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Open File Report 120

Geochemical exploration in regolith dominated terrain, North Queensland (CRC LEME - AMIRA P417 Final Report)

R.R. Anand, S.J. Fraser, M.R. Jones, Li Shu, T.J. Munday, C. Phang, I.D.M. Robertson, K.M. Scott, P. Vasconcelos, J.E. Wildman, J. Wilford

Extensive, variable, and generally thick regolith is a major impediment to mineral exploration in parts of northern Queensland, as well as in many other parts of Australia. The Mount Isa and Charters Towers - north Drummond Basin regions host numerous base-metal and gold deposits and have a long history of mineral exploration. The well-exposed parts of these regions have been effectively explored by traditional methods. Exploration is now concentrating on areas obscured by deep weathering or by transported cover.

The main aim of this project was to develop suitable geochemical exploration methods for regolith covered areas based on an improved knowledge of the nature and evolution of the regolith and landscape. To achieve this, this project undertook regolith-geochemical dispersion studies in a variety of geomorphological and sedimentary environments with particular emphasis on distribution, characteristics and origin of the regolith. The activities of the project ranged from regional to district-scale investigations, with more detailed studies at specific sites or prospects. Sites were selected to address specific problems and, in many cases, mapping was extended to place them in their regolith-landform context. The outcomes of district-scale and specific studies are available as investigation reports. The purpose of this report is to summarise the results and to develop models, conclusions and recommendations. Regolith maps and geochemical data are available on a compact disc (Appendix III).



As an aid to geochemical dispersion studies, a regolith-landform framework was established for the Mt Isa region. There has been complex erosion, deposition and weathering during the Mesozoic and Cainozoic, forming complex landscapes and regoliths. Mesozoic sediments were deposited on a land surface of broad river valleys with low hills and interfluves. By the early Cainozoic most of the Mesozoic sediments had been eroded except in the south-east and north of the region studied. Field relationships and dating of Mn oxides strongly suggest that evolution of the weathering profiles spans the Tertiary, possibly extending into the Cretaceous. Weathering of Cambrian, Mesozoic and Proterozoic bedrocks left lateritic profiles capped, in places, by ferruginous or siliceous duricrust. The depth of weathering varies and is controlled largely by landscape position, bedrock, structural features and any overlying sediments at the time of weathering. Palaeoplains and topographic lows are more deeply weathered than the erosional plains and hill belts. At many locations, Proterozoic bedrocks are weathered to greater depths where overlying Cambrian or Mesozoic sediments have been removed or were never deposited. In places, remnant river channel and sheet wash deposits have been silicified and ferruginised.

Massive, fragmental and nodular duricrusts have formed in situ on Fe-rich weathered rocks by accumulation of ferruginous materials from mottled saprolite and were left by down-wasting of the profile as clays and soluble elements were removed. Slabby duricrust formed on lower slopes by induration of locally derived colluvium and saprolite with lateral accumulation of Fe. Slabby duricust can be distinguished by its landscape position on plateau edges, micromorphology (platy), geochemistry (Mn and P-rich) and its goethite-rich mineralogy.

Silcretes have formed on a variety of bedrocks but are most common on siliceous materials. In places, silica has cemented alluvial sands or sheet wash sands and gravels. Silicified alluvial sands and gravels now occupy topographically higher areas, because of relief inversion since induration.

The plains feature variable thicknesses of Cainozoic and Mesozoic sediments, underlain by weathered or fresh Proterozoic bedrock. Soils have formed on fresh and weathered Mesozoic and Cainozoic sediments and Proterozoic bedrock. Lithosols are associated with resistant rocks, areas of high relief and steeper slopes. In depositional areas, the soils vary from black through brown to grey sandy clay, sands or clays and generally contain polymictic gravels. Some soils have been weathered (mottling, silicification) since their deposition. Black and brown soils have developed extensively on Cainozoic and Mesozoic sediments. Black soils were developed progressively from brown soils where the alluvium was fine, water was retained and kaolinite was transformed to smectite.

