methods make use of gravitational, magnetic, electrical, as well as electromagnetic fields of the earth. On the other hand, artificial source methods, get use of generation of local electrical or even electromagnetic that are analogously used in natural fields, or in most important single group of geophysical surveying techniques.

Studies have shown that, natural field methods provides data based on earth properties to a significantly greater depth, on top of being more simple when carrying out, as compared to artificial source methods. On the other hand, artificial methods have the ability of producing a more detailed a better resolved pictures of the subsurface geology as compared to natural methods.¹

There are many geophysical surveying methods which can either be used in air or in the sea. However, higher expenses associated with airborne or marine operations, works as an offset by the increased speed of operations as well as the advantage of having the capability of surveying fields that ground access is either difficult or too hard.

Studies have indicated that, the physical property of an ore, clearly determines the range of surveying technique applicability. Other determinants include speed of operations, as well as environmental considerations among other reasons².

Geophysical Surveying

Gravity method

The measurement of gravity defines the anomalous density within the earth surface. More often than not, application of ground based gravity meters are used to help measure differences in gravity fields. After this, irregularities of gravity are calculated this is done normally by taking away from field of gravity values the regional fields.

It is worth noting that a positive result shows that the density body is shallow and a lower gravity depicts shallow density body. As a result, deposits of high density like hematite yield gravity highs.

This method on the other hand, enables a prediction of total anomalous mass, which has been linked to anomaly. This gravity technique, sense only contrasts that are lateral in density or magnetization respectively.

The use of gravity method is dictated environmental factors for instance presence of lithologies. It is worth remembering that such bodies do have tiny anomalous and when one try to work underground, it is a challenge to trace them when using survey. It is only possible if the distance is shallow.

Magnetic Techniques

This method exploits small changes in magnetic mineralogy in magnetic iron, as well as iron-titanium oxide minerals, such as magnetite. There measurements are made by the use of fluxgate, proton-procession, optical absorption magnetometers, as well as overhauser.

Mostly, total magnetic field information is needed; vector measurements are in some instances made. It has been established that rocks having magnetic characteristics that can disturb the primary fields of the planet earth.

The intensity of the remanent as well as the magnetization heavily depends on quality as well as constituent of mineral grain. The irregularities in magnetism are closely associated with igneous as well as sedimentary process used mainly to establish minerals³.

It is worth noting that it is of significance to factor in geo-environmental as well as mineral exploration. These are deemed to be of a secondary impact on rocks due to the hydrothermal mechanisms. As a result, magnetic surveys have the ability of outlining fosil hydrothermal activity zones. This is based on the fact that, rock alteration has the ability of effecting changes in bulk densities as well as magnetization directions⁴.

This method has the ability of detecting some ore deposits like magnetite or iron formation. Use of geo-environment encompasses identify of magnetic minerals correlated with that are capable of releasing harmful products. “Such associations have been allowing indirect identification of hazardous materials such as those found in lots of nickel-copper, or even in serpentine-hosted asbestos deposits”⁵.

Seismic Methods

These techniques have had not been fully utilized as a result of their high costs, as well as the difficulties encountered during seismic data acquiring as well as interpretations, in strongly faulted as well as altered igneous terrenes, in the assessment of minerals along with the exploration at the scale of deposit.

Though the entire method has been said to be expensive, shallow seismic haven proved to be inexpensive sources and smaller surveys as compared to typical regional surveys. This has been due to the fact that, the cost of studying certain geo-environmental problems in the near subsurface may not be high-priced. The advantage of reflection seismic is that, it provides fine structural details⁶. While refraction technique, provides estimates of depth to lithologies of differing acoustic impedance precisely.

Refraction method has been applied in the investigation of minerals with the aim of mapping low-velocity alluvial deposits like those containing hematite. Applications in geo-environmental work include; the study of structure, thickness, as well as hydrology of tailing, along with the e4xtent of acid mine drainage around mineral deposits.

Thermal Methods

There are two different methods under thermal techniques, namely; borehole also known as shallow probe technique. This technique is used in thermal gradient measurement, which in itself is much useful, as having thermal conductivity knowledge provides a measure of heat flow.

