The Valhalla U deposit, Queensland, Australia Paul Polito 2 and Kurt - - PowerPoint PPT Presentation

December 2012 1Queen’s Facility for Isotope Research, 2Anglo American

The Valhalla U deposit, Queensland, Australia

Paul Polito2 and Kurt Kyser1*

Where is Valhalla?

Stratigraphic Column for the Calvert and Leichhardt Superbasins, Western Mt Isa Basin

Valhalla hosted here

200 m 0RL 100RL 200RL 300RL 400RL DDH V39

Valhalla Uranium deposit Cross Section

93m @ 1950 ppm U

• Incl. 10.2m @ 7190 ppm

360.0m

Mineralised shear zone Valhalla Mineral Resource as of September 2010 (at 230ppm U3O8 Cut-off) Mt Grade ppm U3O8 Tonnes Measured Resources 16.02 819 13,116 Indicated Resources 18.64 840 15,662 Total Measured + Indicated 34.66 830 28,778 Inferred Resources 9.1 643 5,824

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Alb = albite; Re = reibeckite; Cc = calcite; Mt = magnetite.

Photographs of polished thin sections showing three stages of alteration.

Early Main Late X-cutting main Early

Mineral Paragenesis

Al Am Albite

(1) Alteration minerals associated with early/main stage of U

Earliest alteration comprises amphibole, albite, calcite, titanomagnetite ± brannerite Br = branerite Zr = zircon Al = albite Cc = calcite Ap = apatite Am = amphibole (reibeckite) Mt = magnetite Br Zr Al Al Al Cc Ap Ap Am Am Br Br Zr Zr Ap Ap Main stage involves formation of most of the brannerite, apatite, U-zircon, calcite, reibeckite, albite. Am Am Am Al Al Cc Mt Chlorite, pyrite, galena and Cu-Fe sulfides were not part of the original

• assemblage. Hematite

was a trace component. Al

1 2 3 4 5 6 4 5 6 7 8

Na2O wt.% CaO wt.%

Outer rims of arfvedsonite Inner core of arfvedsonite

Electron microprobe analyses demonstrate that the amphiboles are magnesio- reibeckite. Mg substitutes for Fe, and Ca substitutes for Na in rims. Fluorine ranges from 0.09 % to 0.36 %. V2O3 ranges from 0.13 % to 0.26 %.

Valhalla Reibeckite

V39 234.1m

5 10 15 20 25 32 37 42 47

ZrO2 UO2

Substitution of U for Zr in uranian zircon

Electron microprobe analyses reveal that the brightest zircon areas contain up to 20.4 wt. % UO2 whereas the dull areas contain down to 2.9 wt. % UO2 Zircon veins Zircon aggregates Zircon clusters

Hydrothermal Zircon

5 10 15 20 25 30 32 37 42 47 52

ZrO2 (wt.%) Sum (CaO+FeO+UO2+TiO2+MnO+P2O5)

Substitution of elements for Zr in uranian zircon

Further data analysis shows that Ca, Fe, Mn, Ti and P substitute for Zr in the zircon as well as U. Uranian-zircon also contains up to 1.9 wt. % Al2O3 and 0.31 wt. % V2O3. Some late zircon veinlets are characterised by 1 to 3 micron inclusions of PbS and CuS U-zircon PbS CuS

A later phase of U mineralisation comprised uraninite-coffinite veins (see left), dolomite, calcite, chlorite, red hematite, pyrite, galena and trace Cu- sulfides Py Py Py Py Py Py Ur Ur Ur Ur Ur Ur Ur Ur Ur Chl Cc Cc Gn Gn Gn Hem Hem Ur Cof Cof Cof Ur = uraninite Cof = coffinite Py = pyrite Cc = calcite Chl = chlorite Hem = hematite Gn = galena Dol = dolomite Dol

(2) Alteration minerals associated with late stage of U

Mineral Paragenesis

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5 10 15 20 25

Nb (ppm) TiO2 (%)

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

Other mineralised = Skal, Bikini, Pile, Perseverance, Mighty Glare, Easter Egg and Batman prospects

Lithogeochemical whole rock analysis

2 4 6 8 10 12 14 16 18 20 10 20 30 40 50 60 70 80

(Si+Al)/Nb Al/Nb

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

Alkali Feldspar Control (m = 1/4)

arenites and quartzites wackes (shales and siltstones) Quartz control: m = 0

Lithogeochemical whole rock analysis

Valhalla and most other Mt Isa U deposits are hosted in shales and siltstones

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

Al/Nb K/Nb

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

Muscovite Control (m = 1/3) K-Feldspar Control (m = 1) Albite, reibeckite, chlorite control (m = 0)

Lithogeochemical whole rock analysis

All mineralised samples have experienced K (and Ba) loss.

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

Al/Nb Na/Nb

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

Albite Control (m = 1) Muscovite, K-feldspar, chlorite (m = 0)

Lithogeochemical whole rock analysis

Mineralised samples are dominated by albite. Samples that plot above m = 1 also contain reibeckite.

