Micromorphology and chemical weathering of a K-rich trachyandesite and an associated sedimentary...

Micromorphology and chemical weathering of

a K-rich trachyandesite and an associated

sedimentary cover (Parkes, SE Australia)

Eric Tonuia,*, Tony Eggletonb, Graham Taylorc

aCooperative Research Center for Landscape Evolution and Mineral Exploration (CRC LEME),

University of Canberra/Geoscience Australia, P.O. Box 378, Canberra, ACT 2601, AustraliabCooperative Research Center for Landscape Evolution and Mineral Exploration (CRC LEME),

Department of Geology, Australian National University, Canberra, ACT 0200, AustraliacCooperative Research Center for Landscape Evolution and Mineral Exploration (CRC LEME),

University of Canberra, Canberra, ACT 2601, Australia

Received 7 November 2001; received in revised form 14 November 2002; accepted 9 December 2002

Abstract

The micromorphology, composition and sequential formation of weathering products of a K-rich

trachyandesite and an associated sedimentary cover at Parkes, SE Australia are described. This has

been achieved using a combination of mineralogical (optical, X-ray microdiffraction and SEM–

EDXA microanalysis) and chemical (X-ray Fluorescence) techniques. The weathering profiles have

developed in a low-altitude, relatively flat landscape setting. The following processes are dominant:

(a) weathering of primary minerals notably feldspars and micas to transient ‘poorly crystalline’

oxyhydroxide patches, smectite and ultimately kaolinite; (b) weathering of sediments in a zone of

intense solution activity and redox reactions resulting in development of mottles and ferruginous

nodules; and (c) greater thickness (up to 350 cm) and compositional differences between the soil

over weathered sediments and in situ weathered bedrock. The dominant profile microsystems

evolved under a past humid climate later modified by the prevailing semi-arid to arid conditions.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Micromorphology; Chemical weathering; Residual/transported regolith; Profile microsystems

0341-8162/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0341-8162(02)00200-X

* Corresponding author. Current address: California Institute of Technology/Jet Propulsion Laboratory,

Division of Geological and Planetary Sciences, Mail Code 170-25, Pasadena, CA 91125, USA. Tel.: +1-626-395-

3452; fax: +1-626-796-9823.

E-mail addresses: [email protected] (E. Tonui), [email protected] (T. Eggleton),

[email protected] (G. Taylor).

www.elsevier.com/locate/catena

Catena 53 (2003) 181–207

1. Introduction

Much of the available literature dealing with secondary mineral genesis from basic

volcanic rocks has concentrated on basalt weathering (e.g. Siefferman and Millot, 1969;

Eswaran, 1979; Eswaran and Coninck, 1971; Velde, 1985). A limited number of

researchers have described the weathering products of andesites (e.g. Hendricks and

Whittig, 1968; Colman, 1982; Glassman, 1982; Dubroeucq et al., 1998). Very limited

research has been reported on the weathering of andesites from Australia, which is

surprising given the antiquity of the continent and the dual role that weathering processes

have played in shaping its current landscapes and economic pursuits. Most of the existing

studies have concentrated on the weathered mantle (regolith) at the landscape scale with

little or no attention to the processes involved in the underlying bedrock. In addition, these

studies have been carried out mainly on the western side of the continent despite features

that differ significantly from the less studied eastern side. A proper understanding of the

weathering paragenetic sequences in underlying rocks is pertinent to formulation of

regolith evolution models and understanding of element mobility and distribution at the

landscape scale.

This study focussed on the Northparkes Cu–Au deposits, located near Parkes, south-

eastern Australia (Fig. 1). The landscape around the study area is flat with little or no

outcrop and hence, the weathering profiles are well preserved. This paper aims to: (1)

examine the micromorphology, mineralogy and chemistry of the secondary products; and

(2) establish their genesis in terms of weathering and landscape setting.

Fig. 1. Regional geomorphic setting of the study area (adapted from Chan, 1998).

E. Tonui et al. / Catena 53 (2003) 181–207182

2. Geomorphic and geological setting

The climate within the Parkes area is arid to semi-arid with daily maximum and

minimum temperatures of 22.9 and 11.3 jC, respectively, and an annual rainfall of 586

Fig. 2. Geological setting and location of the Northparkes Cu–Au deposits (modified from Heithersay et al.,

1990).

E. Tonui et al. / Catena 53 (2003) 181–207 183

mm (approximately 85 mean rain days). Vegetation is extensively cleared woodland

comprising of grey box, rosewood, white cypress pine and casuarinas.

The study area is situated on the western slopes of the northerly aligned Eastern

Highlands, which divide the coastal Tasman Sea drainage to the east from the inland

drainage to the west. This area sits within the headwaters of the northerly flowing Bogan

River just a few kilometers to the north of the Canobolas Divide, a regional northwest

aligned drainage divide between the Darling River catchment to the north and the Lachlan

River catchment to the south (Fig. 1). Multiple weathering profiles on bedrock and/or sand

and gravel occur near and on the Canobolas Divide (Chan, 1998). The former are mainly

associated with extensive Tertiary lava flows and the latter with the Lachlan River and its

tributaries. The study area is located in the northeast portion of the Lachlan Fold Belt

approximately 25 km from the southern edge of the Surat Basin (Fig. 2). Relief on the

western slopes is subdued with plains to rises ( < 30 m local relief) and a few north

trending low hills to hills (30–300 m). Relief and elevation increase towards the east

(Chan and Tonui, 2003).

