An investigation of the geochemistry of the Middle Group of the eastern Bushveld complex,South Africa Part 1

Abstract

This paper presents whole-rock geochemical and mineral chemical data for chromitite layers of the Middle Group of the eastern Bushveld complex. It reports on compositional variations of chromite from the chromitite layers, and special attention is paid to the platinum-group element (PGE) content and mineralisation within the chromitite layers. The discussion is focused on possible mechanisms by which the association of PGE enrichment within the chromitite layers can be modelled. The MG chromitites from bottom (MG0) to top (MG4C) are characterised by progressive melt evolution, showing decreasing Mg# and Cr/(Cr+Fe) ratio and increasing Cr/(Cr+Al) ratio, TiO₂ and V concentrations. An increase in the Mg# together with a drop in the Cr/Cr+Al) and an increasing Cr/(Cr+Fe) ratio at the base of the MG4A chromitite strongly suggests the addition of hot and primitive magma. High Cu concentrations in the chromitite layers with coincident low S values suggest the application of Naldrett et al.³⁶ Fe-loss model during cooling. With the help of the La/Cu and Cu/S ratios it could be shown that Cu is concentrated neither in base metal sulphides nor in trapped silicate melt and thus the model is not applicable. Laser ablation inductively coupled plasma mass spectrometry studies of single chromite grains also have not revealed Cu to be in solid solution and thus another Cu concentrating phase must be found. The PGE patterns of the MG chromitite layers are very similar to the one of the UG2 suggesting that they derived from the same magma and the same style of mineralisation did apply. It furthermore implies the presence of one parental magma only for the entire Critical Zone. Due to low S contents in the MG chromitite layers, it seems unlikely that the PGE have been concentrated by base metal sulphides only. About a sixth of the platinum-group mineral observed is associated with As, Bi and Te, and therefore PGE concentration by the cluster model is favoured. Enrichment of the high-temperature PGE over the lower-temperature PGE in the lowermost MG chromitite layers and the MG4A is probably due to the presence of high temperatures of the chromitite forming melt, and thus temperature could play a role in the fractionation of the two PGE groups.

Keywords

MIDDLE GROUP CHROMITITE LAYERS PGE MINERALISATION BUSHVELD COMPLEX

References

Anhaeusser

C. R.

: ‘Ultramafic and mafic intrusions of the Kaapvaal craton‘, in ‘Geology of South Africa‘, (ed. Johnson

M. R.

et al), 95–134; 2006, Pretoria, Geological Society of South Africa, Johannesburg/Council for Geoscience.

Barnes

S.-J.

and Maier

W. D.

: ‘Platinum-group element distribu-tions in the Rustenburg Layered Suite of the Bushveld complex, South Africa‘, in ‘The geology, geochemistry, mineralogy and mineral beneficiation of platinum-group elements‘, (ed. Cabri

L. J.

), Special Volume 54, 431–458; 2002, Montreal, Canadian Institute of Mining, Metallurgy and Petroleum.

Barton

P. B.

Jr : Sulfide petrol.. Miner. Soc. Amer. Spec. Pap., 1970, 3, 187–98.

Cawthorn

R. G.

: ‘The platinum and palladium resources of the Bushveld Complex‘, S. Afr. J. ScL, 1999, 95, 481–489.

Cawthorn

R. G.

: ‘Platinum-group element mineralization in the Bushveld Complex—a critical reassessment of geochemical mod-els‘, S. Afr. J. Geol., 1999, 102, 268–81.

Cawthorn

R. G.

and Walraven

: ‘Emplacement and crystal-lization time for the Bushveld Complex‘, J. Petrol., 1998, 39, 1669–1687.

Cawthorn

R. G.

, Merkle

R. K. W.

and Viljoen

M. J.

: ‘Platinum-group element deposits in the Bushveld complex, South Africa‘, in ‘The geology, geochemistry, mineralogy and mineral benefication of platinum-group elements‘, (ed. Cabri

L. J.

), Special Volume 54, 389–429; 2002, Montreal, Canadian Institute of Mining, Metallurgy and Petroleum.

