Handbook of Exploration Geochemistry
VOLUME 4 Regolith Exploration Ge Tropical and Su btropica
Edited by
C.R.M. BUTT Division of Exploration ,doscience CSlRO Floreat Park, WA 6014, Australia
H. ZEEGERS D6partement Exploration BRGM F- 45060 Orli5ans Cedex 2, France
1992 ELSEVIER Amsterdam - London - New York Tokyo
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 211 1000 AE Amsterdam, Netherlands
ISBN O 444 89095 5
Library of Congress Cataloging-in-Publication Dala
Regolith exploration geochemistry in tropical and subtropical terrains / edited by C.R.M. Butt, H. Zeegers.
p. cm. -- (Handbook of exploration geochemistry ; v. 4) Includes bibliographical references and indexes. ISBN 0-444-89095-5 (acid-free) 1. Geochemical prospecting--Tropics. I. Butt, C.R.M.
II. Zeegers. H. (Hubert), 1942- . III. Series. TN270.R45 1992 622 '. 13'0913--dc20 91-34317
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,
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Chapter 1.3
THE FERRUGINOUS LATERITES
D. NAHON and Y. TARDY
INTRODUCTION
Laterites are widespread in the intertropical belt. The depth of weatherin ranges from a few metres to over 150 m depending on the age of the laterite, the regional tectonic activity, the climate, the climatic history and the nature of the parent rock. Although exhibiting a wide variety of colours, textures and petro- graphic features, laterites have markedly homogeneous mineralogical and chemi- cal compositions and do not obviously reflect the parent rocks from which they are derived. They therefore may mask the underlying geology and are a consider- able hindrance in mapping and mineral exploration. Nevertheless, many ele- ments, including Al, Fe, Mn, Co, V, P, Cr, Ni, Cu and Au, may be concentrated to ore grade in the lateritic mantle and become significant targets for exploration.
The term “laterite” is commonly attributed to Buchanan (18071, who de- scribed naturally hardening surficial materials that were used as bricks (Latin: later, a brick) from Malabar, southern India. In some African dialects, red surfcial materials are called brick earth (Maignien, 1966) and laterite refers to blocks used for construction (Prescott and Pendleton, 1952).
For 150 years, a considerable and controversial literature has been devoted to the definition of the term latente and two essentially different positions have emerged. Firstly, many scientists have used ‘ I laterite” to designate those Fe- and Al-rich weathering products, generally formed under tropical conditions, that are either hard or become hard upon exposure to alternate wetting and drying (Pendleton, 1936; Kellogg, 1949). This definition also includes certain highly weathered material with sesquioxide-rich, humus-poor nodules, even though they may be surrounded by earthy material that does not harden (Sivarajasingham et al., 1962). Thus, from this point of view, laterites include bauxites, “ferricretes”, Fe- or Al-duricrusts, I ‘ carapaces”, I ‘ cuirasses”, Pisolith- or nodule-bearing forma- tions and also clay-rich (kaolinite) horizons in which concretions and mottles are present. Secondlg, Maignien (1958, 1966) and Millot (1964) have proposed that “laterite” should be extended to all weathering products that have those chemi- cal and mineralogical characteristics specific to tropical environments, rather than be restricted to those that are hard or potentially hard. In this definition, the term includes materials commonly associated with indurated ferricretes such as red or yellow ferralitic soils, tropical ferruginous soils, kaolinitic saprolites and lithomarges, all of which are soft anh cannot harden.
43
44
tures or change of volume. Petrographic studies show that primary minerals are pseudomorphically replaced by weathering products which may in turn be further fragmented-yet for a single parent grain these may exhibit parallel extinction. These observations are the basis of the concept of isovolumetric weathering (Millot and Bonifas, 1955), namely the chemical replacement of a unit volume of parent rock by an equivalent volume of the weathering product.
Upwards through this horizon, the progress of weathering is expressed petro- graphically by (i) an increase in yellow to red colouration; (ii) partial then complete dissolution of the main primary minerals; (iii) decreasing induration of the rock and a marked increase in porosity; (iv) neoformation of authigenic smectites and kaolinite at the base of the horizon and, above, of predominantly well-crystallized Fe3’-bearing kaolinite associated with amorphous or poorly crystallized Fe hydroxides. Throughout the horizon, however, Fe hydroxides are essentially restricted to the sites previously occupied by Fe-bearing primary minerals. Kaolinites derived from feldspars remain white even though they contain ferric iron.
