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6 Rock weathering and
structure of the regolith
Ken G McQueen and Keith M Scott
6.1 INTRODUCTION
The previous two chapters have considered the min-
eralogical and chemical changes that occur during
weathering. As rocks weather chemically, and their
mineral constituents change to new, more stable
assemblages, the contained elements are preserved in
resistate minerals, partly redistributed into new min-
erals or taken into solution – in some cases to be
incorporated in other parts of the weathering profile.
Such elemental changes within the weathering profile
may be large and, in some cases, result in economic
mineral deposits (for example, the concentration of
Al in bauxite deposits; Chapter 1). This chapter con-
siders the mineralogical and geochemical changes
that accompany progressive rock weathering and dis-
cusses the typical weathering profiles for common
rock compositions.
6.2 STRUCTURE OF THE REGOLITH
The structure of the regolith at any particular site
depends on the extent to which chemical weathering
has transformed the bedrock composition, as well as
the degree of physical and chemical addition and
removal of materials. Well-developed profiles show a
vertical zonation, which may include from depth to
surface:
a zone of partially weathered bedrock that retains
• the primary rock fabric
a clay-rich or sandy plasmic/arenose zone in which
• the primary rock fabric has been destroyed
a ferruginous mottled zone; a ferruginous, baux-
• itic or siliceous duricrust/residuum
a soil layer
• a surface lag of chemically and physically resistant
• materials (Figure 6.1).
The zone in which the primary rock fabric is preserved
is referred to as the saprolith. The zone in which the
parent fabric has been destroyed, new fabrics formed
or soil developed is termed the pedolith. Weathering
occurs throughout the profile down to the weathering
front, which is defined as the boundary between fresh
rock and saprolith (that is, rock that shows some sign
of chemical weathering). Depending on bedrock type
and landscape setting, various parts of this mature
zonation may be absent, eroded or buried. Across a
landscape (or paleolandscape), there is generally sig-
nificant lateral variation in the regolith and its chemi-
cal structure, and these variations may be down to a
060802•Regolith Science 1pp.indd 103 24/06/08 11:00:34 PM
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104
scale of metres or tens of metres. Typically weathering
will penetrate preferentially and deeper along frac-
tured or more permeable zones. Less-weathered blocks
(lithorelicts) may be preserved well above the weather-
ing front and even to the surface (for example, granite
tors; Figure 3.1). Importantly, the most recently weath-
ered material is that closest to fresh rock. Generally,
this means that in a profile the youngest weathering
zone is at the base and the oldest at the top.
Within the saprolith, the lowermost saprock layer is
the least chemically altered zone, with less than 20%
of weatherable minerals altered. It will generally retain
evidence of the most recent weathering. Under humid,
oxidising and acid conditions, most sulfides and
carbonates are the least stable minerals and hence the
first to breakdown. Consequently S will be depleted,
or present as sulfate either in solution or as precipi-
tates, and elements hosted by the sulfides (As, Cd, Co,
Cu, Mo, Ni, Zn) pass into solution or are incorporated
into neo-formed (secondary) minerals (Section 5.3.8;
Appendix 2) . Elements present within carbonates
(Ca, Fe, Mg, Mn and Sr) will also be released and may
be depleted. Initial alteration of ferromagnesian min-
erals will release some Fe and Mg and contained trace
elements (Figure 5.3). Changes in chemical conditions
at the weathering front may also cause some dissolved
elements – derived from more extensive weathering
higher in the profile – to be precipitated or substituted
into existing minerals (for example, supergene Au
enrichment; Figure 6.2; Figure 5.14)
Chemical modification is more extensive in the
saprolite, where more than 20% of weatherable miner-
als are altered, although the primary bedrock fabric is
maintained. Progressive destruction of ferromagne-
sian minerals and feldspars results in depletion of Mg,
Ca, Na, K, some depletion in Si and retention of Al
and Si within the main weathering products of kaoli-
nite, other clay minerals and secondary silica. Hosted
trace elements that are released include Co, Cr, Cu,
Mn, Ni, Ba, Cs, Rb and Sr. Muscovite will generally
persist through the saprolite and retain some of the K,
Rb and Cs of the parent rock composition. Under
reducing conditions (such as below the water table),
the Fe2+ and Mn2+ released from weathered ferro-
magnesian minerals are mobile, but will precipitate as
insoluble Fe and Mn oxides where conditions become
more oxidising. Trace elements (particularly As, Co,
Cr, Cu, Ni and Zn) that are adsorbed or incorporated
in these stable oxides will be partially retained in the
profile, or even relatively enriched. More extensively
altered parts of the saprolite (typically the leached
upper saprolite) are marked by alteration of all but the
most chemically resistant primary minerals – as well
as progressive destruction of the less-stable secondary
minerals (such as smectites). This leads to further
release and potential leaching of Mg, Fe, Co, Cu, Cr,
Ni and Zn and relative enrichment in Al and Si. The
REE, which have generally been considered immobile,
can show significant depletion and enrichment within
the saprolite depending on their primary host (such as
Soil
Lag
Saprolite
Saprolith
Pedolith
Saprock
Bedrock
Lateritic gravels
Lateritic duricrust
Mottled
zone
Plasmic (clay)
or
arenose (sandy)
zone
Loose
Cemented
Weathering front
Pedoplasmation front
Cementation front
Lateritic residuum
or ferricrete
Regolith
Figure 6.1: Idealised regolith profile (after Eggleton 2001).
060802•Regolith Science 1pp.indd 104 24/06/08 11:00:34 PM
Rock weathering and structure of the regolith 105
highly weatherable pyroxenes and apatite versus
resistate zircon) and the weathering conditions. The
light REE – particularly Ce, (probably as Ce4+) – are
mobile under strongly oxidising conditions (see
Section 6.8 below).
Major primary mineral alteration, chemical leach-
ing and secondary mineral growth can eventually
destroy the primary rock fabric to produce the pedo-
lith(Figure 6.1). This typically contains a clay- or
quartz-rich plasmic/arenose zone (strongly enriched
in Al and Si relative to the parent bedrock) and a
mottled zone(in which darker Fe oxides are segre-
gated from the more pallid clay minerals). Under
some conditions, weathering in the plasmic zone can
result in the alteration of kaolinite to gibbsite and
leaching of Si. Most primary quartz and resistate
accessory minerals are retained – resulting in residual
enrichment in Si and elements such as B, Cr, Hf, Nb,
Rb, REE, Th, Ti, V, W and Zr. The mottled zone in the
upper part of the pedolith with Fe- and clay-rich zones
is enriched in Fe relative to the zone below. Marked
accumulation of the Fe oxides – particularly over
mafic and ultramafic rocks – can produce a very fer-
ruginous zone, which is typically composed of ferri-
crete(if cemented by Fe oxides) orferruginous
residuum(if less consolidated). This very ferruginous
zone is enriched in elements associated with hematite
and goethite, as well as those present in highly resistate
accessory minerals. Accumulation of Al oxides, such
as gibbsite and boehmite, can form a bauxitic zone
and precipitation of dissolved silica can produce an
almost pure SiO2 accumulation of silcrete. These
chemically stable and, in most cases, physically resist-
ant materials can harden to form a duricrust and
protect this zone of the regolith from erosion and
further major chemical alteration.
