research news no.34 -- wygralak etal · the macfarlane peak group consists of mafic volcanics,...

Gold mineral systemsin the Tanami region New insights from NTGS–AGSOresearch

AS Wygralak, TP Mernagh, G Fraser, DL Huston, G Denton, B McInnes, A Crispe & L Vandenberg

Prelminary results of a joint NTGS–AGSO research program todocument gold deposits of the Tanami region in the NorthernTerritory raise important new questions regarding ore genesis.Deposits in the Dead Bullock Soak and The Granites goldfieldsare related to regional D5 or D6 structural events. The ore fluidswere low to moderate salinity (4–10 eq wt % NaCl), moderate tohigh temperature (260–460°C) and gas-rich. Ore depositionoccurred at depths of three to eight kilometres. In contrast, orefluids in the Tanami goldfield (also related to D5 structures)were low temperature (120–220°C) with minor CO2, and oredeposition occurred at depths of less than 1.5 kilometres.Preliminary 40Ar/39Ar studies of biotite tentatively suggest thatmineralisation at Callie occurred at 1720–1700 Ma. Lead isotopedata indicate that the Pb in the deposits was not derived solelyfrom nearby granites. In the Dead Bullock Soak and TheGranites goldfields, O and H isotopic data are consistent witheither a metamorphic or magmatic origin for the ore fluids; inthe Tanami district these data require an additional meteoricfluid input. These results illustrate the complexity of goldmineralisation in the Tanami region, and raise importantquestions about the origin and timing of these deposits.

The Tanami region, located 600 kilometres north-west of AliceSprings, is one of the most important new gold provinces inAustralia. It straddles the Northern Territory–Western Australian

border along the southern margin of the Palaeoproterozoic NorthAustralian Craton. The Tanami region contains more than 50 goldoccurrences including three established goldfields (Dead BullockSoak, The Granites and Tanami), as well as several significant goldprospects (Groundrush, Titania, Crusade, Coyote and Kookaburra).The region has produced 4.5 Moz Au. The remaining resource is 8.4Moz (260 t) Au, but this figure is steadily growing because ofextensive exploration.

Outcrop in the area is sparse (5%), requiring the development andfine-tuning of new geophysical and geochemical techniques to allowthe detection of ‘blind’ gold mineralisation under the regolith cover.Understandably, successful explorers safeguard their methodologyand data to maintain their perceived competitive advantage. This hasresulted in a lack of a regional understanding of geological factorscontrolling mineralisation.

Being aware of this problem and recognising the economicimportance of the Tanami region, the Northern Territory GeologicalSurvey invited the Australian Geological Survey Organisation tocommence a joint National Geoscience Agreement project in 1999.The purpose was to study hydrothermal systems and develop modelsthat facilitate mineral exploration.

Regional geologyRecent stratigraphic1, structural2 and igneous3 studies indicate that theTanami region has a similar geological history to the Pine CreekOrogen, Tennant Creek Province and eastern Halls Creek Orogen ofWestern Australia (Eastern Lamboo Complex).

AGSO Research Newsletter

May 2001, no. 34

Editor: Julie Wissmann

Graphic Designer: Karin Weiss

Cover photo: Matt Peljo, Lynton Jaques & Dean Hoatson

This publication is issued free ofcharge. It is published twice a yearby the Australian Geological SurveyOrganisation. Apart from any usepermitted under the Copyright Act1968, no part of this newsletter is tobe reproduced by any processwithout written permission. Requestsand enquiries can be directed toAGSO’s Chief Executive Officer at theaddress shown below.

Every care is taken to reproducearticles as accurately as possible, butAGSO accepts no responsibility forerrors, omissions or inaccuracies.Readers are advised not to rely solelyon this information when making acommercial decision.

© Commonwealth of Australia 2000

ISSN 1039-091X

Printed in Canberra by National Capital Printing

Australian Geological Survey OrganisationGPO Box 378, Canberra ACT 2601cnr Jerrabomberra Ave & Hindmarsh DrSymonston ACT 2609 AUSTRALIA

Internet: www.agso.gov.au

Chief Executive OfficerDr Neil Williams

Editorial enquiriesJulie WissmannPhone +61 2 6249 9249Fax +61 2 6249 9977E-mail [email protected]

AGSO Research Newsletter isavailable on the web atwww.agso.gov.au/information/publications/resnews/

Readers please note: AGSOResearch Newsletter ceasespublication with issue number 34(May 2001). AGSO is no longerpublishing research papers under itsown imprint. Papers that wouldnormally appear in the ResearchNewsletter will be sent to otherjournals.

