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Journal of Trace and Microprobe Techniques Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ltma20
HIGH DENSITY MINERAL INCLUSIONS IN ROCKS EVIDENCED BY γ-RAY TOMODENSITOMETRY a
a
Costel T. Rizescu , Gheorghe N. Georgescu & Octavian G. Duliu
b
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Research and Development Institute for Electrical Engineering, 313 Splaiul Unirii, Bucharest, RO-74204, Romania b
Department of Atomic and Nuclear Physics, University of Bucharest, Mãgurele, P.O. Box MG-11, Bucharest, RO-76900, Romania Version of record first published: 16 Feb 2007.
To cite this article: Costel T. Rizescu, Gheorghe N. Georgescu & Octavian G. Duliu (2001): HIGH DENSITY MINERAL INCLUSIONS IN ROCKS EVIDENCED BY γ-RAY TOMODENSITOMETRY, Journal of Trace and Microprobe Techniques, 19:1, 119-129 To link to this article: http://dx.doi.org/10.1081/TMA-100001467
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J. TRACE AND MICROPROBE TECHNIQUES, 19(1), 119–129 (2001)
GEOCHEMISTRY, SOIL, AND ENVIRONMENTAL SCIENCE
HIGH DENSITY MINERAL INCLUSIONS IN ROCKS EVIDENCED BY γ-RAY TOMODENSITOMETRY Costel T. Rizescu,1 Gheorghe N. Georgescu,1 and Octavian G. Duliu2,∗ 1
Research and Development Institute for Electrical Engineering, 313 Splaiul Unirii, RO-74204 Bucharest, Romania 2 Department of Atomic and Nuclear Physics, University of Bucharest, M˜agurele, P.O. Box MG-11, RO-76900, Bucharest, Romania
ABSTRACT A fragment of a polysulfide rock containing quartz and high density mineral inclusions has been investigated by using a 192 Ir γ -ray dual computer tomograph. The tomographic image revealed the existence of three different mineral fractions, whose densities and effective atomic numbers, as calculated by means of the image histograms, were equal to 2.57 ± 0.24 × 103 kg/m3 (Z eff = 11.6 ± 2.1), 4.01 ± 0.22 × 103 kg/m3 (Z eff = 25.1 ± 2.1), and 6.01 ± 0.90 × 103 kg/m3 (Z eff = 68.5 ± 6.0), respectively. Subsequently, these minerals were identified as quartz, a complex mixture of medium density polymetallic sulfide (pyrite, blenda, chalcopyrite, and sphalerite), and galena as heavier component. A detailed mineralogical analysis confirmed these data. Key Words: Tomography; Densitometry; γ -ray; Mineral; Rock
INTRODUCTION CAT represents a nondestructive physical method of investigation based on the attenuation of nuclear radiation (X- and γ -ray, electrons or neutrons) (1,2). At their passage through matter, γ - or X-rays and, in some circumstances, β-rays as
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Corresponding author. E-mail:
[email protected] 119
C 2001 by Marcel Dekker, Inc. Copyright
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well as neutrons are attenuated following Beer’s law
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I = I0 e −µx
(1)
where I stands for the transmitted current of radiation, I0 represents the incident current of radiation, x is the sample width, and µ is the LAC. In the case of X- or γ -rays, the LAC depends on both sample density and sample effective atomic number, Zeff . By passing through the investigated object a great number (105 –106 ) of coplanar, thin beams of X- or γ -rays, it is possible to measure experimentally the attenuation along each ray direction. Therefore, the multitudes of exploring beams determine through the sample a planar section whose thickness is roughly equal to the beam diameter (usually fractions of millimeters). By using appropriate reconstruction algorithms (3–5), it results in a 2D map of the LAC over this section. Because the number of projections is finite it follows that the number of LAC values onto the investigated section will be also finite. Thus, the investigated section is decomposed into a number of volume units, having a prismatic shape, called voxels (acronym for volume element). Each voxel, whose dimensions are roughly equal to one half of the beam diameter (6), is characterized by a single value of the LAC. Subsequently, to each voxel is attributed a pixel of image whose shades of gray are proportional to the corresponding numerical value of the LAC or of the Zeff . This picture, which represents the reconstruction of the distribution function of the LAC or the effective atomic number by means of its projection, is the final tomographic image. This technique, known as the reconstruction of an object by its projection, has been first of all proposed by Radon (7) and later developed to the modern CAT by Oldendorf (8), Cormack (9), and Hounsfield (1). In the case of high-energy electromagnetic radiation such as X- or γ -ray, the LAC of any material represents the product of two terms: the density ρ and the mass attenuation coefficient µm . The second term, in the case of low energies (below 100 keV) and low atomic numbers (less than 26), depends on the average atomic number of the sample, but becomes almost constant for energies higher than 100 keV (10). In the case of heavy elements such as Sb, Pb, or U, this independence appears at higher energies (usually greater than 0.5–1 MeV). At the present moment, there are in use two types of axial CT: medical CT equipped with X-ray tubes and CT provided with isotopic γ -ray sources. CT equipped with intense X-ray tubes (equivalent to 12–15 kCi or 450–550 TBq) has the advantage of an extremely short running time (a few seconds or even less) but presents some disadvantages known as beam hardening and absorption edge effects (3). These effects, intrinsically related to the polychromatic nature of the X-rays generated by classical tubes, needs special mathematical correction (2). In the second case, the monochromatic γ -ray generated by radioisotopic sources such as 169 Yb (50.4 keV), 241 Am (59 keV), 192 Ir (316.5 and 468.1 keV), or 137 Cs (662 keV) used in combination with energy-dispersive detectors naturally exclude
