A characterising of the ore minerals due to mineralogical, chemical and textural properties in Malmberget.

Transcription

1 A characterising of the ore minerals due to mineralogical, chemical and textural properties in Malmberget. Cecilia Lund 1, Olof Martinsson 1 Luleå University of Technology 1, Luleå, Sweden ( cecilia.lund@ltu.se) Abstract This study was an attempt to find a way of characterising an iron ore body both mineralogical and textural in a quantitative manner by using analytical methods like optical microscopy, microprobe (EMPA) and an automatic SEM based system, Particle Texture Analysis (PTA). The source of this study is an iron ore body, called Fabian, located in Malmberget, Sweden. Two types of ores were identified and analysed in this study named orebreccia and ore. The Particle Texture Analysis was made on two fractions of crushed ore. The mineralogy was evaluated and characterized as mineral liberation and mineral association. Magnetite has a simple outline and straight grain boundaries and the gangue minerals have a finer particle size with a more complicated texture. The liberation of magnetite in ore and ore breccia is high. The ore quality for both ore and ore breccia does have similarities in a process technique perspective. Introduction LKAB is today one of the world s leading producers of upgraded iron ore products. The customers expect good qualities of the products which demand a detailed knowledge of the processes from mine to customer. To achieve this it is important to get a traceability of ore feed and control over the production process but it will also demand a good knowledge of the raw material. Concept of process mineralogy Through studying the process mineralogy of iron ore it is possible to understand the behaviour of the ore feed during the processing plant and also later on the sintering and pelletising processes. It is needed to know what kind of minerals and textures the ore is consisting of due to the fact that the liberation characteristics are intimately related to the mineralogical texture (Lorenzen and van Deventer 1994). Optical microscopy has traditionally been the instrument used for the identification and quantification for both mineralogical and texturally properties, which is a time consuming process (Petruk 2000). A number of different techniques according to image analyses system based on SEM techniques have been developed during the three last decades for a more rapid quantitative description of mineralogy and particle textures (Jones and Gravilovic 1970; Gottlieb, Wilkie et al. 2000; Petruk 2000; Gu 2003). In this study which is a part of a larger thesis, a determination of the mineral properties of one specific iron ore body from LKAB Malmberget was carried out. This was an attempt to find a way of characterise the mineralogy and textures of the iron ore body. Three methods were used; optical microscopy, microprobe and automatic SEM based system, Particle Texture Analysis (PTA) (Moen 2006). 71

2 The Fabian ore body is major Fe resource at Malmberget since the last drill program has shown an expanded ore body. The ore body plunge in ~ 80 to a depth of at least 1250 meters. The length and width is approximately 700 to 900 m and 50 to 150 m respectively. The ore quality is also better then average for the Malmberget deposit, due to the higher Fe-content (fig. 2). Geological setting Regional geology: The geology of the ore province of northern Norrbotten is compound of bedrock sequences with different ages. The oldest rocks are an Archaean granitoid-gneiss basement with dated ages of 2.8Ga. Unconformably overlaying sequences are supracrustal successions of Palaeoproterozoic age Ga, followed by Svecofennian volcanic and sedimentary units, dated c. 1.9 Ga. Forty apatite-iron deposits are known from the northern Norrbotten ore province and they are hosted and probably genetically related to the volcanic rocks in the Svecofennian succession (Martinsson 2004). The two economically most important deposits are mined by LKAB at Kiruna and Malmberget. These two apatite-iron ore deposits have a similar origin and were formed by magmatic-hydrothermal process at Ga (Bergman, Kübler et al. 2001). However, there are major discrepancies in character between them, due to later overprinting by metamorphoses, deformation and granitic intrusions, which are stronger for the Malmberget ore. Deposit geology: More than 20 different tabular to stock shaped ore bodies are known in the Malmberget ore field, spread over an area of 2.5 x 5km. There exist only one generally description of Malmberget deposit written by (Geijer 1930). The Malmberget deposit were probably from the beginning a continuous ore lens which were exposed for at least two phases of folding and metamorphism. These events torn the ore lenses apart by strong ductile deformation and today they occupy a large-scale fold structure were the individual ore bodies stretch parallel to the fold axis, which plunge 40º-50º towards SSW (Bergman, Kübler et al. 2001) (fig. 1). Fig. 1 A simplified geological map of Malmberget. From Bergman et al. (2001). 72