2 AGSO Research Newsletter MAY 2001

Research News No.34 18/7/01 12:36 PM Page 2

MAY 2001 AGSO Research Newsletter 3

Figure 1. Geology of the Tanamiregion with the location of goldoccurrences

The oldest Tanami rocks are isolated inliers of Archaean gneiss and schistsuch as the Billabong complex (60 km east of The Granites mine; figure 1),and the Browns Range Metamorphics (southern flank of Browns RangeDome4). SHRIMP based U-Pb zircon geochronology indicates protolith agesaround 2510 Ma and high-grade metamorphism at 1882 Ma.4 The 1882 Maevent is assigned to the Barramundi Orogeny in the Pine Creek Orogen. It isthe maximum age for rift initiation, basin formation and deposition of theoverlying orogenic sequences.

0 30 km

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These orogenic sequences comprise the multiply deformed volcanics andsediments of MacFarlane Peak and Tanami Groups. The MacFarlane PeakGroup consists of mafic volcanics, turbiditic sandstone, siltstone and minorcalc-silicate. The Tanami Group comprises basal quartzite, the Dead BullockFormation—which consists of carbonaceous siltstone, graphitic shale, minoriron-rich BIF and chert—and the turbiditic Killi Killi Formation. Dolerite sillsintrude both MacFarlane Peak and Tanami Groups and predate deformation.The Tanami Group signifies rapid marine transgression, then deep marinestarved sedimentation, followed by rapid prograding wedge sedimentation.

Tanami Group sedimentation is terminated by the onset of the TanamiOrogenic Event (TOE) between 1848 and 1825 Ma.2 Recent geochronologicalevidence (detrital zircons as young as 1815 Ma in the Killi Killi Formation)suggests that this event is younger than the previously accepted age of theBarramundi Orogeny. The TOE involved multiple deformation (D1–3) and synto post D1 greenschist to amphibolite facies metamorphism (M1). The TOE isprobably equivalent to the Eastern Halls Creek Orogeny, which was related tothe collision of the North Australian and Kimberley Cratons at around 1835 Ma.5,6

The Pargee Sandstone, a coarse siliciclastic molasse, unconformably overlies theKilli Killi Formation and represents a sub-basin formed during the TOE.

The period immediately after theTOE is characterised by D4

extensional rifting, sedimentation,felsic volcanism and granite intrusion.Aeromagnetics indicate thatintercalated basalt and turbidite of theMount Charles Formation lie abovedeformed MacFarlane Peak andTanami Groups.7,8 Geochemistryindicates a continental rift setting forMount Charles basalt.9 The age ofMount Charles Formation is poorlyconstrained, but it is intruded byCoomarie Suite aplite dykes dated at1824±12 Ma (AGSO OZCHRON).Extrusion of the Mount WinneckeGroup felsic volcanics during D4

rifting (1825–1815 Ma) is alsocoincident with widespread granite



biotite assemblages at depth give wayto chlorite at shallower levels.However, the ore is uniformlyassociated with decarbonised zoneswithin the originally carbonaceoussiltstone. Callie mineralisation consistslargely (70%) of free gold and minorauriferous arsenopyrite.

The Granites goldfield (figure 3),which has a total resource of 1.6 MozAu, comprises strataboundmineralisation within intensely folded,amphibolite facies iron-rich rocks ofthe Dead Bullock Formation. Thehost unit to the ore is amphibole-garnet-magnetite schist with up to 15per cent sulfides (pyrrhotite, pyrite,arsenopyrite and chalcopyrite).

intrusion. Five granite suites have been identified: Winnecke Suite, FrederickSuite, Inningarra Suite, Coomarie Suite, and The Granites Suite.3 Most granitesare I-type with similar characteristics to those in the Halls Creek Orogen. D5

faults formed after emplacement of the Winnecke, Fredrick, Inningarra andCoomarie granite suites.