Hem
Hem
Goe Goe
Goe
Qtz
Kaol
Qtz
Feld
Mica
Pedogenic calcrete
Soil
Lateritic ferruginous zone
Clays with ferruginous mottles
Saprolite
Saprock
Fresh rock
Zone of gold enrichment
Smec
Chl
Kaol
(Qtz)
Smec
Serp
Talc
Goe
Goe
Goe
Goe Qtz
Qtz
Qtz
Kaol
Kaol
Smec Smec
Talc
Serp
(Qtz)
Amph
Pyrox
Plag Chl
(Qtz) Serp
Amph
Talc Pyrox
PERIDOTITEBASALTPORPHYRY
Au-depleted Zone
Figure 6.2: Supergene enrichment in Au caused by concentration of Au derived from higher in the weathering profile (after
Butt 1989).
060802•Regolith Science 1pp.indd 105 24/06/08 11:00:35 PM
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106
A soil profile will generally develop close to the
surface in the zone of greatest biological activity and
accumulation of organic detritus. Decay of organic
matter and the activity of microorganisms and veg-
etation result in concentration of humic acids,
organic complexes, carbonates and nitrates. The soil
chemistry will reflect climate and water content as
well as the composition of the underlying regolith/
bedrock and any colluvial, alluvial and aeolian addi-
tions. See Section 13.2.2 for more detail on soils as a
sampling medium.
A mature soil in equilibrium with the local envi-
ronment is typically differentiated into a number of
horizons. These are soil layers that are approximately
parallel to the land surface and differ from adjacent,
genetically related, layers in their physical, chemical
and/or biological properties, or in characteristics such
as colour, pH, structure, fabric, texture, consistency,
and types and number of organisms. Soils may vary in
character from thin, coarse-grained lithosols – that
have only one or two poorly differentiated horizons –
in some deserts, to thick, organic-, silt- and/or clay-
rich soils differentiated into several horizons, in more
humid regions. Common soil horizons (Figure 6.3)
include:
Litter (or O1): organic matter on the ground
• surface
O (or O2): fibrous (peaty) or massive organic
• matter
A: near-surface mineral horizon containing humi-
• fied organic matter
E (or A2): pale, commonly sandy, eluvial horizon
• with little organic matter. Iron and Mn oxides and
clays leached or translocated to lower horizons
B: illuvial horizons enriched in clay, and/or Fe and
• Mn oxides and/or organic matter derived from
overlying horizons
C: mineral horizon from which the overlying
• horizons are presumed to have been derived. It is
only slightly affected by pedogenic processes so
that remnant geological structures or fabric may
be retained
D: layers below the C horizon unaffected by the
• pedogenic processes that formed overlying hori-
zons, such as previously formed saprolite or trans-
ported overburden
R: continuous fresh rock.
•
The A, E and B horizons are referred to as the
solum.
This terminology is ideally suited to well-differen-
tiated soils, such as podzols, but less readily applied to
arid-zone soils that have weak horizon differentiation.
In soils in semi-arid Australia, the B horizon may be
differentiated from the A horizon by a contrast in
texture and/or being more sodic or alkaline, rather
than the characteristics noted above. For more detail
on soil types see CSIRO (1983) for Australia and FAO-
UNESCO (1988) for a more global view.
A surface lag may form above the soil if the finer or
less dense material is preferentially removed by sheet-
wash or wind erosion. As quartz and hematite are the
most chemically and physically stable minerals under
most surface conditions, this lag is relatively enriched
in Fe and Si. The presence of stable accessory minerals
01 Litter
02 Peaty horizon
A Humic horizon
E Pale eluvial
horizon
B Illuvial
horizon
C Mineral
horizon
D Weathered
rock
E Fresh rock
Saprolite
Bedrock
Organic
Solum
(pedogenic
processes)
Unconsolidated
parent
material
Figure 6.3: An idealised soil profile (after Butt et al. 2005).
060802•Regolith Science 1pp.indd 106 24/06/08 11:00:35 PM
Rock weathering and structure of the regolith 107
in the lag or underlying duricrust can also result in
relative enrichment in Ba, Cr, Mn, Nb, Ti, W and Zr.
Other minor and trace elements can be accumulated
and concentrated in the Fe and Mn oxides that persist
in the lag (such as As, Bi, Co, Cr, Ga, Ni, Pb, Sc, Th
and V).
The water table is an important chemical and phys-
ical boundary within the weathering profile – marking
the interface between the zone of water saturation and
the overlying zone of partial water content. In weath-
ering profiles that are in a steady state, the water table
will generally occur in the saprock or lower saprolite
zone. However, its position can change seasonally, or
over longer time periods, with climate change and
landscape evolution. Under wetter conditions the
water table can rise up through the weathering profile.
As conditions become drier, the water table may pro-
gressively fall. The depth to the water table will also
depend on the geomorphic setting and local hydro-
logic factors, including vegetation pumping. Below
the water table, conditions are typically reducing (low
Eh), and water movement is generally slow through
the water-filled fractures and pore spaces (Figure 6.4).
Permeability and hydraulic gradient control water
movement. Above the water table, gases (including
oxygen from the atmosphere) have greater access to
the regolith via voids and fractures and conditions are
typically more oxidising (higher Eh). Water generally
moves rapidly through this zone transporting dis-
solved and suspended components and leading to
strong leaching with further void formation. As well
as moving downwards and laterally, water and dis-
solved components can also move upwards through
this zone by capillary action – particularly where
evaporation exceeds rainfall. The change in Eh near
the water table is commonly sufficient to produce a
redox zone marked by the precipitation of insoluble Fe
and Mn oxides. The pH may also be reduced in this
zone due to the release of H+ when divalent cations
dissolve:
M2HO M(OH)2H
2
22
+= +
++
(Eqn 6.1)
Acid production is even greater if the cation is Fe2+,
which may be oxidised and hydrolysed (sometimes
referred to as ferrolysis):
Fe Fe e
23
=+
++-
(Eqn 6.2)
and
Fe 3H OFe(OH)3H
3
23
+= +
++
(Eqn 6.3)
Thus acidity is generated during chemical weather-
ing even in the absence of sulfides (see Chapter 5.3.1
eqns 5.22–5.25 and Chapter 10).
