Journal of Geochemical Exploration 104 (2010) 105–117

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Journal of Geochemical Exploration

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Biogeochemical sampling for mineral exploration in arid terrains: Tanami GoldProvince, Australia

N. Reid a,b,⁎, S.M. Hill a

a School of Earth and Environmental Sciences, University of Adelaide, Adelaide, S.A. 5005, Australiab CSIRO Exploration and Mining, ARRC Building, 26 Dick Perry Avenue, Kensington, W.A. 6151, Australia

⁎ Corresponding author. CSIRO Exploration andMininAvenue, Kensington, W.A. 6151, Australia. Tel.: +61 88559.

E-mail address: nathan.reid@csiro.au (N. Reid).

0375-6742/$ – see front matter. Crown Copyright © 20doi:10.1016/j.gexplo.2010.01.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 June 2009Accepted 15 January 2010Available online 29 January 2010

Keywords:BiogeochemistryGeobotanySpinifexSnappy gumTriodia pungensEucalyptus brevifolia

Vegetation sampling is an effective mineral exploration technique in areas of transported cover in theTanami Gold Province where other techniques have been of limited success. This research tests the ability ofplants to show signatures of mineralisation as well as the optimum scale of sampling for first-pass mineralexploration surveys. The semi-arid Tanami Gold Province in northern Australia encompasses nearly160,000 km2 and is a highly prospective yet under-explored region due to transported cover maskingmineralisation. Various dominant plant species were sampled along transects across four sites of Aumineralisation in the Tanami (Larranganni, Hyperion, Coyote and Titania). Snappy gum (Eucalyptusbrevifolia) gave a geobotanical (plant distribution) and lithological (S and Zn) signature of an underlyinggeological structure known to host Au mineralisation at the Coyote Prospect. Soft spinifex (Triodia pungens)provided an Au, As, ±Zn, ±S, and±Ce expression of buried Au mineralisation at Coyote and Titania. At theHyperion prospect, mineralisation is located at the contact between granite and dolerite, and biogeochemicalsignatures from snappy gum and dogwood (Acacia coriacea) show elevated±Au, Ce, S and Zn thatcorresponded to the contact. Biogeochemistry is able to determine the location of mineralisation at each site,except at Hyperion where the sample density was too low. It is able to identify underlying substratedifferences, however, background knowledge relating to regolith, geology, hydrology and geophysics areimportant in aiding the interpretation of the elemental data. It was found that having too few samples (atHyperion) there was insufficient useful information for mineral exploration. Grid coverage of an area (atTitania) provided information on mineralisation and groundwater dispersion plumes; however, in this case asingle transect over the same area would have been sufficient for a first-pass mineral exploration survey.

g, ARRC Building, 26 Dick Perry6436 8675; fax: +61 8 6436

10 Published by Elsevier B.V. All rights res

Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction

The search for new mineral resources is commonly hampered bytransported cover and there is the need for low-cost methods that canbe applied by mineral explorers to effectively explore through thecover. Biogeochemistry has been used in many terrains and climatesto define surficial chemical signatures from buried mineralisation(Cole et al., 1968; Brooks et al., 1995; Hill, 2002; Dunn, 2007). As partof regional exploration programs, this process needs to be refined tosampling several species across prospect/tenement surveys that canbe presented as an efficient, effective and robust approach for mineralexplorers. Within Australia there have been few plant species thathave been investigated for this purpose and there has been littleprogress into adapting this methodology into orientation programsthat exploration companies can use to survey their tenements.

Ore deposits can be masked by varying thicknesses of transportedcover. In Australia, transported cover material is typically extensive,red, ferruginous, sands and silts that include a mixture of windblowndetritus (aeolian: sand dunes and sand plains), and/or materialsreworked by surface water flow (alluvial, fluvial, or sheetwash). Largecomponents of this material are not derived from the immediatelyunderlying bedrock and therefore it can host geochemical/geophys-ical signatures that are different from the underlying bedrock. Drillingthrough transported cover material has been the best way todetermine the underlying geological substrate, however, it isexpensive, time consuming and can have an environmental impactin sensitive sites (Cole et al., 1968; Brooks et al., 1995; Dunn, 2007).

The biogeochemical method of mineral exploration is used toprovide an expression of buried geological features, and utilises theability of plant roots to penetrate surficialmaterials and then translocategeochemical signatures through the roots to other plant organs. It hasbeen effective at detecting buried mineralisation targets and has beenused extensively in Canada (Dunn, 1981, 2007), and Russia (Malyuga,1963; Kovalevsky, 1987), and trialled in Australia (Cole et al., 1968;Brooks et al., 1995;Anandet al., 2007; Reid et al., 2008; Reid et al., 2009).

erved.