The bedrock underlying most of the Parkes area is Late Ordovician Wombin and

Goonumbla Volcanics to the east, and Siluro-Devonian Derriwong Group sediments to

the west (Fig. 2). A Late Ordovician monzonite to syenite unit intrudes the Wombin

Volcanics and Goonumbla Volcanics. The Northparkes Cu–Au deposits are hosted by a

sequence of Late Ordovician to Early Silurian volcanics that comprise K-rich trachyan-

desites and associated quartz monzonite intrusives. These intrusives occur as small bodies

rarely protruding into the weathering zone. This study has been carried out over two such

bodies containing mineralization: the Endeavour 22 (E22) and Endeavour 27 (E27)

deposits (Fig. 2).

3. Materials and methods

3.1. Regolith distribution

The distribution of the regolith materials and the major regolith profiles at Northparkes

E22 deposit is presented in Fig. 3. The following units constitute the regolith profile (see

Eggleton, 2001 for glossary of regolith terms).

3.1.1. Fresh rock

Most of the minerals remain unaltered. A distinct change marks the weathering front,

above which is the saprock.

3.1.2. Saprock

The saprock has a thickness of 4 to 6 m. It is greenish-grey, slightly weathered, hard

and jointed.

3.1.3. Saprolite

It is very friable, deeply weathered (thickness of 20–30 m) and retains the textures and

fabrics of the host rock including quartz veins. Three major varieties have been identified


Fig. 3. The distribution of the regolith at the Northparkes E22 deposit and the two main types of regolith profiles: (A) Transported/residual regolith profile; (B) Residual

regolith profile. Unconformity refers to the boundary between transported and residual regolith.

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based on the degree of weathering and differences in colour, texture and fabric (Fig. 3).

The greenish-grey (8/5 GY) saprolite occurs in transition from the saprock and is overlain

in most parts by orange-pink (2.5YR 8/4) saprolite. The mottled saprolite is relatively Fe-

rich and occurs below the boundary with the transported regolith. The weathering styles

and products that account for these differences will become more apparent in the following

sections.

3.1.4. White clay units

These occur as restricted pockets on the flanks of the mottled clay units overlying the

orange-pink saprolite. They consist of dense, massive white clay with mesoscopically

homogenous fabrics. Thickness varies from 5 to 10 m.

3.1.5. Mottled clay

The mottled clay is characterized by a contrast between clay-rich domains and Fe

segregations (mottles), which can be easily distinguished on a centimeter scale. It

consists of whitish grey to pale green to pink clay with the Fe oxides occurring as

diffuse impregnations within it or as secondary structures such as rounded to

subrounded nodules. The mottled clay has been subdivided based on the mottle size

into the mini- (>5 mm), medium- (20–50 mm) and mega-mottled (>50 mm) clay. A

zone of rounded to subrounded Fe–Mn-rich nodules occurs in transition with under-

lying saprolite. These nodules are also interspersed within the mega-mottled clay.

Elongate opaline Si nodules occur as a narrow band (2–4 m) between the medium-

and mega-mottled clay (Fig. 3) or as friable ‘sugary’ aggregates within the mega-

mottled clay. Carbonate (notably dolomite) is present as cement or coating on these Si

nodules.

3.1.6. Soils

The soil varies in thickness from 0 to 200 cm over saprolite to 0 to 350 cm over mottled

clay (Appendix A). These soils can be broadly classified as red brown earths based on the

classification system for Australian soils (McDonald et al., 1990). Commonly used soil

terms (Soil Survey Staff, 1990) have also been incorporated into the descriptions in

Appendix A. The topsoils (A-horizon; up to 100 cm) are similar, i.e. they consist of red to

reddish brown hard setting silty to sandy clay loams. Large cracks (20–50 mm) can extend

up to 100 cm during the dry season. Structure is predominantly massive (earthy). There is

a distinct boundary to the underlying B-horizon. The B1- and B2-horizons are distinct in

soil over mottled clay and indiscernible in saprolite-derived soil. The B1-horizon is

typically reddish brown and consists of medium to heavy clay. Pedality is 20–50 mm

platy. The B1-horizon can extend up to 110 cm.

The B2-horizon (100–350 cm) is reddish brown to yellow in the soil over mottled clay.

It is generally 200–250 cm thick and has strong pedality (20–50 mm) prismatic. Elongate

(5–10 mm) gypsum crystals and carbonates, occurring as veneers, coatings or friable

powdery masses are common. The B-horizon of the saprolite-derived soil can extend up to

110 cm and seldom contains carbonate and gypsum. The pH varies from slightly acid 6 at

the surface to 7.5 in the B-horizon in soil over saprolite and 5 to 9 in the B2-horizon over

mottled clay.