Peneplanation of the Tanami basement and deposition of the BirrinduduGroup siliciclastic platform cover sequence occurred sometime after intrusionof the Granites Suite (post 1800 Ma). Regional correlation with the TomkinsonCreek Group and Upper Wauchope subgroup in the Davenport Province andTennant Inlier suggests a 1790–1760 Ma age.

Younger Proterozoic sequences include the Redcliff Pound Group, AntrimPlateau Volcanics, and siliciclastics of the overlying Lucas Formation, PedestalBeds and Larranganni Beds. Redcliff Pound Group quartz arenite looselycorrelates with the Amadeus Basin Heavitree Quartzite.10

D6 faults cut all earlier generations of structures and are mainly definedfrom aeromagnetics.

Deposit geologyThe Dead Bullock Soak (DBS) goldfield is the largest goldfield in the Tanamiregion, with a total resource of 6.3 Moz Au. Stratabound mineralisation ishosted in carbonaceous siltstone, iron-rich rocks and chert of the DeadBullock Formation, which have been metamorphosed to greenschist facies.Although most deposits are hosted by BIF (e.g., Villa, Dead Bullock Ridgeand Triumph Hill), by far the largest deposit, Callie (3.9 Moz), is hosted bycarbon-rich siltstone at a stratigraphically lower position. All deposits occur inan east plunging anticlinorium (figure 2a). Unlike other goldfields in theTanami region, there is not a close spatial association between goldmineralisation and granitoids.

The BIF-hosted deposits are localised in iron-rich parts of the Orac andSchist Hill formations (local mine terms) in the upper part of the localstratigraphy (part of Dead Bullock Formation). The iron-rich units have anamphibolite-chlorite±sulfide±magnetite assemblage. Although the distributionof the iron-rich units is a first-order control on mineralisation, high gradeshoots are localised in zones of parasitic folds or structural thickening.11

Mineralisation in the Callie deposit is localised in F1 fold closures and iscontrolled by the intersection of a corridor of D6 veining and carbonaceousrocks (figure 2b). Hydrothermal alteration varies with depth in the mine;

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Figure 2a. Geology of Dead Bullock Soak (DBS) goldfield

Figure 2b. A cross-section of Callie inDead Bullock Soak



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Although the orebodies are broadlytabular and steeply pitching,lenticular ore shoots are also present.At East Bullakitchie deposit, these oreshoots follow a prominent foldplunge.12 Gold occurs in disseminatedsulfides (arsenopyrite, pyrrhotite,pyrite), and is also associated withquartz-carbonate veining. Unlike theDBS goldfield, there is a close spatialassociation of mineralisation andgranites. Inningarra and The Granitesgranite suites (1815±4 and 1795±5 Marespectively) lie in close proximity tomineralisation. The geometry of thehost sequence is related to D5

deformation, which occurred in theinterval between the Inningarra andThe Granites granite intrusions.

The Tanami goldfield (figure 4),which has a total resource of 2.1 MozAu, consists of quartz veins hosted byweakly deformed basalt and medium-to coarse-grained clastic sediments ofthe Mount Charles Formation. Theseunits exhibit little metamorphism(sub-greenschist facies).

Mineralisation is controlled bythree sets of D5 faults striking350–010º, 020–040º and 060–080º,and dipping east and south-east. Thequartz veins are associated with aninner sericite-quartz-carbonate-pyritealteration zone (within one meter oflodes) and an outer chlorite-carbonate zone (to 20 m).13 Feldsparis locally a gangue mineral in theauriferous veins. Gold occurs in

Bedding with youngingS1 slaty cleavageS2 slaty cleavageS0/S2 intersectionShear zoneUnconformity Pits

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Figure 3. Geology of The Granites goldfield

Figure 4. Geology of Tanami goldfield



sulfides (pyrite, arsenopyrite and pyrrhotite). Veintextures such as colloform banding indicate high-level mineralisation. Like The Granites goldfield,there is a spatial relationship with the Coomarie andFrankenia granites (1815±4 Ma and 1805±6 Marespectively). This association has been used to infera genetic link with the granites, either directlythrough a magmatic-hydrothermal origin of the fluids,or indirectly through contact metamorphic fluids.13

The Groundrush prospect (resource 0.7 Moz Au)is characterised by dolerite-hosted quartz veinmineralisation adjacent to a chilled margin of theGroundrush (Talbot South) granite dated at 1812±5Ma. The majority of the ore is free gold, but someauriferous arsenopyrite is also present. The orebodyis open at depth. Mineralisation and deformationrelationships remain to be determined.

