GEOCHEMISTRY OF BAUXITE

Transcription

1 Laterite and bauxite are the residual product of extreme chemical weathering of pre existing aluminous rocks under favorable geomorphic, climatic and hydro-geologic conditions. In Mainpat plateau, basalt of Deccan volcanism is the rock which had undergone chemical weathering. Highly mobile and soluble constituents like Si, Mg, Ca, Na and K are leached out from basalt and least mobile chemical constituents like Al, Fe and Ti remain at place. Mobility of an element is a function of solubility, sorptive phenomenon and translocation (Dennen et al., 1). Thus geochemical processes of lateritisaton and bauxitisation can be summarized as a selective mobilization, leaching and partial reprecipitation going hand in hand with hydration of the elements that remain in the weathering profile. It is, therefore, quite essential to study the geochemistry of parent rock and residue in respect of major and trace elements which will ultimately lead to understand the process of laterisation/bauxitisation in that particular area. Geochemistry of basalt can be better understand by the major oxide and trace element distribution. Table.1 shows the major oxide concentration of basalt of the area and their corresponding normative mineral composition (Table.). Based on these data different diagrams are prepared to characterize the basalt of the area. The Table.1 shows that the basalt occurring in the area is enriched in titanium content. Normative mineral composition suggest that the basalt is diopside bearing. On the LeBas et al. (184) classification diagram, all the points of the collected samples belongs to andesitic basalt and basalt category (Figure.1). Miyashiro s (14) classification diagram (Figure.) suggest that the basalt of the area is tholeiitic basalt which fall in continental plateform type of Pearce et al. (1) diagram (Figure.). Mineralogically i.e. quartz-anorthite plot (Cation norm) based on Strekeisen and Maitre (1) classification (Figure.4) further confirms that these rocks are basalt. 0

2 Table.1 Major Oxide Composition of Basalt Element SiO Al O Fe O MgO CaO Na O K O MnO P O TiO V O Cr O B B1R B BR B BR Table. Normative Mineral Composition of Basalt Mineral/ Sam no. Q Or Ab An Di Hy Mt Il Ap Ol B B1R B BR B BR

3 Na O+K O 8 4 Foidite Tephrite Basanite Trachybasalt Phono- Tephrite trachyandesite Basalt Tephriphonolite Trachyandesite Basaltic Picrobasalt Phonolite Basaltic andesite Andesite Trachyte Trachydacite Dacite Rhyolite SiO Figure.1 LeBas et al (184) Classification based on SiO and Total alkali content. 4 FeO*/MgO Tholeiitic 1 Calc-alkaline SiO Figure. Basalt discrimination between tholeiitic and calc-alkaline (Miyashiro, 14)

4 FeO* Con. Ocean Island SCI Ocean Ridge Orogenic MgO Al O Figure. Tectonic classification of Basalt (Pearce et al, 1) Q' a b 4 a b * * 8* * 10a* 10b* alkali rhyolite rhyolite 4 dacite (dacite) trachyte Q trachyte 8 Q latite andesite 10 basalt ANOR Figure.4 Streckeisen and Le Maitre (1) classification based on mineral quartz and anorthite percentage (cation norm).

5 In order to classify the different varieties of laterite and bauxite occurring in the laterite profile, a chemical criteria is used in this chapter. Based on the major chemical constituents of laterite and bauxite, particularly Al O and Fe O are used for nomenclature: Table.: General chemical criteria used for classification of different geochemical groups of bauxite/laterite of Mainpat area: Groups/types Al O % Fe O % Bauxite >4 1- Bauxitic Laterite Lateritic bauxite Laterite -1 >40 Soil/Alluvium < <1 Geochemical behaviors of major and trace element of laterite/bauxite of Mainpat deposits have been studied with respect to fresh bed rock. Bed rock samples were collected from exposures. Total 8 samples were analysed for all ten major constituents (Al O, SiO, Fe O, TiO, CaO, Na O, K O and LOI and presented in table.), and trace elements (V, Cr, Ga, Ni, Mn, Co, Cu and Zr are given in table.). The sample location map and table are as follows: 4

