CRC LEME
Open File Report 138
ABSTRACT
Regolith-landforms of the Mt Isa Geodynamic Transect
S. J. Fraser
In March 1994 the Australian Geodynamics CRC (AGCRC) and the Australian
Geological Survey Organisation (AGSO) conducted a seismic survey
across the Proterozoic Mt Isa Inlier in Northern Queensland. The
objectives of that study were to determine the regional and local
structure of the Mt Isa region, particularly the nature of its boundaries
and internal structure. The survey resulted in some 1200 seismic
shot holes being drilled along the full extent of the transect.
Coincident with this study, the CRC for Landscape Evolution and
Mineral Exploration (CRCLEME) in collaboration with the CRC for
Australian Mineral Exploration Technologies (CRCAMET), initiated
an AMIRA Project (P417) which was concerned with developing our
understanding of the nature and evolution of the regolith and landscape
of the Mt Isa region.
The large number of seismic shot holes along the seismic transect
presented a unique opportunity to study the nature of regolith materials
and regolith stratigraphy associated with the principal lithologies
and tectonic units of the Inlier and adjoining basins. To that end,
the three CRCs entered into a collaborative arrangement whereby
the lifts from each shot hole were logged from a regolith perspective.
The observed regolith stratigraphic relationships were placed in
context by mapping the regolith landforms along a 5 km swath for
the length of the transect. This report presents the results from
this study and represents a successful outcome from that collaboration.
Weathering Characteristics
The complex geology observed within the drill spoil along the Geodynamic
Transect is reflected in the development of a highly variable, complex
regolith profile. The observed complexities are related to a varied
tectonic history, complex boundary relationships, sudden changes
in lithology the steep dip and gradational metamorphic grade.
The rocks observed along the Geodynamic Transect and in particular
across the Inlier itself are characterised by relatively thin regolith
profiles. The nature of the present-day regolith and more notably
the thin profiles can be attributed to the present erosive geomorphic
phase, which has persisted through the late Tertiary and Quaternary
(Blake, 1987). This regime coupled with the present climate in the
study area is not favourable for deep (>50 metres) regolith development
or the preservation of older weathering profiles.
The removal of the regolith mantle is facilitated by the relatively
high relief of the inlier, coupled with heavy seasonal rains which
act to prevent the 'blanket' lateritic profiles which have developed
within similar aged rocks in areas of western and south-eastern
Australia.
Remnant mantles of black soils and patches of ferruginous alluvium
and colluvium elevated above the present basin and are interpreted
as being indicative of past stable landscapes are observed being
actively eroded from the Proterozoic rocks of the inlier. The removal
of this material is aided by the development of active new drainage
networks, which have captured older drainage systems.
Evidence for past deep weathering episodes are preserved within
mesa structures, developed over Proterozoic and Mesozoic rocks and
within the basinal sedimentary assemblages present within the Eromonga
and Georgina basins. Other more fragmental evidence for past deep
weathering events is present beneath isolated siliceous, ferruginous
caps, Tertiary sediments or alluvial soil profiles along the Geodynamic
Transect. At these locations the relatively resistant materials
have preserved a more complete weathering profile with deeper saprolite
development than surrounding rock units and the development of mottling,
strong iron staining and silicification of the upper profile.
Thick regolith profiles are generally restricted to areas of subdued
relief; unconsolidated or poorly consolidated sediments, where shearing
or faulting is present or where significant permeability contrasts
along bedrock contacts are present. In these situations regolith
thickness exceeds 40 metres.
Silicification
Silicification is observed in numerous locations and geomorphic
settings along the Geodynamic Transect. It occurs as laterally extensive
siliceous duricrusts capping Mesozoic sedimentary successions and
Proterozoic basement, and as variably silicified horizons within
saprolite, saprock, bedrock, alluvial or colluvial profiles. The
silicification of the rocks observed within the inlier can be attributed
to intrusion of igneous bodies and associated contact metamorphisism
and metasomatic fluids, regional scale metamorphism and to groundwater
and weathering processes. Silicification resulting from groundwater
and weathering processes is commonly spatially related to ferruginisation.
Silicic horizons are both overlain, or in places underlain by iron
enriched or cemented material. The spatial relationship of ferruginisation
and silicification suggests that siliceous and ferruginous duricrusts
can form in similar settings.
In areas where there has been considerable stripping of weathered
material these siliceous mantles are elevated in respect to the
rest of the landscape. This phenomenon is best illustrated by the
mesas to the south east of Cloncurry and by the silicic capped metasedimentary
successions south east of Mt Isa which rise up to 200 metres above
the undulating granitic and porphyritic plains below.
