Kaolin Deposits of Charentes Basin, France
Encyclopedia
Kaolin Deposits of Charentes Basin, France are clay
deposits formed sediment
arily and then confined by other geological structures.
and Oligocene
deposits, laid above karst
ic limestone
formations of the Campagnien, in the North of the Aquitaine Basin
. The kaolin clays of Charentes belong to this mainly continental, tertiary formation often referred as “siderolithic”, of which the principal outcrop is situated in the South of the Charente Maritime department, 35 miles going North-East from Bordeaux city. The quarries are scattered along a 20 mile long, 7 mile wide, North - South band.
The clay concentrations of economic value are composed of a succession of clays, sands and pebbles. This torrential-stream deposit, close to enlaced rivers, laid to the deposition of sandy-clayey materials, with a variable iron content, coming from a lateritic weathering, of the French “Massif Central” granites. The presence of numerous lignite
rich levels indicates that the deposit was performed in the presence of a lot of organic matter, leading to important pedogenetic and diagenetic possibilities of evolutions. These chemical and mineralogical evolutions (dissolution–crystallization) allow the neo-formation of kaolin and gibbsite
, as well as iron sulphide.
At their basement, highly enlaced and with channel shapes, those deposits often fill karstic depressions, leading to the formation of “clay wells”. The juxtaposition of features are some times without explanations using the deposition laws, probably in relation with post-sedimentary strain phenomena, eventually linked to substratum collapse”. In the upper part of the series, the deposits are more regular, with lateral extensions up to several hundreds of meters.
Those complex geometries, with structures smaller than 20 meters, lead to particularly difficult recognition, estimation and exploitation phases. To this complex geometry, one should notice important lithology
variations. The AGS company uses no less than 24 description codes and 8 colour codes, for its samples description. Those classes are subdivided to take into account the grade in organic matters, iron, titanium, potassium, the colour, the aptitude to flow.
In deposits with sharp contacts, the geometry may be relatively simple, nevertheless, there is always uncertainty caused by lack of information and large drill hole grid.
Generally, these boundaries are determined by mineral grade rather than geological properties: deposit boundaries are chosen based on the cut-off grade. Changing the important factor of cut-off grade, the boundaries of the deposit can be extended or contracted.
For this reason, even for the deposits with sharp boundaries, a clear definition of the cut-off grade and distinction between ore and gangue
due to dilution during mining, the presence of intermediate layer and the limitation of mining in a selective way are essential.
However, in the case of the exploitation of soft materials, extraction can be done more selectively and it would be easier to take into account the geological and geometrical limits.
On the other hand, sometimes the uncertainty on the estimation of grades is bigger than the uncertainty on the boundaries definition. Estimation is then performed inside predefined boundaries.
One can imagine that the anisotropy and structural complexity of the deposit are due to its geometrical form, while the geometrical dimension of the deposit help us to guess about its economical value.
Geometrical features can appear in variographic studies and usually they affect, or hide, grade distribution structures. The presence of a series of nearly homogeneous kaolin areas, linked together in “zones“, creates a mosaic effect. This phenomenon is due to the existence of periodical settling regimes of the rivers. The size of these “zones” can affect the form of the variogram
and increase the nugget effect due to high differences of values in the edge of the “zones”. A hole effect is one the other known phenomena caused by the presence of two or more separated lenses with low difference in grade and shape. The distance between these lenses can thus be estimated.
formations are relatively frequent in Charentes clay deposits. Their thickness changes between some decimeters in lenses, to metric scale in continuous forms. These organic materials had some influences on kaolin deposited layers. Some of the observed influences are as follow:
In gathered samples close to these organic materials, clays generally do not contain mica minerals, and especially in the neighborhood of Cuisian lignite, kaolinite is very well-ordered and the clay does not contain swelling clays with hydrazine.