Horizon 3 The fine grained saprolite is the horizon in which most primary minerals have
been altered to secondary minerals such as aoliite, goethite or amorphous iron oxyhydroxides. Only quartz and resistan5 minerals (e.g. zircon, tourmaline, chromite) remain unweathered or only partly weathered. The horizon has a considerable range in thickness, from a few metres to more than 100 m. It is a product of isovolumetric weathering, so thak the original rock fabric is perfectly preserved and recognizable. There is some redistribution of Fe hydroxides but these are diffuse and no mottles are formed. The fme saprolite is also referred to as lithomarge, “argiles bariolées” and pallid zone. However, the latter t e m can be misleading, as the horizon may exhibit a wide variety of colours, including pale green, yellow, pink, purple, red and brown as well as white.
At the base, the fine saprolite has a fine grained, slightly porous texture, but it becomes massive at higher levels as the clay fraction increases. This increase is due essentially to the secondary precipitation and accumulation of authigenic kaolinite in pores that were created during the dissolution of primary minerals. This is evident petrographically by the presence of clays coating and filling microvoids. The kaolinite consists generally of small platy crystals (0.5 pm) together with minute crystals of goethite which impart an orange colour to the coatings and filings. Because the clay fraction is so abundant, internal tensions due to shrinkjng and swelling resulting from interaction with water may rework the weathering matrix. However, kaolinite is not a highly expandable clay and in general only microstructures are affected to any degree, with macrostructures such as quartz veins preserved high in the profite. This second generation of kwthite is at abmhtte accumulation within part of an isovolumetrically weath- ‘ered horizon (Ambrosi and h’ahon, 1986);. The presence of two types of kaolinite indicates that two slightly different geochemical weathering systems are acting
F
z ~ -
I
I
simultaneously to form two different facies within the one horizon, i.e. a porous and a massive facies, Kaolinite and goethite coexist in these facies as long as th’ey remain below the water-table. No ferruginous concretions are observed to form or develop in the permanently saturated (phreatic) zone (Nahon, 1976; Muller et al., 1980).
Horixon 4 The mottled zone overlies the fme saprolite above the water-table, in the
unsaturated (vadose) zone. Here, the pre-existing macrostructure of the parent rock is progressively destroyed. Two major types of reorganization affect this horizon. Firstly, vertical and lateral percolation of water in the unsaturated zone leads to the formation of a network of channels and tubular voids of large diameter (average 10 mm). Secondly, ferruginous spots (5-150 mm) and nodules (10-30 mm) develop, becoming both more abundant and more indurated to- wards the top of the mottled zone. This implies that iron is mobilized from areas around large (> 1 mm) pores and reprecipitated and concentrated in clay-rich areas (Tardy and Nahon, 1985).
The channels and tubular voids act as receptacles for the secondary accumula- tion of kaolinite. The kaolinite occurs as coatings and fillings, and consists of minute, randomly distributed particles of detrital or authigenic origin. It has been derived from overlying horizons or upslope, either by physical translocation or dissolved in solution, and then deposited or reprecipitated lower in the profile or downslope. Because the kaolinite void fillings contain small pores, they can subsequently be ferruginized into indurated nodules.
The ferruginous segregations that accumulate in clay-rich areas commonly consist of purple-red Al-hematite. As such, they are the product of the epigenetic replacement of kaolinite by hematite (see Fig. I.3-2), that is, the simultaneous
I
1 2 3 Fig. 1.3-2. Sketch showing the epigenetic replacement of kaolinite ”booklets” by hematite (from Ambrosi et al. (1986)). I : kaolinite booklets; 2: relicts of the same partially replaced booklet, all retaining the same orientation (arrows); 3: purple-red hematitic plasma.
46
dissolution of kaolinite and precipitation of hematite (Nahon, 1976; Tardy and Nahon, 1985; Ambrosi et al., 1986). Indurated nodules become more numerous towards the top of the mottled clay horizon so that the pre-existing lithostructure is progressively obliterated and replaced by a newly formed pedostructure. However, original volumes are commonly preserved and any quartz veins that are present are strongly corroded but not displaced.
Horizon 5 The iron crust horizon overlies the mottled zone and consists of several
different facies. It develops where the purple-red hematitic nodules that form in the mottled zone become more numerous, finally. coalescing into an indurated cuirasse (ferricrete). Dissolution and replacement of the kaolinite is at a maxi- mum when hematite is most abundant and most aluminous (7-15 mole % Al,O,).