The second thermal method technique is airborne, also referred to as satellite-based measurements. This has been used in the in the determination of Earth’s surface temperature, as well as thermal inertia of surficial materials of thermal infrared radiations that the Earth surface radiates⁷. Some of the thermal noises include variations in thermal conductivity, topography, as well as intrinsic endothermic along with exothermic sources.

Borehole thermal techniques have been used in geothermal exploration; however, have rarely been used in the exploration of minerals. On the other hand, have some potential advantages in the exploration and in the geo-environmental investigations. The “causes of heat flux anomalies include oxidizing sulfide minerals and high radioelement concentrations”⁸.

Some of the conditions that concentrate, or disperse, the heat flow occur due to hydrologic along with the topographic influences, as well as anomalous heat conductivity. In the application of geo-environment, sulphide oxidation bodies’ in-place on waste piles, in case, is sufficiently rapid, can lead to the generation of measurable thermal anomalies, which has the ability of providing a measure of metal amounts that are being released to the environment.

In addition, the measurements of airborne thermal infrared, are applied in geothermal exploration, however, it might have potentiality in the exploration as well as in geo-environmental applications whenever ground surface temperatures have anomalous due to sulphide oxidation, or even heat flow perturbations due to structure or lithology.

Electrical Methods

This technique comprises of several techniques that uses differing instrument as well as procedures, have varying depth along with lateral resolutions. These methods have been divided into five groups namely direct current resistivity technique, electromagnetic, mise-a-la-mase, induced polarization and self polarization.

The ones deemed effective in exploring magnetide and hematite comprise of; “Direct current resistivity method; this measures Earth resistivity by the use of either pa direct or low frequency alternating source of current”⁹. Research has shown that these minerals (magnetite as well as hematite) can be conductive in presence of electricity.

However, most conduction in rocks is as a result of water content as well as dissolved ironic species. It has been indicated that, high porosity leads to low resistivity in water saturated rocks. It is worth mentioning that the technique has been applicable in different process of exploring minerals¹⁰.

Additionally, broader resistance of the earth material makes the mechanism to be able to identify lithologies as well as other attributes that dictate mineral composition. It has been documented that wastes that are acidic and originate from mining processes are the best since they have H+ thus capable of being a better conductor un like when a concentration of salt are used¹¹.

Electromagnetic technique; this technique uses alternating magnetic fields with the aim of inducing measurable currents in the Earth. The usual methods of applying electromagnetic technique in the exploration of minerals, has been in the search for low resistivity, or high-conductivity immense sulphide deposits.

Airborne methods might be applied in the screening of large areas, and as an effect, the method has been providing a multitude targets for ground survey. Electromagnetic techniques including airborne, have been widely used in the mapping of lithologic as well as structural features, from which several mineral explorations and geo-environmental inferences are possible¹².

Conclusion

Though there are several geophysical surveying used in the Search for minerals, the number is limited on methods that are applied in the searching of hematite and Magnetite Irons Ores. This is based on their physical properties like the crystal nature and lustre on one hand; and chemical properties like magnetism along with conductivity of hematite and magnetite. For instance, hematite is ant ferromagnetic material; though it appears to be ferromagnetic at Currie temperatures of about 1000k, though with a very tiny moment, on the other hand, magnetite is considered as being highly ferromagnetic¹³. Another factor that has hindered the application of other methods includes the cost incurred during exploration, along with environmental concerns, though the last two are not of main concern as compared to the first factor.

References

Criss, Erick., and Champion, Den, Magnetic properties of granitic rocks from the southern half of the Idaho batholith–Influences of hydrothermal alteration and implications for aeromagnetic interpretation: Journal. Of Geophysical Research. 89 (1984): 7061-7076.

Durrance, Maron, Radioactivity in geology, principles and applications: London: Ellis Horwood Ltd, 1986.¹⁴

Jessop, James and others. 1995, Evaluation of geophysical methods for locating subsurface hazards in the Shawnee National Forest: DoI Conference on the Environment and Safety. Colarado: Colorado Springs CO. 1995;

Khan, Jacobson, “Remote Sensing and Geochemistry for Detecting Hydrocarbon croseepages”. Geological Society of America Bulletin 120(2008): 96–105.