Groundwater compositions can also have a signifi-
cant affect on the chemical conditions above and below
the water table, including on a regional scale. For
example, in the southern Yilgarn Craton of Western
Australia, conditions in the upper 5–30 m of the rego-
lith are commonly acid and oxidising, whereas in the
northern Yilgarn Craton they are commonly neutral
and weakly to moderately oxidising (Gray 2001).
6.3 FIELD EXAMPLES OF WEATHERING
PROFILES ON COMMON ROCK TYPES
Particular zones within a regolith profile are com-
monly recognised by their colour, induration, textural
O2O2
Soil
Voids abundant
Water moves laterally
and vertically (capillary)
Slow water
movement Saprock
Saprolite
Redox zone Fe/Mn oxides
pH lowered by Fe2+
oxidation/hydrolysis Water table
Weathering front
Base of oxidation
Fresh rock
Few voids
Oxidising
Eh higher
Reducing
Eh lower
Figure 6.4: Eh and pH conditions about the water table.
060802•Regolith Science 1pp.indd 107 24/06/08 11:00:36 PM
Regolith Science
108
features and mineralogy (such as recognition of mot-
tling). Some of these features can be difficult to deter-
mine in pulverised and homogenised drill chips – a
common means of observing and sampling weather-
ing profiles during mineral exploration. Field descrip-
tions of zones within profiles based on information
obtainable from drill chips are therefore simplified
from those in Figure 6.1. Thus, in the descriptions of
weathering in common rock types (below), the weath-
ered materials are divided into lower saprolite (which
also includes saprock) and leached upper saprolite
(which may include some pedolith material), respec-
tively. Examples are for complete, well-developed and
largely intact profiles. Such profiles are more com-
monly preserved in areas with a long history of weath-
ering, tectonic stability and low relief or where they
are protected from erosion by well-developed
duricrusts or other deposits (such as lava f lows). As
well as climatic conditions and geomorphic setting,
the degree and depth of weathering are influenced by
rock type, structure and the presence of sulfide min-
eralisation. Studies in the Yilgarn Craton of Western
Australia indicate that the extent and depth of weath-
ering commonly follow the succession: granite and
mafic rocks<ultramafic rocks<sediments and fine-
grained felsic rocks (Anand and Paine 2002). Over
areas of strong fracturing and sulfide mineralisation,
the depth of weathering can be considerably greater.
6.3.1 Weathering in granitoids
Granitoids typically have a massive fabric, and weath-
ering is generally initiated by water penetration along
regular unloading joints and other fractures. The
major minerals in unweathered granitoids are quartz,
K-feldspar, plagioclase, biotite and, in some cases,
muscovite and amphibole. With the commencement
of weathering, the ferromagnesian minerals (biotite
and amphibole) and feldspars start to breakdown to
form goethite and kaolinite so that Al and Fe are
retained but more soluble components (Mg, Na, K
and Ca) are lost from the lower saprolite. With more
intense/longer weathering these processes continue so
that feldspars and ferromagnesian minerals are com-
pletely destroyed and kaolinite becomes a major com-
ponent of the leached upper saprolite – resulting in
relative enrichment of Al (Aspandiar 1998). Minor
amounts of other clays, such as halloysite and smec-
tite, may also form. Further weathering – particularly
under humid climatic conditions –, can lead to break-
down of kaolinite and removal of silica in solution to
produce Al hydroxides (gibbsite and boehmite): that
is, a bauxite zone above the kaolinite-rich saprolite.
Some elements that are retained in resistate minerals
– such as Zr and Ti, in zircon and rutile – are residu-
ally concentrated in the upper part of the profile (Butt
1985; Figure 6.5). Soluble Na, K and Ca are strongly
leached from the upper saprolite. The K present in
muscovite is, however, retained. In the example of
granitic weathering from the Darling Ranges, WA
(Figure 6.5), the formation of a surficial siliceous
capping (duricrust) has diluted all other components,
but this horizon, and the thin soil formed above it,
contain the same minerals as in the leached upper
saprolite – though not in the same proportions.
Weathering profiles in felsic volcanic rocks show a
similar pattern of mineral development to weathering
in granitoids, but with stronger ferruginous mottling
where they have a higher original Fe content (Anand
and Paine 2002).
6.3.2 Weathering in mafic rocks
Most mafic rocks are igneous or metamorphosed
igneous rocks and generally contain dominant plagi-
oclase, ferromagnesian minerals (such as olivine,
pyroxenes, amphiboles, biotite and chlorite) and vari-
able amounts of other minerals, including quartz.
Muscovite, carbonates and sulfides (mainly pyrite or
pyrrhotite) may be present in alteration and mineral-
ised zones. With the commencement of weathering,
the carbonate minerals (dolomite and calcite) rapidly
dissolve, pyrite and pyrrhotite weather to form
goethite, and the ferromagnesian minerals alter to
smectite and kaolinite. This results in loss of Ca, but
retention of Al, Fe and Mg (the latter two may even be
slightly residually enriched), at the base of the lower
saprolite. With more intense/longer weathering, pla-
gioclase and chlorite also break down to form smec-
tites and more kaolinite and goethite, with Mg being
lost from higher in the lower saprolite. Further weath-
ering results in the complete loss of plagioclase and
chlorite and the formation of more clay minerals
(kaolinite and smectites) and goethite/hematite, with
060802•Regolith Science 1pp.indd 108 24/06/08 11:00:36 PM
Rock weathering and structure of the regolith 109
soluble elements – particularly K and Mg – being lost
from the leached upper saprolite. Aluminium and Fe
(plus insoluble trace elements, such as Cr) become
enriched in this zone. The formation of soil continues
the process of enrichment in clay minerals and Fe
oxides, although the formation of near surface cal-
crete/sulfates in the soil may dilute other components.
Quartz and muscovite are retained through the
weathering cycle. Figure 6.6 depicts a generalised
weathering profile on mafic rocks from Mt Magnet
(WA), showing the mineralogical variation.
Weathering of Cenozoic basalts (containing
olivine, pyroxene and glass) under more temperate
conditions in south-eastern Australia, has produced
55 2.0 0.5
Duricrust
Yellow-brown
Brown
Colour
White
Grey
Grey
Grey
Grey
Lower
Saprolite
Key to Figures 6.5 - 6.9
Calc
Chl/V
Dol
Goe
Gyp/Ba
Hem
Kaol
Kspar
Magh
Musc
Parag
Plag
Qtz
Smec
Calcite
Chlorite/vermiculite
Dolomite
Goethite
Gypsum/barite
Hematite
Kaolinite
K-feldspar
Maghemite
Muscovite
Paragonite
Plagioclase
Quartz
Smectite
Fresh
Rock
Leached
Upper
Saprolite
Soil
400
20
0.9
10
<0.1
260
288
0.7.7
122
<0.1
Kspar
Qtz
Plag
Biotite
Musc
Goe
Kaol
Gibbsite
Hem
Magh
Zr ppm
Al2O3 %
TiO2 %
Fe2O3 %
Na2O3 %
Thick-
ness (m)
300
35
0.6
3.0
0.1
300
17
0.4
2.9
0.9
230
17
0.4
3.0
2.6
150
16
0.3
3.1
3.9
160
15
0.3
2.9
4.3
Figure 6.5: A generalised profile through weathered granite, Darling Ranges, WA (after Gilkes et al. 1973; Anand 1984).