Cawthorn

R. G.

, Lee

C. A.

, Schouwstra

R. P.

and Mellowship

: ‘Relationship between PGE and PGM in the Bushveld complex‘, Can Miner., 2002, 40, 311–328.

Cawthorn

R. G.

, Eales

H. V.

, Walraven

, Uken

and Watkeys

M. K.

: ‘The Bushveld Complex‘, in ‘Geology of South Africa‘, (ed. Johnson

M. R.

et al), 261–281; 2006, Pretoria, Geological Society of South Africa, Johannesburg/Council for Geoscience.

10.

Coertze

F. J.

, Burger

A. J.

, Walraven

, Marlow

A. G.

and Maccaskie

D. R.

: ‘Field relations and age determinations in the Bushveld Complex‘, Trans. Geol. Soc. S. Afr., 1987, 81, (1), 1–11.

11.

Dare

S. A. S.

, Pearce

J. A.

, McDonald

and Styles

M. T.

: ‘Tectonic discrimination of peridotites using fO₂—Cr# and Ga—Ti—FeIII systematic in chrome—spinel‘, Chem. Geol., 2009, 261, 199–216.

12.

Davies

and Tredoux

: ‘The platinum-group element and gold contents of the marginal rocks and sills of the Bushveld complex‘, Econ. Geol., 1985, 80, 838–848.

13.

de Waal

S. A.

, Maier

W. D.

, Armstrong

R. A.

and Gauert

C. D. K.

: ‘Parental magma and emplacement of the strati-form Uitkomst Complex, South Africa‘, Gait Miner., 2001, 2, 557–571.

14.

Eales

H. V.

: ‘Implications of the chromium budget of the western limb of the Bushveld complex‘, S. Afr. J. Geol., 2000, 103, (2), 141–150.

15.

Eales

H. V.

: ‘A first introduction to the geology of the Bushveld Complex and those aspects of South African geology that relate to it‘, Popular Geoscience Series, vol. 2, 84; 2001, Pretoria, The Council for Geoscience.

16.

Eales

H. V.

and Cawthorn

R. G.

: ‘The Bushveld Complex‘, in ‘Layered Intrusions‘, (ed. Cawthorn

R. G.

), 181–230; 1996, Amsterdam, Elsevier.

17.

Eckey

H.-F.

, Kosfeld

and Tarck

: ‘Deskriptive Statistik: Grundlagen — Methoden — Beispiele‘, 5th, revised edn, 283; 2008, Wiesbaden, Gabler.

18.

Gain

S. B.

: ‘The geological setting of the platiniferous UG-2 chromitite layer on the farm Maandagshoek, eastern Bushveld complex‘, Econ. Geol., 1985, 80, 925–943.

19.

Gould

, Rathbone

P. A.

, Kimbell

G. S.

, Burley

A. J.

, Peart

R. J.

, Parker

M. E.

and Pease

S. F.

: ‘The geology of the Molopo Farms Complex, southern Botswana‘, Bull. Geol. Surv. Botswana, 1987, 21, 178.

20.

Harmer

R. E.

and Sharpe

M. R.

: ‘Field relations and strontium isotope systematic of the marginal rocks of the eastern Bushveld complex‘, Research Report 50, Inst. Geol. Res. Univ., Pretoria, South Africa, 1984.

21.

Hatton

C. J.

and Schweitzer

J. K.

: ‘Evidence for synchronous extrusive and intrusive Bushveld magmatism‘, J. S. Afr. Earth Set, 1995, 21, 579–594.

22.

Hill

and Roeder

: ‘The crystallization of spinel from basaltic liquid as a function of oxygen fugacity‘, J. Geol., 1974, 82, 709–729.

23.

Hulbert

L. J.

and von Gruenewaldt

: ‘Textural and composi-tional features of chromite in the lower and Critical Zones of the Bushveld Complex South of Potgietersrus‘, Econ. Geol., 1985, 80, 872–895.

24.

