Detrital chrome-spinel grains in heavy-mineral sand deposits from southeast Africa

Transcription

1 Mineralogical Magazine, February 2006, Vol. 70(1), pp. 51±64 Detrital chrome-spinel grains in heavy-mineral sand deposits from southeast Africa M. POWNCEBY* AND P. BOURNE CSIRO Minerals, Box 312, Clayton South, VIC 3169, Australia ABSTRACT Detrital chrome-spinels are contaminant grains within ilmenite concentrates produced from heavymineral deposits along the coast of southeast Africa. The presence of even minor levels of chromia in the predominantly ilmenite-rich concentrates, downgrades their market value as potential feedstocks for the production of titania pigment. An understanding of their composition can assist in their removal from the ilmenite concentrates. Compositions from a database of close to 900 chrome-spinel analyses shows the major element components and their ranges (in wt.%) are: Cr: 0.4ÿ45.3, Al: 0.0ÿ31.0, Fe: 8.5ÿ69.6 and Mg: 0.0ÿ12.2. Minor components include Ti: 0.1ÿ11.4 and Zn: 0.0ÿ13.7. The chrome-spinel data fall into two compositionally distinct groups. The first group of spinels is dominated by a strong trend reflecting the mutual substitution between Al 3+ and Cr 3+ in the spinel structure. The second group of spinels is characterized by compositions containing abundant Fe 3 O 4 magnetite component. The clear division between chrome-spinel compositional types indicates the grains are derived from at least two chemically dissimilar provenances. The compositional differences between the chrome-spinel groups has a positive impact on subsequent ilmenite upgrading treatments as the spinels which contain the highest magnetite component are easily removed via low-intensity magnetic separation procedures. KEYWORDS: chrome-spinel, southeast Africa, ilmenite concentrates. Introduction THE southeast coast of Africa has been a commercially important source of Ti minerals since widespread aeolian dune placer deposits rst came to prominence in the early- to mid-1970s (Fockema, 1986; Force, 1991; Wipplinger et al., 1999). Since then, a large number of economic to sub-economic deposits has been located along the coastline stretching from the vicinity of East London (South Africa) in the south to Mombasa (Kenya) in the north (Fig. 1). At present, all Timineral production in southeastern Africa comes from the large Quaternary beach sand deposits currently being mined along the Kwazulu-Natal coast of South Africa at Richards Bay and Hillendale. Recently however, exploration focus * Mark.Pownceby@csiro.au DOI: / has moved further north along the coast into Mozambique, Tanzania and Kenya where additional large reserves of ilmenite and associated heavy minerals, rutile and zircon, have been identi ed. Many of these deposits such as the Corridor Sands project in southern Mozambique and the Kwale project in Kenya are in the process of detailed planning and feasibility studies with the aim of coming on-line as commercial producers within the next 2ÿ5 y. The heavy-mineral sand deposits of southeast Africa are characterized by assemblages dominated by ilmenite (70ÿ90%) but with minor, variable amounts of leucoxene, rutile and zircon. Typical gangue minerals associated with the deposits include magnetite (an Fe-rich spinel), silicates (garnet, pyroxene, andalusite) and chrome-bearing spinels (Hammerbeck, 1992). Of the range of gangue minerals, the presence of chrome-bearing spinels (typically at levels of # 2006 The Mineralogical Society

2 M. POWNCEBY AND P. BOURNE FIG. 1. Location map showing the extent of mineral sands occurrences along the southeast coast of Africa (shaded area), together with sample localities for the ilmenite concentrates examined in this study. <3ÿ5%) is particularly problematic as chromium is an undesirable impurity element in ilmenite concentrates. The pigment industry requires ilmenite feedstocks with <0.1 wt.% Cr 2 O 3 (Beukes and Van Niekerk, 1999); however, it is not uncommon for ilmenite concentrates from heavy-mineral deposits in northern Kwazulu- Natal and Mozambique to contain as much as 0.3 wt.% Cr 2 O 3 (Hammerbeck, 1992; Nell and Den Hoed, 1997). Secondary bene ciation of the ilmenite concentrates is therefore required to remove the spinel contaminant grains. Background Previous work on characterization of Cr-bearing spinels within African southeast coast ilmenite concentrates has largely been con ned to studies de ning their separation behaviour when subject to an applied magnetic eld. For example, the distribution of Cr 2 O 3 (and TiO 2 ) as a function of magnetic susceptibility was measured by a number of workers including Bergeron and Prest (1976), Lee and Poggi (1978), Nell and Den Hoed (1997), Beukes and Van Niekerk (1999) and Gouws and Van Dyk (2001), using ilmenite concentrates sourced from the northern Kwazulu-Natal region. All studies showed that the Cr 2 O 3 distribution as a function of magnetic susceptibility was bimodal, re ecting the presence of chrome in at least two distinct spinel phases (Fig. 2). None of these studies, however, attempted to de ne chemically these compositions besides the obvious inference that in the more magnetic fractions, Cr was contained mainly in magnetite-rich spinels; whilst in the relatively non-magnetic fractions it was present in magnetite-poor spinels. More recently, Steenkamp and Pistorius (2003) quantitatively analysed chrome-bearing spinel grains in the low-magnetic susceptibility waste stream fraction of an ilmenite concentrate from the Kwazulu-Natal region (i.e. the material remaining after the removal of the magnetiterich spinels). The spinels in the fraction were characterized by having low total Fe and relatively high Cr, Al and Mg contents, consistent 52

