Hydrogeochemistry New Developments and Exploration ImplicationsDavid J Gray and Ryan R P Noble CSIRO Exploration and Mining

PlanBackgroundDiscussion on Southern Yilgarn FerrolysisAu-in-carbonateDepletion fronts Looking for Sulpides (barren & mineralised)Indices for SBarren vs economicIndices for NiS and VMSHydrogeochemical mapping

Yilgarn Regolith - Temporal OverviewOlder humid weathering during TertiaryPresumably geographically homogeneous lateritic surfaceArid over-printing in last 3M yearsGeographical differentiation: South Eucalypts / carbonate-rich soil North Acacias / carbonate-poor soil, valley calcretes

Modelled groundwater salinities in the Yilgarn Craton

(Commander, 1989) Key Kilometres

Yilgarn Craton Groundwater pHClear and sharp NS pH differentiationSouthern Yilgarn - surface groundwaters pH 3-4, deep groundwaters Fe-richApproximately 250,000 km2 pervasive acidityWhy? Bottom-up? Top-down?05010015020025Kilometers

Ferrolysis and Supergene Gold DispersionWater tableDissolution of Au,Ag AuCl2- + Fe2+ + 2H2O Au(S) + FeOOH + 2Cl- + 3H+2Au(S) + 4Cl- + 2H+ + O2 2AuCl2- + H2OReprecipitation of Au (Neutral pH / Fe-rich)Fe2+Fe2+O2O2Weathering front(Ferrolysis devised by Brinkman, 1977; Revised from Mann, 1983)Fe2+2Fe2+ + O2 + 5H2O 2Fe(OH)3(S) + 4H+But pH 6.5, Fe, Si, K, Mg, Na~~~~~~~~Alunite, dis-ordered kaoliniteCaCO3

Yilgarn Craton Groundwater pH05010015020025Kilometers

Yilgarn Soils (modified from Northcotte)Close correlation of carbonate soils (green) with acid groundwatersCounter-intuitiveWhat process can drive this?Note Au:Ca correlation in soils

Main Flora (GA)Clear spatial correlation - Eucalypt sp, carbonates & acid groundwatersAre mere plants causing fundamental chemical shifts how we explore?Change previous bottom-up models for top-down?

Correlations + Strong Regional S additionCould this be the cause of groundwater acidity in S of Yilgarn Craton? CaSO4 + CO2 + H2O CaCO3 + SO42- + 2H+ 1m soil with 2% CaCO3 could cause 40m of groundwater at pH 1

Sequential bio-mediated Reactions?Could we have a sequence of reactions that cause micro-alkalinity, but result in acidity?E.g. Reith & Schmidt Mumm CO(NH2)2 + 2H2O HCO3- + 2NH4+ + OH- CaSO4 + HCO3- +OH- CaCO3 + SO42- 2NH4+ + 1O2 N2 + 3H2O + 2H+E.g. Carbonate replacement of Fe oxides CH2O + 2Fe2O3 + 3H2O HCO3- + 4Fe2+ + 7OH- CaSO4 + HCO3- +OH- CaCO3 + SO42- 4Fe2+ +O2 + 10H2O 4Fe(OH)3 + 8H+

Could acidification come from above?Correlation between soil carbonate and acid groundwater Regional NS differencesFerrolysismay be occurring, Fe2+ from soil not weatheringKaolin alunite shallow groundwaters pH 3-4Interaction with lower saprolite model pH rise to 6.5, Fe dissolveModel can explain data buteffect of windblown vs authigenic Lithology botanyPast plant distributionsDoes this effect how we explore?1 m

Carbonate-rich1.61.20.80.4002004006008001000Au (ppb)Depth (m) I extractable (coarse) TotalMt Hope Profiles1.41.21.00.80.60.40.20.003006009001200Au (ppb)Depth (m)Carbonate-poor

Soil GoldCarbonate formation partially to wholly reworks AuIn top 1 m, alkaline conditions precipitate Au, mostly recycled. Below carbonate zone Au is highly mobile depletionPrimarily (past?) flora controlled. Can walk across different zonesAcid conditions beneath carbonate enhances Au solubility may enable tree roots to develop signatureSome elements moving downwards will be captured by pervasive alunite

Supergene depletion of Au in the Yilgarn CratonKilometres

Gold geochemistry - Karari section>0.1 ppmHigh grade>1 ppmAuintervalsFreshrockTransitionMottledzoneLateritic gravelsCalcreteFesaprolite Lower- saprolite andsaprock100 mRed claysClay-saprolite04080120Depth(m)

Use of groundwater for exploration Southern YilgarnFor Au: Use shallow groundwaters (check for Fe). Deep groundwaters will show little [Au] but may show other pathfindersFor NiS, VMS: At this stage, no method for using shallow groundwaters, due to very high background metal contents. High dissolved Fe in deep groundwaters will suppress sulphide oxidationUranium: will be complex, but again high backgrounds in shallow groundwatersParadoxically, very large Au, base metal, U concentrations doesnt mean good exploration technique

