Thursday, February 25, 2016

Silicification in the Laterite Profile

Generally speaking, silica is associated with the laterite profile in two distinct ways : a) As massive silica; forming thick silica cap or silcrete, and b) As boxwork; silica sheets and lenses that are emplaced within limonite, saprolite or clay zones.

Massive silica cap or silcrete is generally present in “fossil” laterite deposits that have developed over ultramafic rocks of Precambrian age. Good examples come from certain laterite deposits of Brazil and Australia. Here, the silica content in the silcrete can reach +80% level and may require blasting during stripping operation.

Such silica caps are not always limited to laterite deposits but also cover sediments and other rock types. It is debated whether the source of silica is the pedogenic (laterisation) process or whether it has a more regional source. While currently occupying positions of high topographic levels, such silica caps are actually believed to have formed in basins and areas of low topography. Eventual denudation of the land lowered the surrounding topography, thereby elevating the silcrete-protected laterite.

According to Thiry and Millot (1987), thick silica caps or silcretes are only preserved in arid or seasonal wet/dry climates. In humid tropical climates, the water circulation is too high and silica caps are generally removed.

While occasionally massive silica can be deposited in the saprolite zone, much of the silica found in the limonite, saprolite and bedrock zones is of the boxwork or sheet type. The sheets are generally oriented sub-horizontally (or follow the topography) at the time of formation. This indicates precipitation of silica at the water table level.

Boxwork silica in the limonite is actually inherited by this zone as the process of laterisation matures. The original silica accumulations occur in the saprolite or hard saprolite zone where abundant free space is available either along natural joint and fracture openings or through extensive leaching along these surfaces.

When the surrounding saprolite becomes limonised (after further leaching and collapse), the original silica deposition in this zone is inherited by the limonite. If the silica veins in the boxwork are thick, the box-work may retain its original shape and geometry. If the silica walls are too delicate, the boxwork will suffer and show the consequences of collapse.

In practically all cases, silica in the form of boxwork, sheets and lenses is derived from the leaching of ferromagnesian minerals in the ultramafics. The total amount of silica involved in such cases is small and a source of silica outside the ultramafics is not required.

Depending on the rainfall quantity and its seasonality, silica released from the chemical weathering of olivines (and pyroxenes) may not leave the laterite profile entirely. In areas of wet-dry seasons, the flushing of silica from the laterite environment is not complete. The silica can be temporarily fixed in the profile in the form of clays or as crystalline/colloidal silica deposit.
Silicification in the Laterite Profile
Laterisation trend in wet humid climates (left) & Laterisation trend in dry laterites with silica deposition in the profile (right).
The occurrence of clays such as nontronite is very common in the intermediate zones of many laterite profiles in the world. Nickel also enters the nontronite crystal structure and much of the nickel present in the intermediate zone of the laterite may be in the nontronite mineral.

Much of the silica deposited in the laterite profile-whether massive, boxwork or sheet type-is amorphous in nature and results in the formation of chalcedonic or opaline deposit. Occasionally, however, microcrystalline quartz (microquartz) may be formed. In this context, Milnes, Wright and Thiry (1991) observe that:

“The occurrence of opal denotes solutions with a high silica content, high rates of precipitation, and the presence of impurity ions. Solutions with a relatively low silica content and low concentrations of impurity ions usually precipitate quartz, the largest crystals being favoured by the lowest concentration of contaminants.”

The two figures above show the marked differences between normal laterite profile developed in wet humid climates where all the magnesia and silica is eventually leached out of the limonite zone versus laterite profile developed in dry laterites with considerable silica deposition in the form of silica cap and zones of siliceous limonite and saprolite. (Reference: Waheed Ahmad, 2008, Fundamentals of chemistry, mineralogy, weathering processes, formation, and exploration).

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