Geochemical prospecting in lateritic terrain

Interest in tropically weathered terrains has grown considerably in the past few years, especially on the part of North American mineral exploration companies. This increasing interest has come about as a result of the expanded international search for base and precious metal deposits in these environments.

The economic importance of gold and copper deposits in deeply weathered terrain has dictated a need for the understanding of the weathering processes that have contributed to the development of these terrains. One of the most effective exploration tools in tropically weathered and arid terrains has proven to be geochemistry. However, its proper application requires some understanding of past and present weathering and erosional processes that have created the landforms and soil types that make up these terrains. These erosional and weathering cycles have a long geological history, often in the order of tens of millions of years or more. This is in contrast to the 10,000-year-old glaciogenic surfaces and soils with which most Canadian geologists are familiar. Effective geochemical exploration in deeply weathered terrains requires a knowledge of weathering processes, soil formation and landscape development.

For a more complete review of these criteria, definitions of terms used in the classification of deeply weathered terrains and geochemical case histories, the reader is referred to a recent text edited by Butt and Zeegers (1992), and to the proceedings of a short course on prospecting in tropical and arid terrains (March, 1994) available from the Prospectors and Developers Association of Canada.

One may divide the tropical parts of the world into the following three regional zones:

– Humid tropics — the equatorial rain forests;

– Seasonal wet-dry tropics — savannas; and

– Warm arid zones — semi-deserts and deserts.

The principal climatic elements that influence weathering processes are temperature and moisture. In general, high temperatures and precipitation enhance weathering processes. However, variables such as topography, forest cover, rate of precipitation, soil permeability and evapotranspiration can influence the amount of soil moisture available for leaching and removal of soluble components or for concentration of elements.

Paleoclimates since late Cretaceous have contributed to the development of deeply weathered surfaces around the world. In Australia, for instance, climates in the mid-Mesozoic were probably humid and temperate to warm — becoming humid, sub-tropical or tropical in the Oligocene-Miocene. Thereafter, there has been a trend to aridity which has continued to the present. The result of this long weathering cycle has been formation of a deep weathering lateritic profile which, in part, has been eroded. A similar climatic history can be applied to most tropical-arid terrains throughout the world.

Climatic zones relevant to the present tropics and subtropics are shown in Fig. 1. The inner tropical zone of partial planation defines, albeit roughly, the areas of equatorial rain forests. In this zone, weathering is intense and, as a result of dense vegetation, erosion may not be severe. The seasonal tropics or savannas which occur in the peritropical zone are characterized by deep, intensely weathered regolith (up to 40 metres or more in depth) which formed over long periods of tectonically stable landmasses. Following these long periods of weathering, climatic changes to drier conditions have resulted in dehydration of the near-surface regolith and lowering of the water-table. This has caused hardening of iron and aluminum oxide accumulations, leading to the formation of cuirasses (armored surfaces or laterite duricrusts) which, in effect, tend to preserve the soft, easily erodible, underlying, weathered material. With ongoing erosion, the peneplained surface is characterized by plateaux and mesas capped by lateritic duricrust.

In warm arid zones, the major relief and weathering features are inherited from pre-Pleistocene savanna climates, with the plateau surfaces being preserved in semi-arid environments. In more recent times, however, fluvial and eolian processes have dominated, leading to the erosion and concealment of paleo-weathered surfaces in desert environments.