3 The iron ore minerals are both magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) but the magnetite is more common of the two. Hematite forms several separate ore bodies and portions of others (Geijer 1930). The main gangue minerals are apatite, amphibole, pyroxene, feldspars, quartz and biotite. Among the accessory minerals are pyrite, chalcopyrite, titanite, zircons and calcite most common. Every ore body is characterised by there own mineral, chemical and textural properties. Due to the strong metamorphic recrystallization of the area, the minerals are recrystallised, coarse grained, and elongated in the direction of the lineation of the rocks. Fig. 2. A schematic picture of the Fabian ore body. Analytical methods The analytical methods which were used are optical microscopy, microprobe (EMPA) and Scanning electron microscopy (SEM-PTA). Sample preparation For optical microscopy and microprobe, samples of intact ore were used and for the SEM-(PTA) method, the samples were crushed and sieved into sized fractions. Drill core samples from various parts of the ore were taken to characterise different textures. Polished thin sections were made of both intact ore and two particle sizes from sieved fractions of crushed ore (tab. 1). The sized samples were of a larger volume since the sample should be representative for a specific ore type and not only for a specific texture. 73

4 Ore types: Two types of ore were identified and used in this study, called ore breccia and ore. The ore breccia is largely consisting of magnetite but do have gangue minerals like quartz, amphibole, pyroxene, apatite and feldspars in different proportion occurring as breccia infill in the wall rocks. The ore is more massive magnetite that contains gangue minerals like amphiboles and apatite in fewer amounts. The ore breccia is bordering the massive ore, but occurs also partly as inclusions in the massive ore (fig. 3). Table 1. The samples which is used in the study. Sample Intact ore 150 µm 75 µm (AP1) x (AP3) x (AP4) x (B32585) x (A32584) x (C32586) x (Ore breccia) x x (Ore) x x At the mineral processing laboratory at LTU the samples were crushed in a Retsch jaw crush, +3 mm, split by a Jones splitter and sieved with a Ro-Tap shaker at the fraction 150 μm and 75 μm. Optical microscopy Polished thin section were optical examined in transmitted and reflected light on a standard petrographical microscopy (Nikon Eclipse E600). Fig. 3. Two ore types. The red spots are defined textures and parageneses. 74

5 To characterise all the different mineral associations, textures and parageneses, a mineral identification were made of both the silicates and oxides. 23 different spots were analysed on the major minerals in both ore types, ore breccia and ore (Fig. 3) Microprobe Mineral analyses were performed on a JOEL JXA-8500F electron microprobe at NTNU, Trondheim, Norway. For the microprobe analyses an accelerating voltage at 15.0 kv, a probe current at 95 μa and a < 1μm beam diameter were used. Totally 166 analyses were made, representing both silicates and oxides on the major minerals in 23 spots to cover all different textural and mineral assemblage variations, observed in the samples. Beside this mineral identification, it also verified that the used sized fraction samples were representative. Scanning electron microscopy The particle analyses were performed on a Hitachi S-4300SE scanning electron microscopy equipped with Oxford Inca Feature software NTNU, Trondheim, Norway. For the particle analyses an accelerating voltage at 20.0 kv and a probe current at ~0.5nA were used. Particle texture analysis PTA By using Back Scattered Electron (BSE), images are analysed by means of grey levels and every grain of interest was analysed by X-rays. Every analyzed grain size fraction is imported to the PTA software. Images analysis is performed offline to process and evaluate if grains occur liberated or in composite particles. Standard queries can be performed from the output results in a new database such as the mineral liberation of any mineral, mineral association of any mineral and miniature images of particles of a certain texture category (Moen 2006). To reduce the unclassified group of minerals an extensive identification of minerals and phases for classification should be done. Results Mineralogy The textures of the ore breccia is characterised by magnetite grains with a simple outline and straight grain boundaries, either as single grains or as a particles in a matrix of quartz and feldspar. This matrix has a myrmecitic texture and the grains shows anhedral granular outline with complicated grain boundaries (fig 4). 75

