Chromite Deposits Introduction Classification Stratiform deposits

Transcription

1 Chromite Deposits This text is based mostly on the chapter of J.M. Duke in Geoscience Canada (see the reference list). This publication can be freely downloaded from internet. Other sources are given in the references. Introduction: The ore deposits of chromite are the single source of this mineral. Chromite is a mineral from the spinel group with the nominal chemical composition FeCr 2 O 4. The mineral can be used as is, if concentrated, for the manufacture of refractory bricks for furnace linings and as foundry sand. At the same time, chromite is the only source of the element chromium which is an essential component of many types of stainless steel, other types of steel, and some non-ferrous alloys. The composition of chromite is variable because of extensive solid solutions with other minerals from the spinel group, e.g., magnetite, Fe 3 O 4, or hercynite, FeAl 2 O 4. Although Cr-rich chromite was required for many of its applications in the past, chromite with lower Cr/Fe ratios can be used today. There are other minerals of chromium but all of them are exceedingly rare and find their place only at the shelves of museums and private collections. Classification: Chromite deposits can be broadly classified into two categories, bearing in mind that some deposits may contain features from both types. The two categories are stratiform and podiform deposits. Elluvial and alluvial deposits which originate by weathering of the rocks and transport of chromite from the primary occurrences are relatively unimportant. Stratiform deposits: Stratiform deposits are large, sheet-like bodies in layered mafic to ultramafic igneous complexes. Stratiform deposits produce slightly less than a half of the globally mined chromite but their chromite reserves are about 98 % of the total chromite reserves. Therefore, it is very likely that they will place a major role for the chromite mining industry in the future. General geology: Their age is typically Precambrian and they are found in within cratons, that is, large, stable cores of the continents. The large mafic to ultramafic igneous provinces intruded the continental crust which contained or may have contained more acidic rocks, such as granites or gneisses. Morphology of these intrusions can be described either as flat and tabular or funnel-shaped. The flat and tabular bodies were emplaced in the form of sills into the pre-existing rock complexes and the layering in these intrusions is parallel throughout the intrusion body (of course, may be disrupted by later tectonics). Examples of such intrusions are Stillwater, Kemi, and Bird River. The magmatic layering in the funnel-shaped intrusions dips gently toward the center of the intrusion, giving the layers a geological form of a syncline. Examples are Bushveld complex, Muskox, and Great Dyke. Geometry and mineralogy: Stratiform chromite deposits consist of multiple layers (rarely one layer) of chromitite, a rock made mostly of chromite. The layers are thin, with thickness of 1 cm to 1 m, but laterally very extensive, with lateral length up to 70 km. The layers are parallel to the primary igneous layering and the barren rocks between the layers contain a small amount of disseminated chromite. Chromitites contain % of chromite, the other minerals are olivine, orthopyroxene, plagioclase, clinopyroxene, and their alteration products. A common but minor mineral is brown mica, indicating that the magma contains some water. The chromite crystals are euhedral to subhedral, with the silicates located in the interstitial space between the chromite crystals. Despite the apparent similarity and simplicity of these deposits, the differences between the deposits or between the layers within a single deposits may be striking (Duke 1983).