060802•Regolith Science 1pp.indd 109 24/06/08 11:00:36 PM
Regolith Science
110
similar suites of minerals and elemental depletions/
enrichments in saprolite and soil (Loughnan 1969;
Moore 1996). The importance of water in weathering
is illustrated by comparison with the weathering of
basalts on the Moon where only physical, rather than
chemical, weathering occurs (Section 14.2).
6.3.3 Weathering in ultramafic rocks
Ultramafic rocks consist mainly of olivine and
pyroxenes, but they are commonly serpentinised or
altered by metamorphism to amphibole–chlorite or
talc-bearing assemblages. They are rich in Mg and
poor in Al relative to other rock types. Chemical
weathering of olivine- and serpentine-rich rocks pro-
duces smectitic clays, such as nontronite and saponite,
in the saprolith. These are altered to kaolinite in the
upper saprolite, with loss of Mg. Some of t his Mg may
be precipitated as magnesite nodules in the lower
saprolite. Iron – originally present in olivine and
disseminated magnetite – forms goethite in the
saprolite, which is dehydrated to hematite near the
top of the weathering profile. Excess Si released
during the weathering commonly forms deposits of
opaline silica within the profile. Under free-draining
conditions in high-relief terrains, Ni released from
the weathering of olivine and serpentine can substi-
tute for Mg in serpentine close to the weathering
front to produce garnierite. In low-relief and
poor-draining environments Ni may be taken up in
smectitic clays or goethite in the lower saprolite.
1025
Pink-brown
Colour
Buff to
red-brown
Brown
Brown to
brown-grey
Grey
Lower
Saprolite
Fresh
Rock
Leached
Upper
Saprolite
Soil and
calcrete
1
K2O %
Qtz
Plag
Calc
Dol
Hem
Goe
Musc
Parag
Chl/V
Kaol
Smec
Gyp/Ba
Pyrite
Cr ppm
Al2O3 %
MgO %
Fe2O3%
600
11
0.5
16
0.3
450
15
0.2
18
1.5
350
14
0.4
19
6
250
13
0.2
16
5
300
13
0.5
10
Thick-
ness (m)
approx 3.0
Figure 6.6: A generalised profile through weathered mafic rocks, Mt Magnet, WA (after Scott and Martinez 1990).
060802•Regolith Science 1pp.indd 110 24/06/08 11:00:37 PM
Rock weathering and structure of the regolith 111
Significant concentration of Ni by these mechanisms
can produce lateritic nickel deposits.
Unweathered serpentinised ultramafic rocks at
Panglo (Yilgarn Craton of WA) are dominated by
chlorite and talc, with only trace quartz (Figure 6.7).
With the commencement of weathering, chlorite
readily breaks down to form goethite, kaolinite, ver-
miculite and smectite in the lower saprolite. Thus,
although the Mg in talc is retained, Mg from chlorite
is gradually depleted up the profile. Aluminium and
Fe from chlorite are retained in the neo-formed clays
and Fe oxides. Ferruginous duricrust above the upper
saprolite is particularly Fe-rich, indurated by calcrete,
and commonly incorporates a transported compo-
nent (indicated by the presence muscovite) so that Mg
and Cr contents are diluted. The soil formed above
ferruginous duricrust shows further dilution of the
diagnostic ultramafic components (Mg and Cr),
although these are still present in significant amounts.
Chromium present in spinels is retained during
weathering, but that in chlorite (up to 3500 ppm) is
transferred to goethite (8700 ppm) formed from the
chlorite breakdown (Scott 1990).
6.3.4 Weathering in detrital
sedimentary rocks
Detrital sedimentary rocks, and their low-grade
metamorphosed equivalents, contain mainly quartz,
muscovite, chlorite and, in some cases, plagioclase,
carbonates, epidote and minor sulfides, together
2
Brown
Brown-
yellow
Yellow to
light brown
Colour
Yellow to
khaki
Khaki
Khaki to
brown
Saprolite
Fresh
Rock
Soil
1.1
K2O %
Qtz
Calc
Dol
Hem
Goe
Musc
Chl/V
Cr ppm
Al2O3 %
MgO %
Fe2O3%
1100
7.8
0.5
1.6
3.8
1800
8.6
0.4
27
13
2500
10
<0.1
13
16
2300
8.4
<0.1
12
18
2200
6.9
<0.1
11
18
2800
9.1
<0.1
14
Grey
20
2800
7.5
<0.1
12
Magh
Talc
Kaol
Ferruginous
duricrust
Thick-
ness (m)
0.560-70
Calcrete
Figure 6.7: A generalised profile through weathered ultramafic rocks, Panglo, WA (after Scott 1990).
060802•Regolith Science 1pp.indd 111 24/06/08 11:00:37 PM
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112
with trace amounts of accessory minerals (zircon,
rutile, ilmenite and apatite) and carbonaceous
matter. As many of the accessory minerals are the
weathering products of other rocks, they are less
affected by further chemical weathering under a new
weathering regime. In profiles on weathered sedi-
mentary rocks (especially sandstones), it can be dif-
ficult to distinguish the weathered and unweathered
zones. Physical processes generally play a more
important role in the development of weathering
profiles on these rocks types – including increased
fracturing and destruction of the rock fabric by stress
relief, drying and wetting, freeze/thawing and
veining by secondary minerals (such as Fe oxides).
Mottling and strong bleaching are other clues to
identifying the weathered zone.
In unweathered Archean shale (interbedded with
mafic volcanic rocks) in the Panglo area (Yilgarn
Craton of WA), the major minerals are quartz, plagi-
oclase, mica (muscovite + paragonite), carbonates,
chlorite with or without pyrite. Weathering com-
mences along cleavage/bedding planes in the shale,
with the carbonates, pyrite and feldspar breaking
down to form goethite and kaolinite. After this initial
mineralogical change there is little further change
throughout the lower saprolite (Figure 6.8). In the
upper leached saprolite, smectite and hematite are
developed, but chemical compositions are not mark-
Brown
Colour
White to
pale brown
White to
pale brown
Yellow
Black to
grey
Lower
Saprolite
Fresh
Rock
Leached
Upper
Saprolite
Soil and
calcrete
White to
grey
Qtz
Plag
Calc
Dol
Hem
Goe
Magh
Musc
Parag
Chl/V
Talc
Kaol
Smec
Pyrite
Cr ppm
Al2O3%
K2O %
Fe2O3%
Thick-
ness (m)
<1.0
approx 25
>30
10
0.9
7.6
320
16
2.6
1.6
230
19
2.3
0.8
210
18
2.6
1.7
230
20
1.6
1.5
170
17
1.8
1.2
160
Figure 6.8: A generalised profile through weathered shale, Panglo, WA (after Scott and Dotter 1990).