Irvine

T. N.

: ‘Crystallization sequences in the Muskox intrusion and other layered intrusions-II. Origin of chromitite layers and similar deposits of other magmatic ores‘, Geochim. cosmochim. Acta, 1975, 39, 991–1020.

25.

Kinnaird

J. A.

, Kruger

F. J.

, Nex

P. A. M.

and Cawthorn

R. G.

: ‘Chromitite formation—a key to understanding processes of platinum enrichment‘, Trans. Inst. Min. Metall (Sect. B: AppL Earthsci.), 2002, 111, B23—B35.

26.

Kottke-Levin

, Tredoux

and Grabe

P.-J.

: ‘An investigation of the geochemistry of the Middle Group of the eastern Bushveld complex, South Africa. Part II: the silicate cumulates’, in prep.

27.

Kruger

F. J.

: ‘Filling the Bushveld complex magma chamber: lateral expansion, roof and floor interaction, magmatic unconfor-mities, and the formation of giant chromitite, PGE and Ti-V-magnetite deposits‘, Miner. Depos., 2005, 40, 451–472.

28.

Lee

C. A.

and Parry

S. J.

: ‘Platinum-group element geochemistry of the Lower and Middle Group chromitites of the Eastern Bushveld Complex‘, Econ. Geol., 1988, 83, 1127–1139.

29.

Lodders

: ‘Solar system abundances and condensation tempera-tures of the elements‘, Astrophys. J., 2003, 591, 1220–1247.

30.

Maier

W. D.

and Barnes

S.-J.

: ‘Concentrations of rare earth elements in silicate rocks of the Lower, Critical and Main Zones of the Bushveld Complex‘, Chem. Geol., 1998, 150, 85–103.

31.

Merkle

R. K. W.

: ‘Platinum-group minerals in the middle group of chromitite layers at Marikana, western Bushveld Complex: indications for collection mechanism and postmagmatic modifica-tion‘, Can. J. Earth Sci, 1992, 29, 209–221.

32.

Mondal

S. K.

and Mathez

E. A.

: ‘Origin of the UG2 chromitite layer, Bushveld Complex‘, J. Petrol, 2007, 48, (3), 495–510.

33.

B. W. and Campbell

I. H.

: ‘The effects of temperature, oxygen fugacity and melt composition on the behaviour of chromium in basic and ultrabasic melts‘, Geochim. Cosmochim. Acta, 1986, 50, 1871–1887.

34.

Naldrett

A. J.

and Lehmann

: ‘Spinel non-stoichiometry as the explanation for Ni-, Cu- and PGE-enriched sulphides in chromitites‘, in ‘Geo-platinum ‘87‘, (ed.

Prichard

H. M.

.); 1987, London, Elsevier Science publishing Ltd.

35.

Naldrett

A. J.

and von Gruenewaldt

: ‘Association of platinum-group elements with chromite in layered intrusions and ophiolite complexes‘, Econ. Geol., 1989, 84, 180–187.

36.

Naldrett

A. J.

, Wilson

, Kinnaird

and Chunnett

: ‘PGE tenor and metal ratios within and below the Merensky Reef, Bushveld Complex: implications for its genesis‘, J. PetroL, 2009, 50, (4), 625–659.

37.

Pactunk

A. D.

and Cabri

L. J.

: ‘A proton- and electron-microprobe study of gallium, nickel and zinc distribution in chromian spinel‘, Lithos, 1995, 35, 261–282.

38.

Rice

and von Gruenewaldt

: ‘Shear aggregation (convective scavenging) and cascade enrichment of PGEs and chromite‘, Miner. Petrol, 1995, 54, 105–117.

39.

Schürmann

L. W.

, Gräbe

P.-J.

and Steenkamp

C. J.

: ‘Chromium‘, in ‘The mineral resources of South Africa, Handbook 16‘, (ed. Wilson

M. G. C.

and Anhaeusser

C. R.

); 90–105; 1998, Pretoria, Council for Geoscience.

40.