Occurrence of gibbsite is always associated with these well-ordered kaolinites. Normally occurrence of hyper-aluminous clays due to the existence of gibbsite is one of the interesting subjects in the history of these kaolins. This causes many discussions about the origin of this mineral. The existence of gibbsite has been mentioned in the studies of Languine and Halm (1951), Caillere and Jourdain (1956) , Kulbickie (1956), Dubreuilh et al. (1984) and Delineau (1994)
The high energy current can interrupt the continuity of the settled layers of kaolin and reduce the simplicity of the estimation methods.
Dissolved aluminium can be transported with complex to a less acidic medium.
This theory alone cannot explain what is observed in-situ in the some samples of the “BD” deposit, where gibbsite was found in sandy layers containing quartz.
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
deposits formed sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
arily and then confined by other geological structures.
Overview
The geological unit called Charentes basin is composed of EoceneEocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
and Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
deposits, laid above karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
ic limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
formations of the Campagnien, in the North of the Aquitaine Basin
Aquitaine Basin (geology)
The Aquitaine Basin is after the Paris Basin the second largest Mesozoic and Cenozoic sedimentary basin in France, occupying a large part of the country's southwestern quadrant. Its surface area covers 66,000 km2 onshore. It formed on Variscan basement which was peneplained during the Permian...
. The kaolin clays of Charentes belong to this mainly continental, tertiary formation often referred as “siderolithic”, of which the principal outcrop is situated in the South of the Charente Maritime department, 35 miles going North-East from Bordeaux city. The quarries are scattered along a 20 mile long, 7 mile wide, North - South band.
The clay concentrations of economic value are composed of a succession of clays, sands and pebbles. This torrential-stream deposit, close to enlaced rivers, laid to the deposition of sandy-clayey materials, with a variable iron content, coming from a lateritic weathering, of the French “Massif Central” granites. The presence of numerous lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
rich levels indicates that the deposit was performed in the presence of a lot of organic matter, leading to important pedogenetic and diagenetic possibilities of evolutions. These chemical and mineralogical evolutions (dissolution–crystallization) allow the neo-formation of kaolin and gibbsite
Gibbsite
Gibbsite, Al3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al3 . It is also sometimes called hydrargillite ....
, as well as iron sulphide.
At their basement, highly enlaced and with channel shapes, those deposits often fill karstic depressions, leading to the formation of “clay wells”. The juxtaposition of features are some times without explanations using the deposition laws, probably in relation with post-sedimentary strain phenomena, eventually linked to substratum collapse”. In the upper part of the series, the deposits are more regular, with lateral extensions up to several hundreds of meters.
Those complex geometries, with structures smaller than 20 meters, lead to particularly difficult recognition, estimation and exploitation phases. To this complex geometry, one should notice important lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
variations. The AGS company uses no less than 24 description codes and 8 colour codes, for its samples description. Those classes are subdivided to take into account the grade in organic matters, iron, titanium, potassium, the colour, the aptitude to flow.
Geometry of the retaining structures
The uncertainty in estimating the tonnage of mineral resources or ore reserves depends on a number of factors, and the uncertainty of definition of the deposit boundaries is one of them.In deposits with sharp contacts, the geometry may be relatively simple, nevertheless, there is always uncertainty caused by lack of information and large drill hole grid.
Generally, these boundaries are determined by mineral grade rather than geological properties: deposit boundaries are chosen based on the cut-off grade. Changing the important factor of cut-off grade, the boundaries of the deposit can be extended or contracted.
For this reason, even for the deposits with sharp boundaries, a clear definition of the cut-off grade and distinction between ore and gangue
Gangue
In mining, gangue is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. The separation of mineral from gangue is known as mineral processing, mineral dressing or ore dressing and it is a necessary and often significant aspect of mining...
due to dilution during mining, the presence of intermediate layer and the limitation of mining in a selective way are essential.
However, in the case of the exploitation of soft materials, extraction can be done more selectively and it would be easier to take into account the geological and geometrical limits.
On the other hand, sometimes the uncertainty on the estimation of grades is bigger than the uncertainty on the boundaries definition. Estimation is then performed inside predefined boundaries.