Fig. 1. Location of the four Tanami Prospects examined in this study.

106 N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

This study presents results from four orientation studies ofdifferent designs over known Au mineralisation (Coyote, Titania,Larranganni and Hyperion prospects) from the Tanami Gold Province,northern Australia. The implications for using small-scale biogeo-chemical sampling programs for mineral exploration tenementsurveys in areas of transported cover are presented.

2. Study area

2.1. Climate

The Tanami region is semi-arid with irregular, summer-dominat-ed, monsoonal rainfall patterns (Gibson, 1986; Wilford, 2000, 2003).The mean annual rainfall is 357 mm (Rabbit Flat, central Tanami;Fig. 1; Australian Bureau of Meteorology, 2009). The average dailymaximum temperature rises to 39 °C in January and the average dailyminimum is 6 °C in July (Gibson, 1986). The average annualevaporation potential is 3750 mm (Wilford, 2000).

2.2. Landscape

Bedrock exposure across the region is sparse (<1% of thelandscape) and instead the landscape is dominated by transportedcover. The transported cover is mostly between 10–20 m thick withsome palaeo-drainage systems having a thickness of over 100 m(Wilford, 2003). This cover results in large areas of the Tanami GoldProvince being ineffectively explored even though they are highlyprospective for mineralisation (Crispe et al., 2007).

The Tanami Gold Province has low topographic relief, with mostslopes <0.5°, and with large areas without recognisable drainagechannels (Wilford, 2000). The main landscape process responsible fortransporting the surface materials here are shallow overland waterflow dominated by sheetwashmostly derived from low hills and rises,

Fig. 2. Geological cross sections for (a) Larranganni (Kookaburra and Sandpiper adapted fromBagas et al., 2009), and (d) Titania (adapted from Readford, 1999).

and aeolian saltation, particularly associated with the prevailingeasterly winds (Wilford, 2000).

2.3. Vegetation

The vegetation of this region consists of dry savannah woodlandtypical of semi-arid northern Australia (Gibson, 1986). Hummock-grasslands, dominated by spinifex (Triodia spp, including the softspinifex, shown to be deep rooted), are widespread and abundantacross the arid Australian landscape and are the dominant vegetationcommunity in this region. Other species include Eucalyptus and Cor-ymbia trees (including snappy gum and desert bloodwood, thought tobe deep rooted) that are locally abundant, along with small trees andshrubs of Acacia (including dogwood, with taproot and feeder roots;and two-nerved wattle, shallow rooted) and Melaleuca, includinginland tea tree and sandhill paperbark (which have feeder and taproots, Gibson, 1986; Wilford, 2000).

2.4. Geological setting

The Tanami Gold Province encompasses nearly 160,000 km2 fromthe central west of the Northern Territory and into north-easternWestern Australia (Huston et al., 2007). The region is highlyprospective for Au mineralisation and hosts the 6.0 Moz Callie Au-deposit (Cross et al., 2005) and other smaller deposits and prospects.Most of these deposits, however, were found in outcrop, or asextensions from adjacent outcrop (Perry, 1962).

The locations of the four sites investigated here (Larranganni,Hyperion, Coyote and Titania) are shown in Fig. 1, and geological crosssections of each site are shown in Fig. 2. The Larranganni prospectcomprises several small Au mineralisation occurrences, the largestbeing Kookaburra and Sandpiper (Doust, 1997; Bagas et al., 2007).They have strong surficial expressions of Au (soil and lag) associatedwith structurally complex zones in areas of shallow cover. These

Bagas et al., 2007), (b) Hyperion (adapted from Power, 2004), (c) Coyote (adapted from

107N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

108 N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

expressions are strong because the cover thickness is <1 m(Glengarry, 2000). The site is underlain by bedrock of the StubbinsFormation (1864 Ma metawacke, pelite and dolerite sequences,Fig. 2a, T. Beardsmore, 2005, written comm.). This is sometimesreferred to as the Bald Hill sequence (the oldest rocks in the Tanami),which is in contact with the Killi Killi Formation (Bagas et al., 2007;Huston et al., 2007).

The Hyperion prospect is within the Groundrush dolerite withscattered monzogranite dykes (Fig. 2b, Power, 2004).

The Coyote and Titania study sites are underlain by Killi KilliFormation bedrock within the Granites–Tanami Block of the NorthAustralian Craton (Fig. 2c and d, turbiditic sandstones and siltstones).The Killi Killi Formation is dominated by thick sequences ofPalaeoproterozoic turbidites and greywackes (1848±22 Ma) and ispart of a widespread northern Australian turbidite package (Vanden-berg et al., 2001; Cross et al., 2005; Petts and Hill, 2005). Most of theknown Au mineralisation in the area is hosted by this unit and theolder Proterozoic Dead Bullock Formation (Mernagh and Wygralak,2007).