The Titania (Oberon) prospect (resource 0.48 MozAu) is hosted by turbidites in extensively folded,lower greenschist facies Killi Killi Formation. The orebodies are structurally controlled by D5 structures andare localised in anticlinal closures. Like Callie,mineralisation appears to be controlled by the amountof carbon in the host rock. Ore minerals includearsenopyrite, pyrite and free gold.

The Crusade prospect (resource 0.1 Moz Au)contains quartz vein mineralisation associated with afaulted rhyolite-basalt contact within the Nanny GoatVolcanics. The location of auriferous quartz veins iscontrolled by reverse thrusting related to D5

deformation. A significant part of the ore comprisesfree gold.

Little public information is available on theCoyote and Kookaburra prospects that lie on theWestern Australian side of the border, although theyare currently being evaluated.

Age of mineralisationConsiderable uncertainty surrounds the age of goldmineralisation. The spatial relationship betweenmineralisation and granitoids (many of which arenow dated) has led to proposed genetic linksbetween granitoid intrusion and mineralisation; apostulation that remains to be further tested. Incontrast, Callie is not obviously spatially related togranitoid, raising questions about any genetic role ofgranitoid in gold mineralising events.

A pilot 40Ar/39Ar study has been conducted toassess the potential of this method for improving ageconstraints on mineralisation. Samples for 40Ar/39Aranalysis were selected after petrographic examinationthat showed a variety of mineral phases and textures,including multiple vein generations. Interpretation ofgeochronological results is critical to understandingthese geological relationships.

Wall-rock biotite of an ore-stage vein from Callie(sample 11666) yields a discordant spectrum withages of less than 1000 Ma from the first two steps,giving way to ages between 1680 and 1950 Ma forthe final 70 per cent of the gas release (figure 5a).The older part of the age spectrum exhibits an agepeak at around 1850 Ma. Subsequent steps yieldprogressively younger ages leading to a relatively flatportion of the age spectrum at 1700±20 Ma between47 and 67 per cent of the gas release. The spectrumthen rises to a maximum age of 1950 Ma beforeprogressively ‘younging’ to around 1700 Ma. Coarse,unfoliated biotite from the vein margins in the samesample yields a relatively flat age spectrum in whichall but two steps over approximately 80 per cent ofthe gas release, give ages between 1690 and 1730

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Figure 5. 40Ar/39Ar age spectra from gold deposits in theTanami region: a. biotite age spectra from Callie deposit; b. sericite age spectra from the Galifrey (11648) and Titania(11502) deposits; c. sericite age spectrum from the Carbinedeposit; d. sericite age spectra from the Callie deposit

5a.

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Ma. The exceptions are two steps with slightly olderapparent ages of 1752 and 1774 Ma (figure 5a).

Another biotite sample (11669) from ore-stage quartzvein at Callie returned relatively flat spectrum, with 70 percent of the gas release yielding ages in the range 1666 to1711 Ma (figure 5a). A sample of mixed, fine sericite andcoarser muscovite associated with an auriferous vein fromCallie (sample 11480) returned a discordant age spectrum.It rose from initial ages of 900 Ma to a maximum age of1450 Ma at about 20 per cent of the gas release, followedby progressively younger ages spanning a range toaround 1200 Ma (figure 5d). The non-ideal behaviour ofthis age spectrum is suggestive of 39Ar recoil duringirradiation, making interpretation problematic.

Hydrothermal sericite from the Carbine deposit(Tanami goldfield, sample 11481) yields an age spectrumin which 75 per cent of the gas yields ages between 1840and 1865 Ma, before exhibiting considerably younger agesof 1500 Ma in the final few per cent of gas release (figure5c). The convex upwards shape of this spectrum may alsobe indicative of 39Ar recoil—a possibility supported by thetotal gas age of 1810 Ma, which is slightly older than theconventional K/Ar age of 1772±20 Ma.

