7.1 INTRODUCTION

Many types of mineral deposit form by weathering. For discussion in this

chapter we have selected bauxite, nickel laterite and kāolin; we also describe

supergene manganese and the supergene enrichment of sulphides. These mineral

deposits are among the most important ores formed by weathering and supergene

enrichment is one of the best understood aspects of weathering. The selections were

made to illustrate the several different chemical processes taking place during

wcathering.

Weathering is the breakdown and alteration of rocks by physical and chemical

processes, both of which may be aided by organic activity. The fractionation of the

rock occurs in response to changes in environmental conditions since the rock was

formed. The products of weathering are materials more nearly in equilibrium with

their environment than those from which they are derived. For example, the minerals

of igneous rocks, formed at high temperatures in the absence of abundant water, are

unstable in the cooler, wet conditions at the earth's surface, the minerals of these

igneous rocks are altered to low-temperature water-bearing phases or dissolved and

removed. Physical weathering disaggregates the rock creating large surface areas and

greater access by fluids. This increases the susceptibility to chemical weathering.

Chemical weathering usually is in two stages, alkaline then acid.: The behaviour of

the different elements within thesilicate minerals is differentially affected by

changesin pH. During the alkaline stage K, Na, Ca may be removed and the

remaining rock material may be relatively enriched in Fe, Si and Al. When conditions

become acid the hydroxides of aluminium and iron can migrate to a limited extent.

Rose et al. (1979) list the major processes of chemical weathering as

hydration, hydrolysis, oxidation and solution. The earliest evidence of rock

weathering is oxidation of Fe2+ to Fe3+ and removal of Na, Ca and Mg in solution

(Dennen and Anderson, 1962). Na is particularly mobile during weathering but

release of K into solution is restricted by its adsorption on to kaolinite.

As might be expected, the relative stabilitics of the common rock-forming

minerals in igneous rocks bear an antithetic relationship to the order in which they

crystallize from a melt as a result of falling temperature. Thus the arrangement of

minerals in order of increasing stability during weathering (Goldich, 1938)

corresponds to Bowen's (1922) reaction series in order of decreasing temperature of

crystallization (Fig. 7.1). The relationships are not so clear when the wide-ranging

compositions of metamorphic rocks are considered. It might be anticipated that those

minerals which formed at the surface would be the most resistant to weathering but

chemical precipitates in sedimentary rocks suffer severe weathering under changing

climatic corditions. Factors which influence the nature and rate of chemical

weathering include permeability, climate, relief and drainage (Ollier, 1969).

Weathering is the initial stage in the cycle of events leading to the formation of

sediments. Without some form of weathering rock material cannot be removed by

transport to be deposited elsewhere to form alluvial deposits. Also the products of

weathering may give rise to mineral deposits formed in situ; these are called residual

deposits.

The production of an insitu residual deposit requires deep and long-continued

weathering followed by lack of erosion. Residual deposits are therefore usually

absent from glaciated regions and mountain belts where erosion is active. Two

distinct types of residual deposits may be recognized. The first results from the

accumulation of pre-existing mineral species in the source rock; concentration takes

place by the removal of many other components from the source rock. The eluvial

accumulation of cassiterite (page 178) is an example of this type. The economic

concentrations of gold which occur in the weathering mantle over some porphyry

copper deposits, as at Ok Tedi in Papua New Guinea, occur in this way. The second

type, and more widespread economically, are the residual deposits where the valuable

mineral comes into existence as a result of weathering and the economic mineral

continues to accumulate. Bauxite, some of the World's china clay deposits and

nickeliferous laterites are examples of this second type.