2.5. Mineralisation

Tanami Gold NL's Larranganni prospects have Au hosted in quartzveins within bedrock of the Stubbins Formation (Fig. 2a). NewmontAsia Pacific's Hyperion prospect contains approximately 100,000 oz ofAu. It was considered originally as blind to traditional surficialexploration methodologies and was discovered by drilling (Power,2004). The mineralisation is situated at the contact between theGroundrush dolerite and an un-named monzogranite dyke (Fig. 2b,Power, 2004).

The mineralisation at Coyote is hosted by quartz veins within theturbiditic sediments of the Killi Killi Formation (Bagas et al., 2009).The mineralisation is mostly associated with a tightly folded anticline,defined by a thick siltstone layer known as the ‘marker silt’comprising the southern limb of the anticline dipping steeply to thesouth and the northern limb dipping at around 40° to the north(Fig. 2c, Bagas et al., 2009). Weathering extends to a depth of 170 mover the mineralisation, which complicates the alteration signature ofthe ore (Bagas et al., 2007). This prospect is presently held by TanamiGold NL.

The Titania prospect, presently held by Newmont Asia Pacific,includes an estimated 600,000 oz of Au mineralisation, hosted within

Table 1Summary of species sampled at each site and analysis laboratory.

Field site Target species Cover(m) n

Coyote Soft spinifex (Triodia pungens) 5–15 39Snappy gum (Eucalyptus brevifolia) 31Silver box (Eucalyptus pruinosa) 15Desert bloodwood (Corymbia opaca) 11Dogwood (Acacia coriacea subsp. sericophylla) 10

Titania Soft spinifex 15–30 76Inland tea tree (Melaleuca lasiandra) 77sandhill paperbark (Melaleuca glomerata) 17Two-nerved wattle (Acacia bivenosa) 39Dogwood 31Corkwood (Hakea macrocarpa) 39Beefwood (Grevillea striata) 11Red bud mallee (Eucalyptus pachyphylla) 11Desert bloodwood 8

Larranganni Snappy gum <1 47

Hyperion Soft spinifex 7–8 9Snappy gum 8Silver box 4Dogwood 9Turpentine (Acacia lysiphloia) 6

the Killi Killi Formation intruded by dolerite sills (Fig. 2d, Gibbons andWebb, 1997; Wygralak and Mernagh, 2001; Huston et al., 2007). Oreminerals include arsenopyrite with traces of chalcopyrite, sphaleriteand pyrrhotite. There is a close association between Au andarsenopyrite.

2.6. Transported cover

The transported cover at the Larranganni sites is generally <1 mthick, mainly consisting of aeolian and localised sheetflow sediments.The Hyperion prospect has 7–8 m of transported cover includingaeolian sands and sheetwash gravels (Worrall et al., 2005). Thetransported cover over the Coyote site varies in thickness between 5–15 m and includes mixed aeolian and sheetflow deposits (Fig. 2c). Thecover thickness increases towards the south of the sampled areaextending from the margins towards the axis of a major palaeo-drainage system. The Titania prospect is covered by approximately15–30 m of transported cover associated with the western margins ofa palaeo-drainage, with cover thickness increasing from the north-west towards the southeast (Fig. 2d, Gibbons and Webb, 1997). Themain differences between the sites is the shallowerweathering profileat Larranganni and Hyperion compared to the deeper transportedcover and weathering profiles at Coyote and Titania.

3. Methods

Vegetation samples were collected along single transects, or in thecase of the Titania prospect within a grid pattern (Table 1). The Coyoteand Larranganni transects each consisted of 30 sampling sites; theHyperion transect 9 sites; the Titania grid included 7 transects, 500 mapart with 9 samples along each transect at 250 m spacings (Table 1).The Titania sample spacing of 250 m was greater than the other sites(100 m) in order to cover the entire site in the time allotted. Sampleswere collected prior to the wet season (January: before rains startedfor Coyote and Larranganni, and October: for Hyperion and Titania) asthis collection time has previously been shown in other semi-aridregions of Australia to be associated with biogeochemical character-istics derived from deeper parts of the plant's root system (Southwestern Australia, Pate et al., 1998; Broken Hill, New South Wales,Hulme and Hill, 2004). Leaves or phyllodes (green, leaf-like branch-lets) were chosen as the target plant organ because they have giventhe best results in previous studies (Cohen et al., 1999; Hulme and