060802•Regolith Science 1pp.indd 112 24/06/08 11:00:37 PM
Rock weathering and structure of the regolith 113
edly different from those in the lower saprolite, except
for residual concentration of Cr in neo-formed
goethite and resistate rutile (Scott and Dotter 1990).
The soil – with higher Fe but lower K and Al than in
the leached upper saprolite – probably has a trans-
ported component at Panglo (Figure 6.8).
Non-Archean shales tend to have higher K con-
tents than their Archean equivalents. The K may be
present as K-feldspar and/or biotite, both of which
weather out leaving lower K through the saprolite (for
example, Dickson and Scott 1997). Chromium con-
tents in shales in less mafic environments may be
lower than at Panglo. Commonly, up to 1% organic
matter may be present in black carbonaceous post-
Proterozoic shales, with higher abundances in min-
eralised shales (Appendix 2). This organic matter
(kerogen) has variable H/C and O/C ratios (Saxby
1976) which affects its weathering susceptibility, but
ultimately it is broken down during weathering
(Section 7.3.1) to produce a pallid saprolite: that is,
black shales weather to white saprolite.
In arenaceous rocks, the feldspars and phyllosili-
cates (micas, chlorite) weather to kaolinite and
goethite/hematite, but the abundant quartz compo-
nent and accessory resistates remain largely unaf-
fected. Quartz-rich sandstones break down
mechanically, but do not change chemically during
weathering – apart from the formation of minor Fe
oxide coatings in fractures/voids and, in some cases
of extreme weathering, precipitation of additional
secondary silica. There are often silty interbeds,
which weather as described above.
6.3.5 Weathering in carbonate-rich rocks
Limestones contain abundant calcite, with only minor
silica/quartz and clays. Weathering involves dissolu-
tion of the calcite – with the loss of the Ca – and accu-
mulation of the insoluble components to form a thin
soil. (Dolomite tends to dissolve less rapidly than
calcite, but, when it does, it frees Ca and Mg.) If there
are significant clay and silt impurities or interbeds in
the carbonate rock, these can be weathered and resid-
ually concentrated to form thicker (up to 30 m) accu-
mulations of smectite, kaolinite or bauxite (gibbsite/
boehmite). Minor Fe and Mn released from the car-
bonates during dissolution are generally oxidised to
form insoluble goethite, hematite and Mn oxides. The
Fe oxides produce deep red-coloured residual soils
known as terra rosa.
Profiles on carbonate-rich rocks are very distinc-
tive: with sharp contacts between the unweathered
(undissolved) rock and the weathering residue (Rob-
ertson et al. 2006). Unless there is a significant, non-
carbonate component, there is generally no saprolith.
Percolation of water along fractures and accompany-
ing dissolution of the carbonate may lead to spectacu-
lar karst topography, with jagged and pinnacled rock
surfaces forming the weathering front (Figure 6.9).
Quartz, residual clays, insoluble Fe and Mn oxides
and wind-blown dust commonly accumulate in
hollows and cavities in the unweathered limestone
and may form irregular and thick deposits. Zirco-
nium, Ti and Si – present as minor components in the
original rock – are residually concentrated, although
much of the quartz and clay is derived from the more
silty interbeds. Other primary constituents may be
residually concentrated (such as. at Laowanchang in
Guizhou Province, China, gold has been concentrated
in this way). Further concentration of resistate miner-
als/elements occurs during the formation of the soil.
The presence of chlorite in the soil (Figure 6.9) also
makes the presence of an introduced component in
the soil at Laowanchang obvious.
6.4 EFFECTS OF EROSION AND
DEPOSITION ON PROFILE DEVELOPMENT
The weathering profiles described above are mature
profiles for which there has been sufficient time and/
or intensity of weathering for the zones to become
well developed. However, in the case of recent expo-
sure, the profiles may be immature, with only some
features of the saprolite present and little or no surfi-
cial ferruginisation and soil development.
Mature weathering profiles may be partly eroded
and a new weathering profile developed across the
unconformity. In inset valleys (paleovalleys), clay-
rich sediments commonly develop ferruginous mega-
mottles (more than100 mm across) as a result of later
weathering (Ollier et al. 1988; Figure 6. 10). These
mega-mottles are formed by the mobilisation and
concentration of Fe oxides often around roots or
060802•Regolith Science 1pp.indd 113 24/06/08 11:00:38 PM
Regolith Science
114
around shrinkage cracks (Anand 2001; Johnson and
McQueen 2001).
Commonly, after a period of erosion – especially
where a channel has developed – renewed deposition
may commence, with accumulation of vegetable
matter, which, in some cases, is pure and thick enough
to ultimately become lignite, which is enriched in ele-
ments such as Mo and V. Such a reducing horizon can
cause precipitation of secondary sulfides and U min-
eralisation such as at Mulga Rock, WA (Figure 6.11;
Douglas et al. 2005).
The addition of transported material to weather-
ing profiles can particularly affect the chemical com-
position of the soil layer. Most soils are affected by
some colluvial movement of adjacent material (metre
scale movement), but, unless this movement intro-
duces material derived from a different rock type or
from material with a different degree of weathering,
it is unlikely to be recognised. Aeolian additions to
profiles are obvious where they are present as sand
dunes, but they may not be so obvious when the
introduced material is finer grained silt and clay. This
extraneous component can also be worked into the
profile by soil churning or bioturbation (Sections
3.4.4 and 8.3.2). This material can be recognised in
the soil by detailed analysis of the particle size distri-
bution, particle morphology and chemical and min-
eralogical characteristics (Scott 2005; Dickson and
Scott 1998; Tate et al. 2007).