6 Figure. Location Map GEOCHEMISTRY OF BAUXITE

7 Table.4: location, elevation and rock types. No. Latitude Longitude Elevation Rock Type (in meter) Bauxitic laterite Bauxitic laterite Lateritic bauxite Bauxite Bauxite Bauxitic laterite Lateritic bauxite Bauxite Bauxite Bauxitic laterite Lateritic bauxite Laterite Lateritic bauxite Bauxitic laterite Bauxite Laterite bauxitic Lateritic bauxite Bauxitic laterite Bauxite Bauxitic laterite Lateritic bauxite Lateritic bauxite Laterite Lateritic bauxite Bauxitic laterite Bauxitic laterite Bauxite Lateritic bauxite Bauxitic laterite Lateritic bauxite Lateritic bauxite Laterite Bauxitic laterite Bauxite Bauxitic laterite Laterite Lateritic bauxite Bauxitic laterite

8 The analytical results reveal that the presume absence of each individual element plays a significant role in the process of formation of bauxite/laterite which is discussed as follows:.1 GEOCHEMICAL BEHAVIOR OF MAJOR ELEMENTS.1.1 SILICA (SiO ) Behavior of silica in laterite/bauxite profile of Mainpat area is very significant. This mode of occurrence of silica indicates a very fast leaching and removal of the same during lateritisation/bauxitisation process, which may be due to high precipitation and good drainage conditions. Higher temperatures also promote silica depletion by causing silica absorptions on Al-hydroxides (Valeton, 1). The behavior of the silica during laterisation was also explained by Krauskopf (1) in term of absorption or replacement of Al and Fe hydroxides. Bauxite it s associated in laterite/bauxite profile shows very low concentration of silica where as laterite has little higher concentration of silica (Figure.1). Behavior of silica with respect to fresh bed rock is not as per principle of depletion and enrichment. However quartz is almost absent or rarely occurs in bauxites. As per XRD mineralogical and chemical result silica in bauxite and laterite is mainly contained in clay minerals, and it is negligible in the form of quartz as also revealed from SiO Vs Al O plot (Figure.). On the other hand silica gradually decreases from % in basalt, 0% in laterite and then up to % in bauxite (Table.1). Krauskopf, (1) suggested that silica is no more soluble in near neutral condition than in acid. Absorption of this silica on Al-hydroxides gave rise to formation of kaolinite in saprolite. Then further desilication of saprolite and laterite ultimately led to the formation of bauxite. Incomplete desilication or differential leaching due to change in Eh-pH condition is responsible for the appearance of kaolinite in bauxite. The sudden increase in silica in top most ferruginous laterite horizon at Mainpat may be attributed to the formation of clay mineral by silica fixation brought about by certain water consuming vegetation (Valeton, 1). Gradual upward depletion of silica and its fluctuation in laterite profile of the study area.

9 .1. ALUMINA (Al O ) Alumina is relatively less mobile than iron and much less than silica and titania, therefore, it undergoes relative residual enrichment during process of lateritization. It is the ionic potential i.e. ratio of the ionic charge and ionic radius (z/r), that determines the leaching or retention of elements (Goldschmidt, 1). Elements with ionic potential between to. are enriched during lateritization. Ionic potential of Al is., therefore it is considered less mobile elements. It has a limited vertical and lateral mobility and most of these remains within the laterite profile. Laterite profile shows a high enrichment of alumina during lateritization process. Fresh bed-rock i.e. basalt contains average 8% alumina. Bauxite containing high alumina occurs here in the form of isolated patches or lenses forming irregular distribution. The highest alumina is recorded in gibbsite crystal indicate limited Al-migration and absolute enrichment. The mobilized aluminium might have moved through the cavities, deposited and later developed into fine crystals of gibbsite..1. IRON (Fe O ) Ferrous iron is soluble and mobile during surficial weathering however ferric iron is very stable under oxidizing conditions, and is therefore practically immobile during weathering (Mc Farlane, 1). As per Valeton (1), Fe + compounds such as goethite, lepidocrosite and hematite may crystallizei. Directly from solution, by oxidation of Fe + solution, by hydrolization of Fe + compounds, or by oxidation of Fe + organic complexes. ii. From amorphous phases. The Fe O content in basalt (host rock) is 1-0% (Table.) which may be mainly in the form of primary opaque minerals. The Fe O content increases 10-1% in saprolite, -40% in laterites and it drops to 4-0% in bauxites. This behavior shows a strong enrichment of iron with respect to bed rock. However, exceptionally high Fe O content of certain samples dose not appears to be the result of relative enrichment. It may be an absolute enrichment of iron, probably 8

10 Figure. Geochemical Anomaly Map for AlO in Bauxite/Laterite bearing area of Mainpat Plateau, District, Surguja (FeO value at each sample location is given for Comparative study) GEOCHEMISTRY OF BAUXITE