Sample spacing(m) Analysis/Laboratory

100 ICP-MS/ICP-OES — Genalysis, PerthINAA — Becquerel, CanadaInter-laboratory duplicates ICP-MS — ACME, Canada

250 ICP-MS — ACME, Canada

100 ICP-MS/ICP-OES — Genalysis, PerthINAA — Becquerel, CanadaInter-laboratory duplicates ICP-MS — ACME, Canada

100 ICP-MS/ICP-OES — Genalysis, PerthINAA — Becquerel, CanadaInter-laboratory duplicates ICP-MS — ACME, Canada

109N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

Hill, 2003). Samples of leaves from the dominant species of each areawere collected. Every sample was collected using a new pair ofpowder-free, latex gloves (or nitrile gloves for spinifex samples) toensure there was minimal contamination between samples. Sampleswere placed in brown paper bags to allow air exchange with thesamples to prevent rotting.

Sampleswere laid out in awell ventilated setting overnight and laterdried (45 °C for 48 h) in an oven while still in closed bags. The driedsamples were ground to a fine powder using a rotating stainless steelblade mill (Breville® Coffee ‘n’ Spice Grinder), and then the milledsamples were split for duplicate analyses. The grinderwas cleanedwithethanol and compressed air between each sample to ensure that alltraces of the previous samplewere removed. The grinder was then pre-contaminated with a small amount of the ensuing sample then cleanedagain before grinding the sample for analysis.

For the Coyote, Larranganni and Hyperion Prospects the sampleswere subjected to a cold, concentrated nitric acid digest overnight, thena concentrated perchloric acid attack to fully dissolve the tissues.Analysis was by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and Optical Emission Spectrometry (ICP-OES; Genalysis Laborato-ries, Western Australia; Table 1). These samples were also analysed by

Table 2Summary statistics of species sampled at each site and 95% confidence errors calculated fro

Site Species Element Det lim

Coyote Soft spinifex Au (ppb) 0.2As (ppm) 0.1S (ppm) 10Zn (ppm) 1

Snappy gum Au (ppb) 0.2As (ppm) 0.1S (ppm) 10Zn (ppm) 1

Larranganni Snappy gum Au (ppb) 0.2As (ppm) 0.1S (ppm) 10Zn (ppm) 1

Hyperion Snappy gum As (ppm) 0.1Ce (ppm) 0.01S (ppm) 10Zn (ppm) 1

Soft spinifex As (ppm) 0.1Ce (ppm) 0.01S (ppm) 10Zn (ppm) 1

Turpentine As (ppm) 0.1Ce (ppm) 0.01S (ppm) 10Zn (ppm) 1

Dogwood As (ppm) 0.1Ce (ppm) 0.01S (ppm) 10Zn (ppm) 1

Titania Soft spinifex Au (ppb) 0.2As (ppm) 0.1Pb (ppm) 0.01S (ppm) 100Zn (ppm) 0.1

Inland tea tree Au (ppb) 0.2As (ppm) 0.1Pb (ppm) 0.01S (ppm) 100Zn (ppm) 0.1

Two-nerved wattle Au (ppb) 0.2As (ppm) 0.1Pb (ppm) 0.01S (ppm) 100Zn (ppm) 0.1

Dogwood Au (ppb) 0.2As (ppm) 0.1Pb (ppm) 0.01S (ppm) 100Zn (ppm) 0.1

Instrumental Neutron Activation (INAA) at Becquerel Laboratories,Canada, with 8 g of sample compressed into a briquette, shrink-wrapped then irradiatedwith a neutron source. For the Titania prospectsamples were digested in nitric acid then aqua regia and analysed byICP-MS at ACME Laboratories, Canada (Table 1). Duplicates were run ofevery 30th sample, and laboratory standards (mountainhemlock foliagefor samples that were analysed at ACME Laboratories, Vancouver and amixed leaf internal standard for samples analysed at GenalysisLaboratories, Perth) were run to determine analytical bias, accuracyand precision. Detection limits and the calculated errors for eachelement used in this study are presented in Table 2.

Soil data was used from Coyote and Titania as a comparison. Thesoil data from Coyote were part of the original exploration and muchof the methodology is confidential. The soils from Titania were a 1 kgsample collected from 2–10 cm depth. The soils were then sieved to−75um and analysed by ICP-MS at ACME Laboratories, Canada.

