Late-stage apatite: a potential HREEenriched. minerals in carbonatites

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1 Late-stage apatite: a potential HREEenriched co-product of LREE minerals in carbonatites Sam Broom-Fendley Camborne School of Mines, University of Exeter, UK Frances Wall Gus Gunn Aoife Brady Will Dawes Jens Andersen I,A 2014

2 , Malawi Also see poster GP1-D UK Mauze Nepheline syenite Malawi South Africa Calcite carbonatite Ankerite carbonatite Chilwa Alkaline Province, Malawi Late Jurassic/Early Cretaceous Mkango Resources Ltd. REE-resource, % TREO, % TREO. (Cut-off, 1 %)

3 Drill core results show Y P 2 O 5 correlation Increasing REE with Fe All lithogies show increasing HREE with P 2 O 5 FeO + MnO Y (ppm) Calcite-carbonatite Ankerite carbonatite TREE (ppm) P 2 O 5 (%) Higher REE concentration in the later-stage ferruginouscalcite-carbonatite (as could be expected) Correlation between P 2 O 5 and Y in all lithologies Evidence apatite (Ca 5 (PO 4 ) 3 F) is the control on the HREE concentration

4 LA-ICPMS of apatite confirms M/HREEenrichment Y concentration of fluorapatite from is higher than data from other carbonatites and granites Y (ppm) Granite data data (Ap-3) 100 Carbonatite data Typical carbonatite La (ppm) Data compiled from 24 different published studies

5 Why is apatite M/HREE-enriched? Questions: What is the paragenesis of the apatite at? At which temperature did crystallisation take place? What is the source of the crystallising fluid? Is it meteoric or magmatic? Techniques: Detailed mineralogy, coupled with laser ablation data Fluid inclusion analyses O and C isotope analyses of carbonates and apatite Also see talk by Safaa Al-Ali, next

6 Paragenesis 3 main carbonatite stages Magmatic Hydrothermal Alteration Hydrothermal stage subdivided into Calcite rich apatite Ankerite rich synchysite Apatite occurs in 4 stages Ap 1, 2 magmatic Ap 3,4 hydrothermal How does the apatite vary with the paragenesis?...

7 Earliest Apatite (Ap-1) Medium-grained, calcite carbonatite Isolated occurrences in breccia Laser ablation data, compared with data from other carbonatite-apatite Ap Ap Calcite 500 μm Apatite habit lozenge shaped, eu-subhedral Similar to typical carbonatite apatite Apatite chemistry LREE Rich, around 3 % REO substituting High Sr, low Mn Normal for igneous apatite

8 Earliest Apatite (Ap-2) Closer look at an ovoid grain (BSE) BSE Laser ablation data, compared with data from other carbonatite-apatite 50 μm Apatite habit: Zoned and fractured Inclusions are rare First signs of HREE enrichment at :REE distribution Slight Dy enrichment on rim

9 Magmatic Hydrothermal apatite (Ap1,2 Ap-3) Early ovoid apatite (Ap-1,2) Later anhedral apatite (Ap-3) Transition to non-magmatic textures CL 500 μm

10 Magmatic Hydrothermal apatite (Ap1,2 Ap-3) Early ovoid apatite (Ap-1,2) Later anhedral apatite (Ap-3) BSE CL Y (counts) 500 μm Later apatite is more Y-rich 50 μm

11 Cathodoluminescence images Hydrothermal apatite (Ap-3) Calcite-carbonatite Ankerite-carbonatite Ap Flr Stron & syn Carb Carb Ap Ap & stron Carb 5 mm Most common apatite type at Stringers, anhedral, banded Same apatite habit in all carbonatite types Different REE distributions

12 Hydrothermal apatite Ap-4 Found outside the main carbonatite body Abundant fluorite Very HREE-rich Xenotime overgrowths M/HREE peak Carbonate Fluorite Apatite CL Xenotime overgrowths Typical carbonatite apatite Apatite 10 μm

13 Apatite crystallisation temperature V LVCO 2 LVCO 2 Ap-1 Ap-3 Ap-4 LV, V, No CO 2 Increasing inclusion abundance Decreasing T Decreasing CO 2 (?) Mostly LV inclusions Lower T Many open, hard to analyse Small data set, wide spread of Th Ap-3 Ap Homogenisation temperature

14 Fluorite post apatite LV Fluorite/Chondrite Y anomaly No CO Fluorite La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Fluorite provides a more reliable T constraint on apatite crystallisation Homogenisation temperature

15 Stable isotope evidence for hydrothermal alteration - carbonates Fractionation Ap-4 Ap-1 Ap-3 Ap-3 Primary Igneous Carbonatite field Hydrothermal alteration Carbonate isotope data indicates a hydrothermal influence --- meteoric or deuteric?

16 New isotope data in collaboration with NIGL, UK Apatite: Not influenced by diffusion Records conditions during REE mineralisation Stable isotope evidence for hydrothermal alteration - apatite More meteoric values Ap-1,2 Ap-3? Ap-4 Apatite isotope data suggests a meteoric influence Paragenesis

17 Trends in REE concentration Magmatic apatite Chenga apatite Calcite-carbonatite Ankerite carbonatite? C? Early carbonatite Magmatic: Calcite Zircon Apatite La/Yb (CN) C Late carbonatite La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 180 C Sm/Dy Fluorite, baryte calcite Xenotime formation 10 La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ~300 C? No CO 2 inclusions found δ 18 O ap : 0-1 δ 18 O ap : 2 3 Transitional/hydrothermal: Ovoid and anhedral apatite Synchysite-(Ce) Florencite Ferroan calcites CO 2 -bearing inclusions δ 18 O ap : 1 2

18 Conclusions Questions: What is the paragenesis of the apatite at? At which temperature did crystallisation take place? What is the source of the crystallising fluid? Is it meteoric or magmatic? Answers: Complicated! But dominantly anhedral and hydrothermal. Varied REE distribution Uncertain, but limited FI data suggests low-t, but above 160 o C Isotope data suggest interaction with meteoric water caused crystallisation