Read Guides for mineral exploration through and within the regolith -- central Gawler Craton and Curnamona Province text version

Explorers' guides

Guides for mineral exploration through and within the regolith -- central Gawler Craton and Curnamona Province

Adrian J Fabris and Malcolm J Sheard (Geological Survey Branch, PIRSA)

Introduction

A series of six mineral explorers' guides for some of Australia's major metallogenic provinces are one of several key legacy products produced by the Cooperative Research Centre for Landscape Environment and Mineral Exploration (CRC LEME). Two of the guides are for regions within South Australia -- the central Gawler Craton and Curnamona Province (Fabris et al. 2009; Sheard et al. 2009; Fig. 1). The guides provide an introduction to the regolith and landscape history of the regions, together with advice on the challenges, strategies and methods that can assist exploration for mineral deposits within or beneath the regolith (weathered in situ basement ± transported cover). They include a field guide to key regolith sites, case studies, relevant regolith maps and other useful data/papers/reports.

Content

Each guide begins by describing the physical setting and outlining the geological framework for the region. The mineral potential is then summarised and includes details of key mineralisation styles and deposits.

MUSGRAVE PROVINCE

" )

Marla

CRATON

Tarcoola

" )

CURNAMONA PROVINCE

Broken Hill

" )

CENTRAL Kimba " ) GAWLER CRATON

) Port Augusta "

SA

GAWLER

Coober Pedy

" )

Whyalla

" )

Port Lincoln

0 100 200 km Equidistant Conic

" )

) ADELAIDE "

PIRSA 203771_019

Figure 1 Locality map of the central Gawler Craton and Curnamona Province.

A detailed account of the landscape evolution throughout the Phanerozoic is provided and incorporates descriptions of sedimentation, tectonics, weathering, palaeoclimate and vegetation. These establish a framework for subsequent detailed regolith material descriptions containing information on distribution and landscape positions (architecture) across the region. Examples of indurated regolith materials are shown in Figure 2. Those materials can be represented by a number of variants and typically have long and/or complex developmental histories. Schematic regolith models in 3D are used to represent some of the common regolith materials and their architectural relationships (Fig. 3). Two models were developed for the Curnamona Province to account for differences from north to south: northern Flinders Ranges, and Olary Ranges and surrounds. Common regolith relationships for each region are characterised using schematic profiles (Fig. 4). These are useful for the conceptual understanding of regolith materials, as well as regolith profile history and geochemical dispersion patterns derived from weathering mineralisation. Throughout each guide there is a common exploration theme -- the importance of having a sound understanding of regolith materials and the evolution of the landscape. Such an understanding is fundamental to knowing what can be sampled and what should be avoided, as well as being an aid in designing geochemical sampling surveys and interpretation of assay results. Mapping a prospect's regolith-landforms is the best way to gain that necessary knowledge, and the guides contain descriptions of regolith mapping methods and available examples, as well as useful datasets for their creation. Within the Gawler guide, particular attention is given to mapping hydrothermal alteration and palaeodrainage­palaeocoastal features.

(a)

(a)

(b)

5 cm

(c)

(d)

NSW QLD

Figure 2 Examples of indurated regolith materials. (a) Groundwater silcrete with typical ropy surface (iron-stained), Stuart Creek. (b) Sectioned pedogenic silcrete mini columns displaying basal pebble growthnuclei, Mulligan Dam Regolith type section, NE of Mount Babbage. (c) Sliced calcrete cobble revealing complex aggregated interior, from an area ~25 km NE of Wudinna. (d) Conglomerate of ferruginised rock fragments, western Kalkaroo Station. (Photos 407498­501).

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Gawler Craton, Curnamona Province

(a)

Bedrock/saprock outcrop (inselbergs/ranges) highlands Modern drainage, eroding Lower erosional plain + channel + alluvium colluvium + thin soil Erosional plain (piedmont slope) Exhumed bedrock Upper plain with seif + colluvium + lag (whalebacks, tors, low dunefield + sandplain outcrop) and soils Alluvial fan Lower level escarpment in saprolith, truncated Duricrust cap profile Landscape Leached Recent low occupied (pallid) alluvium by ephemeral upper salt pan/playa saprolite Saprolith Old channel, infilled and duricrust capped Inverted relief

MS f00 2-0

(b)

Flinders Ranges, eroding highlands (bedrock ± saprock) Saprolith remnant preserved in lower range lands

Peneplain remnant on ranges upland (? Mesozoic surface) Deep gorge incision into bedrock