3.2. Methods

Unoriented and oriented samples for bulk mineralogy and optical microscopy,

respectively, were collected from the face of the open pit. Close-spaced vertical

sampling at a preferred distance of 1 m was carried out to ensure good coverage of

all regolith profiles. Care was taken to ensure that only the inner core of the regolith

units was sampled. Bulk samples, clay extracts and individual mineral grains were

analysed for mineralogy using a Siemens D501, h–2h powder diffractometer using

CuKa radiation and a graphite postsample monochromator, collimated by 1j divergence

slits. The samples were run from 2j to 70j at steps of 0.02j 2h and a counting time of

1.2 s/step. The clay extracts were run from 2j to 42j with the same steps and counting

time as the bulk samples. A modern Rietveld computer software package (SIRO-

QUANT) was used to obtain the quantitative mineralogy data, the foundations of which

are reported by Taylor and Clapp (1992). Much of the sample preparation and analysis

of clay extracts was accomplished following the procedures developed by Moore and

Reynolds (1997) and Brindley and Brown (1984). Ethylene glycol solvation (to separate

the smectite from chlorite phases) was achieved by subjecting the clay-bearing slides to

glycol vapours at 60 jC for over 12 h in a desiccator. Optical microscopy was carried

out using oriented polished and unpolished samples mounted in polyester resins.

Cambridge S360 and Jeol 6400 Scanning Electron Microscopes (SEM) were used in

combination with energy dispersive X-ray analysis (EDXA) to determine the morphol-

ogy and chemistry of the mineral grains. Major element geochemistry on bulk samples

was obtained using the XRF analysis method of Chappel (1991). Most of the

descriptions of micromorphology are adopted from Brewer (1964), Nahon (1991),

Delvigne (1998) and Stoops et al. (1979). Munsell colour codes of the dry samples

were also recorded.

4. Micromorphology, mineralogy and chemistry

The saprolite and saprock have been interpreted as having resulted from in situ

weathering of bedrock (residual regolith), while the mottled clay resulted from the

weathering of sediments (transported regolith).

4.1. Residual regolith

4.1.1. Micromorphology

4.1.1.1. Fresh rock. Hydrothermal alteration has obscured some of the primary rock

fabrics (Fig. 4A). The trachyandesite is aphanitic and composed of K-feldspar, plagioclase,

muscovite and quartz with minor amounts of pyroxene, other ferromagnesian minerals

(notably hornblende, diopside and epidote) and opaques (oxides and sulfides). SEM min-

eral chemistry confirms albite (Na0.95Al0.98Si2.9O8) and orthoclase (K0.91Al1.02Si3.05O8) as

the major feldspars. The micas mainly occur as pseudomorphs of partially altered

plagioclase in a quartzo-feldspathic matrix.


4.1.1.2. Saprock. Weathering has imparted a brownish grey impregnation on the mineral

surface. The earliest stage of alteration of the feldspars consists of the development of

brown, cloudy and ‘turbid’ patches of low interference colours, initiated at the loci of twin

planes in plagioclase or cross hatches in K-feldspars (Fig. 4B and C). This material

becomes more pervasive with progressive weathering and later envelopes the entire

primary plasma. It is characterized by a decrease in Ca and minor presence of Fe as

compared to the parent material. The presence of small amounts of Fe and high Al

suggests it is either Fe beidellite or nontronite. X-ray diffraction shows it is nontronite,

Fig. 4. Fabrics and textures associated with the weathering of the trachyandesite. (A) Hydrothermally altered

trachyandesite showing the plagioclase feldspar laths (PL), muscovite (MS) and opaques in the matrix. (B)

Development of brown ‘cloudy’ (BC) material and smectite (SM) on loci of twin planes of K-feldspar in the

saprock. (C) Mixtures of mica/kaolinite (SM) with low interference colours in the lower saprolite. (D) Kaolinite

with low interference colours set with clusters and distorted stacks of kaolinite/mica (KM) in the middle saprolite

forming ‘accordion’-like fabrics. (E) Globular blasts of kaolinite/mica (KM) in a matrix of very fine-grained

kaolinite (KL) in the upper saprolite. All scale bars = 50 Am.


differentiated using the b-cell dimension technique of distinguishing trioctahedral (Mg)

from dioctahedral (Al, Fe3 +) smectite (d060 of nontronite ranges from 1.521 to 1.523).

Feldspars are etched by dissolution (Fig. 5A and B). Apart from the smectite, the < 2

Am clay fraction (Fig. 6) shows the presence of minor amounts of kaolinite and illite. The

14-A smectite peak is sharp implying well-crystalline clay. The brown cloudy material

appears to be a precursor to the development of smectite and Fe oxides in the matrix.

4.1.1.3. Saprolite. Three distinct weathering stages occur in the lower, middle and upper

saprolite.