4. Results

Mineral exploration techniques in these covered settings havemostly involved sampling soils or surface gravel lags, and shallow

m duplicate analyses, nd is not detected.

it Error Min Median Max

±0.06 nd 0.4 2.3±0.1 nd 0.5 1.2±55 797 1018 1186±2.7 5 14 34±0.06 nd nd nd±0.1 nd 0.15 0.4±55 668 1028 2502±2.7 6 11 22±0.06 nd 0.75 2.5±0.1 nd 0.3 1.6±55 717 812 999±2.7 5 15 38±0.1 0.3 0.61 0.7±0.02 0.19 0.45 1.40±20 1112 1967 2606±6 13 16 26±0.1 nd 0.12 0.2±0.02 0.10 0.2 0.3±20 807 878 984±6 7 10 14±0.1 0.2 0.4 0.6±0.02 0.29 0.5 0.86±20 1160 1302 1426±6 15 20 38±0.1 0.3 0.56 1±0.02 0.29 1.2 8.16±20 907 1027 1422±6 10 15 24±0.07 nd 0.3 2.9±0.1 nd 0.4 1.9±0.06 0.09 0.24 0.92±100 700 1200 1700±0.2 4 9.2 38±0.07 nd 0.3 3.4±0.1 nd 0.4 1.7±0.06 0.07 0.21 0.85±100 1400 2800 4700±0.2 4.4 7.8 17±0.07 nd 0.3 0.7±0.1 nd 0.5 1.1±0.06 0.02 0.05 0.12±100 20,000 27,600 33,000±0.2 3.9 11.2 18±0.07 nd 0.5 0.8±0.1 nd 0.6 0.9±0.06 0.06 0.12 0.29±100 900 1400 6000±0.2 5.3 11.35 26

110 N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

drilling or geophysical techniques. Reid et al. (2008, 2009) presentedpreliminary results of biogeochemical sampling carried out withinthis region, but there have been no other reports of the use of plantbiogeochemical techniques applied in this region. Metals in plants canbe sourced directly from: the ore deposit; groundwater dispersionaway from the ore; metals that have migrated into transported covermaterials; transported cover materials; and/or, surface materials(including leaf fall, rainfall and bioturbation). The interpretation ofresults has tried to simplify the source to one dominant mechanismfor each important element and plant species.

4.1. Larranganni

Snappy gumwas sampled from a transect over the Kookaburra andSandpiper mineralisation as this was the dominant species. Althoughthere were detectable Au and As concentrations directly above boththe Kookaburra and Sandpiper zones of mineralisation, theseconcentrations were not highly anomalous (Fig. 3). There was alsodetectable Au and As between the two ore bodies. Since thetransported cover in this area is minimal and the groundwaterdepth is deep (40 m;Worrall et al., 2005), these values must either befalse positives or reflect weak dispersion and diffusion from the orebodies. As at Coyote, the Zn results show more diffuse anomalousconcentrations and therefore appear to relate to a dispersed chemicalsignature over the ore bodies. This site highlighted that where thetransported cover was minimal (<1 m), the biogeochemical expres-sion of mineralisation is more direct, rather than via groundwater andsediment dispersion haloes. The concentration ranges for theelements are the same as for the Coyote prospect except that snappygum had detectable Au at Larranganni as the trees were growingdirectly into mineralised bedrock, and the sample with the highest Auvalue (2.5 ppb) was growing directly in a mineralised quartz vein.

Fig. 3. Biogeochemical cross sections (snappy gum samples) across t

4.2. Hyperion

This site was to determinewhether a small sample population couldbe used to determinemineralisation properties. This was in comparisonto the closer sample spacingat theother sites. Five specieswere sampled(Table 1), of which silver box (Eucalyptus pruinosa) was restricted indistribution, and turpentine (Acacia lysiphloia) had sticky leaves,resulting in significant windblown dust contributions to its phyllodeanalyses with As, S, Zn and Rare Earth Elements (REE) showingincreasing concentrations to the west (Fig. 4).

While no species highlighted mineralisation definitively, snappygum and dogwood highlighted changes that could be related to thechanges in substrate and may be related to mineralisation, howeverthe small sample size did not allow for accurate delineation andinterpretation. Snappy gum provided a distinctive S anomaly whichcould relate to buried mineralisation at the site (Fig. 4). There was amajor change in chemical composition near the centre of thetransect, with As, Ba and REE more abundant to the east, and Au(single point, 0.9 ppb), Ni and S in greater concentrations over thecentre of the transect (Ba and Ni unpublished and Ce represents allREE). The geological information available for the site shows that theeast of the transect consists of a granitic bedrock within the doleriteand that the west is underlain by dolerite alone, with the Aumineralisation at the contact between the units (Fig. 2). This relatesto the chemical data where the elements abundant in the east (i.e. As,Ba and REE) are typically higherwithin granites in the region (Power,2004).

