PGEs in the Panxi Fe-Ti ore-bearing intrusions what control them?

Transcription

1 PGEs in the Panxi Fe-Ti ore-bearing intrusions what control them? George Ma Gregory Shellnutt Liang Qi

2 Contents Geological Background PGEs in Emeishan basalts implications for the PGEmetallogenesis potentials in the intrusions Reviews of the PGE distributions in the Panxi oxidebearing intrusions Xinjie, Hongge, Baima and Taihe Summary

3 Geological background Pang et al. (2010) ~260 Ma Emeishan large igneous province (ELIP) covers an area of 0.3 million km 2 in SW China and N Vietnam Two types of mineralised intrusions: Oxide-dominated (Fe-Ti-V) Sulfide-dominated (Ni-Cu-PGE) Five oxide-ore bearing layered intrusion in the Panxi (Panzhihua-Xichang) region: Taihe ( 太和 ) Baima ( 白馬 ) Xinjie ( 新街 ) Hongge ( 紅格 ) Panzhihua ( 攀枝花 )

4 gabbroic gabbroic maficultramafic maficultramafic Pang et al. (2010)

5 Taihe gabbroic intrusion Hou et al. (2012)

6 Panxi intrusions revisited Xinjie Hongge Baima Taihe mafic-ultramafic mafic-ultramafic gabbroic gabbroic Fe-Ti-V oxide + Ni-Cu- (PGE) sulfide PGE-rich horizons near the bottom Fe-Ti-V oxide Fe-Ti-V oxide Fe-Ti-V oxide εnd (T) = 5.3 to +2.8 εnd (T) = 2.7 to +1.0 εnd(t) = +1.6 to +4.2 εnd(t) = +2.3 to +3.3 high (Th/Yb) N = 4.7 to 20.7; high δ 18 O (up to +8.0) (Th/Yb) N = 0.8 to 7.5 in MCZ (Th/Yb) N = 0.8 to 5.1 (Th/Yb) N = 1.8 to 13.2 Fo 88 Fo 84 Fo 76 Fo 76 Low range (Cu/Pd) N High range (Cu/Pd) N High range (Cu/Pd) N High range (Cu/Pd) N Low range (Ni/Ir) N High range (Ni/Ir) N High range (Ni/Ir) N High range (Ni/Ir) N No sulfide seg. prior to emplacement Sulfide seg. prior to/during emplacement? Sulfide seg. prior to emplacement No sulfide seg. prior to emplacement?

7 High-Ti and Low-Ti Emeishan flood basalts

8 Zhou et al. (2008 Lithos) High-Ti: Fe-rich, more fertile source + lower degree of partial melting Fe-Ti oxide deposits Baima, Panzhihua Low-Ti: Fe-poor, more refractory source + larger degree of partial melting Ni-Cu-(PGE) sulfide deposits Jinbaoshan, Limahe In this scenario, more sulfides are retained in the source of the high-ti series, thereby resulting in PGE-poor nature of the high-ti series Vice versa for the low-ti series!!! The PGE budgets are different in the high- and low-ti series!!!

9 No distinction in terms of PGEs and S-saturation between the highand low-ti series (PGE database of Li et al., 2012 GCA)