Lower saprolite. Internal transformation of the primary minerals is more pronounced

than in the saprock. Feldspar alteration proceeds by differential shattering and coalescence

of the etch pits to irregular and elongate microfragments (Fig. 5C and D) resulting in three

distinct secondary products. The first and less abundant is spongy ‘cotton wool’ textured

smectite, which covers the surface of the feldspar (Fig. 5E). The second and equally

abundant secondary product is a platy ‘book’ variety of clay that has formed in the cavities

of the feldspar and contains equal amounts of Al and Si interpreted as kaolinite (Fig. 5F).

The books are oriented randomly with respect to the feldspar crystals. The third most

abundant product is a curly, platy and ‘cornflake’ textured clay (Fig. 5G), which is

characterized by appreciable amounts of Fe and Si and low Ca interpreted as nontronite. It

appears as a reddish brown smear on the surface of a very fine-grained ( < 0.001 mm)

mixture of kaolinite and sericite.

Middle saprolite. Seen under the SEM, the middle saprolite is marked by an increase

in the curly and platy ‘cornflake’ textured smectite. The feldspars have altered to very fine-

grained (0.001–0.005 mm) kaolinite with low interference colours, flecked with patches

of mixed kaolinite and mica (Fig. 5C). The < 2 Am clay fractions (Fig. 6) confirm the

dominant clay to be kaolinite. The kaolinite and smectite are highly stained by Fe oxides.

Intermineral and transmineral fissures are sparse in the saprock and abundant within the

lower and middle saprolite. The spacing and density of these fissures increases towards the

upper saprolite. They are lined by thin ( < 5 Am) to fairly thick (5–10 Am) ferriargillans,

which often engulf fragments of the secondary minerals. The orientation of the clay

particles is generally perpendicular to the fissure walls.

Upper saprolite. Progressive weathering produces a dominant clay phase pseudo-

morphing the feldspar consisting of a mat of very fine-grained (0.005–0.10 mm) kaolinite

of low interference colours, set with clusters, larger flakes, plates, books and distorted

stacks of a kaolinite–muscovite mixture of higher interference colours (Fig. 4D). It is

highly stained by Fe, but SEM EDXA shows that it is composed of K, Al and Si, i.e.

sericite. The close association of the clay and sericite has resulted in a material with a

moderate yellow to white interference colours, resulting in the development of an

accordion-like structure of mixed kaolinite and mica (Fig. 5H). The cornflake-textured

smectite decreases as kaolinite becomes more abundant.

The plagioclase laths and smectite gradually disappear upward through the profile and

the matrix acquires a characteristic orange-pink colour in the process. The secondary

products here consist of pseudohexagonal plates, books and vermiforms of kaolinite with

larger plates of sericite (Fig. 7A and B). Intense fabric reorganization occurs towards the

upper part of the profile, defined by the presence of small ‘blasts’ of very fine-grained


sericite–kaolinite mixture (Fig. 4E). The loss of lithic fabric is most pronounced within the

white clay unit as the clay becomes more homogenous, bleached and massive.

4.1.2. Bulk mineralogy and chemistry

There is a general depletion of the feldspars with progressive weathering (Fig. 8). Loss

of plagioclase occurs at the lower saprolite, while K-feldspar persists to the upper levels of

the saprolite. A small amount of plagioclase occurs in detrital rock fragments within the

soil. Apart from quartz, muscovite is the most persistent with its loss occurring at the white

clay unit. The low contents of quartz in the lower and middle saprolite suggests that most

of the silica occurs within the primary silicates. Kaolinite and Fe oxides (as hematite and

goethite) increase towards the upper levels of the profile and apart from smectite are the

main products of the weathering of the primary minerals. Smectite is present within the

lower and upper levels of the profile and is absent within the orange-pink saprolite.

Carbonate (as calcite) occurs within the soil.

The chemical composition (Fig. 8) shows a general depletion of silica towards the

orange-pink saprolite and a subsequent increase within the white clay unit and soil.

Aluminium and Fe oxides show a general increase towards the upper saprolite associated

with formation of clay (notably kaolinite) and Fe oxides with progressive weathering.

Calcium and magnesium oxides show an increase in the white clay unit and soil due to the

presence of carbonate and gypsum. Potassium and sodium oxides decrease towards the

upper saprolite corresponding to progressive depletion of muscovite, K-feldspar and

plagioclase with progressive weathering.

Fig. 9 shows variations in Zr/Ti ratios along the residual regolith profile. The trend of

Zr/Ti ratio is erratic, with a subtle upward decreasing trend in the saprolite followed by an

increase in the white clay unit and soil. This is not typical of residual regolith profiles (see

discussion).

4.2. Transported regolith

4.2.1. Micromorphology

The fabric of the ‘transported regolith’ is characterized by the occurrence of diffuse

segregations of Fe as mottles or nodules in a predominantly clay-rich matrix. The major

characteristics are described below.