6 Fig. 4. Photographs of the mineralogy and textures from ore breccia, (upper photos) and ore (lowest photo). Mag, magnetite, ap, apatite, qtz, quarts, amp, amphibole, bt, biotite. In the ore the texture of magnetite is dominated by grains having simple outlines with straight grain boundaries. The grains are of different size with coarser grains often elongated in the direction of lineation occurring in a finer grained matrix. Gangue minerals like apatite, amphiboles and biotite are elongated in a linear direction (tab. 2). Table 2. Sample descriptions of the mineralogy and texture. Mag Hem Ap Pl Mc Qtz Amp Cal Bt Py Texture x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Single simple outlined mag grain or aggregate in a myrmecitic matrix of fsp-qtz. Single simple outlined mag grain or aggregate in a myrmecitic matrix of fsp-qtz. Single simple outlined mag grain or aggregate in a myrmecitic matrix of fsp-qtz. Massive mag. fine grained at the contact to amphibole. Massive mag, homogenous, fine grained matrix, elongated coarse grains. Massive mag, elongated coarse grains. Ap and amp grains elongated in the direction lineation. Mag, magnetite, hem, hematite, ap, apatite, pl, plagioclase, mc, microcline, qtz, quarts, amp, amphibole, cal, calcite, bt, biotite, py, pyrite. Particle texture analysis (PTA) The modal mineralogy for the ore breccia samples shows slightly different results when comparing the fractions 150 µm and 75 µm. Magnetite decreases with 8.2 % in the 75 µm fraction. The classes Mg(Ca)- silicate and quartz/feldspar are also decreasing in the smaller fraction. Instead the albite and actinolite class increases, about 14 %, however, actinolite is not represented at all in 150 µm (fig. 5), (tab. 3). 76

7 Fig. 5. The modal mineralogy of the different mineral classes divided in particle size fractions. The two particle fractions in the ore samples do have the same mineral classes represented but in different volume proportion. The magnetite class decreases 5 % from the 150 µm to the 75 µm fraction. The amphibole - pyroxene minerals (actinolite) and (Mg(Ca)-silicate) constitute together 5 % in the 150 µm fraction and increases to 7% in the 75 µm fraction which also become the second largest group (fig. 5). In both ore types there is an unclassified class that include particles that could not be determinate because of an incomplete x-ray analysis. Table 3. Mineral classes and number of grains in each fractions. Ore breccia 150 µm Ore breccia 75 µm Ore 150 µm Ore 75 µm Mineral Number w% Number w% Number w% Number w% Apatite 3 0,01 8 0, , ,71 Magnetite/hematite , , , ,31 Quartz 592 2, , , ,17 Calcite 46 0, , ,05 Biotite , , ,03 Pyrite , ,11 Titanite 145 0, , , ,18 Ortoklas/microcline 614 2, , ,02 Actinolite/Augit , , ,68 Dolomite Albite 157 3, , , ,6 Fe-Ti-oxide 133 0, , , ,91 Quartz/Feldspar 911 9, , ,03 Mg(Ca)-silicate , , , ,18 Calcite/apatite ,01 Zircon ,98 6 0, Unclassified , , , ,39 Total At the liberation analysis, classes of similar minerals were fit into broader groups. Amphiboles*: actinolite + Mg(Ca)-silicate, Feldspar*: Qtz/Fsp + orthoclase/microcline + albite and the remaining classes are Magnetite och Quartz. 77