10 PGEs and S- saturation seem to be controlled by crustal contamination

11 More evidence for crustal contamination

12 Panxi intrusions revisited Xinjie Hongge Baima Taihe mafic-ultramafic mafic-ultramafic gabbroic gabbroic Fe-Ti-V oxide + Ni-Cu- (PGE) sulfide PGE-rich horizons near the bottom Fe-Ti-V oxide Fe-Ti-V oxide Fe-Ti-V oxide εnd (T) = 5.3 to +2.8 εnd (T) = 2.7 to +1.0 εnd(t) = +1.6 to +4.2 εnd(t) = +2.3 to +3.3 high (Th/Yb) N = 4.7 to 20.7; high δ 18 O (up to +8.0) (Th/Yb) N = 0.8 to 7.5 in MCZ (Th/Yb) N = 0.8 to 5.1 (Th/Yb) N = 1.8 to 13.2 Fo 88 Fo 84 Fo 76 Fo 76 Low range (Cu/Pd) N High range (Cu/Pd) N High range (Cu/Pd) N High range (Cu/Pd) N Low range (Ni/Ir) N High range (Ni/Ir) N High range (Ni/Ir) N High range (Ni/Ir) N No sulfide seg. prior to emplacement Sulfide seg. prior to/during emplacement? Sulfide seg. prior to emplacement No sulfide seg. prior to emplacement? Crustal contamination, gauged from the εnd proxy, seems to be the key to PGE mineralization

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14 Panxi intrusions revisited Xinjie Hongge Baima Taihe mafic-ultramafic mafic-ultramafic gabbroic gabbroic Fe-Ti-V oxide + Ni-Cu- (PGE) sulfide PGE-rich horizons near the bottom Fe-Ti-V oxide Fe-Ti-V oxide Fe-Ti-V oxide εnd (T) = 5.3 to +2.8 εnd (T) = 2.7 to +1.0 εnd(t) = +1.6 to +4.2 εnd(t) = +2.3 to +3.3 high (Th/Yb) N = 4.7 to 20.7; high δ 18 O (up to +8.0) (Th/Yb) N = 0.8 to 7.5 in MCZ (Th/Yb) N = 0.8 to 5.1 (Th/Yb) N = 1.8 to 13.2 Fo 88 Fo 84 Fo 76 Fo 76 Low range (Cu/Pd) N High range (Cu/Pd) N High range (Cu/Pd) N High range (Cu/Pd) N Low range (Ni/Ir) N High range (Ni/Ir) N High range (Ni/Ir) N High range (Ni/Ir) N No sulfide seg. prior to emplacement Sulfide seg. prior to/during emplacement? Sulfide seg. prior to emplacement No sulfide seg. prior to emplacement? Crustal contamination, gauged from the εnd proxy, seems to be the key to PGE mineralization

15 Generalised model for the formation of Panxi intrusions

16 Conclusions keys to PGE mineralisation PGE-mineralisation requires a combination of such factors as: emplacement of unevolved, S-undersaturated PGE-undepleted magma, immediately followed by effective S-saturation driven by crustal assimilation at the very early stage of fractional crystallisation, and perhaps in a dynamic environment such that the sulfides can effective sequester the PGEs (high R- factor)

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18 More evidence for crustal contamination

19 Ni Ir Ru Rh Pt Pd Cu Ni Ir Ru Rh Pt Pd Cu

20 Baima gabbros Low and almost constant PGEs in the Baima rocks Silicate fractionation with concurrent sulfide segregation would have produced a range of PGE variations correlative with silicate or oxide fractionation proxy (e.g. MgO and Cr)

21 Taihe gabbros Ni Ir Ru Rh Pt Pd Cu Ni Ir Ru Rh Pt Pd Cu Note the different patterns for samples collected from the eastern and western portions of the pluton

22 Taihe gabbros (eastern) Near-unity (Ni/Ir) N and (Cu/Pd) N : magma did not undergo sulfide segregation Presence of interstitial sul.: magma was eventually saturated with S No correlation between Cu and PGEs: PGEs are held in the earlier crystallized silicates, spinel and Fe-Ti oxides High PPGE/IPGE: removal of IPGEs by ol ± chr ± Fe-Ti oxide fractionation Ni Ir Ru Rh Pt Pd Cu

23 Taihe gabbros (western) Ni Ir Ru Rh Pt Pd Cu ppm Cu + wide range of PGEs = sulfide control uneven distribution of sulfide controls the PGE abundances in the rocks

24 PtTe Taihe gabbro (western) silicate corundum chalcopyrite