3 CONTAMINANT CHROME-SPINELS IN AFRICAN ILMENITE CONCENTRATES FIG. 2.Cr 2 O 3 distribution data (wt.%) in unroasted ilmenite concentrates sourced from the northern Kwazulu-Natal region of South Africa. The data from Bergeron and Prest (1976) are in Gauss, whilst the data from Nell and Den Hoed (1997) are presented as a function of reciprocal magnetic susceptibility (expressed in log 10 units). with the interpretation that these residual spinels were low in magnetite (Fe 3 O 4 ) component. Based on the analyses, the non-magnetic spinels were considered to belong to the magnesiochromite (MgCr 2 O 4 ) spinel series although the data also indicated a signi cant amount of solid solution towards a magnesioaluminate (MgAl 2 O 4 ) spinel component. Unfortunately however, the results of Steenkamp and Pistorius (2003) were only reported as an averaged bulk spinel composition and therefore provide no information with regard to the range in composition of the spinels. The prime aim of this paper is to present a comprehensive electron microprobe study of chrome-bearing spinel contaminant grains in titanium deposits from southeast Africa. Constructing and analysing a database of chrome-spinel analyses sourced from a number of deposits along the coast in order to quantify their maximum compositional range achieves this. Information regarding compositional variation in the chrome-bearing spinels provides greater understanding of the compositional variability of the spinels and identi es how these impact on their physical properties, in particular their magnetic separation characteristics. Spinel group chemistry and mineralogy All spinels contain two differing cations, or at least two different valence states of the same cation, in the ratio 2:1. This gives the general crystallographic formula AB 2 O 4 where the tetrahedrally coordinated sites are labelled A, and octahedrally coordinated sites, B. In spinel types commonly associated with ilmenite concentrates, the major cations substituting into the A site are the divalent cations Mg, Fe, Mn (minor) and Zn (minor) whereas substitution within the B site involves the cations Al, Cr, Fe and Ti. Aluminium, Fe and Cr are present as trivalent cations. However, substitution of Ti 4+ into the octahedral B site (as in the case of ulvoèspinel ÿ Fe 2 TiO 4 ), relies on a coupled substitution 2B 3+ =Ti 4+ + A 2+ mechanism, relative to the general AB 2 O 4 formula. This gives rise to a range of spinel solid solutions within the system (Fe 2+,Mg)(Al,Cr,Fe 3+ ) 2 O 4 ± (Fe 2+,Mg) 2 TiO 4 meaning that most spinels fall within a compositional space enclosed by the six spinel end-member components Fe 3 O 4, FeCr 2 O 4, MgCr 2 O 4,FeAl 2 O 4,MgAl 2 O 4 and MgFe 2 O 4.The positions of these end-member compositions within the quaternary Cr-Fe-Mg-Al system are shown in Fig. 3a. 53