NiS & VMS Exploration Northern Groundwaters

S SpeciationHS-H2S(aq)SS2-Geochemists Workbench (thermo.com.v8.r6+.dat) + Warner et al. (1996)[SO4] = 0.01 M[Fe] = 10-20 M[Ni] = 10-25 M[HCO3] = 0.01 M

(T = 25 C, P = 1.013 bars)024681012140.500.51.0pHEh (volts)

Ni Speciation[Fe] = 10-6 M[Ni] = 10-6 M[SO4] = 10-2 M

(T = 25 C, P = 1.013 bars)Trevorite (NiFe2O4)Pentlandite (Fe4.5Ni4.5S8)Violarite (FeNi2S4) NiSO4 (aq)Vaesite (NiS2)Ni2+Geochemists Workbench (thermo.com.v8.r6+.dat) + Warner et al. (1996)024681012140.500.51.0pHEh (volts)

Adding MgSO4 / CO2 groundwater to pyrrhotite/antigorite (barren S)Pyrrhotite pyrite/hematite oxidation can occur with SO- as oxidantPyrrhotiteTalcBruciteMagnetiteMagnesiteAntigoritePyrite01020304000.20.40.60.81Reaction progressMinerals (grams x 100)HematiteC2FeS + 0.4SO42- + 1.6 H2O 0.8Fe(OH)3 + 1.2FeS2 + 0.8OH-

Minerals - Pentlandite / Antigorite (Ore)Pentlandite violarite oxidation can occur with SO- as oxidant010203040TalcPentlanditeViolarite00.20.40.60.81Reaction progressMinerals (grams x 100)HematiteBruciteMagnetiteAntigoriteMagnesitePyrite

Deep AlterationPrimary sulphides oxidised to secondary sulphides by SO- pH INCREASE - strongest for barren sulphidesDissolved Ni release lowSO- depletion

Addition of NO to pyrite/talcNO is the oxidant, SO- is dissolved0100200300400Minerals (grams)00.20.40.60.81Reaction progressPyriteTalcHematiteChalcedony

NO oxidation of Violarite/Talc (Ore)TalcTrevorite (NiFe2O4)ChalcedonyNi2SiO4ViolariteHematite0100200300400Minerals (grams)00.20.40.60.81Reaction progressPolydymite

Ni and Fe - Violarite/TalcNote that Ni and Fe do not necessarily correlate00.20.40.60.81Reaction progressConcentration (g/L)NiFe

pH Shallow OxidationOxidation of pyrite produces serious acidity (AMD)00.20.40.60.81Reaction progress345678pHPyriteViolarite/ pyriteViolariteOxidation of violarite subdued acidification

Shallow OxidationRelease of SO-Strongest pH decrease for barren sulphides will give high dissolved metalsDissolved Ni release high for violarite, but also for sub-economic SCreate indices to minimise barren sulphide signature and maximise NiS

Dissolved NiHighly anomalous concentrations at Harmony Other Sporadic highs may represent minor NiS pods. Knights Regionally anomalous (> 0.03 mg/L)However, critical to distinguish Ni-rich vs Fe-rich

Convert skewed raw data to scaled Index 040801201600.10.20.30.40.50.60.70.80.91Ni Index RangeFrequency0%20%40%60%80%100%FrequencyCumulative %0

Ni IndexNormalised for entire regionNi anomaly obviousFalse positive at Endurance

FeS IndexOptimised for FeSBest signature for apparently barren sulphidesWeaker anomaly for NiS

Mineralisation IndexAdd normalised Ni, Co, W, PtClearer anomaliesEndurance still false positive

Min - FeSSubtracting the FeS index from the Min index tends to accentuate NiS relative to FeS

Hydrogeochemical Map of the NE YilgarnProof of concept for broad scale HydrogeochemistryProspectivity mapping, especially outside recognized beltsCommon database for environmental / health studiesOverlap with Laterite AtlasNon-spatially biased background dataSize of alteration systems

DeliverablesDirect concentrations of target metals U, Au, Ni, Cu, Zn. Known pathfinders but anomaly size small Anionic chalcophiles (As, Sb, Mo, W). Useful pathfinders with larger anomaliesMineral Saturation Indices valuable for UraniumUse of SO42-, NO3-, Cl, Br to show sulphidesMetal indices (e.g. Ni, Co, Pt for NiS; Zn, Cu, Pt, Ag for VMS) larger anomaliesAs, Cr, NO3 etc environmental / health

Acknowledgements

CSIROCRCLEMEAMIRA, Industry SponsorsCharles Butt, Cliff Stanley, Mel LinternQuestions ?

Nickel Hydrogeochemistry - David J. Gray CRCLEME, 2006Nickel Hydrogeochemistry - David J. Gray CRCLEME, 2006Nickel Hydrogeochemistry - David J. Gray CRCLEME, 2006