Sericite sample (11502) from Titania yields an agespectrum that rises gradually from initial ages around 1200Ma to a flat section in which 60 per cent of the gas givesages between 1660 and 1710 Ma (figure 5b).

Hydrothermal sericite from the ore zone at theGalifrey prospect (Tanami goldfield, sample 11648) yieldsan age spectrum which rises initially from 1200 Ma to aflat section, in which 60 per cent of the gas release yieldsages between 1600 and 1660 Ma (figure 5b).

The three biotite-age spectra from the Callie depositare plotted together in figure 5a. The two vein-biotitesamples yield very similar age spectra with most of thegas having apparent ages in the range 1670 to 1720 Ma.Textural evidence clearly suggests that these biotitescrystallised synchronously with the quartz veins in whichthey are hosted. This in turn is interpreted as synchronouswith ore mineralisation. With typical values for argonclosure in biotite around 300°C, and evidence from fluidinclusions that suggests temperatures in the quartz veinsexceeded 300°C (see below), the biotite ages presentedhere should be regarded as minimum ages for the timingof ore mineralisation. However, ambient country-rocktemperature before vein formation and mineralisation, atthe estimated depth of between three and six kilometres,is likely to have been considerably less than the biotite

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closure temperature (assuming near-surface geothermalgradients of less than 50°C/km). If mineralisation wasrelatively localised, the thermal effects could have beenrelatively short-lived—especially as maximum vein-formation temperatures were probably in the range310–330°C. Given these considerations, the argon agespreserved in biotite give a close estimate of the timingof vein crystallisation and associated goldmineralisation.

The age spectra from Titania and Galifrey (twodeposits 50 km apart) are remarkably similar in formand age, but their ages are 50 to 100 Ma younger thanthose derived from Callie biotites. These younger agescould represent cooling and isotopic closure ages ofsericite significantly later than ore mineralisation.

In summary, the limited amount of 40Ar/39Ar datacurrently available is consistent with at least some ofthe Tanami gold mineralisation significantly post-datingthe TOE in the region. In the case of the Callie deposit,the data may suggest a link with the Strangways Event(1720–1730 Ma14) that was responsible for widespreaddeformation and metamorphism in the Arunta Provinceto the south-east of the Tanami region.

Fluid inclusions studiesThe physico-chemical character of the ore-bearing fluidshas been investigated by microthermometric and laserRaman microprobe analysis of fluid inclusions.

At Callie the ore-stage, gas-rich, fluid inclusionshomogenise over a temperature range of 310–330ºCand have salinities of 7–9 wt % NaCl eq. The laserRaman microprobe did not detect any CH4 in theseinclusions, but both CO2-rich and N2-rich inclusions arepresent. Two-phase aqueous inclusions also coexistwith the gas-rich fluid inclusions and homogenise overthe range 220–360ºC. Their salinities range from 8–22wt % NaCl eq. The estimated depth of formation of thegas-rich inclusions is 3.2–5.8 kilometres.

The Granites goldfield contains abundant CO2-bearing, primary fluid inclusions with salinities rangingfrom 4–8 wt % NaCl eq. Although most inclusions areCO2-rich, laser Raman microprobe analysis has shownthat the vapour phase of some inclusions contains CO2

and up to 73 mole % N2, while others contained CO2,small quantities of N2, and up to 50 mole % CH4. Thisindicates significant local variations in fluidcomposition. Some inclusions homogenised to thevapour phase. The majority, however, homogenised toa liquid over the range of 260–300°C. Two-phaseaqueous inclusions are also present in lesser amountsand they coexist with the CO2-bearing inclusions. Theaqueous inclusions have salinities ranging between 18and 20 wt % NaCl eq and homogenise to the liquidphase over the range 240–260°C. The estimated depthof formation for the above inclusions range from3.8–7.5 kilometres.

Rare, primary, CO2-rich fluid inclusions are found ina few deposits in the Tanami goldfield, but two-phaseaqueous inclusions are the dominant type. No gaseshave been detected in the vapour phase of theseinclusions. They can be grouped into threepopulations: (i) relatively saline inclusions with 18–23wt % NaCl eq; (ii) moderately saline inclusions with11–17 wt % NaCl eq; and (iii) low-salinity inclusionswith 0–10 wt % NaCl eq.