4 M. POWNCEBY AND P. BOURNE FIG. 3. Tetrahedral plots for the major elements Fe(t), Mg, Cr and Al showing (a) the positions of important endmember compositions and, (b) the entire chrome-spinel dataset (898 analyses). In many publications dealing with ilmenite concentrates contaminated with Cr-bearing spinels, the spinels are nearly always classi ed under the generic term `chromite'. The term `chromite', however, strictly refers to the mineral phase chromite ÿ FeCr 2 O 4 ÿ the (Fe 2+ Cr)-rich end-member of the spinel solid-solution series. To avoid possible confusion, throughout this paper all spinels in the ilmenite concentrates are referred to collectively as `chrome-spinels' unless a unique end-member phase composition is otherwise clearly identi able. Sampling criteria Heavy mineral concentrates (HMC) were obtained from a range of deposits along the southeast coast of Africa (Fig. 1). Samples were sourced from companies currently mining or actively exploring in the region and were mainly derived from either test sample pits (e.g. Kwale, Moma, Moebase) or from current mine production (e.g. Richards Bay, Hillendale). In addition, several concentrates from Holocene dune deposits (Sodwana Bay, Morgan's Bay), or from presentday shoreline deposits (Umgababa) 1 were also analysed. A list of samples examined is included as Table 1. To minimize possible bias due to sample preparation, the only company-supplied samples included in the dataset were those that had undergone `standard' processing treatments. These include desliming to remove the nes (mainly clays) followed by gravity separation using spirals or heavy liquid media to produce a bulk HMC. For the Holocene dune material, the samples had been wet sieved and then screened to remove the nes. Even after these preliminary processing treatments, the HMC may still contain a signi cant proportion of other components besides ilmenite and chrome-spinel (rutile, zircon, aluminosilicates), which dilute the sample. Therefore, upon receipt of the samples by CSIRO Minerals, a split portion (50ÿ100 g) of each HMC was magnetically separated at a eld strength of 3.3 kgauss to separate the ilmenite and chrome-spinels from rutile, zircon, leucoxene and other non-magnetic gangue minerals. This 0ÿ3.3 kgauss fraction should comprise predominantly the ilmenite (+ chrome-spinel) fraction and was the material used in all subsequent analytical testwork. Data collection Electron microprobe (EMP) analyses were obtained using a JEOL Superprobe electron microprobe analyser (Model JXA 8900R) equipped with ve wavelength dispersive spectrometers (WDS). For the analyses, grains were 1 Dr V. Hugo, formerly of the University of Natal, supplied the Richards Bay, Sodwana Bay, Morgan's Bay and Umgababa bulk samples. 54

5 CONTAMINANT CHROME-SPINELS IN AFRICAN ILMENITE CONCENTRATES TABLE 1. List of heavy-mineral deposits examined in this study together with their measured bulk Cr 2 O 3 content (wt.%). Sample Locality Country/province %Cr 2 O 3 # Analyses* 1 Morgan's Bay** South Africa, Eastern Cape Province (8+14) 2 Xolobeni South Africa, Eastern Cape Province Umgababa# South Africa, Kwazulu-Natal Province Hillendale South Africa, Kwazulu-Natal Province Richards Bay South Africa, Kwazulu-Natal Province (94+74) 6 Sodwana Bay{ South Africa, Kwazulu-Natal Province (47+54) 7a Corridor Sands (Xai Xai) Southern Mozambique, Gaza Province b Corridor Sands (Chongoene) Southern Mozambique, Gaza Province c Corridor Sands (Chibuto) Southern Mozambique, Gaza Province Moma Northern Mozambique, Namplua Province (9+6) 9 Moebase { Northern Mozambique, Zambezia Province Kwale Kenya, Coast Province * Numbers in parentheses indicate results from two individual sample blocks. ** Ilmenite concentrate from old mining operation of Holocene dune deposit. # Magnetic concentrate from present-day beach sample. { Heavy mineral concentrate from Holocene dune deposit. { Sample appears to be missing the high-fe 3 O 4 spinel component. dispersed in epoxy resin and mounted into 2.5 cm round blocks and polished at at a nal diamond paste cutting size of 1 mm. The samples were coated with a 25 nm layer of carbon prior to analysis. In order to locate individual chrome-spinel grains, each sample block was step-scanned on a grid of points with a 10 mm step size between points (total coverage 16,000 mm616,000 mm or 256 mm 2 ). At each point a 10 ms analysis was made for Cr X-rays using the Ka line. The Cr distribution map showed the positions of chrome-spinel grains as an array of bright spots, the coordinates of which could be retrieved and stored. Using this procedure, up to 10,000ÿ12,000 mineral grains in total could be scanned over a period of ~9 h. Once the coordinates of individual chromespinel grains were stored, the elemental analyses proceeded automatically for each position. For the quantitative analyses, the microprobe was operated at 20 kv and 40 na and the electron beam defocused to 5 mm. The suite of elements analysed included Cr, Fe, Mg, Al, Zn and Ti. Standards used were spinel (MgAl 2 O 4 ), hematite, ZnS, rutile and chromium metal. Detection limits for the analysis conditions used were (in ppm); Cr 300, Fe 140, Mg 65, Al 70, Ti 80 and Zn 200. In this study, only analyses from the clear and optically homogeneous part of each chromespinel grain were included in the dataset. Data analysis Major compositional features of spinels are traditionally illustrated by plotting element ratios such as Cr/(Cr+Al) against Mg/(Mg+Fe 2+ ) and Fe 3+ /(Cr+Al+Fe 3+ ) against Mg/(Mg+Fe 2+ ). These form the basis for projecting compositional data elds onto the ``spinel prism'', a 3-D plot used for discriminating spinels from various tectonic provenances (Stevens, 1944; Thayer, 1946). To derive the Fe 3+ from the total Fe measured, the common procedure is to recalculate the ferric Fe content based on the assumption of spinel stoichiometry (e.g. Irvine, 1965). Unfortunately, stoichiometry-based calculation methods result in the Fe 2+ :Fe 3+ ratio being the major source of error 55

