Invisible gold revealed in supergene and hypogene environments

Invisible gold revealed in supergene andhypogene environmentsMinerals Down UnderRob Hough, Ryan Noble, David Gray

Hidden depositsInsert presentation title

Yilgarn Craton – SalinityModelledgroundwatersalinities in theYilgarn Craton(Commander, 1989)KilometresInsert presentation title

Dissolved Au Concentration – Yilgarn Craton706050Frequency(%) 40NorthernCentralKalgoorlie3020100.005 - 0.010.01 - 0.020.02 - 0.040.04 - 0.10 - 0.00500.1 - 0.20.2 - 0.40.4 - 1> 1Insert presentation titleDissolved Au range (ppb)

Supergene depletion of Au in the Yilgarn CratonKilometresInsert presentation title

Complex regolith materialsInsert presentation title

Laser ablation transect of Au distributionInsert presentation title

Alunite rich veins within slabby ferricrete (transported) Mount GibsonBulk analyses of: Au 0.1 ppm, As 36 ppm, Cu 85 ppm.Au: 2.9 ppmCu: 320 ppmAs: 430 ppmPb: 420 ppmAu: 1.5 ppmCu: 280 ppmAs: 340 ppmPb: 560 ppmFine-grained, Fe-stainedkaolinite/aluniteAu: 1.6 ppm, As: 360 ppm, Cu:290 ppm. Pb: 700 ppm(EMPA/LA-ICPMS)300 μmHough et al., 2005; 2007Insert presentation title

4Dissolved Au vs. Fe (pH < 5.4)AuCl 2- + Fe 2+ + 2H 2 O ⇒ Au (S) + FeOOH + 2Cl - + 3H +Dissolved Au (ppb)321pH 3.4,Eh 600 mVNorthernCentralKalgoorlieEastern215 ppmFe00 10 20 30 40 50 60Insert presentation titleDissolved Fe (ppm)

Golden Virgin Pit, Parker Range (WA)Insert presentation title

Primary gold in quartz, iron oxide (after sulphide).13-15 % Ag in the Au-Ag grains4 mmsulphidefine Au in qz1 mmInsert presentation title

Quartz vein block from 30 m depthIron oxide rich fracture surface100 μm1cmInsert presentation title

SEM: Back-scattered electron imagesInsert presentation title

Thin films – overlap illustrates thickness effectson atomic number contrastInsert presentation title

Electron transparent goldInsert presentation title

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Octahedra precipitated later (drying phenomenon)Insert presentation title

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Faraday 1850 – changed gold sols blue from red‘known phenomena seemed to indicate that a mere variation in thesize of [gold] particles gave rise to a variety of resultant colours’Insert presentation title

Nanoparticulate gold• Colloidal suspension• Affects UV-Vis and colour ofsuspensions• Controlled growth of size andshape of particleTsuji et al., 2005• Stable form of transport: colloidcan remain stable up to 400°C(relevant to hydrothermaltransport)Insert presentation title

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Gold nanoparticles in refractory oresPalenik et al., 2004Insert presentation title

Gold chloride evaporation: Extractable goldInsert presentation title

Hough et al. 2008 GeologyAfter Anand. 2007Insert presentation title

arytes – evaporite ‘rose’Insert presentation title

Using LA ICP MS to study nature of gold in calcreteLintern, 2008LA ICPMS traverse across polishedsection of Au-rich calcrete (arrowed)• 20 samples studied to date• Au is “nuggetty”• Au-rich zones not related to other geochemistrye.g. Ca• Au peaks appear not related to micro fabricfeatures• Results consistent with evapotranspiration modelfor Au accumulation in calcrete120000counts 197 Au80000400000Insert presentation title0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000distance (μm)

Maia-96 at the Australian Synchrotron – XFM lineRecent data from XFM beamline, AS December 2008 trialsRun (#304): Bounty deposit, WARegolith, calcreteMel Lintern, CSIRO 2009Au correlated with BrAuBrAPS: 117 x 203 pixels(0.7 x 1.1 mm 2 )1 s dwell, 6 x 6 µm 2 pixelsTranslucent to high-energyX-rays: Rb K, Sr K, beamDr Mel LinternCSIRO Exploration and MiningBr-Au-Fe RGB compositeXANES: Au+ ionicXFM beamline, 20.1 keVBeam size φ ~ 1 µm 210 x 10 mm 2 , 8000 x 8000 pixels (1.25 µm, 0.6 ms “dwell” per pixel)Sr-Fe-Rb RGB compositeGeoPIXE 4.6 Dynamic AnalysisInsert presentation title

Colloidal nanoparticulate gold and sulphates:Evaporation• Stable colloidal transport of gold, some differences inprecipitate from different ligands but still {111}crystals.• Seeing ‘Invisible’ gold.• Gold transport in areas where saline groundwatersinteract with gold deposits worldwide.• Sulphates an important mineral host for gold in theregolith including in weathered sedimentsInsert presentation title

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