Only type iii, low-salinity inclusions are associatedwith mineralisation. Necking down of inclusions hascaused a wide spread in homogenisation temperatures,but there is a distinct mode at 160ºC. Depth estimationsfor the aqueous inclusions range between 0.4 and 0.8kilometres. It is estimated that the CO2-rich inclusions

5d.



formed between 1.3 and 1.5 kilometres. These shallow depths are consistentwith a number of high-level features (comb quartz and chalcedonic textures).

Ore-stage fluid inclusions from the Groundrush deposit have a distinctlydifferent character. The inclusion population is dominated by high-temperature, gas-rich inclusions. These inclusions homogenised over atemperature range of 220–460ºC with a mode at 420ºC. Their salinity rangedbetween 4–10 wt % NaCl eq. Laser Raman microprobe analysis indicates thatthe vapour phase is typically dominated by methane with lesser amounts ofCO2. The estimated depth of formation of these inclusions ranges between 5.5and 8.3 kilometres.

Figure 6 summarises physico-chemical properties of fluid inclusions fromthe above deposits. Groundrush appears to have formed at the greatestdepths and has the most reduced (CH4-rich) fluids. The Granites goldfield andthe Callie deposit formed at shallower depths. The Granites fluids were CO2

rich, but variable in N2 and CH4. The Callie fluids show only small variationsin temperature and salinity and are more oxidised with only CO2 and N2 beingdetected. The Tanami deposits appear to have formed at the shallowest levelsand are dominated by low-salinity aqueous fluids, although some CO2-bearingfluids have also been detected.

Source of metalsδ18O values calculated for ore fluids using mean temperatures obtained fromfluid inclusions and δ18O values of quartz are 5.9±2.0 ‰ at Callie, 4.6±2.0 ‰at The Granites, and range from 1.0 to 5.0 ‰ at the Tanami goldfield. Therelevant fluid inclusion δD values are -85‰ to -58‰ (median -72‰)at Callie,-71‰ at The Granites and -69‰ to -37‰ (median -52‰) at the Tanamigoldfield. The corresponding mean fluid inclusion δ13C values are -6.2‰ atCallie and -8.1‰ at The Granites. There was insufficient CO2 to measure δ13Cvalues at the Tanami goldfield.

The above data indicate involvement, at the ore stage, of metamorphic ormagmatic fluid at Callie and The Granites, and a notably different exchangedmeteoric fluid at the Tanami goldfield.

Further information on the provenance of metal-bearing fluids wasobtained by comparing lead isotope ratios of hydrothermal K-spar andauriferous sulfides with lead isotope ratios of K-spar from granites spatiallyassociated with mineralisation. The results obtained to date (figure 7) indicatethat the initial lead isotopic ratios of sulfides from The Granites goldfield,Tanami goldfield and Groundrush deposit probably were similar to eachother. They are significantly different, however, to the initial ratios of thegranites as determined from K-feldspar separates. The initial ratio of theGroundrush granite is unique compared with the other granites. It may beindicative of derivation by partial melting of Archaean basement. Doleritehosting the Groundrush deposit has lead isotopic values similar to thesulfides. However it is unclear whether this feature is primary, or if theisotopic ratios were re-equilibrated by subsequent hydrothermal activity.

A combination of stable isotope data and lead isotopic ratios—as well assignificantly younger Ar-Ar ages of ore-related minerals, when compared withthe age of granitic intrusions—suggest that these intrusions were not thesource of ore-bearing fluids.

Tanami region gold mineral systemResults of this study indicate that gold deposits of the Tanami region aresuprisingly diverse, with some being basalt and dolerite hosted, some in BIF,and some being sediment hosted. Moreover, fluid inclusion data suggest thatthe four main deposits all formed at different depths with Groundrush beingthe deepest and the Tanami deposits being the shallowest (figure 8, page 14).Hydrogen and oxygen isotope data are consistent with either a magmatic ormetamorphic origin for the ore fluids, but the Pb isotope data show that thePb in these deposits was not derived from a magmatic source.