Tallus apron Pleistocene alluvial fan being incised by Holocene drainage Gibber clad eroding Mesozoic sediment Inverted relief, duricrust capped saprolite in Mesozoic and/or Cenozoic sediment Erosional escarpment Landscape low occupied by ephemeral playa (salt crusted) Palaeo-lake beach ridges, gravelly Modern alluvial fan

Thru

st

fault

Proterozoic bedrock

Debris flow

Saprolith, truncated profile Bedrock Truncated and modified profile Evaporative playa/pan in topographic low

Upland-facing escarpment caused by ramp faulting of Mesozoic-Cenozoic regolith Major faulted edge to bedrock

t en m lt lle au co p f De ram

Drainage sink Seif dunes

8

Breakaway/escarpment cut into upper saprolite Lower plain capped by duricrust seif dune Festoon dunefield in topographic low

Depressed weathering zone Incised and infilled palaeovalley

Basinal sediment (Mesozoic ± Palaeozoic) Diachronous faulting with subtle surface trace Listric faulting associated with basin compaction/extension

Palaeogene fluvial channel and fill (sandy) Neogene fluvial channel and fill (silty, clayey) Listric faulting of sediment, basin compaction/extention Palaeogene fluvial channel and fill (sandy) Bedrock (Benagerie Ridge, Proterozoic)

Lacustrine sediment Palaeosol ± deep weathering ± duricrust Basinal sediments (Palaeogene - Neogene)

203771_022

Lunette Gibber clad alluvium Quaternary alluvium Duricrust

Basinal sediments (Mesozoic ± Palaeozoic)

Figure 3 Block diagrams summarising major regolith-landform components. (a) Gawler Craton. (b) Northern Flinders Ranges, Curnamona Province.

Weathering, as a process, includes a series of chemical and mineral changes involving progressive loss of Na+, K+, Ca2+, Mg2+ and retention of Si4+ (in part), Al3+ and Fe3+, as well as the mobilisation of minor and trace elements. Factors controlling element dispersion during weathering, with details on the key minerals in which they are incorporated or absorbed, are discussed in a section

(a)

Variable presence of nodular, platy, powder or hardpan carbonate (calcrete)

on regolith geochemistry. Examples of the behaviour of specific elements in regolith are provided and element associations specific to each region are listed (based on CRC LEME and industry case studies). A key chapter discusses exploration challenges and strategies in regolith dominated terrains. It begins by outlining the general exploration approach, based upon integrating

(b)

an understanding of regolith into exploration programs. A summary of appropriate regolith materials to be sampled is given, with examples drawn from significant research conducted in each of the regions over the last two decades. Regolith sampling media discussed includes lags, soil, regolith carbonate, ferruginous materials, siliceous materials, saprolith, vegetation and groundwater.

Surface lag, residual ± transported depending on setting Soil, commonly calcareous and alkaline, with low to high aeolian component Ferruginous residuum, goethite ± hematite, pisolithic to massive Collapsed plasmic (clay-rich) or arenose (grit-rich) zone Collapsed megamottle horizon Mottle zone: dominantly red-brown mottles-megamottles, ferruginised horizon Pedoplasmation front Ferruginous mottles ± megamottles ± stains ± fracture infill-linings Pallid zone Low competency, pale coloured, most or all weatherable minerals are weathered Moderate competency, some less weathered corestones may occur Saprock-saprolite transition Weathering front Trace alteration along fractures in zone near weathering front

Pedolith #4 Saprolith #4 Pedolith #3 Saprolith #3

Aeolian dune - sandplain partial cover, contains younger regolith carbonate accumulations. Palaeosol below dunes, contains older calcrete + gypcrete, sodic and alkaline. Claypan sediment, playas and salinas. Channel sands, cross-bedded, loose, weakly Fe-stained at top. Unconformity Clayey to silty overbank floodout sediment. Unconformity, Pliocene - Pleistocene Silcrete/porcellanite, encapsulates megamottles, Late Neogene. Silicification front, sharp or diffuse Ferruginous mottling and megamottling, goethite ± hematite staining and fracture infill. Clayey to silty transported regolith, Neogene. Unconformity, Neogene Palaeogene-Neogene silcrete, massive to nodular. Silicification front, sharp or diffuse Ferruginous zone, mottled or stained yellow or reddish. Channel sands, may be gravelly towards base, may be carbonaceous (lignitic) in lower half. Groundwater silcrete overprint, massive, grey, typically affects channel sediment + bank regolith. Palaeogene channel unconformity Palaeogene clay-silt-sand regolith.