Scoon

R. N.

and Teigler

: ‘Platinum-group element mineraliza-tion in the critical zone of the western Bushveld Complex; I, Sulfide poor-chromitites below the UG2‘, Econ. Geol., 1994, 89, 1094.–1121.

41.

Scoon

R. N.

and de Klerk

W. J.

: ‘The relationship of olivine cumulates and mineralization to cyclic units in part of the upper Critical Zone of the western Bushveld complex‘, Can. Miner., 1987, 25, 51–77.

42.

Schoenberg

, Kruger

F. J.

, Nagler

T. F.

, Meisel

and Kramers

J. D.

: ‘PGE enrichment in chromitite layers and the Merensky Reef of the western Bushveld Complex; a Re—Os and Rb—Sr isotope study‘, Earth Planet. Sci. Lett., 1999, 172, 49–64.

43.

Sharpe

M. R.

: ‘Noble Metals in the Marginal Rocks of the Bushveld Complex‘, Econ. Geol., 1982, 77, 1286–1295.

44.

Spandler

, Mavrogenes

and Arculus

: ‘Origin of chromi-tites in layered intrusions: evidence from chromite-hosted melt inclusions from the Stillwater Complex‘, Geology, 2005, 33, 893–896.

45.

Spandler

, St.H. O'Neill

and Kamenetsy

V. S.

: ‘Survival times of anomalous melt inclusions from element diffusion in olivine and chromite‘, Nature, 2007, 447, 303–306.

46.

Sharpe

M. R.

and Hulbert

L. J.

: ‘Ultramafic sills beneath the eastern Bushveld complex: mobilized suspensions of early Lower Zone cumulates in a parental magma with boninitic affinities‘, Econ. Geol., 1985, 80, 849–871.

47.

Stosch

H.-G.

: ‘Geochemie der Seltenen Erden‘, Lecture script of the Department of Mineralogy and Petrology at the University of Cologne, Germany, Version 3.1, 2008, 209.

48.

Teigler

: ‘Platinum-group element distribution in the lower and middle group chromitites in the western Bushveld Complex‘, Miner. Petrol, 1990, 42, 165–179.

49.

Teigler

and Eales

H. V.

: ‘Correlation between chromite composition and PGE mineralization in the Critical Zone of the western Bushveld Complex‘, Miner. Depos., 1993, 28, 291–302.

50.

Tredoux

, Lindsay

N. M.

, Davies

and McDonald

: ‘The fractionation of platinum-group elements in magmatic systems, with the suggestion of a novel causal mechanism‘, S. Aft J. Geol., 1995, 98, 157–167.

51.

von Gruenewaldt

, Hatton

C. J.

, Merkle

R. K. W.

and Gain

S. B.

: ‘Platinum-group element — chromitite associations in the Bushveld Complex‘, Econ. Geol., 1986, 81, 1067–1079.

52.

Walraven

F. L.

: ‘Notes on the age and genetic relationships of the Makhutso Granite, Bushveld Complex, South Africa‘, Chem. Geol., 1988, 72, 17–28.

53.

Walraven

, Retief

E. A.

, Burger

A. J.

, de Swanepoel

D. J. V.

: ‘Implications of new U-Pb zircon age dating on the Nebo Granite of the Bushveld Complex‘, S. Afr. J. Geol., 1987, 90, 344–351.

54.

Walraven

, Armstrong

R. A.

and Kruger

F. J.

: ‘A chromostrati-graphic framework for the north-central Kaapvaal craton, the Bushveld complex and Vredefort structure‘, Tectonophysics, 1990, 171, 23–48.

55.

Walraven

and Hattingh

: ‘Geochronology of the Nebo Granite, Bushveld Complex‘, S. Afr. J. Geol., 1993, 96, 31–41.

56.

White

J. A.

: ‘The Potgietersrus project: geology and exploration history‘, Proc. 15th CMMI Cong., Johannesburg, South Africa, 1994, S.Afr. Inst. Min. Metall., 173–182.

An investigation of the geochemistry of the Middle Group of the eastern Bushveld complex,South Africa Part 1 – the chromitite layers

Abstract

Keywords

References