KAOLIN DEPOSITS

Kaolinite is the chief economic mineral of the recondite group of clay

minerals. Its basic structure two-layer lattice, a gibbsite sheet and a silica tetrahedral

sheet; kaolinite, does not expand with decreasing water content, one of the

characteristics which distinguishes it from the smectite group. Its crystal structure is

generally resistant to attack by most corrosive fluids, making kaolinite inert, not

readily reacting with media in which it is placed; this is an important commercial

property of china clay. Commercial-quality china clay is used in the paper industry, in

china manufacture, for ceramics and refactories. Diverse other markets exist

including the paint and rubber industries. In the paper industry china clay is used to

fill the interstices of the pulp fibres and as a surface coating to produce a smooth,

bright, often glossy finish. As this represents one of the major markets for the product

the brightness of the clay is a very important criterion when a kaolin depositis being

evaluated. China clay lacks sufficient plasticity and grecn strength to make some

ceramic products. Ball clay, a variety of kaolin having plasticity and high-strength

characteristics, is commonly added to china clay to improve workability. (Fig 7.12).

7.4.1 Genesis of kaolin deposits

The hydrous aluminosilicate, kaolinite, is formed from the destruction of

aluminium silicates, principally feldspars. The feldspar lattices are wrecked by ionic

solution, hydration and hydrolysis. K, Na and Ca are extracted and reaction of the Al

and Si with OH results in formation of kaolinite. The conversion of orthoclase to

kaolinite may be illustrated as follows:

4KAISi3O8 + 2CO3 + 4H2O → 2K2CO3 + Al4 Si4 O10 (OH)8 + 8SiO2

This can occur under late-stage hydrothermal conditions or under chemical

weathering conditions in tropical or subtropical climates. A combination of these two

environments has been invoked for at least one major group of deposits, those in

south-west England (Bristow, 1977).

The chemical weathering conditions that produce kaolin deposits are

considered broadly similar to those of bauxitization (Section 7.2.3). Frequently

lateritic bauxites have major, if impure, kaolin deposits associated with them.

Kaolinization often occurs as an intersheiliatestage in the formation of lateritic

bauxites. The presence of oxidizing sulphides hastens kaolinization. Deposits that

result from supergene weathering contain kaolin stacks that are fragile and

undeformed; the china clay is usually of low density and high porosity.

Kaolinization resulting from hydrothermal activity is likely to be controlled

by fluid paths such as jointing and brecciation and will probably be best developed

near major conduits. Deposits arising in this way will tend to be more restricted than

the weathering blankets formed by supergene chemical weathering but they may

extend much deeper.

Material that has suffered in situ kaolinization may be eroded and transported

to be deposited elsewhere as any other detrital mineral. Sedimentary kaolin deposits

are major sources of production in the USA and the ball clay deposits of England are

thought to have originated in this way (Fig. 7,12). Sedimentary sorting processes will

have been responsible for creating sufficiently concentrated deposits to be

economically viable. Some material in these deposits may have undergone

kaolinization within the basin of deposition.

7.4.2 Classification of kaolin deposits

We propose to divide kaolin deposits into three groups, based upon their

mode of formation: (1) residual kaolin deposits; (2) sedimentary kaolin deposits; (3)

hydrothermal kaolin deposits. Large concentrations of pure white kaolinite may be

found within each of these groups.

7.4.3 General characteristics of kaolin deposits

(a) Distribution in space and time

Approximately half the World's production of kaolin comes from extensive

china clay deposits in the USA and the UK. Major deposits occur in Czechoslovakia,

Guyana and Brazil. Australia and South East Asia also have large quantities of kaolin.

The kaolin deposits of south-west England are dated at 273 millior years (Bray and

Spooner, 1983) whereas the major kaolin-producing area of the USA, in South

Carolina and Georga, has deposits of Upper Cretaceous to Middle Eocene age.

Deposits of kaolin resulting from chemical weathering have formed where climatic

conditions were suitable. Those in Western Australia, which may have begun

formation in the Proterozoic, have experienced many periods of kaolinization

between then and the present day.