THICKNESS OF MtNERALOGlCAL MICROPROFILES ASSOCIATION
i
I
; 28.531 F
+ SIBBSITE
ì E T H I T E +
(AOLINITE
I
GETHITE
LAOLINITE
JUARTZ
+
7
!
JONTRONITE
KAOLINITE t
OUARTZ
+
MONTMORILLONITES
+ OUARTZ
+ MICROCLINE
+ CALCITE
GRAN I TE
Fig. 1.3-3. Profile formed by artificial weathering under conditions simulating seasonally humid climates (Fritz, 1975).
TABLE 1.3-1 . - _
Element gains and losses during transformations involved in the development of some horizons of the laterite profile
Horizon formed Leached Accumulated
Pebbly layer - Alumina Iron Pisolitic cuirasse Silica Alumina, iron Nodular horizon Alumina, silica Iron Saprolite Alkalis, alkaline
earths, silica
The iron crust is thus nodular, with a conglomeratic structure, but has formed by in-situ accumulation of iron oxides and is not detrital in origin.
Towards the top of the conglomeratic iron crust, a cortex develops along microcracks and microfissures through the purple-red nodules. The cortex con- sists of aluminous goethite containing 15-25 mole % AlOOH and grows in a concentric, centripetal manner by the rehydration and replacement of Al-hema- tite by Al-goethite, resulting in the formation of a pisolitic structure. Initially, the pisolitic iron crust remains indurated. However, towards the periphery of the cortex, the goethite becomes progressively poorer in aluminium and at the same time the pisoliths diminish in size and become separated due to the development of cracks and dissolution features, so that the cohesion of the horizon is reduced. Finally, almost pure goethite, associated with minor kaolinite, forms in large voids between pisoliths and in cracks and fissures. Thus, the ferruginous pebbly horizon can be considered to be the result of the geochemical degradation of the indurated, cemented iron crust which it overlies.
The formation of the hematite nodules and the mineralogical succession goethite-hematite-goethite observed from the mottled zone to the cuirasse and pebbly layer is due to fluctuations in water activity in the profies during the alternation of dry &nd humid seasons (Tardy and Nahon, 1985). Profies similar to those described have been formed experimentally by simulating weathering under such seasonal conditions (Fig. I.3-3; Flitz, 1975).
The lateritic profie is the result of the progressive development of each horizon by a transformation of that immediately underlying it by the differential mobility of the elements of which it is composed (Table 1.3-1). When seasonally humid climates are long established, the downward movement of the transforma- tion fronts (Chapter 1.4) leads to progressive deepening of profies and sequences at the expense of fresh rock (Nahon, 1976).
I
I
CHEMICAL MASS BALANCES AND DIFFERENTIATION OF LATERITIC HORIZONS
The evolution of a typical lateritic profie takes place in two stages. The first stage is isovolumetric and preserves pre-existing rock structures and volumes. It
48
extends through the saprock and saprolite and, in some situations, to the mottled zone and even to the soft, nodular facies of the iron crust, the carapace. The second stage destroys pre-existing rock structures, commonly with some loss of volume. It affects the uppermost horizons of the profile, particularly the cuirasse.
A quantitative mass balance of the isovolumetric stage of weathering clearly displays the relative accumulation of Al and Fe as a consequence of
(1) complete leaching of alkaline and alkaline earth elements, and (2) partial dissolution of silica (Lacroix, 19 14). The residual elements Si (as quartz), Al, Fe and Ti are considered immobile,
but nevertheless may be partly removed (e.g. 25% reductions of both Si and Al) or weakly enriched (increases of 30% in Fe, 50% in Ti) in the fine saprolite and, particularly, the mottled zone (Tardy, 1969). Although thé relative loss or concentration of Fe depends on the prevailing redox conditions, both Fe and Al are almost entirely retained in the saprolite.
In the uppermost horizons, isovolumetric mass balances can only be calcu- lated between individual primary minerals and their pseudomorphic weathering products. The hematitic nodulation that develops at the expense of kaolinite in the mottled horizon and leads to the formation of the carapace and cuirasse is typical of the processes involved. Ambrosi and Nahon (1986) calculated mass balances from the mineralogy and porosity of specific features. Mineral composi- tions were established by estimating the percentage of each phase by thin section petrography and X-ray diffraction. Porosities were estimated from thin sections by optical and scanning electron microscopy and compared with values calcu- lated from the difference between particle and bulk densities. The resultant mass balances are only approximations because they do not account for ions that have to be released into the solution to equilibrate with the newly formed minerals. The reactions are:
7.37 A12Si,05(OH)4 + 1.28 Fe,O, 4- 21 -74 Fe,O, --$
initial kaolinite initial hematite incoming in solution
0.13 Al2Si2O5(0H), + 23.98(0.96 Fe,O, + 0.04 Al,O,)
+ 14.48 SiOz 4- 6.38A120, -I- 14.48 H,O
remaining kaolinite 4% aluminous hematite nodule
removed in removed in removed by solution solution evaporation
This mass balance corresponds to:
14.48/14.74 = 98% of silica released;
14.48/14.74 = 98% of water released;
21.74/23.02 = 94% of iron imported;
6.38/7.37 = 87% of aluminium released.