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these unwanted effects. 192 Ir, which presents a high specific activity (∼5 GBq/g) and hence allows to obtain very narrow intense beam of γ -ray with dual energy, is one of the most suitable radionuclide for γ -ray CAT. If two γ -rays with different energies are used simultaneously, it becomes possible to measure at once two parameters as density and effective atomic number of the absorbent (11–13). This technique is called dual-energy CAT. A dual-energy CT is characterized by gathering projection data over a section, using simultaneously two different γ -rays. This peculiarity considerably improves the quality of tomographic reconstruction. At the same time, by slightly changing the reconstruction algorithm, a tomographic image depicting the local values of the effective atomic number over the same section can be obtained too (13). Thus, the CAT image represents de facto the distribution function of the LAC or of the effective atomic number Zeff over the investigated section. Most isotopic γ -sources have an activity a few orders of magnitude lower than that of X-ray tubes that proportionally increase the acquisition time. In spite of this disadvantage, CT equipped with γ -ray sources generate true density maps of the investigated sections (12,14). To be investigated by dual-energy CT, a sample must present some characteristics as the local variation of the density or of the effective atomic number to be no less than 5–10% or the linear dimensions of the domains with different densities to be no smaller than the CT spatial resolution (about 0.5 mm). If the external diameter, d, of the sample satisfies the condition of optimal precision, e.g. 0.8/µ ¯ ≤ d ≤ 4.4/µ ¯ where µ ¯ is the average LAC (15), then this fact will bring an additional of precision in tomographic reconstruction. Rocks represent one of the most suitable category of objects to be investigated by CAT (16). The higher the level of rock inhomogeneity, the better is the quality of the resulting images. CAT has been successfully used to determine the heterogeneity in reservoir rocks (17), to investigate the frequency spectrum of rhytmites in sedimentary cores (18), to detect diamond inclusions in kymberlyte (19), and to study the inner structure of manganese nodules (20), but these are only some examples. In fact there are more than 100 papers devoted to this subject (16). In this paper we report the quantitative results obtained by investigating with a dual γ -ray CT a polymetallic sulfide sample rich in quartz. MATERIALS AND METHODS Sample The investigated rock (Fig. 1) was a polymetallic sulfide presenting a complex structure due to an alternation of metallic and nonmetallic (quartz, calcite)
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Figure 1. Photographic image of the investigated sample. The sections S1 , S2 , and S3 across which CAT images have been obtained are represented by horizontal lines.
minerals. The specimen has roughly a sickle-like shape and measures approximately 5 × 10 × 6 cm3 . It has been collected from the Herja mine located in the Baia Mare metalliferic ore region in north-western Romania. At a careful visual inspection (including microscope examination, acid attack, and porcelain-plate trace), we have identified a succession of four crystallization processes that determined the banded pattern of the sample. The main metallic components of the sample was sphalerite (ZnS), followed by pyrite (FeS2 ) and chalcopyrite (FeCuS2 ). Small aggregates of galena (PbS) with ideomorphic inclusions of quartz have been observed too. Quartz represented the main nonmetallic constituent, strongly interrelated with the other components (Table 1). Mainly in its central part, the quartz has been spotted with blackish deposits consisting of small crystals of galena (PbS) that, in turn, contained small quartz inclusions (Fig. 1). Consequently, we have chosen for examination that part of the sample which was rich in different mineral fractions.
Computer Tomograph The CT used for this study was a first generation dual energy γ -ray, home made, TOMO RAY-1 type constructed at the Research Institute for Electrical
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Table 1. Mineralogical Structure of the Investigated Sample Mineral
Dimensions (mm)
Occurrence
Quartz
0.02–4.00
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Dolomite Sphalerite (blende) Pyrite Chalcopyrite
1.50–2.00 0.02–8.00 0.02–2.00