6.5 WEATHERING REGIMES
Chemical and physical conditions in the regolith can
change significantly through time particularly in
response to erosional, climatic, hydrologic and bio-
logical changes. At any one location, this can be the
result of crustal plate movement, global or local
climate change, as well as local tectonic and geomor-
phic factors. Regolith that has formed and been pre-
served over a long time span may therefore have
Red
Colour
Red-brown
Saprolite
Fresh
rock
Chert
Soil
15
White to
grey
Qtz
Calc
Kaol
Chl
Goe
Hem
Anatase
Zr ppm
Al2O3 %
TiO2 %
Fe2O3%
SiO2%
CaO%
860
3.6
19
50
<0.1
17
420
3.8
13
56
0.2
0.7
-
0.1
<0.1
2.7
55
Figure 6.9: A generalised profile through weathering limestone, Laowanchang Au deposit area, Guizhou Province, China
(data from J Mao pers. comm. 2007)
060802•Regolith Science 1pp.indd 114 24/06/08 11:00:38 PM
Rock weathering and structure of the regolith 115
Lateritic residuum
Saprolite
Ferricrete (inverted
ferruginised Tertiary
sediments)
Basement
BIF
Breakaway
Tertiary
inset-valleys
Modern
drainage
8
3
Dissected
paleolandsurface
on basement
DPB
Plains
Plains Hill belt
6
7
2
1
4
4
5
3
6 7 8
21 4
5
Lateritic
residuum Unconformity
Lag
Lateritic
residuum
Collapsed
mottled
saprolite
Mottled
saprolite
Fe saprolite
Saprolite
Soil
Silty hard-
panised
colluvium
Nodular-
slabby
ferricrete
Gravel
Collapsed
mottled
saprolite
Soil
Silty hardpanised
colluvium
Gravelly hard-
panised colluvium
Gravels
Lateritic
residuum
Collapsed
mottled
saprolite
Mottled
saprolite
Ferricrete
Unconformity
Soil
Mottled
clay
Lateritic
residuum
Mega
mottled
clay
Gravel
Grey
clay
Soil Soil
Hard-
panised
colluvium
Red clay
Gravel
Mega-
mottled
clay
Dolomite
Pisolitic
clay
Gravel &
sand
Quaternary colluvium
and alluvium
Tertiary sediments
Unconformity
Figure 6.10: Formation of mega-mottles as the result of multiple weathering events (after Anand 2001),
060802•Regolith Science 1pp.indd 115 24/06/08 11:00:39 PM
Regolith Science
116
experienced a number of different weathering regimes.
It is important to remember that not only is weather-
ing is a continuous process, but also the rate and type
of weathering can vary depending on the stability of
the minerals in the regolith and underlying bedrock
under the conditions of the prevailing weathering
regime. Changes in weathering conditions can have
important implications for the dispersion of target
and pathfinder elements used in geochemical explo-
ration. Significant changes result in complex, multi-
stage geochemical dispersion effects that may not be
fully explained by the current weathering conditions.
Patterns from different weathering regimes may be
preserved in different parts of the landscape or they
may be superimposed – depending on the history of
landscape development, relative uplift, erosion and
deposition. Figure 6.12 shows how the superimposi-
tion of a second weathering regime may largely
obscure the effects of initial weathering.
6.5.1 Effects of long-term climate change
Climate is a major control on weathering regimes and,
for most weathering profiles, climatic conditions have
changed significantly during their formation (see
Chapter 2). The simplest case is for two markedly dif-
ferent climates, but, for very old weathering profiles –
particularly in some Proterozoic and Archean terrains
– the climatic conditions and weathering processes
have been much more complex. Unravelling this com-
plexity requires detailed studies using techniques such
as paleomagnetic or isotopic dating (McQueen et al.
2007; Anand et al. 1997). For example, over much of
southern Australia, major climatic variations through
the Late Cretaceous and Cenozoic generally resulted
in early deep chemical weathering under predomi-
nantly warm humid conditions (with high availability
of organic material) onto which has been superim-
posed drier chemical weathering under increasingly
arid conditions since the Late Cenozoic. The detailed
picture is more complex, with fluctuations to at least
two cooler-dry episodes before the Oligocene
(McGowran and Li 1998). Paleomagnetic dating of
ferruginous mottling in weathering profiles across the
region indicates a number of periods of major hema-
tite fixation – reflecting episodes of major oxidation
and profile drying following intensive chemical
weathering (Pillans 2006; McQueen et al. 2007; Smith
et al. in press). These dated periods occurred during
35
36
37
38
39
40
U (%) Fe (%) Cu (%) Zn (%) Ce (%) Se (%) Al (%)Ti (%)Zr (ppm)
0 1 0 3 0 1 0 10 0 0.5 0 0.1 0 15 0 1.5 0 200
Silty
Clay
Clay
rich
Lignite
Lignite
Depth (m)
Figure 6.11: Development of a reducing environment leading to sulfide and U precipitation in clay-rich lignite, Mulga Rock,
WA (after Douglas et al. 2005).
060802•Regolith Science 1pp.indd 116 24/06/08 11:00:39 PM
Rock weathering and structure of the regolith 117
the Paleocene (circa 60 Ma), Miocene (circa 16 and
12 Ma) and at some sites in the Late Cenozoic (circa
5 Ma). In some profiles, there is also evidence of
earlier deep oxidation episodes, including in the
Jurassic and Carboniferous (Pillans 2006). See Chapter
2 for further discussion of dating regolith materials.
6.5.2 Geochemical dispersion under
different weathering regimes
Where weathering has occurred under a wide range of
contrasting conditions, some minerals formed under
an early weathering regime have a stability range that
covers the conditions of later weathering. Thus kaoli-
nite, Fe oxides, quartz and secondary silica will persist
over a very wide range of weathering conditions and
may retain minor and trace elements that are incor-
porated into their structures or concentrated in
occluded accessory and secondary minerals. However,
weakly adsorbed elements can be further dispersed
under changed chemical conditions (particularly
changed water activity, Eh and pH). Less-stable weath-
ering products – such as smectitic clays, carbonates,
sulfates, halides that are stable under one weathering
regime – may be completely broken down under dif-
ferent conditions, releasing their contained major,
minor and trace elements.
The availability of water under different weather-
ing regimes is the major control on the resulting rego-
lith geochemistry, element dispersion and fixation.
Weathering regimes can therefore be considered as
part of a spectrum from high water availability (wet,
mainly tropical and temperate, typically with annual
rainfall greater than 500 mm) to low water availability
(dry, mainly arid to semi-arid with annual rainfall
less than 500 mm, or frozen). Seasonal fluctuations
from wet to dry are also a common type of weathering
regime in the subtropics.
Wet conditions – characterised by high water tables
and a typically abundant and active biota – tend to
favour hydration/hydrolysis reactions and mobility of
reduced species, such as Fe2+, and Mn2+. Groundwa-
ter pH conditions are generally neutral to acid – the
latter particularly in zones with abundant decaying
organic material and around oxidising sulfides. High
organic content promotes organo-complexing of
many elements (Section 7.5). Most major elements are
strongly leached – resulting in relative enrichment in
Al, Fe and Ti. Trace elements are also strongly leached
– particularly Cu, Zn, Cd and Ag – but some, includ-
ing As, Bi, Cr, Mo, Pb and V, are retained or relatively
concentrated in Fe-rich zones where these develop
(Lecomte and Zeegers 1992).