(b) Size and grade of deposits

It is not possible to give an average size for a kaolin deposit because of the

different modes of formation. The typical thickness of a chemical weathering profile

is between 35 and 50 m, the larger figure representing more prolonged weathering as

in Western Australia. This does not represent the otiginal thickness as erosion may

have removed large-quantities. The kaolin deposits of sedimentary origin form strata

and lenses up to 10 or 12 m thick and extend for a few kilometres. Kaolin deposits of

hydrothermal origin tend to be more restricted in area, but are much more vertically

extensive.

The annual World production of kaolin is about 18 million tonnes. In 1982 the

USA had about 40% of the market and the UK about 16% (Industrial Minerals,

1983). These two countries dominate the high-value paper-coating clay market. The

largest single producer world-wide is English China Clays (ECC) whose British

operations are centred in south-west England. Their total production amounted to 2.9

million tonnes in 1981. Some 50% of the china clay produced is used for paper filling

while 30% goes for paper coating. Ceramics accounts for another 15% while the

remainder is used for a wide variety of products. The trend in usage seems to indicate

an increase in the paper-coating clay production compared to the filler usage.

(c) Mineralogy

Kaolin deposits that have been formed by chemical weathering_may have

halloysite, siderite, pyrite and limonite as accessory minerals. The kaolinite that

occurs as the pallid zone in bauxite profiles may be associated with one of the bauxite

minerals. Resistate minerals may also be present. Kaolinization associated with

igneous rocks gives jrise to deposits of kaolinite plus one or more of the essential

minerals of the igneous rock. In the south west of England china clay deposits contain

quartz, feldspar, mica and relict tourmaline. Such deposits do not have much

limonite, siderite or pyrite.

(d) Host rock lithology

Residual deposits of kaolin result from the chemical weathering of feldspar-

rich rocks, particularly granites and other aluminous rocks. At present production of

good-quality china clay from the pallidzones beneath bauxite layers is not very

significant.

7.4.4 Residual kaolin deposits

(a) Spruce Pine District, North Carolina, USA

These kaolin deposits were formed from the chemical weathering of small,

irregular, pegmatitic, alaskite stocks (Harben, 1978). The stocks are a few hundred

metres to five kilometres across and are coarse-grained alaskite with about 50%

oligoclase and 25% microcline. The kaolinization took place during the early Tertiary

|and the deposits lie beneath a marked erosion surface which suggests very extensive

weathering. The decomposition of the alaskite extends down as far as 30 m but the

deposits are not worked below 15 m. The deposits are composed of kaolinite and

halloysite mixed with partly decomposed feldspar, fresh quartz and muscovite.

Inclusions of schists and gneiss in the parent alaskite produce waste within the

deposits. The clay is mined by power shovel or hydraulic jet and the district produces

about 160.000 tonnes per year of water-worked clay.

(b) Gabbin, Western Australia

This project, sited 285 km north east of Perth, is in an ancient peneplain

around 400 m in elevation within the Yilgarn block of the Archaean Shield of

Western Australia. The block is composed of elongate greenstone belts surrounded by

large granite masses. Although in general the granite masses have ages of either 3.2

or 2.5 million years, the age of the Gabbin granite is not known. This granite is an

adamellite of quartz, microcline and plagioclase. The plagioclase has weathered more

rapidly than the potassium feldspar as is often the case in kaolinization. Accessory

minerals are rare. Much of the Archaean of the area has been emergent since the start

of the Proterozoic (Walker, 1978). Through this long period of time the area must

have experienced a great variety of climates and since the Tertiary there has been a

drying out of the climate accompanied by deep lateritization. Some of the lateritic

profile has been removed due to post-Tertiary uplift. The weathering front is normally

30 m below the present surface but it may be as much as 50 m. A NE–SW foliation

exists in the Gabbin granite and the deepest weathering and zones of best rheology

appear to follow this trend. A typical section shows the recognizably weathered

granite overlain by a coloured sandy clay which underlies the ore-grade kaolin. The

kaolin deposit maybe 35 m thick with an average of 11 m. The thickness of kaolin

seems to follow the topography but there may be some fault control (Walker, 1978).