One can imagine that the anisotropy and structural complexity of the deposit are due to its geometrical form, while the geometrical dimension of the deposit help us to guess about its economical value.
Geometrical features can appear in variographic studies and usually they affect, or hide, grade distribution structures. The presence of a series of nearly homogeneous kaolin areas, linked together in “zones“, creates a mosaic effect. This phenomenon is due to the existence of periodical settling regimes of the rivers. The size of these “zones” can affect the form of the variogram
Variogram
In spatial statistics the theoretical variogram 2\gamma is a function describing the degree of spatial dependence of a spatial random field or stochastic process Z...
and increase the nugget effect due to high differences of values in the edge of the “zones”. A hole effect is one the other known phenomena caused by the presence of two or more separated lenses with low difference in grade and shape. The distance between these lenses can thus be estimated.
Transformation during and after Sedimentation
Thiry has mentioned that the actual geological setting of kaolin depositions cannot be explained with only transportation and sedimentation cycles. He also stated that the mineralogical sequences cannot be interpreted without local geochemical transformations. Kulbicki has proved the existence of vermicular minerals (kaolinite and dickite) incompatible with normal sedimentary sequences.Influence of the organic materials
LigniteLignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
formations are relatively frequent in Charentes clay deposits. Their thickness changes between some decimeters in lenses, to metric scale in continuous forms. These organic materials had some influences on kaolin deposited layers. Some of the observed influences are as follow:
In gathered samples close to these organic materials, clays generally do not contain mica minerals, and especially in the neighborhood of Cuisian lignite, kaolinite is very well-ordered and the clay does not contain swelling clays with hydrazine.
Occurrence of gibbsite is always associated with these well-ordered kaolinites. Normally occurrence of hyper-aluminous clays due to the existence of gibbsite is one of the interesting subjects in the history of these kaolins. This causes many discussions about the origin of this mineral. The existence of gibbsite has been mentioned in the studies of Languine and Halm (1951), Caillere and Jourdain (1956) , Kulbickie (1956), Dubreuilh et al. (1984) and Delineau (1994)
Sandy overburden and intermediate sands
Generally, kaolin deposits have been covered with colored sequences of sand. In some quarries, we can observe red, green and some times black sands. The black color might be due to the existence of pyrite and organic materials. Sometimes fossil woods (floated branches and trunks of trees) can be found and with the coarse size of pebbles (several millimeters) are evidence of a high energy transportation. This type of sand can have some influences on the leaching by mineral and organic acids produced by pyrite and organic materials, of the lower kaolin deposits. Thiry has found that generally these kaolins contain rather well-ordered kaolinite. Obviously, the level of crystallization can control technical properties of kaolinite as well as the structural impurities.The high energy current can interrupt the continuity of the settled layers of kaolin and reduce the simplicity of the estimation methods.
Gibbsite
Gibbsite is not stable in presence of quartz and it will be changed into kaolinite minerals, so gibbsite has formed after the deposition and we can called it neo-formation gibbsite. Now, the main question is about gibbsite formation in the middle of kaolin series. Due to the pH of leaching, a dissolution of Al2O3 or SiO2 can occur (podzol or laterite profile) The first theory tries to describe this with podzol profiles: it assumes the leaching of silica from minerals and accordingly the gibbsite formation from leached kaolin. We thus should find the hyper-aluminous materials, containing gibbsite in the lower series of kaolin. On the other hand, a second theory proposes the procedure of aluminium leaching in a very acid medium, in deposited organic materials (lignite) with clay. The organic materials can accelerate the solubilization and transportation of aluminium ions with intervention of organic complex. proposed the following scenarios for this dissolved aluminium.Dissolved aluminium can be transported with complex to a less acidic medium.
- 1- if there is any quartz in this medium, it can react, and we obtain well-ordered kaolinite minerals
- 2- In absence of quartz, aluminium will precipitate as a hydroxide mineral: gibbsite.
This theory alone cannot explain what is observed in-situ in the some samples of the “BD” deposit, where gibbsite was found in sandy layers containing quartz.