Dry conditions typically favour oxidation reactions
and a change to more complex, groundwater composi-
tions, particularly with higher salinity, increased activ-
ity of carbonate and sulfate and regional neutral to
alkaline pH. Some major elements (such as Ca, Mg and
Na) may be retained in smectitic clays or enriched by
evaporative concentration and re-precipitation in the
regolith. More alkaline conditions favour mobility of
trace elements such as As, Mo and U. Chloride and
thiosulfate complexing can increase the solubility and
mobility of some elements, such as Au, whereas other
elements – especially Pb, Ag, Ba and Hg – become rela-
tively fixed as insoluble chlorides and sulfates. Marked
pH gradients around sulfide deposits that continue to
weather generally results in dispersion of elements
such as Cu and Zn to form distinct secondary haloes.
Weathering conditions in the Yilgarn Craton of
Western Australia have spanned the range from warm
humid conditions in the Late Mesozoic and Early
Cenozoic to arid conditions in the Late Cenozoic
(Chapter 2). There early-formed lateritic profiles –
which are characterised by a modest amount of strong
element leaching in the upper saprolite – have been
modified under the more arid conditions, with the
leached saprolite becoming more extensive as the water
Regime 1
Regime 2
Overprint surface surface surface
erosional
stripping
uplift
Regime 1 Regime 2
in cover
burial
stable
exposure
level
Overprint
1a 1b 1c
2a 2 etc
3
Figure 6.12: Superimposition of different weathering
regimes.
060802•Regolith Science 1pp.indd 117 24/06/08 11:00:40 PM
Regolith Science
118
table fell (see Figure 6.13). Under these conditions, the
groundwaters became quite saline (Gray 2001).
6.6 DISCRIMINATING PARENT ROCK
TYPES
In deeply weathered terrains, it is generally difficult to
establish the parent rock type for regolith materials
because of the profound mineralogical and chemical
changes related to weathering and, in some cases, to
secondary cementation and induration. In saprock
and saprolite, the original rock fabric may be pre-
served to give clues to the original parent. In many
cases, however, these features are destroyed – particu-
larly in the upper part of the weathering profile. A
number of studies have investigated the use of geo-
chemical criteria – based on the least mobile elements
– to discriminate parent rock types for in situ regolith.
The most widely used technique employs Ti/Zr ratios
to discriminate between regolith derived from igneous
rock types over the range from mafic to felsic (Figure
6.14; Hallberg 1984). These two trace elements are
typically concentrated in rutile and zircon – minerals
that are highly resistant to weathering – and are thus
considered to be relatively immobile. (Some Ti may
also be present in ilmenite, sphene, micas, amphiboles
and pyroxenes and, as these minerals weather, Ti is
released, but re-precipitated as stable anatase [TiO2],
which is effectively immobile (Butt 1985)). Further-
more, because Hf is more concentrated in the zircon
of more fractioned igneous rocks, the Zr/Hf ratio in
zircon can also be used to gain information about the
parentage of highly weathered material (Figure 6.15).
Minor and trace element contents of resistate miner-
als are also useful as guides to mineralisation (for
example, Scott and Radford 2007).
Soil
Time
Ferruginous zone
Mottled zone
Saprolite
Fresh rock
Water table
Water table
Ferruginous zone
Mottled zone
Leached saprolite
Saprolite
A
B
Fresh rock
Continued
slow
weathering
Figure 6.13: Effect of aridity on lateritic profiles of Yilgarn Craton (after Butt 1989).
060802•Regolith Science 1pp.indd 118 24/06/08 11:00:40 PM
Rock weathering and structure of the regolith 119
Of the major elements, Al is generally the least
mobile and plots of Al2O3-TiO2-Zr can also be used to
discriminate regolith from different parent rocks.
Including Al allows characterisation of regolith for a
wider range of rock types, including sedimentary and
metamorphic rocks (for example, Garcia et al. 1994;
Figure 6.16). Interpretation of compositional varia-
tion for these three elements is based on the premise
that sedimentation involves weathering, transport,
mixing from different sources and sorting. In the first
three processes, the contents of the less-soluble ele-
ments, such as Al, Ti and Zr, may vary in response to
the degree of leaching of the soluble elements.
However, their relative proportions are transferred
from the source area into the bulk sediment or rego-
lith without, or with little, modification. This mate-
rial is then sorted according to the hydraulic properties
of its mineral components to produce a chemical frac-
tionation between complementary shales and sand-
stone (Figure 6.16). Other rock types, such as felsic
and mafic igneous rocks or immature volcanic-
derived sediments, will also plot in specific fields.
6.7 CHARACTERISING AND
IDENTIFYING REGOLITH MATERIALS
Major chemical changes that occur during weathering
– particularly progressive loss of Na+, K+, Ca2+, Mg2+
(some Si4+) and retention of Si4+, Al3+ and Fe3+
(Section 5.4.2), can be used to help to characterise
regolith materials that are at different stages of min-
eralogical/chemical evolution. The K/Al and Mg/Al
ratios can commonly be used in this way by compar-
ing two mobilised elements with a relatively immobile
major element. Figure 6.17 shows an example of the
compositional distribution in terms of this index for
three different types of regolith found in both the
Kalgoorlie region of Western Australia and the Cobar
region of eastern Australia (Johnson and McQueen
2001; McQueen 2006). The three different regolith
materials in these examples are in situ saprolite/
saprock, lacustrine clays (difficult to distinguish visu-
ally from weathered saprolite) and younger ferrugi-
nous alluvium/colluvium. These materials have
different regolith histories that are ref lected in their
700
600
500
400
300
200
100
00 2000 4000 6000 8000 10000 12000
Basalt
Andesite
Dacite
Rhyolite
Zr ppm
Ti ppm
Figure 6.14: Ti/Zr plot for discriminating weathered
volcanic rocks (after Hallberg 1984).
60
50
40
30
20
10
01.0 1.5 2.0 27.0
Hf(%)
Gabbro
Carbonitites
Granodiorite
Pegmatite
Pegmatite
Granite
Zr/Hf
Figure 6.15: Zr/Hf for zircons in igneous rocks (after Černý
et al. 1985; Wang et al. 1996).
SPG
CAS
ABS
ABS - Average bulk
sediment
CAS - Calc-alkaline
suite
SPG - Strongly
peraluminous
granite
Zr
Note:
15 Al2O3
Al2O3, TiO2 - wt %
Zr- ppm
sandstone
shales
300 TiO2
Figure 6.16: The 15Al2O3–300TiO2–Zr weathered rock type
discriminant diagram (after Garcia et al. 1994).