6 M. POWNCEBY AND P. BOURNE in the data (Wood and Virgo, 1989; Dyar et al., 1989). This is also the aspect of the mineral chemistry that is most affected by secondary alteration. Since the detrital chrome-spinels found in ilmenite placer deposits have probably experienced some weathering or leaching thereby disrupting the original Fe 2+ :Fe 3+ ratios, the approach used in the current paper is to plot all Fe values as wt.% Fe (t) (i.e. as total Fe). The element composition data initially are presented on a 3D quaternary Fe-Cr-Mg-Al scatter plot showing the variation in major element chemistry followed by a series of ternary x-y-z and binary x-y scatter plots for selected elements. By analysing individual or groups of scatter plots, trends between the various element groupings become apparent. Results and discussion Compositional range ÿ bulk dataset A total of 898 chrome-spinel grains was analysed from the 12 samples. In most spinels, the oxides MgO, FeO (total Fe was reported as FeO), Cr 2 O 3 and Al 2 O 3 summed to between ~91 and 101% (average of 96.3%). The lower totals were re ective of a high Fe 3+ content within some chrome-spinel grains implying a high magnetiterich spinel component. Re-calculating the analyses for these grains, ascribing all the Fe to an Fe 2 O 3 oxide component, produced better totals. The major element components and their ranges (in wt.% element) for the entire dataset was; Cr: 0.4ÿ45.3, Al: 0.0ÿ31.0, Fe: 8.5ÿ69.6 and Mg: 0.0ÿ12.2. The broad variation in composition suggests multiple source areas for the spinels although it is possible that some variation may also be introduced through alteration or weathering processes. Titanium and Zn were also typically present in the chrome-spinels, but generally in only minor or trace amounts. Typical compositional ranges for these minor components were; Ti: 0.1ÿ11.4 and Zn: 0.0ÿ13.7. Combining all the analytical data gives an average bulk composition for the chrome-spinels of: 24.9% Cr; 6.6% Al; 30.6% Fe; 3.4% Mg; 1.4% Ti; and 0.2% Zn. In Fig. 3b, data for the major elements Fe (t), Mg, Cr and Al are plotted on a quaternary scatter diagram. Except for a few outliers, all the data fall within a compositional area enclosed by the spinel end-member components Fe 3 O 4, FeCr 2 O 4, MgCr 2 O 4, FeAl 2 O 4, MgAl 2 O 4 and MgFe 2 O 4. The few data that lie outside this area were spinels that had either been extensively altered or leached or represented more exotic spinel compositions, e.g. Zn-rich. Analysis of the data in Fig. 3b indicates the bulk of the spinels are rich in the FeCr 2 O 4 (chromite ÿ Chr) component though minor trends towards more obviously MgAl-rich (spinel ÿ Sp) and MgCr-rich (magnesiochromite ÿ Mgc) compositions are also evident. The extension of the data points towards the Fe apex indicates spinels containing more ferric-rich magnetite (Mt) compositions (Fig. 3b). To better visualize the compositional trends evident in the dataset, a range of ternary x-y-z and binary x-y scatter plots for various element combinations is provided in Fig. 4. In the Cr-Al- Mg and Cr-Mg-Fe ternary plots (Fig. 4a,b), the data are separated into two compositionally distinct groups. The rst group of spinels is dominated by a strong trend ranging from Al-rich spinel compositions (MgAl 2 O 4 or FeAl 2 O 4 ) to Crrich compositions (MgCr 2 O 4 or FeCr 2 O 4 ) re ecting the mutual substitution between Al 3+ and Cr 3+ in the spinel structure (Fig. 4a). This grouping is denoted as the Al 3+ ÿcr 3+ substitutional series. The second group of spinels lies on a plane extending from spinels that are essentially Cr 3+ (ÔAl 3+ )-rich, towards compositions containing abundant Fe (Fig. 4b). Comparison of the ternary data with the quaternary scatter plot results shown in Fig. 3b indicates that these data lie on a trendline extending towards the Fe 3 O 4 (magnetite ÿ Mt) spinel phase. This grouping is therefore designated as the `magnetite series'. Al 3+ ÿcr 3+ substitutional series The bulk of the data along the Al 3+ ÿcr 3+ substitutional trendline are thickly concentrated in the mid- to high-cr compositional range (~25ÿ32 wt.% Cr) although there are smaller, less dense concentrations of data extending towards low-cr, high-al (3ÿ15 wt.% Cr; Al >20 wt.%) and high-cr, low-al (Cr >35ÿ43 wt.%; Al <10 wt.%) contents, respectively (Fig. 4c). The low-cr, high-al spinels within the series are relatively low in total Fe (<30 wt.% Fe ÿ Fig. 4d,e) and high in Mg (Fig. 4f). This implies that these spinels contain more of an MgAl 2 O 4 (Sp) rather than a FeAl 2 O 4 (Hercynite ÿ Hc) component. In contrast, the spinel data at high-cr, low-al contents are characterized by a slight increase in Mg content relative to the mid-cr spinels (Fig. 4a,b). The increase in Mg is associated with a concomitant 56