Ovnatanov, Stephen. and Tamrazyan, Gren, Thermal studies in subsurface structural investigations, Aspheron Peninsula, Azerbaijan USSR: American Association of Petroleum Geologists Bulletin. 54, (1970): 1677-1685.

Palacky, Gren, Airborne resistivity mapping, in Palacky, Gren. ed., Geological Survey of Canada paper. 1986, 195 p.

Petrovic, Annes, Chafetz, Henry. 2. “Remote detection and geochemical studies for finding hydrocarbon-induced alterations in Lisbon Valley, Utah”. Marine and Petroleum Geology 25 (2008): 696–705.

Reynolds, Leonard, and others, Rock magnetism, the distribution of magnetic minerals in the Earth’s crust, and aeromagnetic anomalies, in Hanna, Willy. ed., Geologic Applications of Modern Aeromagnetic Surveys: U.S. Geological Survey Bulletin. 1924 (1990): 24-45.

Sumner, Julius, Principles of induced polarization for geophysical exploration. Amsterdam: Elsevier. 1976

Van Blaricom, Richard, Practical geophysics: Northwest Mining Association, 1980.

Watson, Kenneth, and Hummer-Miller, Susanne, Thermal infrared exploration in the Carlin trend, northern Nevada: Geophysics. 55-1 (1990): 70-79.

Wright, Peter, Gravity and magnetic methods in mineral exploration, in Skinner, Ben, ed., Economic Geology. 75th Anniversary. 7(1981): 829-839.

Zielinski, George, and DeCoursey, Mill. Localized heat flow and Tertiary mineralization in southern New Mexico: Geophysics. 48, (1983):1212-1218

Footnotes

¹ Jessop, James and others. 1995, Evaluation of geophysical methods for locating subsurface hazards in the Shawnee National Forest: DoI Conference on the Environment and Safety. Colarado: Colorado Springs CO. 1995;

² Reynolds, Leonard, and others, Rock magnetism, the distribution of magnetic minerals in the Earth’s crust, and aeromagnetic anomalies, in Hanna, Willy. ed., Geologic Applications of Modern Aeromagnetic Surveys: U.S. Geological Survey Bulletin. 1924 (1990): 24-45.

³ Criss, Erick., and Champion, Den, Magnetic properties of granitic rocks from the southern half of the Idaho batholith–Influences of hydrothermal alteration and implications for aeromagnetic interpretation: Journal. Of Geophysical Research. 89 (1984): 7061-7076.

⁴ Wright, Peter, Gravity and magnetic methods in mineral exploration, in Skinner, Ben, ed., Economic Geology. 75th Anniversary. 7(1981): 829-839.

⁵ Van Blaricom, Richard, Practical geophysics: Northwest Mining Association, 1980.

⁶ Durrance, Maron, Radioactivity in geology, principles and applications: London: Ellis Horwood Ltd, 1986.

⁷ Ovnatanov, Stephen. and Tamrazyan, Gren, Thermal studies in subsurface structural investigations, Aspheron Peninsula, Azerbaijan USSR: American Association of Petroleum Geologists Bulletin. 54, (1970): 1677-1685.

⁸ Watson, Kenneth, and Hummer-Miller, Susanne, Thermal infrared exploration in the Carlin trend, northern Nevada: Geophysics. 55-1 (1990): 70-79.

⁹ Durrance, Maron, Radioactivity in geology, principles and applications: London: EllisHorwood Ltd, 1986.

¹⁰ Petrovic, Annes, Chafetz, Henry. “Remote detection and geochemical studies for finding hydrocarbon-induced alterations in Lisbon Valley, Utah”. Marine and Petroleum Geology 25 (2008): 696–705.

¹¹ Sumner, Julius, Principles of induced polarization for geophysical exploration. Amsterdam: Elsevier. 1976

¹² Palacky, Gren, Airborne resistivity mapping, in Palacky, Gren. ed., Geological Survey of Canada paper. 1986, 195 p.

¹³ Zielinski, George, and DeCoursey, Mill. Localized heat flow and Tertiary mineralization in southern New Mexico: Geophysics. 48, (1983):1212-1218