4.2.1.1. Mini-mottled clay. The matrix is traversed by numerous root channels, which

have been lined with pale yellowish brown to grey kaolinitic clay, goethite and Mn-rich

coatings. Skeleton grains of Fe oxide illuviated from the soil horizon occur within the

matrix including intercalary of acicular gypsum crystals and numerous subrounded to

subangular quartz grains (Fig. 7F and H). These lithorelicts are composed of muscovite,

Fig. 5. SEM images of residual regolith. (A) Initiation of weathering on the 001 feldspar plane; scale bar = 2 Am.

(B) Fractured surface of K-feldspar; scale bar = 2 Am. (C) Secondary products forming on the fractured surface;

scale bar = 2 Am. (D) Smectite (SM) forming as cellular encrustations on highly fractured feldspar surface; scale

bar = 2 Am. (E) Initial nucleation of smectite as spongy aggregates; scale bar = 10 Am. (F) Kaolinite plates and

books; scale bar = 10 Am. (G) Curly and ‘cornflake’ textured smectite; scale bar = 10 Am. (H) ‘Accordion’ fabrics

of mixed kaolinite and mica; scale bar = 10 Am.


Fig. 6. XRD clay extracts. (A) Upper saprolite. (B) Middle saprolite. (C) Lower saprolite. (D) Saprock (S,

smectite; K, kaolinite; M, mica/illite; Q, quartz; K/S, smectite/kaolinite).


plagioclase and K-feldspar. Voids are abundant, which have in part connected to form

channels (Fig. 7E and G). Micro-laminated infillings of fine and coarse Fe-rich clay

(notably kaolinite) occur adjacent to some of these channels (Fig. 7E). They form deposits

whose grain sizes vary progressively with thickness of the infilling. Iron-rich micropeds

are also present along cracks and clay-coated micro-fractures. The < 2 Am clay extracts

Fig. 7. SEM and optical photomicrograph images of upper saprolite and mottled clay. (A) Orange-pink saprolite

showing pseudohexagonal plates and booklets of kaolinite and mica; scale bar = 5 Am. (B) Pseudohexagonal

plates of kaolinite in the white clay unit; scale bar = 5 Am. (C) Poorly crystalline plates and grains of quartz in the

mottled clay; scale bar = 5 Am. (D) Dolomite crystals from the mega-mottled clay; scale bar = 20 Am. (E)

Channels (CH) and macrovoids (MC) with micro-laminated infillings of illuviated clay (IL) in the medium-

mottled clay; scale bar = 0.5 mm.


Fig. 8. Mineralogical and chemical changes along the residual regolith profile (GG, greenish grey; OP, orange pink; -spar, feldspar).

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(Fig. 10) shows the dominant clay to be well-crystallized kaolinite with small amounts of

smectite. Ferruginous nodules (5–20 mm) also occur in the matrix.

4.2.1.2. Medium-mottled clay. This unit is marked by an increase in ferruginous nodules

and concentration of voids and channels. The dominant clay is well-crystallized kaolinite

(Fig. 10). Characteristic Fe oxide stained clay infillings like those in Fig. 7E are also

common. These infillings appear to be illuvial concentrations of sand, silt and Fe oxides

around ped surfaces, roots or biogenic channels. Ferriargillans line these channels and also

coat a number of detrital quartz and rock fragments. Thin clay neostrians displaying a

vosepic plasmic fabric are also present.

4.2.1.3. Mega-mottled clay. This unit is marked by a significant decrease in ferruginous

nodules. The dominant clay is well-crystallized kaolinite with some smectite (Fig. 10).

Root fragments and channels are abundant with most of these passageways filled with

pseudohexagonal crystals of dolomite (Fig. 7D) or ferruginous clay. The dolomite occurs

mainly as a coating on opaline Si aggregates.

The kaolinite in the transported regolith occurs as thin platy crystals (Fig. 7C) as

opposed to the pseudohexagonal platy morphology of the saprolite. The kaolinite plates

Fig. 9. Variation in Zr/Ti ratios along the residual regolith profile.


Fig. 10. XRD clay extracts. (A) Soil (transported regolith). (B) Soil (residual regolith). (C) Mini/medium mottled

clay. (D) Mega-mottled clay (S, smectite; K, kaolinite; M, mica/illite; Q, quartz).


Fig. 11. Mineralogical and chemical changes along the transported regolith profile.

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adhere closely to the quartz. Vermiform or book-like morphology of kaolinite rarely occurs

in the matrix.

4.2.2. Bulk mineralogy and chemistry

The mineralogy is dominated by quartz, kaolinite, hematite, goethite, dolomite,

smectite, anatase and gypsum (Fig. 11). Goethite is the most abundant Fe oxide and is

present in high contents (>40%) within the Fe–Mn unit and mini- and medium-mottled

clay. High contents of hematite (>10%), dolomite (>10%) and anatase (>5%) occur mainly

within the mega-mottled clay. Smectite (as montmorillonite, i.e. d060 = 1.492–1.504) is

present mainly within the soil and mini-mottled clay.

Chemically, the weathered sediments are mainly composed of SiO2, Al2O3 and Fe2O3

as quartz, clay and Fe oxides, respectively (Fig. 11). The concentrations of these elements

reflect the mineralogical distribution. The high contents of CaO and MgO (2–4%) within

the soil and mega-mottled clay correspond to the presence of carbonate (calcite and

dolomite) and gypsum. Some of the MgO also occurs within smectite.

