The Valhalla U deposit, Queensland, Australia Paul Polito 2 and Kurt Kyser 1* 1 Queen’s Facility for Isotope Research, 2 Anglo American December 2012
Where is Valhalla?
Stratigraphic Column for the Calvert and Leichhardt Superbasins, Western Mt Isa Basin Valhalla hosted here
Valhalla Uranium deposit Cross Section DDH V39 0RL 100RL 93m @ 1950 ppm U Mineralised Incl. 10.2m @ 7190 ppm shear zone 200RL 360.0m 300RL 0 200 m 400RL 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 www.paladinenergy.com.au
Photographs of polished thin sections showing three stages of alteration. Early Early Main Late X-cutting main Alb = albite; Re = reibeckite; Cc = calcite; Mt = magnetite.
Mineral Paragenesis
Al Am (1) Alteration minerals associated Am Cc with early/main stage of U Al Earliest alteration comprises amphibole, albite, calcite, Mt titanomagnetite ± brannerite Al Am Am Main stage involves formation of most of the brannerite, apatite, U-zircon, calcite, reibeckite, albite. Al Am Al Br = branerite Albite Cc Zr = zircon Al = albite Zr Cc = calcite Am Ap = apatite Br Br Am = amphibole (reibeckite) Ap Mt = magnetite Al Br Al Chlorite, pyrite, galena Zr Ap and Cu-Fe sulfides were not part of the original Zr Ap assemblage. Hematite Ap was a trace component.
Valhalla Reibeckite 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 %. V 2 O 3 ranges from 0.13 % to 0.26 %. 6 Outer rims of arfvedsonite 5 4 CaO wt.% 3 V39 234.1m Inner core of 2 arfvedsonite 1 0 4 5 6 7 8 Na 2 O wt.%
Hydrothermal Zircon 25 Zircon veins 20 Substitution of U for 15 Zr in uranian zircon UO 2 10 5 0 32 37 42 47 ZrO 2 Zircon clusters Zircon aggregates Electron microprobe analyses reveal that the brightest zircon areas contain up to 20.4 wt. % UO 2 whereas the dull areas contain down to 2.9 wt. % UO 2
30 (CaO+FeO+UO2+TiO2+MnO+P2O5) Further data analysis shows 25 Substitution of that Ca, Fe, Mn, Ti and P elements for Zr substitute for Zr in the zircon in uranian zircon 20 as well as U. Sum 15 10 5 32 37 42 47 52 ZrO 2 (wt.%) Uranian-zircon also contains up to 1.9 wt. % U-zircon PbS Al 2 O 3 and 0.31 wt. % V 2 O 3 . CuS Some late zircon veinlets are characterised by 1 to 3 micron inclusions of PbS and CuS
Py Dol (2) Alteration Cof Py minerals Ur Gn associated with Ur late stage of U Py Ur Ur Cof A later phase of U mineralisation comprised uraninite-coffinite veins (see left), dolomite, calcite, Cof chlorite, red hematite, pyrite, galena and trace Cu- Gn Cc sulfides Ur Ur Hem Cc Ur = uraninite Ur Chl Cof = coffinite Gn Py Py = pyrite Py Cc = calcite Ur Chl = chlorite Hem = hematite Ur Gn = galena Ur Py Hem Dol = dolomite
Mineral Paragenesis
Lithogeochemical whole rock analysis Other mineralised = Skal, 5.0 Bikini, Pile, Perseverance, Pickwick Sediments 4.5 Mighty Glare, Easter Egg and Cromwell Sediments Batman prospects Other mineralized 4.0 Valhalla 3.5 3.0 TiO2 (%) 2.5 2.0 1.5 1.0 0.5 0.0 0 5 10 15 20 25 Nb (ppm)
Lithogeochemical whole rock analysis 20 Pickwick Sediments Alkali Feldspar Control 18 Cromwell Sediments (m = 1/4) Other mineralized 16 Valhalla Valhalla and most other Mt 14 Isa U deposits are hosted in shales and siltstones 12 10 Al/Nb wackes 8 (shales and arenites and quartzites siltstones) 6 4 2 Quartz control: m = 0 0 0 10 20 30 40 50 60 70 80 (Si+Al)/Nb
Lithogeochemical whole rock analysis 8 Pickwick Sediments K-Feldspar Control (m = 1) Cromwell Sediments 7 Other mineralized Valhalla 6 5 4 K/Nb Muscovite Control (m = 1/3) 3 All mineralised samples have 2 experienced K (and Ba) loss. 1 Albite, reibeckite, chlorite control (m = 0) 0 0 1 2 3 4 5 6 7 8 Al/Nb