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Na/Al K/Al

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

Reibeckite (17.33)

Chlorite Quartzites

Muscovite Albite

Lithogeochemical whole rock analysis

Mineralised samples cluster around the albite node. Valhalla samples trend toward the reibeckite node. Unaltered samples cluster around muscovite whereas weakly altered samples trend toward albite.

2000 4000 6000 8000 10000 12000 14000 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

K/Al (molar) U ppm

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

Musc. K-spar

500 1000 1500 2000 2500 3000 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

K/Al (molar) V (ppm)

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

3000 6000 9000 12000 15000 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

K/Al (molar) Zr (ppm)

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

50 100 150 200 250 300 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

K/Al (molar) Y (ppm)

Pickwick Sediments Cromwell Sediments Other mineralized Valhalla

Lithogeochemical whole rock analysis

Elements added during mineralisation: U, V, Y, Zr, Sr, Fe, Ca, Na, P

Destruction of K-spar and muscovite during formation of albite and reibeckite coincide with the addition of U etc.

Min. δ18O δD Temp

Reib. 8.9

• 103

380oC Calcite 12.3 Reib. 7.1

• 109

340oC Calcite 11.3 Reib.

• 127

Reib.

• 147

What type of fluid is responsible for transporting U to the deposit?

Where did this fluid likely come from?

Formation temperature between 340o and 380oC

Comparison between possible source rocks in the Mt Isa Basin

(mean of values presented)

Red = high Blue = low

Bottletree Formation Mt Guide Quartzite Sybella Granite Na2O (%)

2.23 (n = 36) 0.87 (n = 36) 2.64 (n = 90)

P2O5 (%)

0.15 (n= 36) 0.08 (n= 36) 0.07 (n= 90)

U (ppm)

7.0 (n = 28) 3.5 (n = 37) 6.8 (n = 90)

Zr (ppm)

503 (n = 36) 205 (n = 38) 341 (n = 90)

V (ppm)

12 (n = 13) 78 (n = 38) 19 (n = 76)

Y (ppm)

57 (n = 36) 31 (n = 38) 85 (n = 90)

Sr (ppm)

178 (n = 36) 58 (n = 36) 94 (n = 90)

Questions: Is the Mt Guide low in key elements because it was a source and they have been removed? Which lithology transmits fluid better; granite or conglomerate?

207Pb/206Pb ages from various brannerite and

uraninite grains at Valhalla

1 2 3 4 5 6 7 8 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

Age in Ma Frequency

1543 ± 15 Ma 1510 ± 15 Ma Assemblage of Rodinia

Example of ablation pit

40Ar/39Ar ages from magnesio-arfvedsonite at Valhalla

• Valhalla reibeckite contains between 0.03 and 0.15 wt.% K2O

The oldest 207Pb/206Pb age and the Ar ages above place the timing of the Valhalla mineralizing event between 1551 and 1533 Ma, coincident with peak Isan Orogeny. Younger ages are due to fluid overprints caused by regional tectonic events.

Valhalla versus other styles of U mineralisation

Valhalla Russian Na- metasomatised Unconformity- related Host rocks

Shale, tuff, silty metasediments Mafic volcanics, BIF, silty metasediments Metasediments, sandstone

Main U mineral

Brannerite ± uraninite Brannerite, titaniferous uranates ± uraninite Uraninite

Altern minerals

Amphibole, albite, calcite, (Ti)magnetite apatite, (U)zircon. Amphibole ± aegerine, calcite, albite, apatite, (Ti)magnetite, (U)zircon. Phengitic sericite, chlorite, hematite

Chemical addn

(main stage)

U, Y, V, Sr, Zr, Fe, Ca, Na, P ± REE U, Y, V, Sr, Zr, Fe, Ca, Na, P ± REE U, K, Mg, Fe ± REE

Chemical loss

K, Si K, Si Si

ToC formed

340 to 380oC 300 to 400oC 150 to 250oC

δ18Ofluid

7.8 ± 0.7 per mil ? 4.0 ± 3.7 per mil

δDfluid

• 54 to -98 per mil

(evolved meteoric/magmatic water) ?

• 10 to -50 per mil

(evold marine water)

Age (Ma)

1551 to 1533 Ma 1800 to 1500 Ma 1680 to 1640 Ma

Summary & Conclusions

• Valhalla is characterised by an early assemblage of albite – amphibole –

brannerite – apatite - (titano)magnetite - (uranian)zircon – calcite ± dolomite and later hematite-uraninite-dolomite-calcite-chlorite-coffinite- quartz-galena-pyrite±Cu sulfides.

• Valhalla hosts were enriched in Na, Ca, Fe, U, Zr, Sr, P, Y, Li, Cu, Zn, Nb

and V and depleted in Si, Ba and K similar to metasomatized U deposits in volcaniclastic rocks, and in particular, those from the Ukranian Shield.

• 207Pb-206Pb and 40Ar/39Ar ages indicate that mineralization occurred

between 1555 and 1533 Ma, coincident with the age of peak metamorphism in the Mt Isa area

• Calcite-arfvedsonite O-isotope pairs indicate formation between 340 and

380oC (greenschist facies temperatures)

• O-isotope values are typical of fluids derived from crustal sources during
• metamorphism. δDfluid values are identical to those obtained from the

basal proximal fluvial sediments.