Recommendations for exploration practice

Several geochemical sample media were demonstrated to have specific application in exploration in the Mt Isa region. Appraisal of geomorphology and regolith at a district scale is an important pre-requisite for efficient exploration of a regolith-dominated terrain. Regolith-landform maps and regolith stratigraphy should guide the selection of sampling media, sample interval, sampling procedure, analytical method, element suite and data interpretation. A regolith 'fact map' is produced to describe regolith materials in a landform framework and to divide these broadly into duricrusts, saprolites and colluvium-alluvium. Each, with the exception of colluvium and alluvium, are subdivided according to their bedrock (Proterozoic and post-Proterozoic). The bedrocks have different prospectivities and require different interpretation.

Residual ferruginous materials (massive, fragmental or nodular duricrusts), where they occur, should be collected for district- to prospect-scale surveys. Data from partly transported slabby duricrust should be interpreted with care as their Fe and trace element (Cu. As) content may have been derived laterally.

Soil sampling is effective in areas of shallow overburden (1-5 m). The best materials are mottles or the soil matrix rather than clastic grains. Where Fe and/or Mn oxides have adsorbed significant quantities of indicator elements (e.g., Cu. Zn) multiple regression, followed by a residual treatment of these indicator elements would remove the effects of adsorption and draw attention to anomalies that would otherwise remain hidden.

Areas dominated by thick (>5 m) Cambrian, Mesozoic and Tertiary sediments present significant exploration problems. Coarse sediments should be collected at and just above the unconformity (interface sample) in areas of unweathered or slightly weathered Mesozoic cover to detect a near-miss when drilling a geophysical target. When sediments have been weathered, buried ferruginous bands at palaeosurfaces or at watertables may provide a continuous sampling medium. Horizontal ferruginous bands formed within sediments should be preferentially collected. These are more useful than structurally controlled sub-vertical ferruginous veins within the Mesozoic sediments.



The landscape of the region is a product of several sedimentation and weathering episodes. The dominance of a southerly flowing river system in the early Tertiary, the formation of a large lake system in the middle Tertiary, and the reversal of the river system in the late Tertiary are the main episodes of landscape evolution in the north Drummond Basin. Deposition and erosion in the north Drummond Basin has been dictated by drainage changes. When the southerly drainage was choked during the Tertiary, rapid sedimentation formed the Suttor Formation in the south and the Southern Cross Formation in the north. In view of their extent and fluvial nature, these were deposited over a considerable time span and were not restricted to a single event.

Deep weathering of both the fluvial sediments and the basement formed the duricrust, red earths and, to a lesser extent, yellow earths. Campaspe Formation sediments were deposited on the Southern Cross Formation. In areas where intense erosion of the Southern Cross Formation occurred, Campaspe Formation sediments were deposited in lower levels in the landscape. Yellow and grey earths with ferruginous pisoliths are developed on the Campaspe Formation.

In drill spoil, the Southern Cross Formation is more clay-rich than the Campaspe Formation and contains clasts of the basement rocks. The Campaspe Formation tends to be sand-rich and more sorted than the Southern Cross Formation. The Campaspe Formation may contain detrital nodules and pisoliths throughout the sediment, whereas most of the pisoliths and nodules in the Southern Cross Formation are concentrated near its top. There are no consistent mineralogical and geochemical criteria but hiatuses in feldspar and kaolinite abundances, rounding of quartz grains and geochemical parameters such as hiatuses in Ti/Zr ratios can be used to distinguish the Campaspe Formation from basement volcanics.

Recommendations for exploration practice

The focus of most of the geochemical studies in the Charters Towers - north Drummond Basin was on investigating dispersions in sedimentary cover. Regolith-landform procedures are similar to those described for the Mt Isa region with the exception that regolith units should be divided into Palaeozoic or post--Palaeozoic. The geochemical dispersions appear to be similar to those of the Mt Isa region, with geochemical responses where the cover is shallow (1-5 m). Here, soil sampling (including specific sampling of mottles) would be effective. The probability of hydromorphic dispersion is better in sediments that have been weathered since deposition.

In areas dominated by a thick (>5 m) regolith on Campaspe, Southern Cross and Suttor Formations, dispersion is predominantly mechanical near the base. In places, elevated indicator elements and Au are hydromorphically dispersed with Fe oxides and dolomite-rich bands at least 10 m above the unconformity. Thus, basal sediments and ferruginous bands (redox products) should be sampled preferentially. Extensive sheets of ferruginous pisoliths, developed in the Campaspe Formation, also appear to be a promising sampling medium in the region.


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