Despite the evident differences these deposits have similarities, particularlyCallie, Groundrush and The Granites. Most importantly, ore fluids havesalinities less than or equal to 10 wt.% NaCl eq, and are CO2-rich. The Tanamifluids differ in that they are not as CO2-rich, consisting mostly of aqueousinclusions indicative of high-level fluid mixing. The composition of thesefluids bears some similarities to the fluid chemistry of Archaean lode golddeposits15 that have mainly mixed aqueous-carbonic fluids with somemoderate- to low-salinity aqueous inclusions. Fluid salinities in the Tanamideposits, however, lie in the upper range of those reported from Archaeangold deposits. Groundrush and Callie have greater levels of CH4 and N2,respectively, than that commonly observed in Archaean gold deposits. In this

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respect the fluid chemistry of thesedeposits appears to be more closelyrelated to that reported for turbidite-hosted gold deposits.16

Figure 8 presents a preliminarygold system model based on theseand other data for the entire Tanamiregion. The project’s data conflicts interms of the role of granites in themineral system. Although there is astrong spatial association amongdeposits in the Tanami and TheGranites goldfields and atGroundrush, Pb-isotope and 40Ar/39Ardata presented herein are notconsistent with a direct genetic linkbetween granite intrusion andmineralisation.

The Tanami goldfield has the mostsuggestive evidence for a role forgranites. These deposits are locatedwithin a corridor between two granitedomes, and the 40Ar/39Ar data overlapthe age of intrusion of one of thesegranites. These granites may haveacted as either a heat engine orfocused fluid flow within the Tanamicorridor. Other deposits such as Calliedo not have a close spatialassociation with granites, and the40Ar/39Ar age data do not accord withthe time of granite intrusion. Giventhese uncertainties, the role ofgranites in the mineralising event isan open question.

Structural evidence2 shows that D5

or D6 shear zones are importantcontrols on gold mineralisation withinthe region. These structures facilitatedregional fluid flow into depositionsites. Fluid flow was assisted bybrittle failure and brecciation ofcompetent rocks (e.g., Tanami), theformation of dilatant zones (e.g., TheGranites) and the existence ofstructural corridors (e.g., Callie).

Figure 6. Summary of physico-chemical properties of ore-stage fluidsat Callie, The Granites, Groundrushand Tanami corridor deposits



Alteration assemblages vary as aconsequence of host lithology andalso temperature and depth ofmineralisation. The variety of hostlithologies for these deposits suggeststhat different precipitationmechanisms may have operated indifferent areas. Desulfidation of ore-bearing fluids during interaction withFe-rich host rocks may have led toAu precipitation at Groundrush andThe Granites. A more complexmechanism may have beenresponsible for the sediment-hostedCallie deposit. In this case, relativelyoxidising fluids reacted with graphiticsediments, reducing the fluid,decarbonising the rock, andproducing CO2 gas. H2S was alsopartitioned into the gas phase,destabilising the gold bisulphidecomplexes and precipitating gold.Finally, gold was remobilised by laterfluids that concentrated it near thetop of decarbonised zones in aprocess analogous to ‘zone refining’in volcanic-hosted massive sulfidedeposits. Structural thickening duringF1 folding at Callie may haveenhanced this process by preparing athicker zone of reactive rock. Fluid-inclusion data from the Tanamigoldfield suggest that the ore-bearingfluids mixed with a localised brine,and Au precipitation resulted frommixing and cooling processes.

The data suggest that a number ofprocesses formed gold deposits inthe Tanami region. This directly hasresulted in a diversity of ore depositcharacteristics. Consideration of thisdiversity, from a mineral systemsperspective, highlights the likelihoodthat undiscovered gold deposits inthe Tanami region will have a largerange of characteristics and thatmineral deposition can occur in anumber of different host lithologies.Current evidence indicates that D5

and D6 structures should be thefavoured targets for futureexploration.

Future researchResults of this joint NTGS–AGSOresearch program raise morequestions than they answer. Toaddress these questions, thefollowing research programs will beundertaken over the next year:• further direct dating of

mineralisation (including possiblydating zircon in late ore-stage veins);

• further lead isotope work;• a more extensive program by

AGSO on Western Australianprospects (e.g., Coyote) to detectany regional changes in the ageand character of mineralisation;

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Figure 7. 206Pb/204Pb versus 207Pb/204Pb ratios of hydrothermal ore-relatedminerals, granites (Coomarie, Frankenia, Inningarra, Groundrush) andGroundrush dolerite

and• extension of work to the Birrindudu area (quartz-Au veins in the

Winnecke Granophyre).