2 Example: Bushveld Large Igneous Province (LIP): The Bushveld LIP is located in South Africa. It is a massive complex of mafic to ultramafic rocks of Paleoproterozoic age. The amount of magma delivered to the intrusion was estimated to million km 3 and this amount was intruded within 10 million years. The Bushveld LIP was studied extensively, with a large focus of its potential as an ore deposit of chromite, but also vanadium and platinum-group elements. The Bushveld LIP can be split into the western and eastern lobes and a number of formations with local names which are of lesser interest for the general understanding of the intrusion and the ores. One of the locally recognized and important units is the Rustenburg Layered Suite (RLS). This suite has been further subdivided into the marginal (lowermost), lower, critical, main, and upper (uppermost) zone. The marginal zone is made mostly of norite. The lower zone consists of units with pyroxenite, dunite, and harzburgite, but chromitites are missing. The critical zone is known for its spectacular layering and enormous reserves of chromite. The lower part of the critical zone is entirely ultramafic (rocks contain mostly orthopyroxene) and the upper part, which hosts the chromite ore deposits, comprises harzburgite, pyroxenite, but also norite and anorthosite. The base of the upper critical zone is defined by the first appearance of cumulus plagioclase. Two sets of layers in the critical zone represent most of the chromite reserves. These sets of layers are also called seams. The LG6 seam has an average thickness of 0.8 m and can be traced for over 70 km in the western lobe and a thickness of 0.6 to 1.3 m and lateral extent of 90 km in the eastern lobe. The F chromitite seam has a thickness of 1.3 m and lateral extent of 35 km. The main zone consists mostly of gabbronorites and host the Merensky Reef, a world-class platinumgroup element deposit. The upper zone contains mostly gabbros but the most important economic feature of this zone are layers of Ti-V rich magnetite. They store almost a half of the global reserves of vanadium. This sequence of zones, as well as the entire Bushveld LIP, shows an evolution from ultramafic (orthopyroxenite, harzburgite) to mafic (gabbros, ferric diorites) rocks and eventually granites. It is believed that the complex was formed as a series of pulses of magma from different sources or feeders. This conclusion is also supported by Sr isotopic geochemistry of the rocks (see Kinnaird, 2005). Given the lateral extent and the sill-like nature of the Bushveld LIP, the new batches of magma had to spread laterally and were able to interact, chemically and physically, with the overlying and underlying magmas. A number of different models (the following text after Kinnaird, 2005, see that work for additional references) have been put forward for the formation of thick chromitite seams, based on evidence not only from the Bushveld Complex but also from Stillwater in particular. Previous models for chromitite formation include: (i) gravity-induced separation, crystal sorting and settling, has been discounted both on textural evidence, on the basis of co-tectic proportions, and on the physics of processes in non-newtonian magmas; (ii) immiscibility of Cr-rich liquid which has largely been discounted because of the high temperature (c C) at which Cr 2 O 3 -SiO 2 immiscibility occurs; (iii) increases in oxygen fugacity by country rock degassing seems unlikely because of the difficulty of controlling such changes over the area of the Bushveld and because oxygen fugacity appears to increase systematically from the lowest LG chromitite layer to the uppermost chromitite layers. (iv) contamination by a siliceous component; (v) mixing between resident and new magma; (vi) lateral growth within a stratified magma column; (vii) pressure changes; changes in total pressure within a crystallising magma chamber could change the equilibrium liquidus assemblage. The attraction of this model is that the effects of a pressure change would be felt nearly simultaneously over the whole magma chamber although the magnitude of the pressure change necessary to shift the magma composition from the cotectic into the field of chromite

3 alone is not clear. A pressure change in the order of >> 1kbar would be needed and the general effect of pressure change on mineralogy has been shown to be trivial; (viii) injection of a chromite-phyric magma still requires that chromite is precipitated somewhere else at greater depth in order to be entrained in the ascending magma. Podiform deposits: These deposits occur within ophiolite sequences, that is, fragments of oceanic crust, found on continents mostly because the dense, oceanic crust was obducted onto the lighter continental crust. The typical stratigraphy of an ophiolite sequence includes deep-ocean sediments (uppermost), pillow lavas, sheeted dykes, gabbros, mafic cumulates, ultramafic cumulates, and ultramafic tectonites (see Duke 1983). The contact between the ultramafic cumulates and ultramafic tectonites defines the petrologic Moho. General geology: The podiform chromite deposits are found in the ultramafic rocks, mostly in the tectonites, less commonly in the cumulates. Some studies indicate that the abundance of the podiform deposits increases towards the top of the tectonites. The chromite-rich bodies are hosted by dunite embedded in harzburgite. Geometry and mineralogy: The shape of the podiform deposits is described as irregular and unpredictable. That is why geologists chose the uncertain word pod to express their morphology. The pods may approach lenses or elongated bodies in their form and individual pods may carry from a few kilograms to several million tons of ore, although the very large bodies are rare. The podiform deposits may show foliation, mostly parallel to the foliation or layering of the host rocks. Some authors attempted to describe the podiform deposits as discordant, subconcordant, and concordant. Chromite in the podiform deposits forms often anhedral, cracked grains. A peculiar feature of these ores is the nodular texture characterized by loosely-packed ellipsoidal chromite nodules with sizes between 5 and 20 mm. Minerals other than chromite are olivine, orthopyroxene, clinopyroxene, pargasite, Na-mica, albite, and jadeite. Aqueous inclusions are common in some deposits (Johan et al. 1982), documenting the role of the fluid phase in the ore formation. Formation of the podiform deposits (after Uysal et al. 2009): Many genetic aspects are still not fully understood, there are basically three hypotheses concerning the genesis of podiform chromitites: i) podiform chromitites may represent part of the residuum after extensive extraction of melt from their mantle host, based on their association with the residual mantle rocks such as dunite and harzburgite, ii) podiform chromitites have been interpreted as a cumulate filling of a magma conduit inside the residual mantle, and iii) more recently, it has been stressed that such deposits form as a result of melt/rock or melt/melt interaction (i.e. magma-mingling ). Furthermore, the presence of water in the melt is thought to be necessary for the crystallization of chromium spinel (Edwards et al., 2000). Chemical composition of chromite: As mentioned above, there are several extensive solid solutions between the minerals of the spinel group, to which chromite belongs. Chromite from stratiform and podiform deposits can be, in many cases, distinguishes based on its chemical composition. For some discrimination diagrams, see the slides below. Origin of chromite deposits: Chromitites, either in the stratiform or the podiform deposits, are igneous cumulates. For a long time, it was assumed that they form simply by precipitation of chromite inside a magma chamber and settling of these near the bottom of the chamber. This uncomplicated