7 CONTAMINANT CHROME-SPINELS IN AFRICAN ILMENITE CONCENTRATES FIG. 4. Scatter plots showing major compositional trends. 57

8 M. POWNCEBY AND P. BOURNE decrease in total Fe, suggestive of an increase in the amount of MgCr 2 O 4 (Mgc) component. Note however, the Cr-Mg-Fe ternary plot, whilst con rming the trend toward an increased Mgc component at high Cr contents, also indicates that the high-cr spinels contain a signi cant amount of Fe. This implies the presence of at least some Chr component. The Cr-rich, low-al spinels therefore represent a mixed Mgc-Chr solidsolution series in which there is mutual exchange between Fe 2+ and Mg 2+. Magnetite series Chrome-spinel data of the Magnetite Series lie on a plane extending from the large, dense cluster of mid-range Cr spinels (~25ÿ30 wt.% Cr; ~20ÿ30 wt.% Fe; <5ÿ6 wt.% Mg; and <10ÿ12 wt.% Al), towards spinel compositions that are characterized by extremely high Fe contents. The high-fe spinels are compositionally at the pure Mt (Fe 3 O 4 ) end of the spinel solidsolution series and typically contain up to 68ÿ69 wt.% Fe (stoichiometric Fe 3 O 4 contains 69.9 wt.% Fe). Minor Ti and Al substitutions are common in the most Fe-rich samples although by far, the major compositional variation involves substitution of Cr 3+ (for Fe 3+ ) and Mg 2+ (for Fe 2+ ), i.e. the spinels all tend to lie on a solid solution series between the end-members Mt and (Chr + Mgc). There is no evidence for any signi cant substitutional trend from Mt towards either MgFe 2 O 4 (magnesioferrite ÿ Mgf) or MgAl 2 O 4 (Sp). UlvÎspinel trend The majority of the chrome-spinels in the dataset generally contain <~1 wt.% Ti, although there is a small number of spinel grains containing up to 10ÿ12 wt.% Ti. These high-ti grains represent chrome-spinels containing a high Fe 2 TiO 4 ulvoèspinel (Usp) component and belong to a minor ulvoèspinel series. These spinels also typically contain high Fe (~30ÿ50 wt.%; Fig. 4g) and low Al (<5 wt.%; Fig. 4h), Mg and Cr contents (data for Mg and Cr not shown). Chrome-spinel textures Representative chrome-spinel images are provided in Fig. 5. In general, the chrome-spinel grains are mostly sub-rounded to sub-angular and tended to be slightly coarser grained than the ilmenite. Composite grains are also common in the concentrates (particularly the Umgababa sample). In these composite grains the chromespinel was typically associated with Fe oxides and occasionally with silicates, as well as with ilmenite. Inspection of Fig. 5 reveals that there is a variety of textures exhibited by individual spinel grains ranging from large, optically homogeneous grains (e.g. grains 1a and 1b) through grains showing evidence for the initiation of extensive edge micro-fracturing (grains 1c and 2b) to grains that are extensively fractured and pitted in their entirety (grains 1d, 2d and 4b). Most grains show some evidence of transportation and subsequent reworking. These effects are manifest by abrasive rounding and/or fracturing of grain boundaries. In contrast, there is not a large proportion of grains that appear to have undergone prolonged weathering or leaching either before transport (i.e. hydrothermal alteration of the host rock) or after deposition (i.e. leaching through the action of groundwater). Where present however, the effects of these sorts of alteration processes are made apparent by evidence of leaching along fractures (grains 3b and 5d) or leaching of former inclusions (Figs 3d, 5b). Compositional and textural relationships In order to evaluate possible associations between composition and texture, the individual chromespinel grains shown in Fig. 5 were examined via quantitative EMP analysis methods. Grains shown in Fig. 5 were selected on the basis of composition; in particular their measured (Cr + Al) content and where they plot on the Al vs. Cr data scatter plot previously shown in Fig. 4c. To accomplish this, the Al vs. Cr data was split into ve compositional regions (Fig. 6). Regions 1ÿ3, ranging from high Al/low Cr to low Al/high Cr contents primarily scrutinize the effects on texture of Cr 3+ for Al 3+ solid substitution in the spinels. Conversely, regions 3ÿ4 ostensibly examine the effects arising from an increase in Fe 3+, or magnetite content, in the spinels. The increase in Fe 3+ in the spinels may be inherited from an original magnetite-rich host-rock (primary) or result from oxidation of Fe 2+ through weathering/alteration processes (secondary). The latter mechanism for increased Fe 3+ would be expected to have an obvious effect on the texture of grains such as dissolution textures (pits, pores, channel ways) and/or fracturing and cracking. For the chrome-spinel grains shown in Fig. 5, the corresponding quantitative EMP analyses are 58

