The Formation of Tropical Regolith

8/10/2019 The Formation of Tropical Regolith

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Gold Exploration in Tropical Landscapes

Part 4: The Formation of Tropical Regolith

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Background

Session Headings: Lateritization

You will cover the following points in Part 4: The

Formation of Tropical Regolith.

an overview of the lateritization

process

controls of lateritization

temperatureamount and temporal

distribution of rainfall

groundwater flow and

groundwater table fluctuation

Eh and ph conditions

throughout the regolith

profile

presence of sulphides

parent rock composition

structural deformationphysical erosion

topography

tectonic stability

paleotectonic and

paleoclimatic history

Lateritization

(See Summaryfor main points)

Many areas of the tropics have laterite regolith formed by lateritization, or tropical

weathering, of the bedrock. Lateritization starts at the earth's surface and

progresses downwards, attaining thickness ranging from 1 metre (m) to almost 200

m (Nahon (1991)). The process of lateritization results in the formation of a thick,

layered, and mostly residual weathering rind overlying fresh bedrock. This

weathered crust is composed of laterally extensive, sub-horizontal zones, each

exhibiting characteristic physical, chemical and biological traits. This resulting
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regolith cannot be uniform, because of the wide variety in climate, vegetation,

parent rock material, geomorphology and age. As long as climatic and tectonic

conditions remain stable, the weathering front continues downwards. Bedrock is

consumed and the landscape is progressively lowered by lateritization. This is

termed lateritic lowering. The stratified sequence in the regolith we encounter

today is a geologic snapshot of a dynamic system.

Rainwater falling on the surface of exposed lands is the essential vector of all

interface reactions. Rock weathering reactions are controlled by meteoric (rain)waters and the atmospheric gases dissolved in it; mainly O2and CO2that permeate

rocks and minerals along fractures and mineral grain boundaries. When rainwater

infiltrates soil or rocks and reacts with organic matter or their constituent minerals,

its chemical composition varies during its percolation , establishing a vertically

moving reaction front.

Meteoric waters recharge the groundwater table at depth, bringing the

dissolved gases along with it. The increased porosity and permeability of the

tropical regolith allows groundwater levels to reach to greater depths than the

unweathered bedrock in cooler climates. Most of the tropical rock weatheringoccurs at or near the groundwater table.

Lateritization of bedrock involves a number of geochemical, rock destructive

processes, which are caused by large volumes of infiltrating, oxygen-rich and acidic

groundwater. Meteoric waters, acidic and oxygen-rich, infiltrate the soils and pass

through the laterite to recharge the groundwater system hosted by the regolith at

depth. Within these solutions, rock forming minerals (such as quartz, feldspar,

olivine, hornblende, pyroxene and biotite) break down.

Mineral weathering

Mineral weathering is the chemical alteration of minerals in the laterite regolith

resulting in the complete breakdown of the original mineral. This happens layer by

layer starting at the crystal face, or from the centre of a mineral outwards. Mineral

weathering is most active along mineral grain defects, such as twin planes, pits and

edges. Weathering of minerals causes weakening of the bonds holding the rock

together and it disintegrates. The weathering geochemical system has three main

variables:

rocksmineral constituents that compose the rocks and minerals are

formed in alteration assemblages associated with gold mineralized

systems;

rainwaterslightly acidic, oxygenated and most importantly periodic

(seasonal) and heavy at times; and

dissolved gasesoxygen (O2) and carbon dioxide (CO2) found in rainwater

are strong reactants in the weathering of minerals.

Weathering of minerals releases cations such as Mg2+, K+, Ca2+ and Na+ into the
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Background - Figure 1

Figure 1: Exploration geologist Jawas Dekba stands next to a road cut revealing massive hematite (in

red) at Vangold Resources' Mt Penck gold project, PNG; the hematite formed as a result of the

oxidation of sulphide-rich alteration associated with high sulphidation epithermal gold mineralization

(source: D. Voormeij).

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The Formation of Tropical Regolith