4 view appears to be logical but ultramafic rocks usually contain and precipitate little chromite, perhaps 0.5 vol. %. The ultramafic rocks contain consistently small amount of disseminated spinel with variable concentration of chromium. Then, what processes can collect such small amount and create the vast deposits which we mine? Current perception of this problem indicates that the crystallization happens in situ, at the bottom of the magma chamber, and no gravitational settling is needed. Other processes are required instead and these are illustrated in Figure 1 below. Figure 1. Redrawn after Irvine (1975, 1977). For explanation, see text below. Figure 1a shows a small portion of the triangular diagram where olivine, chromite, and SiO 2 define the apices. The small area shown in Fig. 1a is marked by light gray in Fig. 1b. Note that the area in Fig. 1a represents compositions poor in chromite the ultramafic rocks are usually poor in chromite, as

5 discussed above. An approximate composition of an ultramafic magma is shown in Fig. 1a, c, d by the point A. Because this point is located within the field of olivine, crystallization begins with precipitation olivine and the composition of magma moves along the join A-B. Once the magma composition reaches point B, olivine and chromite will crystallize together but there will be much more olivine than chromite. That is why, as written above, ultramafic rocks commonly contain a small amount of disseminated chromite. The composition of magma moves further along the curve B-D; in the point D, orthopyroxene will appear and the magma can evolve further. As you noticed, there is no massive chromite precipitation event described here. There is a simple way to precipitate only chromite from magma, if a partially evolved magmatic reservoir with a composition in point C is contaminated by extraneous material. This material can be either silica-rich, for example granite or gneiss (Fig. 1c) and can be mixed with the magma by simple contamination from the wall rocks of the continental crust. The other possibility would be the mixing of a primitive magma (point B) with a more evolved magma (Fig. 1d). In both case, mixing shifts the magmatic composition into the chromite field and at the beginning of crystallization, chromite will be the only phase to form. Hence, chromite-rich layers can be formed. This elegant and simple model is supported by observations in the field, mineralogical, petrological, and geochemical work (see Kinnaird 2005, Spandler et al. 2005). A remainder about the ultramafic igneous rocks (simplified, after IUGS classification): References and literature to read Duke, J.M., 1983: Ore Deposit Models 7. Magmatic Segregation Deposits of Chromite. Geoscience Canada, Journal of the Geological Association of Canada 10, Edwards, S.J., Pearce, J.A., Freeman, J., New insights concerning the influence of water during the formation of podiform chromite. In: Dilek, Y., Moores, E.M., Elthon, D., Nicolas, A. (eds.). Ophiolites and oceanic crust: new insights from field studies and the ocean drilling program: Boulder, Colorado. Geological Society of America Special Paper 349,

6 Irvine, T.N., 1975: Crystallization sequences in the Muskox intrusion and other layered intrusions. II. Origin of chromitite layers and similar deposits of other magmatic ores. Geochimica et Cosmochimica Acta 39, Irvine, T.N., 1977: Origin of chromitite layers in the Muskox intrusion and other stratiform intrusions: A new interpretation. Geology 5, Johan, Z., Le Bel., L., Robert, J.L., Vollinger, M., 1982: Role of reducing fluids in the origin of chromite deposits from ophiolitic complexes. Geological Association of Canada, Mineralogical Association of Canada Program with Abstracts, volume 7, 58. Kinnaird, J.A., The Bushveld Large Igneous Province. org/lom.html [May 2005]. Spandler, C., Mavrogenes, J., Arculus, R., 2005: Origin of chromitites in layered intrusions: Evidence from chromitehosted melt inclusions from the Stillwater Complex. Geology 33, Uysal, I., Zaccarini, F., Sadiklar, M.B., Tarkian, M., Thalhammer, O.A.R., Garuti, G., 2009: The podiform chromitites in the Dağküplü and Kavak mines, Eskişehir ophiolite (NW-Turkey): Genetic implications of mineralogical and geochemical data. Geologica Acta 7,