11 Figure. Geochemical Anomaly Map for FeO in Bauxite/Laterite bearing area of Mainpat Plateau, District, Surguja (AlOvalue at each sample location is given for Comparative study) GEOCHEMISTRY OF BAUXITE 0

12 brought about through colloidal transport of iron in ground water. Such pronounced accumulation of iron during bauxitisation of quartz rich rock has been explained by Schellmann (1), as due to the formation of Al-Si compounds which can be easily leached by percolating waters. According to him the process operates only at higher silica concentration and results in removal of aluminium and relative concentration of iron. The considerably higher silica content in bedrock appears to corroborate such a possibility. Iron in the bauxite is mainly found as hematite and goethite. This concept deciphered in the study area as shown in figure.8 and.10. Low iron bauxite ore body is found sandwiched between laterite which indicates process of deferrification by down ward and partial upward movement of iron solution. Laterite variations in iron show a high degree of diversity and are generally of irregular character. In highly ferruginous spongy pockets, which occur within saprolite horizon iron content reaches upto 0%, which indicates downward movement of iron solutions and deposition of the same under oxidation condition..1.4 TITANIUM (TiO ) The titanium content is the characteristic chemical feature of bauxite deposits of Mainpat area. Titanium is the last mobile of the major chemical component of bauxite. Titanium remains in its original place during bauxitisation and very little mobilization and redistribution has been observed (Figure.11). The highest titanium content has been reported by Fox (1) from the basalt derived bauxite deposits of India. The titanium content of Mainpat bauxite ranges between to 1% making it high titanium bauxite (Table.). On the other hand fresh bed rock is low in titanium (-%). This discrepancy makes very difficult to explain the high enrichment of titanium in bauxite with respect to present host rock. However the occurrences of the titanium oxides in bauxite/laterite profile. Shows much positive variation of Fe O from bauxitic-laterite to bauxite while, shows negative relationship with SiO (Figure.1). On the other TiO 1

13 concentration is proportionate to iron oxide and alumina where the original configuration of Fe O /Al O entire laterite/bauxite profile remained unchanged (Figure.1)..1. LOSS OF IGNITION (LOI) At many places LOI is taken as guiding factor for determining the grade of ore, since it can be directly correlated with tri-hydrate content in gibbsite. LOI provide rough estimation of bauxite grades.. TRACE ELEMENTS The geochemical study of trace elements in the laterites/bauxites is of considerable importance in understanding the mechanism of laterisation/ bauxitisation process and deciphering the source rock of such materials. Good deals of trace elements data have been reported from the Trappean bauxite of India. Basalt derived Amarkantak bauxite is characterized by high concentration of vanadium, gallium and copper (Rao, 184). Trace elements studies of some laterite profiles of Vindhyans were conducted by Kolsotra et al (18). Trace element studies of Simaria bauxite deposit was conducted in detail by Kalsotra and Prasad (181). Simaria bauxite which has been derived from shale shows enrichment of Cr, Cu and Zr but concentration is less than the Amarkantak bauxite. To study the behavior of trace elements and their concentration, few selected laterite samples of Mainpat were analysed for main trace elements e.g. V, Cr, Co, Cu, Ga, Ni, Mn and Zr. Analysed data presented in (table.) which provides a comparison of other Indian bauxite deposits of different parentage. In order to determine the enrichment and depletion of minor elements in laterite, their concentration ratios were determined mainly with respect to basalt, as they most suitably explain the chemical and mineralogical characteristics of laterite/bauxite. The behavior of individual, trace elements in the study area laterite can be discussed as follows-