References1. Hendrickx MA, Slater KR, Crispe AJ, Dean AA, Vandenberg LC & Smith JB. 2000.

Palaeoproterozoic stratigraphy of the Tanami region: Regional correlations andrelation to mineralisation—preliminary results. Darwin: Northern TerritoryGeological Survey, record 2000\13.

2. Vandenberg LC, Hendrickx MA, Crispe AJ, Slater KR & Dean AA. 2001. Structuralgeology of the Tanami region. Darwin: Northern Territory Geological Survey,record 2001/04.

3. Dean AA. 2001. Igneous rocks of the Tanami region. Darwin: Northern TerritoryGeological Survey, record 2001/03.

4. Page RW. 1995. Geochronology of an exposed late Archaean basement terrane inThe Granites–Tanami region. AGSO Research Newsletter; 22:19–20.

5. Tyler IM, Griffin TJ, Page RW & Shaw RD. 1995. Are there terranes within theLamboo Complex of the Halls Creek Orogen? Perth: Geological Survey of WesternAustralia, Annual Review 1993–94; 37–46.

6. Sheppard S, Tyler IM, Griffin TJ & Taylor WR, 1999. Paleoproterozoic subduction-related and passive margin basalts in the Hall Creek Orogen, north-west Australia.Australian Journal of Earth Sciences; 46:679–690.

7. Slater KR. 2000. Tanami SE 52-15, 1:250 000 integrated interpretation of geophysicsand geology. Edn 1. Darwin: Northern Territory Geological Survey; 15.

8. Slater KR. 2000. The Granites SF 52–3, 1:250 000 integrated interpretation ofgeophysics and geology. Edn 1. Darwin: Northern Territory Geological Survey; 16.

9. Tunks AJ. 1996. Structure and origin of gold mineralisation, Tanami mine, NorthernTerritory. Hobart: University of Tasmania, PhD thesis.

10. Blake DH, Hodgson IM & Muhling PC. 1979. Geology of The Granites–Tanamiregion, northern Territory and Western Australia. Canberra: Bureau of MineralResources, bulletin 197.

11. Smith MEH, Lovett DR, Pring PI & Sando BG. 1998. Dead Bullock Soak gold deposits.The Australasian Institute of Mining and Metallurgy, monograph 22; 449–460.

12. Mayer TE. 1990. The Granites goldfield. Melbourne: Australasian Institute of Miningand Metallurgy, monograph 14; 719–724.

13. Tunks A & Marsh S. 1998. Gold deposits of the Tanami Corridor. Melbourne:Australasian Institute of Mining and Metallurgy, mongraph 22; 443–448.

14. Collins WJ & Shaw RD. 1995. Geochronological constraints on orogenic events inthe Arunta Inlier, a review. Precambrian Research; 71:315–346.

15. Ridley JR & Diamond LW. 2000. Fluid chemistry of orogenic lode gold deposits andimplications for genetic models. Reviews in Economic Geology; 13:141–162.

➥ Continued page 14



Ore lode

Ore fluidBasalt

Host BIF

Footwall Schist

Callie Host UnitDolerite

Localisedbrine

Hanging WallSediments

Hanging WallSchist

Sedimentaryunit

Figure 8. Schematic diagram showing fluid-system model for the Tanami region

From page 9➥

16. Mernagh TP. 2001. A fluid inclusionstudy of the Fosterville Mine: Aturbidite-hosted goldfield in theWestern Lachlan Fold Belt, Victoria,Australia. Chemical Geology;173:91–106.

➤ Drs Andrew Wygralak, Andrew Crispe & Leon Vandenberg, NorthernTerritory Geological Survey, phone +61 8 8999 5441 or [email protected]

➤ Drs Terry Mernagh, Geoff Fraser & David Huston, Minerals Division,AGSO, phone +61 2 6249 9640 or e-mail [email protected]

➤ Drs Geoff Denton & Brent McInnes, Exploration and Mining Division,CSIRO, phone +61 2 9490 8879 or e-mail [email protected] "


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