4.3. Soils

The soils associated with the residual and transported regolith are slightly different as

already highlighted in Section 3.1. The < 2 Am clay fractions show very broad 001 peaks

of disordered kaolinite and interstratified smectite/mica (Fig. 10). The smectite is mainly

montmorillonite (d060 = 1.491–1.504; contains Al, Ca and Si).

The soil over the transported regolith contains siliceous and ferruginous clay and

organic skeleton grains. The soil over the residual regolith contains abundant detrital rock

fragments, which include partly altered K-feldspar and muscovite. Gypsum and quartz are

more abundant in the soils over the transported regolith than residual regolith.

5. Discussion

5.1. Provenance, profile evolution and microsystems

The micromorphology and composition of weathering products of mottled clay and

saprolite/saprock are different. The boundary between the two sets of materials can easily

be traced in the open pit. This boundary has also been delineated using other contemporary

mineralogical techniques including Portable Infrared Mineral Analyser (R. Jones, Pers.

Comm., 2002). The mottled clay occupies a broad (400–700 m wide) and relatively

shallow (25–30 m deep) paleo-valley in the landscape (Chan and Tonui, 2003). The paleo-

valley longitudinal profile is interpreted from the exposed pit sections as generally falling

towards the northeast. Regional regolith-landscape mapping has revealed that these paleo-

valleys and associated drainage systems are widespread in the region (Chan and Tonui,

2003).

The mottled clay resulted from weathering of transported sediments (transported

regolith) and the saprolite/saprolite from in situ weathering of bedrock (residual regolith).

The transported regolith sustained intense solution activity, as shown by the presence of


numerous voids and channels. Many isolated rounded to subrounded quartz grains and rock

fragments occur within the clay. Paleomagnetic dating (Tonui, 1998; Pillans et al., 1999)

shows preservation of Carboniferous weathering of bedrock and Cainozoic weathering of

mottled clay. Regional stratigraphy and palynological evidence suggests that a shift to a

generally cooler and drier climate in the region occurred during mid-Miocene (Martin,

1991).

The presence of Fe–Mn nodule-rich zones at Parkes reflects this change from

previously wet to drier climates. Iron and Mn can be used to trace paleowatertables

because they precipitate close to the watertables as replacements, coatings and nodular

aggregates (Fitzpatrick, 1980). The presence of Si nodules and aggregates (the term

‘silcrete’ is also commonly used) may also be linked to these climatic changes although

the origin of secondary Si aggregates in the regolith is generally poorly understood. Their

occurrence within the mottled clay as a narrow band suggests that like Fe–Mn nodules,

they would have formed as a result of drop in watertables under conditions that were

favourable for precipitation of Si. The ‘sugary’ aggregates suggest chemical dissolution,

which would have resulted in increased porosity and later infilling by carbonates. These Si

nodules or aggregates can be classified as pedogenic because they precipitated higher up in

the profile as opposed to groundwater types that form lower in the profile, are mostly

massive and retain the fabric of the host rock (Thiry and Milnes, 1991). The carbonate

(dolomite) coatings and predominance of goethite cutans within the mottled clay are an

indication of precipitation from solutions having completely filled the available pore space

and therefore can be considered as signs of past phreatic conditions. The geochemical

dispersion of elements associated with underlying mineralization has also been linked to

these climatic changes, as detailed in the work of Tonui et al. (in press).

The abundance of calcite and gypsum within the soil may also be associated with arid

conditions as a result of retention due to limited leaching. It is important to emphasize

though that development of environmental conditions suitable for formation of pedogenic

carbonates and sulfates is largely controlled by a balance between the amount of rainfall

and landscape setting, which controls the degree of leaching, groundwater pH, rainfall

chemistry, surface runoff and biological concentrations. The greater thickness of the soil

over transported regolith may be associated with high porosity of the deposited material

having allowed soil formation to greater depths than was possible within the saprolite of

the residual regolith.

The weathering of the main components of the trachyandesite (mainly feldspars) is a

solution-controlled process with access controlled by fractures within the loci of twin

planes resulting in formation of etch or dissolution pits. A number of workers have made

similar observations regarding the weathering of feldspars (e.g. Berner and Holdren, 1979;

Nixon, 1979; Parham, 1969; Siefert, 1967), although considerable uncertainty still exists

as to whether secondary minerals precipitate from solution or crystallize from transient

non-crystalline aluminosilicate intermediates (Banfield and Eggleton, 1988). The develop-

ment of oxyhydroxide patches as the first stage of alteration of feldspar suggests the

presence of transient ‘intermediate’ phase prior to secondary clay formation. These

‘primitive’ clay precursors consisting of mixtures of smectite and oxyhydroxides have

been documented during the weathering of basic rocks (e.g. Tazaki and Fyfe, 1987).

However, smectite is not always present as a precursor as evidenced by the development of


an amorphous Si–Al–Ca–Na precursor of halloysite during the weathering of plagio-

clases in rhyodacite (Dubroeucq et al., 1998).