Overlying the kaolin is a varying thickness of silcrete and sandstones. The

distribution of the various ore-grade clays along onesection is shown in Fig. 7.13.

7.4.5 Sedimentary kaolin

Deposits in South Carolina and Georgia, - About 90% of the kaolinᏖᎫᏚᎪ

production of the USA is from sedimentary deposits in South Carolina and Georgia.

The production is based on two centres, one extending from Macon to Wrens in

Georgia, and the other near Augusta, Georgia, and Aiken, South Carolina. Here the

kaolin is in lenses within the Upper Cretaceous to Middle Eocene formations. These

lenses occur, apparently at random, throughout a thickness of more than 50 m of the

formation and may have lengths of several kilometres. The thickness of these lenses

ranges up to 12 m and all contain some detrital sand and mica. Disseminated pyrite is

present in the kaolin under thick cover and as the outcrop is approached the oxidation

of pyrite gives a random yellow stain to the kaolin which affects the grade of clay

produced. It is generally agreed that the source of these kaolinitic sediments lay in

crystalline rocks to the north-west but the presence of the lenses in otherwise coarse,

cross-bedded sands has been the topic of much debate. Initially it was assumed the

crystalline rocks were deeply weathered and the kaolinite was formed in situ and then

washed into the areas where they now occur. On this hypothesis the lenses of

kaolinite are difficult to explain. Kesler (1952) explained the presence of the lenses

by transport of detritus from the crystalline rocks. Weathering offeldspars into

kaolinite occurred within the delta sands, the kaolinite being later washed selectively

to cut off stream segments or ponds (analogous to the settling ponds used in china—

clay processing) containing fresh water.These kaolin-filled ponds were eventually

covered by sands as delta building progressed northwards due to subsidence.

7.4.6 Hydrothermal kaolin deposits

The kaolin deposits of south-west England Some of the finest-quality kaolin

deposits in the World occur in the UK, in south-west England. These are in Devon

and Cornwall with the main economic deposits in the latter county. These deposits

have been described by Bristow (1969), and the kaolinites have been examined in

detail by Exley (1976) and Sheppard (1977). Their genesis has been discussed by

many authors, most recently Durrance et al.(1982), Alderton and Rankin (1983) and

Bray and Spooner (1983).

The granites of south-west England are two feldspar granites and the process

of kaolinization proceeded with the alteration of plagioclase to pure kaolinite, while

the potash feldspar changed into a mixture of kaolinite and some fine-grained mica.

Of the two other essential minerals in the granite, the quartz remained relatively

unaltered while the mica either recrystallized into a finergrained form or remained

unaltered.

Although we include these deposits within the chapter dealing with

weathering deposits, their place here is not fully justified as they have long been

considered to originate from hydrothermal activity (Bristow, 1977). However, there

have been proponents of a supergene origin, the most recent being Sheppard (1977).

Bristow (1977) regarded the hydrotherinal stage as a major preparation process which

produced some interimediate mineral phases and which rendered the granite very

susceptible to alteration when the kaolinite formation took place under supergene

conditions. Following this hydrothermal softening up' process, and possibly when the

granites were essentially consolidated and cooled, great quantities of groundwater

entered the system. This probably occurred duringorafter the late stage of

hydrothermal activity, and, according to Alderton and Rankin (1983), these low-

salinity fluids, with temperatures less than 170°C, were responsible for the

kaolinization. The volume of available fluids accounts for the very large size of the

deposits. Highest quality, in terms of brightness and particle size, is related to the

lithionite granite which was deficient in iron-rich biotite.