49
In the upper part of the iron crust, the Al-hematite is in turn altered to Al-goethite. The mass balance reaction of the alteration of the transformation of a nodule to a pisolith is as follows:
23.98 (0.96 Fe,03 + 0.04 A1203) + 0.13 Al,Si20,(OH)4 + 28.5 H,O 4% aluminous hematite remaining kaolinite rainwater
+ 4.54 Alzo, -+ 57.60(0.80 FeO(0H) + 0.20Al(OOH)) + 0.26 SiO, imported in 20% aluminous goethite in cortex removed in
solution of pisoliths so 1 ut i on
This corresponds to:
100% of silica removed in solution;
79% of alumina imported in solution
99% of water imported as rain;
100% of iron conserved.
A quantitative mass balance calculation proposed by Brimhall and Dietrich (1 987) for weathering and supergene enrichment relates the chemical, physical, volumetric and mechanical properties of the weathering products to those of the parent material ( I ' protolith"). Although this is theoretically a more rigorous approach, it does not take into account the lateral flow of water which is evident in the development of profiles as old as ferruginous laterites. Because primary microfabrics may be preserved essentially undeformed high in the weathering profile, mass balances may be effectively calculated assuming isovolumetric weathering as described above.
RATES OF CHEMICAL WEATHERING AND MECHAh'ICAL EROSION
The rate of formation of profiles and of the progress of chemical weathering as a whole can be estimated from knowledge of the hydrological regime and of overall losses of soluble elements by leaching. Mass balance calculations based on such data may be used to determine the time required to weather a given thickness of rock or to estimate the thickness of rock that can be weathered in a given period of time. Such calculations have been made for a number lithologies and regions (Table 1.3-2). The data suggest an average rate of weathering of about 20 mm/1000 years, i.e. 20 m per million years. In lateritic environments, the rate appears to be about two or three times faster over basic and ultrabasic rocks, as noted by Nahon (1986).
The rate of chemical weathering is largely dependent upon, and directly Proportional to, the quantity of water that percolates through the profile each
-
50
TABLE 1.3-2 Rates of chemical weathering
Location Weathering rate Reference . Lithology mm/1000 years
Granitic rock Ivory Coast 5-50 Leneuf (1959) Granitic rock Norway 12 Tardy (1969) Granitic rock France 10-24 Tardy (1969) Granitic rock Ivory Coast 15 Tardy (1969) Granitic rock Malrrgasey Rep. 25 Tardy (1969) Granitic rock Ivory coast 14 Boulangé (1984) Granitic rock Chad 9.4 Gac (1979) Ultramafic rock New Caledonia 29-47 Trescases (1975)
year. This, in turn, is approximately equivalent to the difference between precipi- tation and evaporation. As this quantity decreases, and the length of the dry season increases, mechanical erosion becomes more significant. Thus, in the Central African Republic, the rates of mechanical erosion and chemical weather- ing are nearly equal, at 8.3 "/lo00 years and 9.4 "/lo00 years respec- tively (Gac, 1979). The region has a rainfall of 1210 mm p.a. and evaporation of 1060 mm p.a., so that only 150 mm percolates through the profile each year.
STABILITY FIELDS OF HYDRATED AND DEHYDRATED MINERALS
The most common minerals in ferruginous and bauxitic laterites are Al- goethite, Al-hematite, gibbsite, boehmite and Fe-kaolinite. Diaspore is relatively uncommon, except in karst and sedimentary bauxites, and corundum, although usually present, is rare (Valeton, 1972; Boulangé, 1984). The distribution of these minerals is a function of (i) climate, (ii) age, (iii) lithology,.(iv) degree of compaction and, where appropriate (v) temperature and grade of metamorphism (Bardossy, 1982).