14 ..1 VANADIUM: It shows that fresh bed rocks in the area have vanadium in the range of 10-00ppm, where as basalt of the area have only ppm vanadium. It has its maximum concentration and ferruginous laterite (up to 40 ppm) and decreases further in aluminous laterite (-40 ppm) and bauxite (18-18 ppm). On the basis of concentration ratios, it may be concluded that vanadium is slightly enriched in bauxite of the area, as their concentration ratio is in between 1-. (Bardossy and Aleva, 10). Vanadium is found highly enriched in ferruginous laterite with respect to basalt. Slight enrichment of vanadium in bauxite with respect to basalt is also notable. Enrichment of Vanadium in ferruginous laterites may probably be attributed to more close relationship of V+ (radius 0.A 0 ) to Fe+ (radius 0.A 0 ) and Cr+ (radius 0.4A 0, Goldschmidt, 14)... CHROMIUM: It shows the behavior of Cr in the laterite with respect to bedrock. Cr has its maximum concentration in bauxite zone (1ppm), followed by aluminous laterite (180ppm), saprolite (140ppm) and ferruginous laterite (10ppm). Basalt of this area has ppm chromium respectively. This shows that there is not much significant enrichment of Cr in bauxites with respect to basalt. However, it is enriched in bauxite and ferruginous laterite with respect to basalt. It is also significant that in some bauxite samples of Mainpat Cr concentration goes upto 0 ppm. It implies the affinity of Cr to Fe and Al. Since there is a relative enrichment of Al and Ti in the weathering profile, and there is maximum concentration of Cr in ferruginous laterite. It may be possible to correlate the distribution of Cr with either of Fe, Al or Ti. The ionic potential does not suggest any special association between Ti and Cr. On the other hand Cr has a close resemblance with ferric iron and aluminium in their chemical properties and ionic sizes. Thus the coherence of Cr with Al in laterite may be attributed to the presence of kaolinite in saprolite unit, and gibbsite and boehmite in the laterite and bauxite zone. Appreciable amount of Cr can occur in kaolinite. A high percentage

15 of kaolinite and iron minerals, together with gibbsite and boehmite may give a high concentration of Cr in ferruginous laterite (Mc Laugnlin, 1)... GALLIUM: Concentration of gallium follows the same pattern as chromium. It shows its strong coherence with Al which is attributable to their similar ionic radii. This coherence is reflected by its higher concentration in saprolite (0-40 ppm), aluminous laterite (0-0 ppm) and bauxite (8- ppm). This indicates that Ga will mostly occur in kaolinite within saprolite unit and in gibbsite within aluminous laterite and bauxite zone (Rao & Murty, 181). Laterite/bauxite of the area have gallium concentration between 10-1 ppm, however Ga is about 1 ppm in basalt. This concentration can be attributed to primary opaque minerals like magnetite...4 COPPER, NICKEL AND COBALT: The ionic potential of Cu+ is less than, hence it is expected to pass into ionic solution and there fore it should be depleted. But it is found that Cu is enriched slightly in many of the samples. Aluminous and ferruginous laterite show a slight enrichment of Cu with respect to parent rock. This concentration may have been brought about by events subsequent to bauxitisation as explained (Gordon and Murata 1) for Arkansas bauxite. It may also be due to other factors such as absorption, ph, oxidation and role of organic matter (Zeissink, 1). This element is known to be a good absorbent and its activity increases with free surface. Banerjee (1) and Balasubramaniam (18) also have the same view. Nickel shows depletion throughout the laterite profile. It is strongly depleted in aluminous laterite (concentration ratio 0.), bauxite (0.4) and pisolitic laterite (0.). However, slight depletion is noted in saprolite zone (concentration ratio 0.). As Ni is a moderately soluble element, it readily passes into ionic solution and its depletion as observed in many bauxite samples is as per normal geochemical behavior. 4

16 Cobalt follows almost the same behavior as nickel in laterite profiles of the study area. The relative enrichment of Cu, Ni and Co towards the basal zone of the profile, confirms their known behavior in many these elements are confined to the lattice structure of ferruginous minerals (goethite and limonite) or their variation may be attributed to the relative abundance of such minerals in different zones of the profile. Sahoo et al. (181) have suggested possible association of Nickel hydroxide, partly in the lattice structure and partly absorbed to the surface of goethite. The latter with the progress of weathering slowly migrates downward to the basal part of the profile. The slight enrichment of Nickel in ferruginous laterite and slight enrichment of Co in saprolite may possibly be accounted to this two fold distribution of these elements... ZIRCONIUM: The behavior of Zr is different percentage in the laterite profile of the area. Zr shows enrichment throughout the profile, although its concentration varies in different layers. It has its maximum concentration in saprolite (4 ppm), followed by its concentration in bauxite (11 ppm), ferruginous laterite (8 ppm) and aluminous laterite (80 ppm). Zircon has ionic potential close to ferric ion, yet its maximum concentration in saprolite and its regular decreases with height in Mainpat profile can not be accounted. The mineralogical studies have revealed the presence of Zr in the form of very finely divided zircon both in the bed rock and laterites. Zircon is known to be relatively insoluble and hard mineral and its consequent in the profile in relation to the bed rock. The relative variation in the distribution of Zr with in the profile, suggests a similar distribution pattern of zircon in the original sediments.. DISCUSSION AND CONCLUSION: Following important conclusions can be drawn from above geochemical studies of laterite profiles of the study area- 1. There is an upward gradual increase of alumina, titania and LOI, and decrease of silica from bed rock to the aluminous laterite/bauxite zone.