The dogwood results provided very similar trends to those of thesnappy gum, with REE being in higher concentrations to the east(Fig. 4). The REE concentrations within this species weremuch greaterthan any others with up to 8 ppm Ce. The reason for this accumulationin REE is unknown and warrants further study.

he Larranganni prospects along 7,833,700 mN to 7,835,000 mN.

Fig. 4. Biogeochemical cross sections across the Hyperion mineralisation along 7,836,630 mN.

111N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

4.3. Coyote

Snappy gum, although widespread in the area did not grow overthe mineralized zone, and the associated area of extensive faultingwhich could indicate snappy gum is a geobotanical (plant distribu-tion) indicator of structure in this region as there is little evidence indifferences in either the landscape type (Petts and Hill, 2005) orcover type in this location (Fig. 2). Snappy gum had greaterconcentrations of S and Zn (Table 2) towards the south of thetransect (Fig. 5; in contrast to the spinifex results) where surfaceflows accumulate and the transported cover thickens towards thepalaeo-drainage system thalweg as seen during flooding. The resultsof the soft spinifex study were presented in Reid et al., 2008: thebiogeochemical cross sections of this species over this site can beseen in Fig. 5. This species had the only detectable Au values of thesite (the other species and soil had concentrations below detection,<0.2 ppb) and while the largest value (2.4 ppb) was only a singlepoint anomaly, it did correspond to the larger 6 point As anomalywhich reinforces the signature of mineralisation (Fig. 5). The results

from both snappy gum and soft spinifex highlight areas of interestwithin a multi-element suite more clearly than the original soil datawhich only highlighted an As anomaly over the mineralisation. Inthis area the plant roots are able to access elements which are notexpressed coherently in the surface soils notably the detectable Au inspinifex assays and not in soils.

4.4. Titania

Nine specieswere sampled at Titania (Table 1),with desert bloodwood(Corymbia opaca), red-budded mallee (Eucalyptus pachyphylla), beefwood(Grevillea striata) and sandhill paperbark (Melaleuca glomerata ) havingrestricted distributions. Figs. 6–8 show that soft spinifex, inland tea tree(Melaleuca lasiandra ), and two-nerved wattle (Acacia bivenosa), respec-tively, had relativelyhighconcentrationsofAu(2.9, 3.4 and0.7 ppb)andAs(1.9, 1.7 and 1.1 ppm)with some associations of Zn and S, extending fromthe surface projection of the zone ofmineralisation and downhydrologicalgradients towards the southeast. This is highlighted by the spatialdistributions of each species not overlapping. The tea tree elemental

Fig. 5. Biogeochemical cross sections across the Coyote mineralisation including soil chemistry from the original AngloGold sampling across 482,325 mE.

112 N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

concentrations are greatest to the southeast of the mineralisationcompared to the other species, and this species is known to be moredependent on water source as it only grows in association with shallowgroundwater, salt lakes and channel systems. Hence it can be inferred thatgroundwater flow is towards the southeast, which correlates with theelectrical conductivitygradientdetermined fromgroundwater indrillholes(Worrall et al., 2005). The Zn and S anomalies are not clear along the singletransect over mineralisation (Fig. 6), the anomaly is subtle in soft spinifexand tea tree, but strong in the topsoil Zn (Swas not detected). Soft spinifexshowed a negative Pb anomaly over mineralisation, which was nothighlighted in results for theother elements investigated (Fig. 6).Dogwood(Acacia coriacea subsp. sericophylla) results did not highlight significanttrends along the single transect or in the gridded array. Two-nervedwattleis shallow rooted (plantswere able to bepulledupandexamined,whereasother species had thicker roots which could not bemoved) and derives itschemical signature from surface dispersion of naturalmaterials and/or drillspoil. This species had very high concentrations of S compared to the otherspecies (up to 3% in leaves), which is suspected to be due to the plantsgrowing in the surface soilwhich is rich ingypsum(seencrystallisingat thesurface). Topsoil samples were taken at the same time as a comparison tothe vegetation. However, thiswas after extensive drilling of the area so thefact that the results show a large Au and As anomaly over themineralisation could be either a reflection of the drill spoil being

incorporated into the soil or a true soil signature. This area was also alocal deposition centre where water would pond in an ephemeral swampwhich would complicate the signature.

5. Discussion

A summary of the species important in detecting lithological,mineralisation, hydrological and regolith changes and the pathfinderelements that correspond to these changes are in Table 3. Thegeobotanical indicator species (in this case snappy gum) havedistribution patterns corresponding to a change in the underlyingsubstrate. The limitations of the species as geobotanical indicators arethat these trends are seen in this area but this may not hold true inother areas and there may be underlying structure or factors affectingthe species distributions that are still not known or inferred, whichcould affect their usefulness in other areas.