9 CONTAMINANT CHROME-SPINELS IN AFRICAN ILMENITE CONCENTRATES FIG. 5. Back-scattered electron images showing the range of textures exhibited by the chrome-spinel grains. Corresponding compositional regions, 1ÿ5, are shown in Fig. 6. In each image the scale bar represents 50 mm. provided in Table 2. The analytical data reveal that even though the chrome-spinels are subdivided on the basis of (Al+Cr) content, within any particular compositional region there may still be considerable variation in other elements. For example in the high-cr region 3, the Cr content remains above ~35 wt.% Cr, though the Mg content can vary from almost 0 wt.% (grain 3b) to ~4.5 wt.% Mg (grain 3a). Similarly, the Al content varies between ~1 to 5 wt.% and the Fe ranges between 21 and 28 wt.%. The range of compositions within each limited (Al+Cr) region re ects the complexity of solid solution that is possible in the spinel structure type. 59

10 M. POWNCEBY AND P. BOURNE FIG. 6. Chrome-spinel dataset divided into compositional regions on the basis of Al and Cr content. The EMP data were also recast into calculated spinel molar components in order to better reveal any associations between composition and texture. For example, chrome-spinels that contain a high MgAl 2 O 4 component are generally considered more mechanically resistant and less susceptible to weathering processes than spinels that contain more of the easily leached components such as FeCr 2 O 4 and FeAl 2 O 4 (Lumpkin, 2001). Spinels from regions 1 and 2 would therefore be expected to show less evidence of alteration effects than spinels from regions 3 and 4, which contain larger Fe(Al,Cr) 2 O 4 components. An examination of the data however, reveals that this assumption does not hold true with grains containing a relatively large Fe(Al,Cr) 2 O 4 component (grains 4c and 4d) exhibiting textures that are essentially indistinguishable from those grains with a high Mg(Al,Cr) 2 O 4 component (grains 1a and 1b). Paragenesis of chrome-spinel alteration The paragenesis of the chrome-spinel alteration is important to determine because the site of alteration will affect the distribution and proportions of different types of altered grains in the deposits. Alteration in the original host rock can potentially produce large variations in the degree and distribution of altered chrome-spinel grains. These variations may ultimately impact on subsequent separation processes. In comparison, in situ alteration in the placer deposits tends to produce a more homogeneous population of altered grains As indicated above, for the most part, examination of chrome-spinel grains by combined SEM and qualitative X-ray analysis techniques reveals that they are relatively unweathered. Where present, alteration is mainly con ned to the margins of well rounded grains suggesting that alteration occurred in situ, or at least after substantial abrasion and rounding of the grains had already occurred, i.e. during the late stages of transportation, or post-deposition. This interpretation is consistent with compositional and textural data for ilmenite grains from the same region (Hugo and Cornell, 1991; Hugo, 1993), which indicate that the majority of ilmenite particles in the deposits are also unaltered or only slightly altered. Magnetic properties of the spinels ÿ implications for processing The intrinsic chemical and mineralogical features of the chrome-spinels in uences their behaviour during subsequent processing treatments. For example, chrome-spinels in ilmenite concentrates from the Murray Basin of southeastern Australia have proved largely impossible to remove (Grey et al., 2003). This is undoubtedly due to the extreme compositional variation displayed by the chrome-spinels with the major factor contributing to the poor magnetic differentiation between the ilmenite and chrome-spinel grains being the large variation in magnetite content between grains (Pownceby, 2005). In comparison, chrome-spinels in ilmenite concentrates from southeast Africa clearly fall into two, compositionally discrete 60