MSf015-08

Pedolith

Upper saprolite

Silcrete #2 Pedolith #2 Saprolith #2 Silcrete #1 Pedolith #1 Saprolith Unaltered sediment

Lower saprolite Saprock Bedrock

MSf003-08

203771_023

Figure 4 Schematic regolith profiles. (a) A near `complete' weathering profile developed in Gawler Craton crystalline basement. (b) A complex weathering profile displaying repeated weathering-induration episodes developed within stacked Cenozoic sediments, northern Curnamona Province.

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Explorers' guides

A discussion on drilling through the regolith follows, having an emphasis on what to look out for, log, and common mistakes such as premature drillhole termination and difficulty in defining important unconformities or zone boundaries from drill cuttings alone. Finally, the analytical approach and data analysis are considered. This emphasises that geochemical data should be examined in context with its relevant regolith-landform(s) and that regolith mapping is crucial to this process. Regolith landform maps can be used to clear up many geochemical uncertainties regarding multiple populations and determine appropriate thresholds for defining what is truly anomalous and what is not.

Central Gawler Craton highlights The use of calcrete and research into the origin of gold-in-calcrete anomalies

Calcrete became the dominant sample medium in southern Australia after the association of calcium carbonate and anomalous gold over concealed mineralisation was demonstrated during the late 1980s in the Yilgarn Craton. Subsequent use of calcrete sampling in South Australia resulted in discovery, in early 1995, of the Challenger gold deposit and renewed exploration for gold over much of the

Gawler Craton. The Gawler guide provides a summary of the more recent research into the origin of gold-incalcrete anomalies. CRC LEME studies in 2004 on the Barns gold prospect demonstrated elevated gold values in the lower section of an 8 m thick aeolian sand dune overlying weathered Tunkillia Suite granitic bedrock. A trench was bulldozed to expose a full section through the dune and sampling confirmed elevated gold contents to 9.2 ppb associated with increased calcium-carbonate levels in the dune; the highest gold concentration being in a calcareous rhizomorph some 5 m above the residual bedrock. The critical role of vegetation in anomaly formation was proposed where biological cycling of gold from the saprolite by deeprooted vegetation translocates gold into the dune via element mobilisation from surface plant litter (Fig. 5). Gold levels in this dune have accumulated since dune formation at ~25 000 years ago. The specific mechanisms for biologically mediated co-precipitation of gold and carbonate have been proposed to explain the association of gold and regolith carbonate at Barns.

related CRC LEME research released elsewhere, have already provided a basis for increased uranium exploration across the Gawler Craton and resulted in the discovery of world-class heavy mineral sand deposits around the eastern margin of the Eucla Basin.

Curnamona Province highlights Using groundwater in exploration

Since groundwater interacts with the minerals that form or line the aquifer system through which it flows, it has the potential to be a direct sampling medium representative of the subsurface. Research into using groundwater in exploration over the Curnamona Province has suggested that purely basing exploration on the concentration of target elements can be misleading because their concentration in groundwater is strongly affected by various processes (e.g. pH, evaporation, evapotranspiration, mixing, precipitation­dissolution and oxidation/reduction), all of which take place during an often complex and, in some cases, long evolution. Several steps are discussed that can be used to develop hydrogeochemistry into a useful tool for base metal sulfide exploration. First determine which samples contain more sulfur than can be accounted for by evaporation or mixing.

Palaeodrainage and palaeocoastal mapping

The Gawler guide provides numerous exploration methods and strategies for palaeochannel and palaeocoastal mapping. These maps and models, and

up to 3.7 ppb Au in biota

1.5 - 9.5 ppb Au in rhizomorphs

0 - 0.2 ppb Au in leached horizon

rhizomorphs

8m

0 - 1 ppb Au in illuvial zone

Up to 36 ppb Au in calcrete Calcareous, silicified leached saprolite

MSf010-08

Leached saprolite

Figure 5 Model for vegetation cycling of gold through sand dune cover and formation of anomalous gold in calcareous rhizomorphs within the sand dune, Barns gold prospect. (After Lintern 2007.)

50 kilometres

Mineral occurrences Groundwater mineralisation indicators

Unrelated to any mineralisation S >13

34

Distal (>2 km) to potential mineralisation 34 S <13 and Cu <10 ppb 34 S <13 and Zn <50 ppb Proximal to potential mineralisation 34 S <13 and Cu >10 ppb 34 S <13 and Zn >50 ppb 203771_025

Figure 6 Groundwater mineralisation indicators, southern Curnamona Province. (Background image is digital terrain model; after de Caritat and Kirste 2005.)

