The Lago Grande Complex, Carajás Province, Brazil: insights into the
remobiliza*on of Pd-Pt through post-magma*c, low-temperature oxidizing fluids 49°50’ W
49°30’ W
49°40’ W
N
5°55’ S
1 2 1 2 3 Maria Emilia S. Della GiusKna , Hazel M. Prichard , Cesar F. Ferreira Filho , Iain McDonald 1 InsKtuto de Geociências, Universidade de Brasília, Brasília, 70719-900, Brazil
2 School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, UK
Au-PGE
SZ C inzen to
6°00’ S
Fe
49°50’ W
49°30’ W
49°40’ W
o ren e S SZ
1 PGE
Fe
2
N
5°55’ S
6°05’ S
Introduc*on
6 km
49°50’ W
3
The Lago Grande Complex (LGC) is a 2.76 Ga layered mafic-ultramafic intrusion located in eno r Au-PGE e SZ S the Carajás Mineral Province, Brazil, and is part of the PGE-enriched Serra Leste MagmaKc SZ C Fe Suit. The LGC is divided into an Ultramafic Zone (UZ), consisKng of interlayered dunite, inzen to harzburgite and orthopyroxenite, and a Mafic Zone (MZ), comprising a sequence of gabbroic Fe rocks. PGE-rich levels are located within the UZ. Three disKnct associaKons are described in this unit, suggesKng that different processes may have operated in the magma causing PGE concentraKon: i) within sulfide-poor chromiKte layers, characterized mainly by IPGE-As-S phases and showing high Pt/Pd and low Pd/Ir raKos (Teixeira et al., 2015); ii) in sulfide-rich levels, with low Pt/Pd and high Pd/Ir raKos (this work), and iii) in sulfide-chromite-poor Diabase dykes intervals, with variable Pt/Pd raKos. Granitic intrusions (ca. 1.88 Ga) Two samples were selected for this study and reveal that at least three episodes of Granitic intrusions (ca. 2.74-2.76 Ga) postmagmaKc alteraKon has affected the LGC and promoted the re-equilibrium of the mineral Serra Leste Magmatic Suite assemblage, including the PGM.
N
Diabase dykes
1
Shear Zone (SZ)
Granitic intrusions (ca. 1.88 Ga)
PGE - Luanga Deposit
Granitic intrusions (ca. 2.74-2.76 Ga)
PGE
5°55’ S
6°00’ S
2763±6 Ma1
Au-PGE - Serra Pelada Deposit
Serra Leste Magmatic Suite
2
Fe - Serra Leste Deposit
Mafic-ultramafic intrusions (1 - Luanga; 2 - Lago Grande; 3 - Vermelho)
3
Itacaiúnas Supergroup
Au-PGE
Rio Novo Group
no e r e SZ S
Xingu Complex
6 km
Au-PGE - Serra Pelada Deposit
Sample / Primitive Mantle
6°05’ S
PGE - Luanga Deposit
PGE
2
Lago Grande Chromitite (Sample123,65) Luanga Chromitite
2722±53 Ma 2553±61 Ma
1000
100
10
1
Fe - Serra Leste Deposit
1
Fe 10000
Shear Zone (SZ)
SZ C inzen to
Fe
6°00’ S
6°05’ S
Gneiss, migmatite, amphibolite
Mafic-ultramafic intrusions (1 - Luanga; 2 - Lago Grande; 3 - Vermelho)
Os
Ir
Ru
Rh
Pt
6 km
Pd
1Machado et al. (1991)
Diabase dykes
Itacaiúnas Supergroup
Methods
49°30’ W
49°40’ W
Shear Zone (SZ)
Granitic intrusions (ca. 1.88 Ga)
PGM assemblage
Rio Novo Group
Xingu Complex
PGE - Luanga Deposit
Granitic intrusions (ca. 2.74-2.76 Ga)
Au-PGE - Serra Pelada Deposit
Gneiss, migmatite, amphibolite
Polished thin secKons were systemaKcally searched for PGM using a ESEM-FEG FEI XL-30 at a magnificaKon of x100. QuanKtaKve analyses of representaKve PGM grains were obtained using an Oxford Instruments INCA EDX analyzer adached to Zeiss Sigma HD SEM. A set of standards from MicroAnalysis Consultants Ltd (St. Ives, Cambridgeshire, UK) was used to calibrate the EDX analyzer. LA-ICPMS analyses were carried out using a New Wave Research UP213 UV lasere coupled to a Thermo X Series ICP-MS and followed the protocol described in Howell and McDonald (2007). All the analyKcal procedures were performed at Cardiff University (UK).