11 CONTAMINANT CHROME-SPINELS IN AFRICAN ILMENITE CONCENTRATES TABLE 2. Selected quantitative EMP analyses showing compositional variation within individual chromespinel grain types. The grains analysed are separated on the basis of composition according to the regions identi ed in Fig. 6. Grain ID Element (wt.%) Calculated spinel components (mole%) Mg Cr Al Ti Fe Mg(AlCr) 2 O 4 Fe(AlCr) 2 O 4 Fe 2 TiO 4 Fe 3 O 4 Region 1: High Al (Al wt.%) 1a RB b RB c SOD d MB Region 2: Med. Cr/Med. Al (20 4 Cr (wt.%) 4 35): (5 4 Al (wt.%) ) 2a RB b RB c SOD d RB Region 3: High Cr (Cr 5 35 wt.%) 3a RB b RB c UM d SOD Region 4: Med. Cr/Low Al (20 4 Cr (wt.%) 4 35): (Al (wt.%) 4 5) 4a SOD b RB c RB d RB Region 5: High Fe (Cr 4 20 wt.%) 5a RB b RB c RB d RB populations. This compositional separation translates into well-de ned differences in magnetic properties (Fig. 2). The current approach to treating southeast African ilmenite concentrates is to initially separate the magnetite-rich spinel grains (which may contain minor amounts of Cr 2 O 3 ), from the bulk using standard magnetic separation procedures. The remaining material, commonly known as crude ilmenite, is then subject to an oxidizing roast at 730ÿ800ëC to increase the magnetic susceptibility of the Fe 2 O 3 -FeTiO 3 solid solution in the titanate grains, relative to the remaining chrome-spinels. The roasting procedure has a limited effect on the spinels enabling the removal of the remaining chrome-bearing spinels from the ilmenite concentrate through subsequent magnetic separation. To investigate the effect of spinel composition on magnetic separation characteristics, the chrome-spinel populations in four crude ilmenite concentrates were examined after the highly magnetic Fe 3 O 4 fraction had been removed. Two samples were from southern Mozambique (SM) and the other two from northern Kwazulu- Natal (KN). In addition to the primary magnetic separation stage, both southern Mozambique samples had also undergone a secondary magnetic separation treatment at a higher magnetic intensity of 5000 Gauss. Aluminium vs. chrome x-y scatter plot results for the four concentrates are shown in Fig. 7, whilst averaged EMP results and corresponding calculated spinel molar components are listed in Table 3. The results show that for all samples, the magnetic separation was successful in removing almost all of the magnetite series spinels. The remaining spinels were essentially Cr-rich, but with signi cant Fe, Mg and Al substitution. Based on the analyses, the non-magnetic spinels are 61

12 M. POWNCEBY AND P. BOURNE FIG. 7. Aluminium vs. chrome scatter plots for concentrates in which the most magnetic magnetite-rich fraction has been removed. Figure 7a shows data from two southern Mozambique concentrates whilst Fig. 7b shows data from northern Kwazulu-Natal. considered to belong primarily to the Fe(AlCr) 2 O 4 spinel series although the data also indicated a sizeable amount of solid solution towards an Mg(AlCr) 2 O 4 spinel component. It is chromespinels of these compositions that are carried forward in oxidizing roast treatments to upgrade the upgrade the ilmenite concentrates. A comparison of the EMP results between samples indicates that the southern Mozambique material has had slightly more of the magnetiterich spinels removed compared to the northern Kwazulu-Natal samples (average Fe 3 O 4 content of ~4 mole% in the SM samples compared to 8ÿ15 mole% in the KN samples). This was presumably because of the higher magnetic eld strength employed to prepare the two SM samples. The increased magnetic eld strength however, does not appear to exert any impact on the other major Fe-bearing spinel component Fe(AlCr) 2 O 4 which is up to 10ÿ20 mole% higher in the southern Mozambique sample. Although information on the magnetic properties of individual spinel compositions is limited, the results are consistent with the interpretation of Thayer (1956) who argued that the magnetic properties of chrome-spinels depend more on the state of oxidation of the Fe rather than the total amount. An unexpected observation from the separated samples was that even after an additional magnetic separation treatment, the average Cr 2 O 3 content of the southern Mozambique concentrates (SM1 and SM2) was signi cantly higher than for the Kwazulu-Natal crude ilmenites (3.14 wt.% and 2.61 wt.% for samples SM1 and SM2, respectively, vs wt.% and 0.36 wt.% for samples KN1 and KN2). Whilst the data in Fig. 7 and Table 3 reveal that all concentrates contain chrome-spinels of approximately the same composition, the large variation in bulk Cr 2 O 3 indicates that the absolute abundance of chrome-spinels can vary signi cantly between regions. This observation is also inferred from the information in Table 1, which generally indicates higher bulk Cr 2 O 3 contents in the more northern deposits (the Moma and Kwale deposits appear to TABLE 3. Quantitative EMP analyses of remaining chrome-spinels in crude ilmenite concentrates sourced from Southern Mozambique and Northern Kwazulu-Natal. These concentrates have had their most magnetic chrome-spinel magnetic fractions removed. See text for details. Sample* Element (wt.%) Calculated spinel components (mole%) Mg Cr Al Ti Fe Mg(AlCr) 2 O 4 Fe(AlCr) 2 O 4 Fe 2 TiO 4 Fe 3 O 4 Southern Mozambique SM-1 (612) SM-2 (472) Northern Kwazulu Natal KN-1 (84) KN-2 (82) * Numbers in parentheses indicate the number of analyses obtained for each sample. 62

