Classification of geochemical exploration models for tropically weathered terrains

Journal of Geochemical Exploration, 32 (1989) 65-74

65

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Classification of geochemical exploration models for tropically weathered terrains C.R.M. B U T T I and H. Z E E G E R S 2

IDivisionof Exploration Geoscience, CSIRO, Wernbley, W.A. 6014, Australia 2Ddpartement Exploration,B R G M , B P 6009, 45060 Orldans Cedex 2, France (Received August 11, 1987; revised and accepted July 14, 1988)

ABSTRACT Butt, C.R.M. and Zeegers, H., 1989. Classification of geochemical exploration models for tropically weathered terrains. In: S. Jenness et al. (Editors), Geochemical Exploration 1987. J. Geochem. Explor., 32: 65-74. Generalized exploration models can be established for deeply weathered terrains,based on the degree of preservation of pre-existing profiles formed under predominantly humid, tropical climates, and the modification due to laterclimatic and tectonic events. The models thus integrate the fullweathering and erosional historiesrather than considering only the present morphoclimatic environment. The following factors are important in classifyingthe models: - preservation of the pre-existingprofile,i.e.whether itiscomplete, or partly or wholly truncated; intermediate and present climates, and related chemical and mineralogical alteration of the former profile; - presence and nature of the overburden. The proposed models allow, to some extent, prediction of the nature of secondary geochemical haloes, including the dispersion characteristicsof given elements or suites of elements. Such information enables the most appropriate operating parameters (e.g.sample media, sample spacing, interpretational criteria) to be selected for geochemical surveys of specific areas and environments. -

INTRODUCTION

An understanding of the geochemical environment is essential for the efficient application of geochemical surveys, whether at regional or local scales. However, the amount of data available on exploration geochemistry and related disciplines such as economic geology, soil science and geomorphology, is increasing at a rapid rate. Accordingly, it is very important that the most relevant information is summarized in a readily accessible and understandable form. This can be achieved by developing conceptual and methodological models that represent data and interpretations for particular generalized situations, 0375-6742/89/$03.50

© 1989 Elsevier Science Publishers B.V.

66 recommend exploration procedures, and provide a framework into which further information may be placed. Although geological, geomorphological and environmental conditions are different for each location, so that the geochemical response to mineralization is always unique in some aspects, many similarities in dispersion characteristics may nevertheless be present over extensive regions. These are the characteristics that can be synthesized on geochemical exploration models to illustrate the nature and origin of the surface expression of mineralization. Ideally, it should be possible to use such models predictively when planning surveys, to anticipate mechanisms of dispersion, select appropriate sample media, and estimate the nature and significance of anomalies. Geochemical exploration models have been derived for some specified geographical and geological regions in North America (Bradshaw, 1975; Lovering and McCarthy, 1978), Northern Europe (Kauranne, 1976) and Australia (Butt and Smith, 1980) based on the intensive exploration activity and experience in these areas. For much of the tropics and subtropics, however, less information is available and useful models are less easy to derive for equivalent regions. However, similarities in the dominant mechanisms of weathering and geochemical dispersion provide a basis for the establishment of exploration models appropriate for all tropically weathered terrains (Butt, 1987). In this paper, a classification of these models is presented, with the objective of permitting useful comparisons between exploration experience from widely separated areas, perhaps in currently very different climatic zones. WEATHERING HISTORY AND LANDSCAPEGEOCHEMISTRY The geochemical expression of mineralization in any environment is the product of the chemical and physical processes of dispersion - essentially the same processes as those of weathering, soil formation and landscape development. Climate is an independent factor in determining the nature of these processes and may become dominant if it remains unchanged over long periods of time. Thus, soils characteristic of a climate may develop in 102-103 years, deep weathering and lateritic profiles in 103-108 years, and landscapes in 107 years. However, climates have changed frequently and often quite profoundly within these time scales, particularly since mid-Tertiary time, so that relic landforms and regolith materials are present (Budel, 1982). In general, only the more recent, extreme or longer established climatic regimes leave relics of geochemical significance. Whereas temperate and highlatitude regions are dominated by the effects of Pleistocene glaciation, the warmer lower latitudes have regoliths and associated landforms related to past (or continuing) periods of deep weathering and lateritization under seasonal tropical conditions. The older, inherited regoliths are important not only because of dispersion associated with their development but also because of the