060802•Regolith Science 1pp.indd 119 24/06/08 11:00:41 PM
Regolith Science
120
mineral and chemical composition. The lacustrine
clays were deposited in the Late Mesozoic to Early
Cenozoic and appear to have been derived from a
deeply weathered landscape. They were well sorted
during erosion and transport to produce a kaolinite–
smectite–quartz dominant sediment. The saprolite/
saprock contains significant muscovite and illite,
which have been variably altered to kaolinite –
depending on degree of in situ weathering and depth
in the profile. The younger alluvium/colluvium was
deposited in the Late Cenozoic when climatic condi-
tions were significantly drier and chemical weather-
ing less intense. Erosion of less altered profiles, more
limited sorting and low levels of post-depositional
weathering produced material that retained signifi-
cant amounts of weakly altered phyllosilicates. These
sediments can also contain local concentrations of
dolomitic calcrete, which will be apparent in their
Mg/Al ratio. There is some compositional overlap
between the three materials, and the clearest distinc-
tion is between saprolite and transported clays. This
approach to chemically distinguishing regolith types
can be applied within particular regions where there
are regolith components characterised by different
parent rock compositions, different degrees of weath-
ering or different histories of sorting or remixing/
homogenisation during transport. The differences
can be established with an orientation survey.
Trace element characteristics – particularly for
those elements hosted by resistate minerals – can be
useful for identifying different types of regolith. For
example, regolith derived from mafic or ultramafic
rocks typically has higher contents of Ti, V and Cr –
hosted in rutile/anatase, ilmenite, magnetite and chr-
omite – than regolith derived from felsic or
meta-sedimentary rocks. Aeolian material in the rego-
lith may have high Zr (and Hf) contents relative to in
situ material due to the greater abundance of zircon
grains in the introduced material (Tate et al. 2007).
6.8 DEGREE OF WEATHERING AND
WEATHERING HISTORY
The Chemical Index of Alteration (CIA) has been
widely used to quantify degree of rock weathering
(Nesbitt and Young 1982). This index, CIA = 100Al2O3/
(Al2O3+CaO+Na2O+K2O), reflects the breakdown of
feldspars and mica to kaolinite, but it has a major
drawback in that it estimates the total history of chem-
ical weathering from the primary source rock: that is,
including that already present in sedimentary rocks
subjected to further weathering. It is thus difficult to
apply this index as a direct measure of the in situ
weathering of a particular regolith sample. However, it
can be useful for comparing samples within profiles
developed on a variably weathered common rock type
(Figure 6.18). Weathering trends within a profile, or
0
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0 0.1 0.2 0.3 0.4 0.5 0.6
K/Al (wt%)
Saprolite on
siltstones/sandstones
Kaolinite
Muscovite
Smectite/chlorite/dolomite
Smectite/chlorite/dolomite
Ferruginous
alluvium
Transported
clays
Saprolite on
mafic rocks
Mg/Al (wt%)
Figure 6.17: Compositional variability of regolith in terms of
Mg/Al versus K/Al (after McQueen 2006).
60 70 80 90 100
CIA (Chemical Alteration Index)
10
0
20
30
40
50
60
70
Depth (m)
Figure 6.18: Chemical Index of Alteration (CIA) variation
with depth in weathered metasediments, Cobar region
(after McQueen 2006).
060802•Regolith Science 1pp.indd 120 24/06/08 11:00:42 PM
Rock weathering and structure of the regolith 121
group of profiles, on common rock types can also be
distinguished using the relative mole proportions of
Al2O3, (CaO+Na2O+K2O) and (total Fe2O3+MgO)
(Nesbitt and Young 1989). Thus an A-CNK-FM plot
for samples from a single weathering profile within
metasediments in the Cobar region (Figure 6.19)
reflects the relative increase in Al and loss of Fe, Mg
and alkalis as weathering proceeds. This approach is
complicated by the inclusion of Fe, which can be accu-
mu lated at redox bou nda rie s or ra pidly mobi lis ed f rom
Fe-bearing carbonates in weathering sedimentary
rocks. Formation of secondary dolomite can also com-
plicate Mg contents of strongly weathered regolith.
The REE are commonly used as petrogenetic indi-
cators because of their similar chemical properties,
typically low solubilities and assumed resistance to
fractionation in supracrustal environments. However,
a number of studies have shown that under some
weathering conditions REE are significantly mobi-
lised and fractionated (for example, Nesbitt 1979,
Duddy 1980, Sharma and Rajamani 2000, Gray 2001,
McQueen 2006). Far from being immobile, REE can
be significantly redistributed during weathering to
the point where their depletions in one part of the
profile, and subsequent enrichments in other parts,
can provide an index for the intensity and style of
chemical weathering. Analysis of REE distributions
through deeply weathered profiles in the Cobar
region indicates significant leaching in the upper
Feldspar Weathering trend
for sedimentary rocks
Most
weathered
saprolite
Saprock
Muscovite
Igneous rock trend
CNK
(CaO+Na2O+K2O) FM
(Tot Fe2O3+MgO)
100
90
80
70
60
50
40
10
20
30
40
50
60
A (Al2O3)
Figure 6.19: A-CNK-FM plot for a weathering profile
through metasediments, Cobar region (after McQueen
2006).
Profile CBAC 215, Cobar region
Down profile
(less weathered)
Note: REE(Y) normalised to North American Shale
Com
p
osite (Gromet et al. 1984)
10
1
0.1
0.01 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
19m
25m
31m
37m
43m
49m
Y
Figure 6.20: REE distribution in a weathering profile
through metasediments, Cobar region (after McQueen
2006).
10 10
20 20
30 30
40 40
50 50
60 60
70 70
002 2143
Approximate
initial ratio
Approximate
initial ratio
648510 612 7
Ce/Nd Ce/Nd
Profile CBAC 215 Profile CBAC 217
Depth (m)
Depth (m)
Figure 6.21: Ce/Nd plots for weathering profiles through metasediments, Cobar region (after McQueen 2006).
060802•Regolith Science 1pp.indd 121 24/06/08 11:00:42 PM
Regolith Science
122
parts, and enrichment in the lower zone close to the
weathering front (Figure 6.20). The light rare earth
elements (LREE) – particularly Ce – appear to have
been the most mobile under the weathering condi-
tions that pertained in this region. This pattern is less
marked in the least-weathered profiles and there is
also some indication that profiles in different settings
have different patterns of REE mobility and enrich-
ment (McQueen 2006). The REE may therefore
provide a basis for evaluating the extent of chemical
weathering and leaching. Comparison of Ce (the
most mobilised LREE) with Nd (another LREE with
simi lar propert ies a nd init ial di stribution) is one pos-
sibility for a REE mobility and weathering index.
Examples of Ce/Nd plots for different weathered pro-
files in the Cobar region are shown in Figure 6.21.
Departure of the Ce/Nd ratio from the initial ratio
(typically around 1.5 to 2.5 for sedimentary rocks)
indicates the relative depletion or enrichment of Ce
and provides an indication of chemical leaching of
the rocks.
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