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Open File Report 138

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 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 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 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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