Bray and Spooner (1983) have examined the evidence for and against a

weathering origin for these leposits. They observed that no vertical profile

development is presentin the south-west of England deposits and the depth of

kaolinization (as much as 250 m) is significantly greater than the 35–50 m

thicknesses which are typical of complete chemical weathering profiles. Furthermore

the Ks Ar age for the secondary muscovites in the deposits indicates that

kaolinization occurred at the time of the sheeted quartz—tourmaline : cassiterite +

wolframite mineralization in the region (267 million years). Bray and Spooner

conclude that the evidence suggests a single-stage hydrothermal process.

The deposits lie on eroded cupolas of an extensive Hercynian granite

batholith, and although five of the six main cupolas have some kaolinization which

has been commercially exploited, the main mass of china clay is located in the

lithionite granite of the St. Austell area (Fig. 27.14). The clay deposits often occur as

funnel shapes opening upwards and extending as deep as 250 m. The funnel-shaped

deposits may occur in zones, or alternatively the kaolinized zones may form

elongated troughs sometimes dipping at high angles into the ground (Fig. 7.15). This

dip may result in unkaolinized granite overlying kaolinized granite. Such shapes

make evaluation difficult, as does the occurrence of ‘core stones' where unkaolinized

granite apparently overlies kaolinized rock. Laterally kaolinized material may pass

into unkaolinized granite over very short distances.

7.4.7 Exploratic for and evaluation of kaolin deposits

If there is sufficient contrast between the china clay and surrounding rocks,

geophysical techniques may be used to delineate the deposit and reduce the amount of

expensive drilling. This is true of some of the deposits of south-west England where

there is sufficient resistivity contrast between the intact granite and china clay to

allow theidentification of hard-rock areas. Gravity has also been used in this area and

from detailed gravity work certain estimates of mass of clay in the ground are

possible. The mode of formation means that it is unlikely that a very good

interborehole correlation will exist in a china-clay deposit, and consequently it has to

be drilled on a grid system at whatever spacing is considered optimum- In the case of

the Gabbin deposit, drilling was carried out on a 200 m grid and more detailed

drilling was undertaken on a 100 m grid in areas of particular interest. The Topira

deposit in the Ituni area of Guyana, however, has been extensively evaluated on the

basis of 42 holes with an average grid spacing of 72 m. in this case proven reserves of

3.4 million tonnes of paper-grade clay have been outlined. English China Clays use a

100 m grid and then undertake infill drilling at 60 m spacings, Elsewhere in south-

west England the normal “stope” in china-clay working is 9 or 12 m (Vincent, 1983)

so evaluation borehole samples are of the same order, and minor variations in

lithology are ignored.

Large-diameter cores are desirable to obtain as large a sample as possible. A

major problem with drilling china clay is the flushing action of the drilling method.

This tends to wash out and erode the core, limiting core recovery. Conventional and

wire-line techniques suffer from this problem and the use of bentonitic drilling muds

is limited by the possibility of contamination of the clays. Core recovery tends to be a

function of core diameter, again favouring larger diameters, but to achieve the better

recovery with larger diameters, the price has to be paid in greater weight and cost.

In evaluating a china-clay deposit the commercial parameters are of most

importance and the economic geologist must be well aware of market requirements

and possible future developments. The commercial parameters usually of most

importance are brightness and fired colour, both having to be white, and particle size

distribution. The rheological properties, linear shrinkage and other fired properties are

of considerable importance. Bulk sampling from trial pits is advisable but even this

may not provide material representative of the commercial-quality clay which will

eventually be supplied to the customer.

Reserve evaluationis necessarily complexas the market specifications may

require blending, and sample data must be collected with future markets in mind. As

blending may involve numerous open pits in operation simultaneously, the geologist

must ensure that none containing potentially marketable clay are allowed to be back-

filled or flooded. Waste-disposal planning must be effective to avoid sterilization of

future reserves and to minimize visual impact. The geologist in the clay industry may

be expected to advise on plant site location and assist in site investigations for waste

tips and mica dam location. As with other mining operations, mine reclamation to

comply with environmental restrictions is very important.