8 Fig. 6. Liberation of minerals. Ore breccia, 1-2 (150μm), 2-3 (75μm) Ore, 3-4 (150μm), 2-3 (75μm). In fig. 6, yellow colour means that 100 % of the magnetite is classified as an apparently liberated grain and orange ditto means that more than 95 % is liberated. The degree of liberation of magnetite is high and quite similar for the both ore types ~ 85 %. It shows a slight increase in the finer fraction. Magnetite with more than 95 % liberated grains will almost be the remaining part in every fraction. The liberation for the different silicate groups is not as high as magnetite. Quartz and Amphibole* classes are more liberated in the finer 75 μm fraction (fig. 6). Fig. 7. Magnetite associated with other minerals. Ore breccia, 1-2 (150μm), 2-3 (75μm) Ore, 3-4 (150μm), 2-3 (75μm). The mineral classes which is associated to magnetite is more diverse in the ore breccia. Amphibole* is associated to magnetite in the ore (fig. 7 & 8). 78

9 Fig. 8. Amphibole* associated with other minerals. Ore breccia, 1-2 (150μm), 2-3 (75μm) Ore, 3-4 (150μm), 2-3 (75μm). Discussion As pointed out in the introduction this study was an attempt to find a way of characterise the mineralogy and textures of an iron ore body. The different ore types were identified during the geological mapping and interpreted in the optical microscopy. Both these ore type were quantified by the mineral content, mineral liberation and mineral association by the particle texture analysis (PTA). The modal mineralogy for this study shows a decreasing magnetite proportion at the finer fraction. This is probably due to the primary grain size in the samples are larger for magnetite compared to the gangue minerals. The liberation of the silicate groups indicates that the primary grain size is finer. The ore breccia has more and larger mineral classes like quartz, feldspar and amphibole fractionated at 75µm particle size. The texture for the silicate matrix is more complicated in comparison to the coarse grained magnetite which has simple grain boundaries. The interpretation is also verified in the optical microscopy This study will be extended to include more samples of the same ore type but also other ore types found in the Fabian ore body. Other ore bodies will also be included to verify that the results could be applicated in a general way for the Malmberget deposit. Conclusions The primary grain size for magnetite is coarser and has a higher proportion at the coarser fraction 150 µm. Differences in the modal mineralogy of magnetite and silicates in different particle sizes of the ore breccia are close connected to the texture. The liberation of magnetite in ore and ore breccia is high and being quite similar. The ore quality for both ore and ore breccia does have similarities in a process technique prospective. 79

10 Acknowledgement Thanks for the financial support by Hjalmar Lundbohm Research Centre (HLRC). I am grateful to Prof. Terje Malvik and Dr. Kari Moen, NTNU for all help during the particle texture analysis and their great knowledge of process mineralogy. References Bergman, S., L. Kübler, et al. (2001). "Description of regional geological and geophysical maps of northern Norrbotten County (east of Caledonian orogen)." SGU Geological Survey of Sweden Ba 56: 110. Geijer, P. (1930). "Geology of the Gällivare Iron Ore field." Geological Survey of Sweden Ca 22: 115. Gottlieb, P., G. Wilkie, et al. (2000). "Using Quantitative Electron Microscopy for Process Mineral Applications." JOM Journal of Metals 52: Gu, Y. (2003). "Automated Scanning Electron Microscope Based Mineral Liberation analysis." Journal of Minerals & Materials Characterization & Engineering 2: Jones, M. P. and J. Gravilovic (1970). "Automatic quantitative mineralogy in mineral technology." Rudy 5: Lorenzen, L. and J. S. J. van Deventer (1994). "The interrelationship between mineral liberation and leaching behaviour." International Journal of Mineral Processing 41: Martinsson, O. (2004). "Geology and Metallogeny of the Northern Norrbotten Fe-Cu- Au Province." Society of Economics Geologists, Guidebooks Series 33: Moen, K. (2006). Quantitative measurements of mineral microstructure. Department of Geology and Mineral Resources Engineering. Trondheim, Norwegian University of Science and Technology. Doctoral thesis: 194. Petruk, W. (2000). Applied Mineralogy in the Mining. Amsterdam, Elsevier 80