67

effect they have on currently active processes. Models must therefore describe the genesis and total geochemistry of the landscape to account for dispersion related to these relic features as well as to active processes. The "landscape geochemistry approach" (Fortescue, 1975) has been adopted in all attempts to derive geochemical exploration models and is particularly important for tropically weathered terrains. Models based on the preservation of characteristics of deeply weathered terrain apply particularly to the region between latitudes 35°N and 35 ° S, although similar features have been observed at much higher latitudes - in places beneath glacial overburden. This corresponds approximately to the inner tropical (equatorial rainforest), peritropical (savanna), and warm arid morphoclimatic zones (Budel, 1982). These zones were all subjected to seasonally humid tropical climates, broadly similar to those of the present wetter savannas, during the time from the Cretaceous period or earlier, until the mid-Tertiary. Some of the geochemical effects of weathering under these conditions are summarized in Table 1. Although local factors such as geology, relief and microclimate influenced the precise nature of the profiles that formed, intensively weathered and leached regoliths (including laterite) having broadly similar characteristics were widely developed in South America, Africa, Australia and southern Asia. Regolith development was accompanied by more or less marked planation of the landscapes. Tectonic events and climatic changes lead to physical and chemical modification of the deeply weathered regolith mantle. Some of the principal effects are listed in Table 2. For example, tectonism commonly changes erosion rates and alters the drainage status (and hence the intensity of leaching) of the regolith, and climatic changes (some induced by tectonic uplift or plate movements) affect the nature of weathering reactions and products, and rates of TABLE 1 S u m m a r y of the chemical effects of deep weathering 1.

Leaching of mobile constituents: - alkalis, alkaline earths.

2.

Formation of stable secondary minerals: - clays (principally kaolinite), T i a n d A1 oxides.

3.

Partial leaching of less mobile constituents: - silica, alumina, titanium.

4.

Mobilization a n d partial re-precipitation of redox-controlled constituents: - iron, manganese.

5.

R e t e n t i o n a n d residual concentration of resistant minerals: - zircon, chromite, quartz.

68 TABLE 2 Effects on the lateritic regolith of changes from the conditions of formation

A. Tectonic activity Uplift: lowering of the water-table; irreversible dehydration and hardening of ferruginous and siliceous horizons; increased leaching of upper horizons under more

oxidizing conditions; -

increased erosion.

Downwarping: -

waterlogging of lower parts of the landscape and imposition of reducing conditions: decrease in erosion, increased sedimentation in valleys.

B. Climatic change To a more h u m i d climate: -

increased leaching and deeper soil development; decreased erosion (due to thicker vegetation);

To a less h u m i d climate: -

decreased leaching; increased erosion;

To a semi-arid or arid climate: -

decreased leaching; retention and precipitation of silica, alkaline earths and alkalis in silcretes, clays, calcretes, salts; increased erosion

leaching and erosion. Despite these modifications, however, many geomorphic and geochemical characteristics of this early weathering have been preserved, either because of the armouring effect of duricrusts or the ineffectiveness of later erosional processes, particularly in continental landmasses of low relief. The erosion that has occurred has resulted in a variety of landforms, many of which reflect the nature and distribution of the regolith in which they are developed and hence may occur in a range of climatic and geological environments. Similarly, since the facets (slope elements ) that comprise individual landforms are defined in terms of the regolith materials themselves (e.g. duricrust, saprolite, colluvium, unweathered rock), they also have equivalents in a similar range of environments. Because, in most instances, the chemical effects of tectonism and climatic change are less severe than those of the earlier deep weathering, this equivalence extends to the geochemical characteristics of the regolith (Butt, 1987). The proposed model systems have essentially the same basis as the models

69 TABLE 3 Classification of models in tropically weathered terrains

Present climate: Savanna (seasonallyhumid) W a r m arid Rainforest (humid) Modifications to pre-existing profile within each climatic zone: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pre-existing profile

Recent leaching

Recent accumulation or-neoformation

Overburden

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A: mostly preserved

B: partly truncated

0: none

0: none

0: none

1: low

AI: Al-silicates or oxides Ca: calcrete

1: in situ

2: moderate 3: strong

C: fully truncated

2: transported

Fe: iron oxide Si: silica Sm: smectite

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Examples:

A 0 0 (0): outcropping lateritic cuirasse. B 0 Ca (2): truncated profile with pedogenic calcrete and transported overburden.

derived for Australia (Butt and Smith, 1980), namely the presence of relics of the pre-existing relief and regolith and the effects (if any) of later physical and chemical modifications. The models are classified (Table 3) according to:

(1) Present climate: this governs active weathering and dispersion processes, including the formation of soil from the exposed horizons of the pre-existing regolith. The amount and seasonality of the rainfall control the intensity of leaching. This is greatest in rainforest regions having high rainfall (> 1600 mm per annum) and a short dry season, and is least in semi-arid to arid regions with low rainfall ( < 600 m m per a n n u m ) and a long dry season ( > 7 months), conditions which permit the retention of alkaline earths within the soil sequence, e.g. as calcretes. (2) Degree of preservation of the profile: this largely determines the nature of the uppermost residual horizon, whether modified as soil or buried beneath transported overburden. Where profiles are fully preserved (A-type models), this is commonly the ferruginous lateritic horizon, or soils developed over or from it. Where the profiles are partially truncated by erosion (B-type models), however, deeper saprolite horizons with quite different geochemical characteristics are exposed. Where the earlier profile has been entirely removed (C-type