The development of the full sequence of horizons requires considerable time. Continued chemical weathering during alternating dry and humid seasons, with little erosion of

the insoluble products, results in the development of the major weathering components which, from the base (fresh rock), includes:

n Saprock (coarse grained saprolite) — a slightly weathered rock in which less than 20% of the weatherable material is altered. Primary minerals may be replaced by weathering products. Upwards through this horizon there is an increase in coloration (yellow and red), and formation of clay minerals (smectites) at the base of the horizon with kaolinite above. Iron hydroxides generally are restricted to sites previously occupied by iron-bearing primary minerals.

n Saprolite (fine-grained saprolite) — This is a hydrated horizon in which most primary weatherable minerals have been altered to secondary minerals such as kaolinite, goethite or amorphous iron oxyhydroxides. Only resistate minerals such as tourmaline and chromite remain unweathered. n Mottled zone — This zone overlies the fine saprolite above the water table. The pre-existing macrostructure of the parent rock is progressively destroyed. Water percolation creates a series of voids and channels which can become filled with secondary kaolinite. Ferruginous spots and nodules are common and become more abundant and consolidated near the top of this horizon so that pre-existing lithostructure is obliterated.

n Lateritic iron crust (ferricrete or cuirasse) — The iron crust overlies the mottled zone and develops where the iron nodules become abundant and eventually coalesce into an indurated, conglomeratic, iron-rich crust. Further hydration and replacement of aluminous hematite by aluminous goethite results in the formation of a pisolitic (nodular) iron crust. The pisoliths eventually diminish in size and become separated to form a pebbly ferruginous layer at surface.

The rate of chemical weathering is largely dependent upon the amount of water that percolates through the profile each year. Data suggest that the average rate in lateritic environments is about 20 mm per 1,000 years, or 20 metres per million years, with rates for mafic and ultramafic rocks being two or three times faster. The time required to form lateritic profiles ranges from a few million to more than 20 million years.

Lateritization can result in the concentration of economically significant elements, including aluminum in bauxites, as well as nickel, iron, manganese and gold in laterite and saprolite. Deep weathering processes can also alter original sulphide minerals so that they become oxides, and they can release metals that may be included in sulphides (for example, gold). Geochemical exploration has proved effective in defining gold deposits in lateritic environments. The weathering and erosional processes in the laterite terrains create gold dispersion patterns many times larger than the primary mineralization. Granier et al. (1963) were amongst the first researchers to show this over the Ity gold deposit in Ivory Coast, West Africa, where the gold anomaly in the near-surface laterite is at least six times wider than the deeper gossan zone associated with primary gold mineralization. Subsequent investigations have identified similar mushroom-like gold patterns in other lateritic terrains in Africa, Australia and elsewhere.

Another example of the mushroom-like pattern is from the Dondo Mobi gold prospect, Gabon (Butt and Zeegers 1992). The gold anomaly in the upper soils is nearly five times larger than the anomaly in the underlying saprolite and bedrock. Microscopic examination of gold grains in the surface soils showed significant corrosion and dissolution, while in the saprolite the gold was similar in appearance to that found in bedrock.

In highly oxidizing weathering environments, gold can be put into solution by a variety of complexes which may include thiosulphate complexes, organic complexes, chloride complexes and cyanide complexes. Generally, the solubilized gold can be precipitated and/or adsorbed by oxides of iron and manganese.

Laterite gold deposits defined by geochemical surveys are being mined today in deeply weathered terrain in various parts of the world, including Australia (Boddington), South America (Bahia, Brazil) and West Africa (Syama, Mali).

Geochemical exploration surveys in deeply weathered terrains should be preceded by carefully planned orientation sampling and geochemical analyses. These procedures will help define the most suitable sample materials and analytical methods and thus optimize sampling and analytical procedures. Also, proper training and supervision of the field samplers will assure that adequate geochemical surveys are carried out.

(References:

(1) Butt, C.R.M. and H. Zeegers (Editors), 1992: Regolith exploration geochemistry in tropical and subtropical terrains. Handbook of Exploration Geochemistry, V4, Elsevier Science Publishers, Amsterdam, 607 p. (2) Granier, C.L., J.B. Lajoinie and C. Vitali, 1963: Gochimie de l’or et du cuivre dans les formations latritiques argileuses du Mont Flotouo, Ity-Cote d’Ivoire. Bull. Soc. Franc. Miner. Crist., 83, pp. 252-258.) — Christopher Gleeson, PhD, P.Eng., is a resident of Iroquois, Ont.