The micromorphology of residual regolith consists of a diverse range of secondary

products. Smectite occurs in appreciable amounts within the saprock and lower saprolite.

The major product in the saprolite is the platy, pseudohexagonal ‘book’ kaolinite. The

kaolinite books are randomly oriented with respect to the etched or pitted feldspar surfaces

indicating an argillic transition where solution in reaction with solid parent phases is

predominant. The fabric relationship apparent in SEM and the dominance of smectite in

lower saprolite and kaolinite in upper saprolite show that the alteration sequence is

feldspar to smectite and then kaolinite. Platy books of kaolinite usually characterize thick

saprolites over feldspathic crystalline rocks (e.g. Keller, 1978). The abundance of this

morphology at Northparkes is associated with the deep weathering that occurred over

these deposits over a long period as evidenced by the Carboniferous age of weathering

from paleomagnetic data (Tonui, 1998; Pillans et al., 1999).

The distribution of Zr/Ti does not fit into the trend of typical in situ weathering profiles.

The erratic distribution can be interpreted as having resulted from repeated dissolution and

precipitation of secondary components. The abrupt change in the white clay unit suggests

sedimentation of some allochthonous material, while increase in the soil might be

associated with input from other sources probably aeolian material. There is evidence

for widespread incorporation of wind-blown dust (silt and clay pellets) in soils in the

Parkes area (Chan et al., 2001), later reworked by colluvial and alluvial processes (parna).

The increased Zr/Ti trend in the soil might also be attributed to Ti loss as anatase, i.e. Ti is

generally lost from ferromagnesium silicates and converted to nanometer-sized anatase

during eluviation (Dixon et al., 1989; Brimhall et al., 1991). The white clay unit appears to

have resulted from high intensity of weathering and loss of material to reduce the

competence of the upper saprolite so that settling of the clays occurs as the rock fabrics

are being destroyed. White clays frequently form in situ beneath the watertable, in anoxic

conditions (Dubroeucq and Volkoff, 1998; Thomas et al., 1999), suggesting that this part

of the profile underwent prolonged water saturation.

Fig. 12 shows the schematic representation of the microsystems of the weathering of

the trachyandesite in transition from the saprock to the upper saprolite and soil. The

dissolution–precipitation mechanism invoked to explain secondary product formation

(Berner and Schott, 1982; Glassman, 1982; Anand et al., 1985; Velbel, 1989) seems to be

the paramount mechanism operating to develop and furnish the alteration products. There

is no doubt that drainage improves in going from the saprock to the saprolite. In the

saprock, solution mobility is reduced because of limited number of conduits leading to a

greater frequency of wet and dry conditions and development of 2:1 clays. Higher up in

the profiles, supersaturation causes a change in the chemical conditions of the micro-

environments leading to the dissolution of 2:1 clays.

Solution also occurs at both the macro and micro scale. Micro influences are observed

in the saprock and saprolite, where K, Ca and Al appear in plagioclase and K-feldspar-

derived smectite and in the concentration of Fe oxides along fissures or adjacent to the

clay. The existence of fissures and argillans indicates that rock structure and individual

minerals influence the breakdown mechanisms, weathering fabric and resulting products.

Banfield and Eggleton (1988) noted that the presence of Fe within the plasma could also


Fig. 12. Schematic representations of the microsystems of the residual regolith from the saprock to the upper

saprolite (SR, saprock; LS, lower greenish-grey saprolite; MS, middle greenish-grey saprolite; US, upper

saprolite).


participate in the disruption of the feldspar lattice by oxidation and hydrolysis of water,

which provides H+ or H3O+, which attacks the feldspar structure releasing K, Na or Ca.

However, for Fe to participate in this manner, episodic redox reactions associated with

Fig. 13. Formation of mottles and nodules in originally kaolinite-rich domains leading to development of

lithorelictual features in primary lithostructures and pedorelictual features in secondary pedostructure. Diagram

also shows formation of bleached domains, macrovoids and channels from originally quartz-rich domains and

secondary formation of pedorelictual mottles in previously formed macrovoids (adapted from Tardy, 1992).

Fabrics of medium- and mega-mottles are also shown.


profile development occurred probably associated with drops in watertable as a result of a

change to more arid climatic conditions as explained earlier.

The evolution of the residual regolith is also a consequence of the reactions between the

weathering minerals. Muscovite, for example, replaced a plagioclase precursor,

pseudomorphing, and hence, preserving the fabric in the process. Alteration of sericitised

plagioclase produced an intimate interlamination of mica and clay to give a homogenous

fabric. Pseudomorphing replacements dominate the lower parts of the profile where the

original rock fabrics tend to be preserved. Continued weathering and reorganization of the

fabric involves dissolution of clay and formation of intimate structures between minerals

including accordion fabrics and clay ‘blasts’ towards the upper saprolite. Pedological

processes represented by development of voids or channels, burrowing by meso- and

micro-fauna and organic coating on skeleton grains mark the culmination of the weath-

ering sequence.