The principal parameters governing the stability of these minerals are water activity, temperature and grain size (Didier et al., 1983; Trolard and Tardy, 1987). In tropical climates with alternating dry and humid seasons, water activity is the crucial factor and can be defined as the relative humidity (RH%) of the air:
u , ~ = [H,O] = p / p , = RH/100 < 1
where p and p , are the partial vapour pressures of the air and of saturated air, respectively, at temperature T. Water activity is always smaller than unity and is related to the radius of the meniscus of water trapped in capillaries of the same radius, as follows:
RT ln[H,O] = 2V cr/r
i
l
!
I
1
i
1 ! 1
,
! 1
!
t
i f i
i
f
Y
52
At 25'C
Moles of Alzo3 per 1 mole of Fe, O, > d.-
.- 5 0.0 0.5 1.0 I
21.0
Goe,, + Gib
22.5 0.8
-
O 113 112 Mole fraction : A'203
(Fe, O3 +Al, O3 1 Fig. 1.3-4. Equilibrium diagram for Al-goethite, Al-hematite, gibbsite and boehmite at 25°C in the system H,O, Al,O, and Fe,O,.
For Al-rich systems:
Al-goethite + gibbsite * Al-goethite + boehmite --j
Al-hematite + boehmite 4 Al-hematite -+ (corundum).
(4) In mineral associations such as (Al-goethite + gibbsite) and (Al-goethite + Al-hematite), the Al-goethite or Al-hematite compositions do not depend on the composition of the system but only on the water activity or temperature. The Al contents increase as water activity decreases or temperature increases.
(5) When Al-goethite and boehmite occur together, the Al-goethite composi- tion is independent of the composition of the system, the temperature and the
53
> > o m al
L .- .- L
L
L s .. n o 0.9 I" U
0.8
0.733 0.7
0.6 0.5 0.3 o .1 0.0
At 5'C
Moles of Al2 O, per 1 mole of Fe2 O3 0.5 1.0
GoeAl
I
13.4 0.9
GoeA,+Gib
15.1 -- 0.8
I 2 I -. I i\
-Hem,,+ Boe
O I f 3 II2 Mole fraction
Fig. 1.3-5. Equilibrium diagram for Al-goethite, Al-hematite, gibbsite and boehmite at 5°C in the system H,O, Alzo, and Fe2O,.
water activity. The Al content of goethite is almost constant at (Alo~,,,Feo~-,,)OOH at 25°C. The Al-hematite and corundum association has a similar relatio,.;hip.
(6) When only Al-goethite or Al-hematite are stable, their Al content is independent of water activity and temperature. The Al-goethite and Al-hematite compositions depend only on the Al,0,/(Al,03 + Fe,O,) ratio of the system.
These observations suggest that water activity and temperature are the two principal factors controlling the distribution of minerals in ferruginous laterites and bauxites. The total composition of the system is also important, particularly because goethites and hematites are Al-bearing minerals as well as Fe minerals. However, particle size may also be important because it affects the solubility products and Gibbs free energy of formation of natural iron oxides, oxyhydrox- ides and hydroxides such as hematite, maghemite, goethite, lepidocrocite and morphous FeO(0H).
I
l
!
54
>. 4- .- '5 0.0 * o m P)
- L
4..
5 .. n 0 0.9- I" U
0.8 -
Her 0.7-
0.6- 0.5- 0.3 -
- - i
At 55'C
Moles of Alz O3 per 1 mole of Fez03 0.5 1 .o
I HemAl+ Boe
11.2 -- 0.4 ~ 12.5
17.7 -. I ( D E 'lh z I ! 0.006 o.o\
O 113 1/2
Mole fraction Fig. 1.3-6. Equilibrium diagram for Al-goethite, Al-hematite, gibbsite and boehmite at 55°C in the system H,O, AJ,O,, and Fe,O,.
In conclusion, the distribution of Fe and Al minerals in ferruginous laterites and lateritic bauxites show that the hydrated minerals (goethite and gibbsite) are more abundant in humid tropical climates (rainforests) whereas dehydrated or partially hydrated minerals (boehmite and hematite) are more abundant in the seasonally humid tropics (savannas). In addition, the dehydrated minerals are more common in the drier, upper horizons than in the lower, wetter horizons close to the water-table. Within the upper mottled, nodular and pisolitic horizons, the mineral distribution is also related to the petrographic fabric, with the dehydrated forms tending to concentrate in the centre of concretions and hydrated minerals on the periphery. Thus, in ferruginous laterites, pisoliths and nodules have a core of hematite and a cortex of goethite, whereas in bauxites, boehmite forms the core and gibbsite the cortex and cement.