Previous work at the Coyote site demonstrated that the softspinifex biogeochemistry delineates the mineralisation within an Au,As, Zn and S halo of high concentrations with the more mobile Zn andS being further dispersed from the mineralised zone (Reid et al.,2008). This is an improvement on the soil results that were used todiscover the mineralisation from only an As anomaly with no otherassociated elements (Fig. 3). The snappy gum results show much

Fig. 6. Biogeochemical cross sections across the Titania mineralisation including surface soil chemistry taken at the same time along 603,000 mE.

113N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

higher Zn and S to the south of the transect compared to the north.Since there is no change in bedrock type over this site the change inchemical concentrations is more likely to relate to the weathering ofsulphides (including minor sphalerite) that move to the south, in thedirection of contemporary drainage and the most likely direction forgroundwater flow. This suggests that the mobile elements from themineralisation (Zn and S from minor sphalerite and S from othersulphides) are dispersed down hydrological gradients and are thentaken up from the groundwater by the snappy gum, and in so doingprovides a much wider pathfinder expression of mineralisation thando soils or the other plant species.

The results from the Larranganni prospects show that since there isminimal transported cover, the snappy gum is expressing a miner-alisation signature directly from the mineralised bedrock below. Thechemical results from this show that the plants are able to directlyuptake elements from mineralised rock interface. This is useful toshow asmost biogeochemical studies have been carried out in areas ofsome cover (Dunn, 2007) as this is the more important application of

biogeochemistry for exploration, but it is also useful to know that theelements are able to be liberated from the mineralisation andincorporated into the plant organs.

The Titania biogeochemical results show that soft spinifex, inlandtea tree and the two-nerved wattle all show an As anomalycorresponding to the surface projection of buried mineralisation.The Au results for these species do not spatially correspond due to theirregular and heterogeneous (i.e. “nuggety”) nature of Au in planttissues, however, the highest values for soft spinifex and inland teatree correspond to the mineralisation but the two-nerved wattle ismore dispersed. The dogwood results do not seem to contain links tomineralisation and the variations observed are more likely to reflectlocal cover material changes. The topsoil results show a distinctive Asand Au halo over the mineralisation with a much greater spatialspread than for the plant species. However, this sampling wasperformed after drilling, so it is not possible to determine whetherthis signature is due to a true soil anomaly or from a soil anomalyenhanced by incorporation of drill spoil material. Another feature

Fig. 7. Arsenic spatial plots for the three dominant species compared to topsoil samples at Titania.

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Fig. 8. Gold spatial plots for the three dominant species compared to topsoil samples at Titania. 115N.Reid,S.M

.Hill

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Table 3Summary of biogeochemical results from the Tanami region.

Chemical change Important species Pathfinder elements

Lithological Snappy gum* REEDogwood REE

Mineralisation Soft spinifex As, Au, S, Zn, REE, ±PbSnappy gum As, Au, Zn, REEInland tea tree As, Au, S, Zn

*Refers to geobotanical indicator.

116 N. Reid, S.M. Hill / Journal of Geochemical Exploration 104 (2010) 105–117

influencing the topsoil and two-nerved wattle results was that thearea directly above the mineralisation is a local depositional area, orephemeral swamp. This means that elements are washed into thisarea accumulating as a surface Au anomaly (possibly from drill spoil)that would mask the underlying mineralisation signature. This siteprovided good spatial coverage of the region and presented multi-element expressions over mineralisation within soft spinifex andinland tea tree results. However, the single transect cross section alsohighlighted the As, Au and negative Pb anomaly at 250 m spacing butdid not highlight features related to the surficial or groundwaterelemental vectors. These trends were more distinctive in the griddedspatial plots presented in Reid et al., 2009. As a first-pass study, asingle transect would have highlighted the anomaly, which could thenhave been further constrained by the gridded array tomore accuratelydefine the extent of the mineralisation expression.