5.2. Redox reactions and secondary structures

The mottled clay is composed mainly of segregations of Fe and clay (i.e. mottles). The

unanimous inference arrived at by most workers investigating mottles is that they form in

response to a fluctuating watertable which induces redox transfers (Nahon, 1986; Ambrosi

and Nahon, 1986; Braun et al., 1993). The mechanisms responsible for mottle and nodule

formation are poorly understood. Tardy (1992) envisaged that bleached domains consist-

ing mainly of quartz and kaolinite (i.e. primary lithostructure and secondary pedostructure)

and exhibiting a white or grey colour (similar to those of this study) occur by de-ferru-

ginization of the previously associated kaolinite and Fe oxyhydroxides (Fig. 13). Original

kaolinite aggregates, free of Fe, can be dispersed and kaolinite particles can migrate and be

leached out. These changes as noted in the mini- and medium-mottled clay are

accompanied by a strong increase in porosity, leading to formation of voids and channels.

Ferruginous nodules containing traces of primary rock clasts with sparse, silt-sized

quartz grains are present within the mottled clay. These nodules are discriminated from the

matrix by striking colour, rounded to subrounded shape, polished surfaces and sharp

boundaries with surrounding matrix. These lithorelicts indicate that most of the sediments

were locally derived. A number of the nodules in this study contained goethite-rich cutans

(rim thickness of less than 20 Am to 1 mm) on the predominantly hematite-rich matrix.

Nahon (1986) postulated that these nodules attest to ancient ferricretes related to ancient

groundwater levels in the landscape and that the coatings were liberated on the soil when

the ferricrete was dismantled. There is no other evidence, however, for presence of an

ancient ferricrete in this study. These types of nodules may alternatively be classified as

degraded mottles having formed from Fe liberated during degradation of hematite mottles

in depositional environments (e.g. Allipour et al., 1996).

6. Conclusions

The earliest stage in the weathering of the K-rich trachyandesite involved the develop-

ment of poorly crystalline precursors of secondary clays and oxyhydroxides. Progressive


weathering resulted in the development of secondary products with distinct morphologies,

which reflect differences in microenvironments of formation.

The evolution of distinct microsystems in the profile is associated with these weath-

ering patterns, which produced the apparent differences in colour, texture and fabric. The

greenish-grey colour of the lower saprolite evolved in a reducing environment under past

phreatic conditions, which produced Fe3 + clays such as nontronite. In the upper saprolite,

more oxidizing conditions resulted in the loss of 2:1 clays producing the characteristic

orange-pink colour. The striking white colour of the upper saprolite has been attributed to

the formation of clay in anoxic conditions beneath a water-saturated zone.

Redox and substitution reactions are predominant in the transported regolith and are the

main factors that lead to porosity differences and development of voids, channels and

macrovoids. The distribution of Fe and kaolinite is mechanical with the development of

secondary structures such as nodules related to redox processes and episodic changes in

climate. Imprints of these redox processes within the regolith include pockets of Fe–Mn-

rich zones and carbonate-coated opaline Si nodules and aggregates. The differences in

weathering patterns in the transported and residual regolith are reflected in the character-

istics of their overlying soils.

Acknowledgements

The research reported here was supported by a grant from the Australian Aid for

International Development (AUSAID) towards postgraduate studies at the Australian

National University. We extend our gratitude to the Research School of Biological

Sciences (RSBS) of the Australian National University, in particular, Roger Heady for

allowing the use of their Scanning Electron Microscope. John Vickers of the Department

of Geology is thanked for preparing the polished thin sections. We are grateful to North

(now acquired by Rio Tinto) for providing the permission to access the mine, publish this

paper and for overall logistical support during the course of the fieldwork. This study was

also carried out as part of the Cooperative Research Centre for Landscape Evolution and

Mineral exploration (CRC LEME). CRC LEME is funded under the Australian

Government Cooperative Research Centers Program.

Appendix A. Soil profile descriptions

Depth (cm) Horizon name Colour (moist)/texture Structure/pedality pH

Transported regolith

0–40 Brown humic Dark brown (10YR4/3);

sandy loam (20%)

Massive/blocky 6–7

40–100 Pale humic Yellowish brown (10YR4/3);

sandy clay (40%)


100–210 Reddish brown

oxic (B1)

Reddish brown (5YR5/4); clayey Fine round grain/platy 7–8.5

210–350 Reddish grey oxic/ Reddish grey (5YR4/2); clayey Fine round grain/prismatic 7–8.5

350–500 Mottled clay Reddish brown (2.5YR4/4);

clayey dark red (10YR3/4) mottles

Fine round grain/prismatic 7–7.5


Key: Brown humic—Acid with organic matter bound to clay minerals; Pale humic—

Pale yellow or brown, bioturbated with 1–2% indiscernible organic matter, soft, 0.5–1.5

mm rounded grain structure; Oxic—Red type or yellow type, soft with 0.05–0.1 mm grain

structure (microaggregates) or fine blocky structure; Pedality—degree of development of

peds.

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