Siliceous mantles are observed within the poorly consolidated sediments
of the Georgina Basin. These silicified saprolitic sandstones and
siltstones and silicified and ferruginised alluvium are observed
continuously for over 1 km at thicknesses up to 10 metres beneath
a thin cover of saprolitic clays, iron-rich colluvium and black
soils. These inclined siliceous mantles sit within the saprolite
below the lower slope of a low hill. This setting has been suggested
as being a favourable sight for the formation of both siliceous
and ferruginous duricrusts (Oilier & Pain 1996).
Ferruginisation
Ferruginisation or more particularly the development of iron-rich
regolith profiles is greatest within the more mafic volcanic, and
schistose and iron-rich sedimentary lithologies. However, some degree
of ferruginisation in the form of iron staining or iron-rich veining
is observed within all rock types. Ferruginisation occurs as pervasive
staining, micro and large-scale veining and spotting and in more
concentrated forms as goethitic and hematite rich ferruginised saprolites
and mottled zones and iron cemented alluvium. Sub-metallic hematitic
accumulations and segregations as early stage mottling were observed
within metasedimentary units and less frequently within metabasalts,
mafic schists. Within the Sybella granites a very well developed
mottled zone up to 10 metres thick is preserved beneath a hard resistant
cap of silicified and ferruginised saprolites.
Iron-rich lags of ferruginous granules, pisoliths, ironstone fragments
and ferruginised bedrock fragments are common over mafic lithologies
and where ferruginous saprolites are exposed at surface. Iron cemented
bedrock fragments and lag occur within drainage depressions or depositional
slopes adjacent to iron-rich litholgies or ferruginous weathering
mantles.
The lack of extensive iron-rich lithologies and ultramafic units
across the inlier has restricted the formation of the deeply weathered
lateritic profiles capped by a hardened ferruginous mantle or 'duricrust',
underlain by mottled saprolitic clays, pallid zone, saprock and
bedrock which are observed in Western and South Eastern Australia.
Calcification
Calcification and the calcareous enrichment of the regolith profile
and underlying rocks is observed as late stage pedogenic calcretes,
calcareous nodules and as pervasive calcareous veining and coatings.
Calcretes are preferentially developed within the calc-silicate
units of the Corella Formation and Corella Group rocks. Pedogenic
carbonates are also observed adjacent to creeks, rivers and other
drainage depressions or poorly drained areas of the landscape. Contemporary
processes have led to the development of calcareous root casts.
These have developed beneath native grasses and are observed within
recent unconsolidated sediments at station 3532. Gypsiferous crystals
and calcification of the upper profile within sediments of the Eromonga
Basin is attributed to present groundwater processes.
Implications for Exploration
Geochemical exploration strategies over much of the Mt Isa Block
are relatively simple due to the extensive outcrop of unweathered
or partially weathered bedrock and skeletal soils. Soil and stream
sediment sampling strategies can be employed over these areas, with
numerous ephemeral streams and creeks providing fresh sample media
after each wet season. Care must be taken to identify colluvial
and alluvial materials, which will dilute or mask the geochemical
signal of the underlying rocks. The Geomorphic domains map shows
the distribution of skeletal soils, colluvium and alluvium across
the study area.
Regional studies undertaken by the CSIRO within the Yilgarn Craton
and the Mt Isa Block and localised studies of the Lady Loretta and
Python Prospects have found that Ferruginous upper profiles, mottled
zone and calcretes provide favourable sampling media for metalliferous
mineral exploration (Anand et. al. 1997; Dell 1992; Lintern &
Butt 1997 and Smith et. al 1992). Both these media would be effective
tools for mineral exploration within the Inlier.
The historically economically important sedimentary and metasedimentary
units of the Urqhart Shales and Mt Isa group rocks show strong bleaching
and deep saprolite development to depths in excess of 40 metres.
In these areas deep drilling below the bleaching is recommended
as weathering may have leached the pathfinder elements. Iron-rich
sedimentary sequences within these units are expressed surficially
by the presence of iron-rich segregations, granules, ferruginous
duricrusts or gossan like features. These massive iron accumulations
effectively scavenge ore related elements and provide a very useful
sampling medium.
The sedimentary dominated areas at the Eastern and Western Extents
of the study area provide the most difficult exploration environments.
Geophysical technologies are proving the most useful tools in these
environments, with supplementary redox front or ferruginous interface
zone sampling. Further work is required on the definitions and development
of effective geochemical strategies in these environments.
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