The Hyperion prospect results showed that there is a minimumnumber of samples required to produce useful information about asite. Any indication of mineralisation from a nine sample transectcould only be inferred in general terms, because most elements onlyproduce single point anomalies. Commonly, these may be falseanomalies as shown by Stanley (2003), where traverses of low sampledensity had a high probability of generating anomalies by randomchance (12.5%) rather than actual representation of mineralisation.Snappy gum and dogwood results showed trends of increasingconcentrations of As, Ba and REE to the east, which implies a greaterproportion of granitic material in the substrate and thereforecorresponds with the geological interpretation. Turpentine resultsshowed the opposite trend and instead showed higher concentrationsof most elements to the west. Since the area of the transect is the onlyarea with any significant clearing of vegetation and that turpentinehas sticky leaves it is surmised that the elemental signature producedby turpentine is due to aeolian particles adhering to the leaves afterbeing transported in the prevailing winds from east to west. Otherstudies have identified elements that are influenced by aeolian inputs(e.g. Ti, Al, Fe, Cr, Hf and Zr) but there have been few studies linkingcontamination to source and hence corresponding to a specific winddirection (Arne et al., 1999; Hill, 2002). This is useful in showing thatthe aeolian particles have little influence on the other species and isworthy of further study.

Overall, each of the sites in the Tanami Gold Province had verysimilar elemental concentrations within each of the species. Theexceptions are the two-nerved wattle, which had a much greaterconcentration of S within the leaves, possibly related to growingwithin gypsum-rich soils, and the dogwood, which had much greaterconcentrations of REE than the other species. This means that theranges for each species can be used as expected values for otherbiogeochemical studies in this region, and variations from theseranges, as well as spatial footprints, can be used to generateanomalous zones in the Tanami Gold Province. The data ranges forAu, S and Zn are comparable to other species sampled over Audeposits with similar cover thicknesses (max Au 3.5 ppb with 3 m ofcover; La Ronge Gold Belt in Canada; max S 0.24%, max Zn 79 ppm,Western Amazon; Dunn, 2007). The As values are not easily comparedto other sites as some Au deposits have high As and some have lower(Dunn, 2007). The Au and As are also comparable to the Moolart Well

mulga leaf results by Anand et al. (2007) and the vegetation samplingcarried out in north-eastern New South Wales by Cohen et al. (1999).

6. Conclusions

Biogeochemistry is a viable first-pass exploration technique thatcan present multi-element haloes at small-scale and then be furtherrefined with either more detailed and/or other exploration techni-ques. Biogeochemical sampling is a relatively cheap, efficient andenvironmentally passive method (compared to drilling) for the initialstages of mineral exploration programs in areas of transported cover,such as across the Tanami Gold Province. Prospect scale biogeochem-ical sampling programs can detect small-scale substrate changes(including mineralisation signatures). The amount of samplingrequired to provide a useful interpretation of substrate should begreater than that used at Hyperion (9 sites at 100 m spacing) but notgreater than the 250 m spacings at Titania as beyond that there is thelikelihood of missing the ore deposit. The grid sampling at Titaniadelineated the zone of mineralisation as well as giving indications ofsurface and groundwater flow, but this would be more useful as afollow up sampling strategy of a target generated by a single transectorientation survey such as was carried out at Coyote.

Soft spinifex was abundant at each site. At the Coyote and Titaniasites it provided a multi-element halo over the mineralised zone,which highlights its potential for mineral exploration in other regionsas this species is very widespread. Snappy gum provided geobotanicalinformation at the Coyote prospect and mineralisation delineation atthe Hyperion and Larranganni prospects. This species provides usefulgeological information relating to element dispersion on a broad scale.Two-nerved wattle is shallow rooted and did not provide usefulinformation about underlying structure or mineralisation. Turpentinehad sticky leaves and provided information on the aeolian dustparticles thereby limiting its use as an exploration tool in coveredterrain. Potentially the other species examined could be used in otherareas but in this study their usefulness could not be establishedbecause of their restricted distributions.

In summary, soft spinifex was the most useful species inidentifying areas of mineralisation through >5 m transported cover.The leaf samples over mineralisation were anomalous in Au, As, Ce, Sand Zn which provided different scaled haloes at each site with littleinterference from other substrate variations.

Acknowledgements

We would like to thank David Lewis for his contribution to NR'sPhD co-supervision for this project. Special thanks to other membersof the Cooperative Research Centre for Landscape Environments andMineral Exploration (CRC LEME) Tanami Regolith Project team: AnnaPetts, Lisa Worrall, Dirk Kirste, Brad Pillans and John Joseph. Thanksfor assistance with sample preparation by Rodney King, Keryn Welk,Jessica Woods and David Kaczan. We also thank CRC LEME, NewmontAsia Pacific, Tanami Gold NL and Australasia Gold for funding andsupport, with particular thanks to Steve Rogers, Karen Hulme, RobertDart, Aaron Brown, Jessie Davey, Michael Neimanis, Damien Honner,Nigel Radford, Simon Bolster, James Anderson, Simeon Robinson andMichael Green. We are very grateful to Ryan Noble and Kevin Cahillfor advice given during the preparation of this manuscript.

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