Distribution of platinum-group minerals in ophiolitic chromitites Jose´ Marı´a Gonza´lez-Jime´nez*1, Fernando Gervilla1, Joaquı´n A. Proenza2, Thierry Auge´3 and Thomas Kerestedjian4 This paper reviews the distribution of platinum-group minerals in ophiolitic chromitites. Our data and literature data, obtained by in-situ investigation of polished sections and techniques of mechanical separation [hydroseparation (HS), or combining electric pulse disaggregation (EPD) plus HS], are contrasted. Finally, in-situ textural data are used as platform criteria to compare the different proposed models that attempt to explain the origin of the platinum-group mineral assemblages found in ophiolitic chromitites. Keywords: Platinum-group minerals, Ophiolitic chromitites, In situ investigation, Mechanical separation Introduction study, we used data from 23 localities hosted in ophiolitic complexes ranging in age from Proterozoic 19,41,42,64,71,73 Pioneering work from the early 1980’s to Cenozoic. In order to provide an up-to-date picture of showed that ophiolitic chromitites contain significant the origin of the different PGM assemblages in amounts of platinum-group elements (PGE), though ophiolitic chromitites, we also contrast different pro- comparatively lower (hundreds of ppb) than chromitites posed models in the literature, on the basis of their 55–57 hosted in layered ultramafic–mafic complexes. textural location of PGM in the chromitite. Platinum-group elements mainly concentrate as specific minerals of these elements known as platinum-group minerals (PGM). The study of PGM assemblages Data selection and methods together with the composition of chromite provides us We compiled a list of PGM in ophiolitic chromitite with valuable information regarding the physico-chemi- worldwide in which the absolute number of grains in the cal nature of parental melt(s) from which chromitite chromitite is reported. For this we took into considera- crystallised.2,5,27,29,51,80 A detailed knowledge of these tion grains obtained both by in situ investigation on assemblages also provides information regarding the polished sections (in which the textural location of PGM post-magmatic events (e.g. serpentinisation, hydrother- is clearly known) and grains obtained by techniques of mal alteration, metamorphism or weathering) affecting mechanical separation [hydroseparation (HS), or electric chromitites during, or after, the emplacement of the pulse disaggregation (EPD) plus hydroseparation (HS)]. ophiolitic complexes.6,24,30,68,71,74,77 The data base from which we worked included 4678 There now exists an ample body of data on PGM in PGM grains in chromitites from 23 localities belonging ophiolitic chromitite. The purpose of this paper is to to ophiolitic complexes of different age and post- present the outcome of a statistical study on the magmatic evolution (Table 1). From a morphological 8,28,47,48,59 distribution of PGM in ophiolitic chromitites taking point of view, both podiform and banded into account both their abundance and their textural chromitites are included. Whereas podiform chromitites location in chromitite ore bodies. Based on our own occur in mantle tectonites or in the Moho Transition research and information from the literature, we Zone (MTZ), banded chromitites are restricted to the contrast data obtained by in-situ examination of upper levels of the ophiolitic sequence (MTZ and/or polished sections and by techniques of mechanical mafic–ultramafic crustal cumulates) (Table 1). separation [hydroseparation (HS), or combining electric Chromitites from which accurate quantitative data of pulse disaggregation (EPD) plus HS]. For this statistical the distribution of PGM are available (Table 1) are hosted in Precambian ophiolitic complexes (Al’Ays, Saudi Arabia; Pampean Ranges, Argentina; and Bou 1Departamento de Mineralogı´a y Petrologı´a-IACT, Universidad de Azzer, Morocco), in Palaeozoic ophiolitic complexes Granada-CSIC, Avda. Fuentenueva s/n, 18002 Granada, Spain (chromitites from Shetland Islands, Scotland; Kraubath 2 Departament de Cristal?lografia, Mineralogia i Dipo`sits Minerals, Facultat and Hochgro¨ssen, Austria; Great Serpentinite Belt, de Geologia, Universitat de Barcelona, Martı´ i Franque`s, s/n, 08028, Barcelona, Spain Australia; Thetford Mines, Canada; Tehuitzingo, 3BRGM (Bureau de Recherches Ge´ologiques et Minie`res), Mineral Mexico; Ray-Iz, Rusia; and Dobromirtsi, Bulgaria), Resources Division, 3 avenue Claude-Guillemin, B.P. 36009, 45060 Orleans, France Mesozoic ophiolitic complexes (chromitites from 4Geological Institute, Bulgarian Academy of Sciences, 24 Georgi Bonchev Troodos, Cyprus; Ohtrys, Vourinos and Pindos, Str., 1113 Sofia, Bulgaria Greece; Oma´n, Sultanate of Oman; Mugla, Turkey, *Corresponding author, email [email protected] Moa-Baracoa, Mayarı´ and Sagua de Ta´namo, Cuba; ß 2009 Institute of Materials, Minerals and Mining and The AusIMM Published by Maney on behalf of the Institute and The AusIMM Received 15 July 2009; accepted 7 October 2009 DOI 10.1179/174327509X12550990457924 Applied Earth Science (Trans. Inst. Min. Metall. B) 2009 VOL 118 NO 3/4 101 Gonza 102 ´ lez-Jime Applied Earth Science (Trans. Inst. Min. Metall. B) ´ nez et al. Distribution of platinum-group minerals in ophiolitic chromitites Table 1 Characteristics of the studied ophiolite complexes. Keynotes: (*) In situ,(**) in situ plus EPD-HS, Prot: Proterozoic, Pa: Palaeozoic, M: Mesozoic, C: Cenozoic. Hz: Harzburgite, D: Dunite, Px: Pyroxenite, W: Wehrlites, G: Gabbro, Srp: Serpentinite. MTZ (Moho Transition Zone) Locality Country Age Host Rock Type Alteration References Al’Ays(*) Saudi Arabia Pro Mantle tectonites (Hz-D) Pod Serpentinisation 66 Pampean ranges(*) Argentine Prot Mantle tectonites (Hz-D-Px) Pod Hydrothermal alteration/serpentinisation 67 Bou Azzer(*) Morocco Prot Mantle tectonites(Hz-D) Pod Serpentinisation/greenschists 30 Shetland(*) Scotland Prot/ Pa Mantle tectonites (Hz-D) Pod Hydrothermal alteration/serpentinisation (?) 61–65, 74 Cumulates (D-W) Pod Kraubath(**) Austria Prot/Pa Mantle tectonites (Hz-D-Px) Pod Amphibolite/greenschist 46–48, 77 MTZ (Hz-D-Px) Band 2009 Hochgrossen(**) Austria Prot/Pa Mantle tectonites (Hz-D-Px) Pod Serpentinisation/eclogite/amphibolite/greenschist 46, 77 Ray-Iz(*) Russia Pa Mantle tectonites (Hz-D) Pod (?) 27 Tehuitzingo(*) Pa Mantle tectonites (Srp) Eclogite/amphibolite/greenschist 84 VOL Great Serpentinite Belt(*) Australia Pa Mantle tectonites (Hz-D) Pod Greenschists/serpentinisation 83 Cumulates (D) Band 118 Thetford Mines(*) Canada Pa Mantle tectonites (Hz-D) Pod Serpentinisation 29, 20 Cumulates (D) Band Dobromirtsi(*) Bulgaria Pa Mantle tectonites (Hz-D-Px) Pod Serpentinisation/greenschists/amphibolite 32, 76, this study NO Troodos(*) Cyprus M Mantle tectonites (Hz-D) Pod Serpentinisation 50 3/4 Othrys(*) Greece M Mantle tectonites (Hz-Lz-D) Pod Serpentinisation 26 Vourinos(**) Greece M Mantle tectonites (Hz-D) Pod Serpentinisation 4, 5, 24, 38 Pindos(**) Greece M Mantle tectonites (Hz-D) Pod Serpentinisation 36, 75 Oman(*) Oman M Mantle tectonites (Hz-Py) Pod Serpentinisation (?) 2, 5 MTZ (Hz-D) Pod Cumulates Band Mugla(**) Turkey M Mantle tectonites (Hz-D) Pod Serpentinisation 80 Moa-Baracoa(*) Cuba M MTZ (Hz-D) Pod Serpentinisation 29 Mayarı´(*) Cuba M Mantle tectonites (Hz-D) Pod Serpentinisation 29 Sagua de Ta´namo(*) Cuba M Mantle tectonites (Hz-D) Pod Serpentinisation 32, this study Loma Peguera(**) Dominican Republic M Pod Serpentinisation 68, 85 Tie´baghi(*) New Caledonia C Mantle tectonites (Hz-Lz-D) Pod (?) 5, 9 Ouen Island(*) New Caledonia C MTZ (D-W-G) Band Hydrothermal alteration/serpentinisation 32, this study Gonza´ lez-Jime´nez et al. Distribution of platinum-group minerals in ophiolitic chromitites 1 Back-scattered electron image of PGM occurring in the different textural positions in ophiolitic chromitites. A and B from chromitites of the Dobromirtsi Ultramafic Massif (southeastern Bulgaria). C and D from chromitites of the Sagua de Ta´namo (eastern Cuba) and Loma Peguera; Dominican Republic), and (ii) in altered inner zones or edges of chromite Ceonozoic ophiolitic complexes (chromitites from crystals Tie´baghi and Ouen Island, New Caledonia). (iii) in fractures of chromite grains As shown in Table 1, most podiform chromitites (iv) in the interstitial silicate matrix (unaltered or occur in dunite hosted in mantle harzburgite. Likewise, altered) between chromite grains (Fig. 1). there are podiform chromitites in mantle sequences that Os-, Ir-, and Ru-rich PGM are relatively more abundant consist mainly of harzburgites and lherzolites (Othrys (up to 52%) in ophiolitic chromitites than Pt-, Pd-, and and Tie´baghi). In contrast, banded chromitites fre- Rh-rich PGM (48%). This includes both podiform and quently occur in crustal dunites (Great Serpentinite Belt banded (Fig. 2A) types. Based on a number of grains and Thetford Mines), or are hosted by units of obtained by in-situ investigation, Os-, Ir-, and Ru-rich harzburgite, dunite-wehrlite or wehrlite-gabbro of the PGM are predominant (56%) (Fig. 2B). In contrast, mantle-crust transition zones as in the Kraubath Massif based on the number of grains recovered by techniques and in the Ouen Island (Table 1). of mechanical separation, ophiolitic chromitites are dominated by Pt-, Pd-, and Rh-rich PGM (52%) Frequency distribution of platinum- (Fig. 2C). However, the latter distribution must be taken with care due to the strong influence of PGM group minerals population recovery in Kraubath, where Malitch et al.46– Platinum-group