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Samples that show a `sulfur excess' can be further sorted (using the stable isotopes of sulfur and oxygen in sulfate) into sulfur from a meteoric source or from sulfide mineralisation. The multielement analyses can then be used to determine those samples with potential for economic significance (Fig. 6).

These are reviewed in the exploration strategies chapter. Coupled with the summary of sampling methodology provided in the appendix, the information provided enables explorers to use biogeochemistry with confidence.

References

de Caritat P and Kirste D 2005. Hydrogeochemistry applied to mineral exploration under cover in the Curnamona Province. MESA Journal 37:13­17. Department of Primary Industries and Resources South Australia, Adelaide. Fabris AJ, Sheard MJ, Keeling JL, Hill SM, McQueen KG, Conor CHH and de Caritat P 2009. A guide for mineral exploration through the regolith in the Curnamona Province, South Australia, CRC LEME Explorers' Guide Series. Cooperative Research Centre for Landscape Environments and Mineral Exploration, Bentley, Western Australia. Lintern MJ 2007. Vegetation controls on the formation of gold anomalies in calcrete

Using plants in exploration

A significant body of research within the Curnamona Province has focused on sampling plants in exploration. While sampling vegetation is not a new concept, the more recent CRC LEME research has successfully demonstrated that vegetation sampling is a viable exploration method and one which should be more routinely used. Case studies have been directed towards investigating a range of vegetation types in a number of landform settings and over a selection of mineralisation styles.

and other materials at the Barns gold prospect, Eyre Peninsula, South Australia. Geochemistry: Exploration, Environment, Analyses, 7:249­266. Sheard MJ, Keeling JL, Hou B, McQueen KG, Lintern MJ and Hill SM 2009. A guide for mineral exploration through the regolith of the central Gawler Craton, South Australia, CRC LEME Explorers' Guide Series. Cooperative Research Centre for Landscape Environments and Mineral Exploration, Bentley, Western Australia. The guides are due for release by mid 2009 and will be available for free download from the CRC LEME and PIRSA websites: <www. crcleme.org.au>, go to the Direct Link on the homepage; and <www.minerals.pir.sa.gov.au>. Limited hard copies are available and have been printed on A5-size paper (appendixes on a mini CD) designed to fit in a vehicle glove box or backpack. For copies contact John Keeling, phone +61 8 8436 3135, email <[email protected] saugov.sa.gov.au>.

Sir Ben Dickinson, former Director of Mines, honoured

The Adelaide City Council has added a plaque to the Jubilee 150 Walkway to commemorate Sir Samuel Benson Dickinson, Director of Mines 1944­56. The walkway, established in 1988, is a series of bronze plaques set into the pavement of North Terrace (between King William Street and Pulteney Street) that contain the names and deeds of the people who made major contributions to the founding and development of South Australia. At the outbreak of World War II in 1939 there was little secondary industry in the state. Mineral production was valued at 3.3m, mostly from iron ore mined in the Middleback Range. In 1940 when Dickinson was appointed to the Department of Mines (now part of PIRSA) he was one of 2 geologists employed in a staff of 20 and was initially directed to investigate water supplies on marginal lands. For the following four years he was engaged in the assessment of wartime strategic minerals, the mapping and supervision of drilling operations for the evaluation of coal at Leigh Creek, and the assessment of potential water supplies in that region. In 1944 Dickinson was appointed Director of Mines. Dickinson, with full political support, was attuned to the requirements for the assessment of mineral resources and their exploitation to enhance growth of secondary industries, population and income in South Australia. His foresight, resolve and energy set a new path for mineral development in this state. In his years as director, South Australia secured an integrated steel works (blast furnace, coke ovens, rolling mills and ship building yards at Whyalla) and become independent of uncertain supplies of coal from interstate with the development of the Leigh Creek coal mine. South Australian uranium deposits were identified as potential sources of supply for use in atomic weapons and to fuel new sources of energy: departmental investigations were carried out at Radium Hill near Port Pirie.

Ben Dickinson using a Geiger counter at Copper Top, a mine in the Peake and Denison Ranges, December 1953. Inset: commemorative plaque.

(Photos 407504, 407510)

Dickinson's legacy also includes the succession of directors and government geologists who succeeded him for the next four decades. Imbued by his culture, vision and dedication, they continued to adapt to the state's needs and were instrumental in revealing oil and gas in the Cooper Basin, one of the world's greatest metal deposits at Olympic Dam and other mineral deposits that continue to make rich returns to South Australia.

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Information

Guides for mineral exploration through and within the regolith -- central Gawler Craton and Curnamona Province

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