70 models), either the rocks are exposed at surface, or soils are forming directly from them. Since the horizons are generally poorly characterized, vary markedly in depth and composition according to lithology, and have ill-defined boundaries, a more detailed classification is not feasible. (3) Chemical modification: marked chemical alteration of the profile may have occurred under the present climate or during an intermediate period since the early deep weathering. It can take the form either of further leaching or, conversely, the retention and/or accumulation of constituents that previously were mobile. Leaching is mostly associated with a change to humid, rainforest climates and the formation of stone-line profiles; however, significant remobilization and leaching of some elements can occur as a response to other environmental change, such as the decline of water tables and the development of saline groundwaters due to the onset of aridity. The dehydration of surficial horizons and the general decrease of leaching that occurs with drier climates results in the formation of different secondary minerals, such as smectites instead of kaolinite, and accumulation of and cementation by precipitates such as silica (as silcrete) and calcite (as calcrete). These chemical modifications have cumulative effects and can produce the most complex geochemical responses. Repeated leaching and/or the absolute accumulation of introduced components can cause strong impoverishment or dilution of most geochemical pathfinders, especially in near-surface horizons. (4) Presence and nature of the overburden: the materials that form the surface layer are commonly critical in determining the most appropriate exploration sample media. There may be no overburden, so that untransformed weathered bedrock (e.g. lateritic cuirasse, saprolite ) or fresh bedrock outcrop, or the overburden may be in situ (as residual soil) or transported (e.g. alluvium, aeolian sand, talus ). This classification (Table 3) is, of course, very simplistic, for where there have been frequent climatic and tectonic changes, complex sequences exist. Nevertheless, it provides a framework that enables valid comparisons to be made between terrains that are now in quite different climatic zones but that have important geochemical similarities. For example, the conditions that face the exploration geochemist in semi-arid parts of Western Australia (annual rainfall 200-300 mm ) are similar to those prevailing in the savanna climate of South Mali (rainfall 1200 mm), because both regions are linked by their dependence on former lateritic weathering episodes. This similarity is shown by a comparison of landform/regolith sections for the northern Yilgarn Block, W. Australia (Butt and Sheppy, 1975) with those for West Africa (Zeegers and Leprun, 1979). Block diagrams illustrating the classification of the landform environments and the dispersion characteristics of some models are shown on Figs. 1 and 2.

71

PREEXISTING PROFILE PRESERVED

A.2.0.(1) ~

Ferrollilicsoil

Portlyde0raded

PREEXISTING PROFILE TRUNCATED B.3.0. (~) B.O.O.(z)

~ ).lljg,:.~o,. ° ~

cOlo)rtz stone-lineli,il~)!J/, ICloy

Humid

(r°int°resi) lii';'/l j

Saprolite

I ~lll t I Saprolile

~ ~

A.O.O.(1) $oiI Nodules Savanna

Cuirosse Mottled clay Saprolite

B.O.O.(1)

I]ii/(ii I Saprollte B.O.O.(2)

Soil Clay Soproiite

A.I. Co,Sin.(1) B.O.Ca,Sm.fl) Pisolites,nodules Colcrete Calcrete Mottled clay Cloy

Semi-arid

Soprolite

.,,ov~um

Soprolite

~ Alluvium I l!l~! ill,JClay

ii~;iiI

Soprolite

B.O. Srn.(2) ~ Alluvium ] ~'1 r I Clay

it);i))jSaprolit i~))l e

Fig. 1. Comparison of some weathering profiles and model classifications in a partly eroded, tropically weathered landscape. Equivalent models in similar landform situations in different climatic zones differ mainly through modifications to the regolith subsequent to deep weathering.

Partial erosion of the pre-existing regolith as a response to uplift a n d / o r climatic change results in similar landforms and hence a similar range of dispersion models being developed in each climatic region. The only differences between equivalent models are due to the mostly minor modifications to the regolith, which tend to have a quantitative rather than qualitative effect on dispersion patterns. Where the pre-existing profile is fully preserved (A-type models), dehydration and hardening of the lateritic ferruginous zone form a cuirasse. This is highly resistant to degradation, especially in the drier savannas and arid zones, where it is commonly exposed or covered by a thin lithosol. Degradation may, however, occur. In arid areas, the expansive growth of calcite during calcrete

72

A..(1) dispersion mode s

B..(1)