Sulphide assemblage
Sample EM44,00 – well preserved orthopyroxenite Sample EM 44,50 – recrystallized harburgite
Primary Pn and Cp are As- and Te-rich (usually higher than 800ppm and up to 50 ppm, respecKvely) and display low S/Se (~1500-2000), below typical mantle values. The tenors for Pd are unusually high and the Pd concentraKons are not obviously correlated with Ni or Co, as expected, and Pd (and to a lesser extent Os, Ru and Rh) is o`en concentrated in chalcopyrite. Also, sulphides are almost completely devoid of Pt and Ir. Sample EM44,00 Well-preserved orthopyroxenite 50 μm
No Pt-Ir Pd-rich Cp
Serra Leste Magmatic Suite Original, primary PGM were not observed in the studied samples. The Fefirst assemblage - SerraPGM Leste Deposit Mafic-ultramafic intrusions were formed together with the recristallyzed sulphides; grains are mainly hosted at Pn grain (1 - Luanga; 2 - Lago Grande; 3 - Vermelho) boundaries or within hydrous silicates. It consists Itacaiúnas Supergroup mainly of Pd-Ni-As, Pd-As-Sb and Pd-Bi-Te phases, with minor sperrylite. Rio Novo Group Pb-bearing PGM (Pd-Bi-Te-Pb and Pd-Pb phases) were formed by a subsequent sulfur loss that Xingu Complex crystallized chiefly within Mt or on Mt-silicate juncKons and ZnS reacted to form a thin ZnGneiss, migmatite, amphibolite silicate rim on Mt borders.
Sample EM 44,50 Recrystallized harburgite
2% 4%
1%
3%
11%
27%
11% Pd-Ni-As
Pd-Ni-As
Pd-Bi-Te±Pb
Pd-Bi-Te±Pb
21% Pd-As-Sb or Sn PtAs2
64%
Pd-Pb
56%
No PGE (-Pd); no As-Cd; lower Te, Bi
1
2
Concluding remarks A combined PGM – sulphide study revealed a complex history for the LGC, involving at least three disKnct recrystallizaKon stages assisted by fluids with disKnct composiKons.
Contact InsKtuto de Geociências, Universidade de Brasília, Campus Universitário Darcy Ribeiro Caixa Postal: 04465 – CEP 70.910-900 – Brasília, DF E-mail:
[email protected]
15% 2% Pd-Ni-As
22%
Pd-Ni-As
Pd-Bi-Te-Pb
48%
Pd-As-Sb or Sn
Pd-Bi-Te-Pb
Pd-As-Sb
10%
53%
Pd-Pb 30%
PtAs2
Pd-Pb
35
Sph Cp PGM PGM Gn PGM Mt (now Hem) Pn 10 μm Further post-magmaKc alteraKon promoted sulphur-loss, transforming Pn to Mt and Cp into Bo, which is accompanied by a low-T silicate assemblage of anKgorite+talc. The high As-Te and PGE contents drop dramaKcally from the igneous to the secondary assemblage, and all the IPGE disappear from the sulphides. However, Pd and Se remain predy comparable, rendering equivalent S/Se raKos in both Cp and Pn. 1
Areas of PGM Numbers of PGM Numbers of PGM Areas of PGM A final alteraKon episode is responsible for the transformaKon of Mt-to-Hem, probably due to redox reacKons under low-T hydrothermal condiKons. During this event, Pd was parKally lost through the recrystallizaKon of Pd-Ni-As phases into maucherite (Ni11As8), a typical low-T hydrothermal mineral, and Pd-Bi-Te-Pb phases were, in turn, replaced by altaite (PbTe), which forms complex intergrowths with anKgorite. Pd-As-Sb Pd-Pb
20%
35
30
30
25
Pd-Pb
20
PtAs2
25
Pd-Pb
20
Pd-As-Sb
Pd-As-Sb or Sn
15
Pd-Bi-Te±Pb
10
Pd-Ni-As
5
15
Pd-Bi-Te-Pb
10
Pd-Ni-As
5
0
Silicate
Jct Pn
Jct Mt
Pn
0
Silicate
Jct Pn
Jct Mt
Pn
Mt
Acknowledgements This contribuKon is part of M.E.S.D.G. post-doctoral study at Cardiff University. M.E.S.D.G. thanks FINATEC for providing travel grants, and CNPq for the post-doctoral scholarship and for conKnuous financial support for field and laboratory work. Duncan Muir (Cardiff University) is acknowledged for lab assistance during SEM sessions. Valuable discussions with Dominique Tanner are also appreciated.