Lithogeochemical whole rock analysis 8 Albite Control Pickwick Sediments (m = 1) Cromwell Sediments 7 Other mineralized Valhalla 6 Mineralised samples are dominated by albite. Samples that plot above 5 m = 1 also contain reibeckite. Na/Nb 4 3 2 1 Muscovite, K-feldspar, chlorite (m = 0) 0 0 1 2 3 4 5 6 7 8 Al/Nb
Lithogeochemical whole rock analysis 0.36 Muscovite Unaltered samples Pickwick Sediments 0.34 cluster around Cromwell Sediments 0.32 muscovite whereas Other mineralized 0.30 Valhalla weakly altered 0.28 samples trend 0.26 toward albite. 0.24 0.22 0.20 0.18 K/Al 0.16 Mineralised samples 0.14 cluster around the 0.12 albite node. Valhalla 0.10 samples trend toward 0.08 the reibeckite node. 0.06 Quartzites 0.04 Reibeckite (17.33) 0.02 Chlorite 0.00 Albite 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Na/Al
Lithogeochemical whole rock analysis 14000 3000 Pickwick Sediments Pickwick Sediments Cromwell Sediments Cromwell Sediments 12000 2500 Other mineralized Other mineralized Valhalla Valhalla 10000 2000 Destruction of K-spar and V (ppm) 8000 U ppm 1500 muscovite during formation of 6000 albite and reibeckite coincide 1000 with the addition of U etc. 4000 500 2000 K-spar Musc. 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 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) K/Al (molar) 300 15000 Pickwick Sediments Pickwick Sediments Cromwell Sediments Cromwell Sediments 250 Other mineralized Other mineralized 12000 Valhalla Valhalla 200 9000 Zr (ppm) Y (ppm) Elements added 150 during mineralisation: 6000 100 U, V, Y, Zr, Sr, Fe, Ca, Na, P 3000 50 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 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) K/Al (molar)
What type of fluid is responsible for transporting U to the deposit? δ 18 O δ D Min. Temp 380 o C Reib. 8.9 -103 Calcite 12.3 Reib. 7.1 -109 340 o C Calcite 11.3 Reib. -127 Reib. -147 Formation temperature between 340 o and 380 o C Where did this fluid likely come from?
Comparison between possible source rocks in the Mt Isa Basin (mean of values presented) Red = high Bottletree Mt Guide Sybella Blue = low Quartzite Granite Formation 2.23 (n = 36) 0.87 (n = 36) 2.64 (n = 90) Na 2 O (%) 0.15 (n= 36) 0.08 (n= 36) 0.07 (n= 90) P 2 O 5 (%) 7.0 (n = 28) 3.5 (n = 37) 6.8 (n = 90) U (ppm) 503 (n = 36) 205 (n = 38) 341 (n = 90) Zr (ppm) 12 (n = 13) 78 (n = 38) 19 (n = 76) V (ppm) 57 (n = 36) 31 (n = 38) 85 (n = 90) Y (ppm) 178 (n = 36) 58 (n = 36) 94 (n = 90) Sr (ppm) 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?
207 Pb/ 206 Pb ages from various brannerite and uraninite grains at Valhalla 8 Assemblage of Rodinia 7 Example of ablation pit 6 Frequency 1543 ± 15 Ma 5 1510 ± 15 Ma 4 3 2 1 0 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 Age in Ma
40 Ar/ 39 Ar ages from magnesio-arfvedsonite at Valhalla •Valhalla reibeckite contains between 0.03 and 0.15 wt.% K 2 O The oldest 207 Pb/ 206 Pb 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- Unconformity- metasomatised related Shale, tuff, silty Mafic volcanics, BIF, silty Metasediments, Host rocks metasediments metasediments sandstone Brannerite ± uraninite Main U mineral Brannerite, titaniferous Uraninite uranates ± uraninite Amphibole ± aegerine, Alter n minerals Amphibole, albite, calcite, Phengitic sericite, (Ti)magnetite apatite, calcite, albite, apatite, chlorite, hematite (U)zircon. (Ti)magnetite, (U)zircon. U, K, Mg, Fe ± REE Chemical add n U, Y, V, Sr, Zr, Fe, Ca, Na, U, Y, V, Sr, Zr, Fe, Ca, Na, P ± REE P ± REE (main stage) Chemical loss K, Si K, Si Si T o C formed 340 to 380 o C 300 to 400 o C 150 to 250 o C δ 18 O fluid 7.8 ± 0.7 per mil 4.0 ± 3.7 per mil ? δ D fluid -54 to -98 per mil ? -10 to -50 per mil (evol d marine water) (evolved meteoric/magmatic water) Age (Ma) 1551 to 1533 Ma 1800 to 1500 Ma 1680 to 1640 Ma
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