B..(2)

dispers on models

dispersion mode s

\,

\

5o <

/

Lc(R,H,M) No So Lc

/

// /

/

Br~ Br = bedrock

So = saprolite

Me : mottled clay

LC : leteritic cuirasse

NO = n o d u l e r horizon

So = soil

Fig. 2. Typical dispersion haloes in a partly eroded, tropicallyweatheredlandscape. Stippling indicatesdispersionhaloes (R = residual;M= mechanical;H= hydromorphic).N.B. Not to scale; anomalysizes and contrasts illustrativeonly). formation disaggregates cemented zones, permitting soil development and erosion. In rainforests, chemical reworking of cuirasse results in the formation of ferrallitic soils and, in highly leaching environments, in the development of lateritic stone-lines overlain by friable, yellow-brown silty soil several metres deep (Lecomte, 1983, 1988). Nevertheless, sampling of the lateritic material, whether intact or degraded, will probably give similar responses in any climatic zone. If soils are sampled, anomalies will be depressed, either by dilution by calcrete (and perhaps some aeolian material) in arid areas, or by leaching in rainforests. Where the pre-existing regolith is partly eroded (B-type models), the ge°chemical responses in the near-surface will closely resemble those of the original saprolite, modified by the same later processes - e.g. calcrete formation or leaching. A cover of transported overburden usually obliterates the surface expression of mineralization, and it may be necessary to drill for samples, the nature of which will depend upon the degree of truncation of the profile before deposition. Hydromorphic and biological activity may, however, give rise to anomalies within the transported overburden, but these are usually a response to present conditions. Deeper in the regolith, changes tend to be less pronounced though there may be changes in leaching rates and the mobilities of certain elements. In drier savannas and arid zones, for example, reduced leaching permits the retention of Mg and the development of smectites, especially in lower parts of the landscape.

73 CONCLUSION

Geochemical exploration models can be established for much of the earth's land surface between latitudes 35 °N and 35 ° S, based on common features of their weathering and erosional histories. These terrains are characterized by the presence of deep, mostly lateritic, regoliths, formed under conditions probably equivalent to the present-day humid savannas, which have been modified in response to subsequent climatic and tectonic changes. The model systems and classifications that have been proposed are, of necessity, very generalized and incomplete. At this stage, they cannot fulfill in any detail the objectives for which models are established, namely to summarize known data and to be used predictively in planning, executing and interpreting geochemical surveys. Nevertheless, they provide a means by which the nature and origin of the landforms, regolith and geochemical dispersion characteristics of a terrain may be understood and by which similar terrains elsewhere may be recognized for purposes of comparison. The establishment of more comprehensive models must await the acquisition of further case histories, and will be aided if these are well documented with respect to geomorphology, regolith and geochemical dispersion so as to enable the classification of the data to be recognized within this or a similar scheme.

REFERENCES Bradshaw, P.M.D. (Editor), 1975. Conceptual Models in Exploration Geochemistry - The Canadian Cordillera and Canadian Shield. J. Geochem. Explor., 4: 1-213. Budel, J., 1982. Climatic Geomorphology. Princeton University Press, Princeton, NJ, 443 pp. (translated by L. Fischer and D. Busche, 1982). Butt, C.R.M., 1987. A basis for geochemical exploration models for tropical terrains. Chem. Geol., 60: 5-16. Butt, C.R.M. and Sheppy, N.R., 1975. Geochemical exploration problems in Western Australia, exemplified by the Mt. Keith area. In: I.L. Elliott and W.K. Fletcher (Editors), Geochemical Exploration 1974. Assoc. Explor. Geochem., Rexdale, Ont., 391-415. Butt, C.R.M. and Smith, R.E. (Editors), 1980. Conceptual Models in Exploration Geochemistry - Australia. J. Geochem. Explor., 8: 89-365. Fortescue, J.A.C., 1975. The use of landscape geochemistry to process exploration geochemical data. J. Geochem. Explor., 4: 3-7. Kauranne, L.K. (Editor), 1976. Conceptual Models in Exploration Geochemistry - Norden. J. Geochem. Explor., 5: 175-420. Lecomte, P., 1983. Profils d'alteration ~ stone-lines au Gabon: premieres hypotheses g~n~tiques. In: Principaux Resultats Scientifiques et Techniques du B.R.G.M. 1983 - P~sumSs. B.R.G.M. (Bur. Rech. Geol. Min.), Paris, p. 211 (abstract). Lecomte, P., 1988. Stone-line profiles; Importance in geochemical exploration. J. Geochem. Explor., 30: 35-62.

74 Lovering, T.G. and McCarthy, J.H. (Editors), 1978. Conceptual Models in Exploration Geochemistry - The Basin and Range Province of the Western United States and Northern Mexico. J. Geochem. Explor., 9: 113-276. Zeegers, H. and Leprun, J-C., 1979. Evolution des concepts en alterologie tropicale et consequences potentielles pour la prospection g~ochimique en Afrique occidentale soudano-sahelienne. Bull. de B.R.G.M. (Bur. Rech. Geol. Min.) (2) II: 229-239.