13 CONTAMINANT CHROME-SPINELS IN AFRICAN ILMENITE CONCENTRATES be the exceptions to this rule). It is important to note however, that whilst the abundance of spinels may be variable, the results of the current study show that the average composition of the chromespinels (after preliminary magnetic separation treatments) remains constant. An increase in bulk Cr 2 O 3 abundances in the more northern deposits should therefore not have any impact on additional oxidizing roast-processing procedures. Summary Detrital chrome-spinel grains in ilmenite concentrates sourced from the southeast coast of Africa were analysed to determine their range in composition. The analysis of a database of 898 chrome-spinel grains shows the major element components and their ranges (in wt.% element) are: Cr: 0.4 ÿ 45.3, Al: 0.0 ÿ 31.0, Fe: 8.5 ÿ 69.6 and Mg: 0.0 ÿ Titanium and Zn are also typically present in the chrome-spinels, but generally in only minor or trace amounts. Typical compositional ranges for these minor components are; Ti: 0.1 ÿ 11.4 and Zn: 0.0 ÿ The chrome-spinel data fall into two compositionally distinct groupings. The rst group of spinels is dominated by a strong trend ranging from Al-rich spinel compositions (MgAl 2 O 4 or FeAl 2 O 4 ) to Cr-rich compositions (MgCr 2 O 4 or FeCr 2 O 4 ) re ecting the mutual substitution between Al 3+ and Cr 3+ in the spinel structure. The second group of spinels lies on a plane extending from spinels that are essentially Cr 3+ (ÔAl 3+ )-rich, towards compositions containing abundant Fe 3 O 4 (magnetite). The clear division between chrome-spinel compositional types indicates that the dataset is made up of detrital spinel grains derived from at least two chemically dissimilar provenances. The individual chrome-spinel grains exhibited a variety of textures ranging from large, optically pristine grains through grains showing extensive edge micro-fracturing to grains that are extensively fractured. Most grains show some evidence of transportation and subsequent reworking; however, there are few grains that have undergone prolonged weathering or leaching. Although there are at least two compositionally distinct chromespinel populations, there is no apparent difference in the degree of alteration experienced by either group. Where present, alteration is mainly con ned to the margins of well-rounded grains suggesting that alteration occurred in situ, after deposition. Analysis of samples after commercial magnetic separation treatments in order to investigate the relationship between the compositions of the chrome-spinels and the observed magnetic response of the samples, con rms that the spinels with the highest Fe 3+ content (i.e. magnetite component) are the most easily removed via low-intensity magnetic separation procedures. The remaining non-magnetic chromespinels belong primarily to the Fe(AlCr) 2 O 4 spinel series, although there is some solid solution towards an Mg(AlCr) 2 O 4 -rich spinel component. Acknowledgements The authors wish to acknowledge the assistance provided by their CSIRO Minerals colleagues Luda Malishev (sample preparation), Cameron Davidson (sample preparation and SEM), Colin MacRae and Nick Wilson (microprobe setup), and Steve Peacock (XRF analyses). This work could not have proceeded without the provision of samples by companies that are involved in the southeast Africa mineral-sands industry. In particular we thank Ticor (South Africa), WMC Resources, Kenmare Resources PLC, BHP Billiton, Tiomin Resources Ltd., and Mineral Commodities Limited. In addition, Ian Grey (CSIRO Minerals) is thanked for making available a suite of samples previously supplied by Dr V. Hugo (University of Natal). PB would additionally like to thank CSIRO Minerals for giving him the opportunity to do this research as a vacation student. References Bergeron, M. and Prest, S.F. (1976) Magnetic separation of ilmenite. US Patent 3,935,094. Beukes, J.A. and Van Niekerk, C. (1999) Chromite removal from crude ilmenite. Proceedings of Heavy Minerals 1999 (R.G. Stimson, editor). Symposium series S23, South African Institute of Mining and Metallurgy, Johannesburg, pp. 97ÿ100. Dyar, M.D., McGuire, A.V. and Ziegler, R.D. (1989) Redox equilibria and crystal chemistry of coexisting minerals from spinel lherzolite mantle xenoliths. American Mineralogist, 74, 969ÿ980. Fockema, P.D. (1986) The heavy mineral deposits north of Richards Bay. Mineral Deposits of South Africa, 2301ÿ2307. Force, E.R. (1991) Geology of Titanium-Mineral Deposits. Geological Society of America Special Paper 259, 112 pp. 63

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