Definitions

Saprolite

Saprolite

[sap-ruh-lahyt]
Saprolite (from Greek σαπρος =putrid, + lite) is the name for a chemically weathered rock. It is mostly soft or friable and commonly retains the structure of the parent rock since it is not transported, but autochthonously formed in place.

Besides resistant relic minerals of the parent rock, saprolites contain predominantly quartz and a high percentage of kaolinite with other clay minerals which are formed by chemical decomposition of primary minerals, mainly feldspars. More intense weathering conditions, exceeding the saprolite stage, give rise to a continuous transition to laterite soils.

Terminology

The saprolite is the part of the regolith which is oxidised and hydrated, the original minerals in the rock weathered and altered by water, oxygen, carbon dioxide and organic acids such as tannins.

The saprolite is divided into three main zones; the upper saprolite, lower saprolite and saprock.

Upper Saprolite

The upper saprolite zone is distinguished by the general absence of fresh rock and the predominance of clay species. The upper saprolite has varigated colors, typically yellow, ochre, orange, tan or light brown in colour. In extreme cases of prolonged weathering, or in particularly iron-poor rocks, the upper saprolite can be bleached pure white.

The upper saprolite is chemically oxidised to the point where few reduced chemical species exist. For instance, iron is present as Fe3+, sulfur is present predominately as sulfates, manganese is present as manganese oxide, silicates are present as clays.

The upper saprolite is structurally and texturally mostly intact and such structures as occur in the fresh rock can usually be traced into the upper saprolite.

The upper saprolite zone may exist down to 300 metres below surface in extreme cases, and throughout large regions of the tropics, may exceed 50 metres. Within the ancient regolith of the Yilgarn Craton, saprolite typically exceeds 100m depth.

Lower Saprolite

The lower saprolite zone is distinguished from the upper saprolite by the generally lower degree of oxidation and the colour. Typically, lower saprolite is greenish in colour and friable, though much more indurated than the upper saprolite.

The lower saprolite sees an oxidation front between chemically oxidised minerals and chemicals above, and chemically reduced minerals and chemicals existing below. This manifests as a change from Fe3+ which lends the red or orange suite of colors to the upper saprolite, to Fe2+ which tends to colour the rock mass green or green-brown.

The oxidation front is a prime marker and an important horizon for ore deposits, especially uranium deposits. The oxidation front and change from upper to lower saprolite sees a change in redox potential and ionic state of most metals, which may prompt downward-travelling oxidised species to leave solution.

The oxidation front is also the point at which sulfur is reduced enough to exist in a sulfide form and hence, there is typically some form of supergene enrichment in metal sulfide minerals. This is an important zone in many ore deposits and an integral part of the ore genesis of Manto ore deposits.

Saprock

The saprock is the zone of rock below the lower saprolite where weathering is restricted to failure systems within the fresh rock mass. Within the saprock zone weathering occurs along joints, foliations, faults and other failures, and projects downwards as a network of weathered fingers.

The top of fresh rock, typically, is taken as the point at which no further visible weathering is evident. However, chemically it is usually able to be shown that weathering and oxidation effects may persist a considerable distance into the Earth's crust, especially around large fault systems.

It is also worth noting that in many hydrothermal fields, weathering and oxidation of the rocks is enhanced by the hydrothermal waters, giving rise to an alteration zone of metasomatic origin. It is arguable whether or not this counts as saprolite.

Formation

The process of forming saprolite is via addition of water, oxygen, nitrogen, carbon dioxide and organic matter into the rock and via removal of magnesium, iron, silica and metals from the rock. Typically, aluminium is immobile within weathering, although this is not always the case.

Saprolite destruction occurs via physical erosion, predominantly via water movement across the surface. However, in eolian dune systems and arid wind-prone deserts scouring of the land surface via wind-blown particles is an important mechanism for removal of saprolite. Within the ice deserts and some periglacial uplands, wind and freeze-thaw may also remove saprolite.

Saprolite is generally not formed in glacial environments as glaciation removes all soft, weathered material readily. It is for this reason that the majority of the Canadian Shield and the Siberian Craton are devoid of significant saprolite.

Other saprolite morphologies

There are several other specific and restricted saprolite morphologies which are recognised from particular environments, but which do not follow the typical and idealised sequence shown above.

Mottled zones

Within the Yilgarn Craton of Australia and elsewhere (?) there is a widespread development of a mottled zone of saprolite consisting of red and white mottled clay-rich saprolite. The mechanism of formation of these mottled zone morphologies was quite contentious, however the general consensus is that these mottled zones are the result of penetration of tree roots into the saprolite during a previous environmental regime which occurred within either the Tertiary or late Cainozoic.

The white clay mottles or 'megamottles' tend to form vertically arrayed tubular conduits of bleached white kaolinite within a hematite-rich clay matrix. In a widely recognised and growing number of sites there have been found fossilised tree roots.

Collapse zones

Within the Yilgarn Craton of Australia and elsewhere that prolonged weathering has occurred, where the rate of erosion is virtually negligible compared to the rate of chemical weathering, it is possible that removal via aqueous solution of the majority of rock components takes place, leaving a laterite profile which consists of highly vesicular hematite-goethite. If well enough developed, the mass of these boxworks will collapse under its own weight, forming a ferruginous hardcap composed of fragments of weathered residual saprolite and laterite materials.

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