17 There is a gradual increase in iron from bottom to top with a minimum in the aluminous laterite/bauxite zone and enrichment in the ferruginous laterite and pisolitic laterite crust. There is high content of titanium in aluminous laterite/bauxite layers. Chemical logs of laterite show two distinct chemical breaks i.e. SiO at the contact of saprolite with laterite, and Fe O at the contact of laterite with bauxite. This reflects different physico-chemical conditions for the formation of saprolite, laterite and bauxite.. In certain sections of bauxite lenses, particularly in clay rich bauxite and aluminous laterite titanium percentage is more than 1. This shows that absolute enrichment has occurred in these cases (Valeton, 1). Several samples of bauxite were analysed to study the nature of silica in them. The study has revealed that major part of silica is in the form of kaolinite and free silica is negligible.. Presents the correlation between alumina-silica, alumina-titania and alumina-iron. A good correlation exists between alumina and iron (r=0.04) in bauxites. This phenomenon indicates that deferrification of laterite was more pronounced in the area. Desilication was more prominent during bauxitisation, which is confirmed by marked correlation between alumina and silica (r=0.1844; fig.d). A poor correlation also exists between alumina and titania in bauxites of the area (r=.00,). 4. General geochemistry of trace elements occurring in laterite profiles indicates that saprolite/clay, laterite and bauxite are enriched in V, Cr, Ga, Zr and Ni. A study of enrichment and depletion behavior of these elements in some other laterite profiles of India. Trace elements show following definite relationships with major elements- Vanadium increases upwards up to the laterite zone but decreases further upwards in the bauxite zone. The maximum concentration of V, thus follows the pattern of Fe O enrichment in the profiles.

18 In general, Cr and Ga show enrichment from bed rock upwards to the bauxite zone, almost similar to the concentration of TiO and Al O. Zr, Cu, Ni and Co decreases upwards from saprolite to the bauxite zone, having maximum concentration in the former. The vertical distribution pattern to a large extent is similar to the SiO (Kaolinite) enrichment in the profile.. The Mainpat laterite/bauxite deposits are characterized by having, 0-8% average Al O and 10-0% Fe O. It implies that they are medium alumina and medium iron containing bauxites. However, high alumina bauxite containing more than 8% Al O and less than 4% Fe O is also found in the area. Few samples are also noticed where the average iron content rises as high as 0-%, slightly surpassing the average Al O content. Titanium in these bauxites varies from -1%, hence they are high titanium bauxites (Table.).. Distribution of bauxite over the plateau shows that some bauxite pockets are confined to palaeo-drainage channels and laterites on both sides. This gives an apparent relationship between iron and silica removal and intensity of the drainage.. Study of the triangular variation diagrams indicates that the alteration initially consisted essentially of desilication followed by relative enrichment of alumina in the succeeding stages. These figures also indicate the differences in chemical compositions of laterite profiles.

19 Symbol for following figures - Soil, - Laterite, - Lateritic Bauxite, - Bauxitic Laterite, * - Bauxite 40 0 Fe O Al O Figure.8 Distribution of Fe O in respect to Al O for the sample collected from profile of Bauxite/Laterite 0 0 Al O SiO Figure. Distribution of Al O in respect to SiO for the sample collected from profile of Bauxite/Laterite 8

20 Figure.10 Distribution of Fe O in respect to SiO for the sample collected from profile of Bauxite/Laterite 0 1 TiO Al O Figure.11 Distribution of TiO in respect to Al O for the sample collected from profile of Bauxite/Laterite

21 Al O SiO Fe O Figure.1 Distribution of Al O, SiO and Fe O for the sample collected from profile of Bauxite/Laterite Fe O Al O TiO Figure.1 Distribution of Fe O, Al O and TiO for the sample collected from profile of Bauxite/Laterite 80

22 Figure.14 Distribution of Fe O, SiO and TiO for the sample collected from profile of Bauxite/Laterite 81

23 Table. CHEMICAL ANALYSIS OF MAJOR OXIDES (%). Oxides/ S. no. Na O MgO Al O SiO P O K O CaO TiO MnO Fe O sum

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25 Table. CHEMICAL ANALYSIS OF TRACE ELEMENTS (%). Elements/S no Co Cu V Cr Mn Ni Ga Zr

26 8 GEOCHEMISTRY OF BAUXITE