Sm/Nd and platinum-group element geochemistry of a late-Precambrian Alaskan-type complex from the Eastern Desert of Egypt H M Helmy Geology Department, Faculty of Science, Minia University, Egypt hmhelmy@yahoo.com A H Ahmed Central Metallurgical Research and Development Institute, Cairo ahmh2@yahoo.com H Kagami Graduate School of Science and Technology Niigata University S Arai Kanazawa University, Kanazawa, Japan ABSTRACT Abu Hamamid intrusion is a late-Precambrian zoned mafic-ultramafic intrusion located along major fracture zone in the Eastern Desert of Egypt It is made up of a peridotite core, enveloped by wehrlite, hornblende-clinopyroxenite and hornblende gabbro, at the margin The different petrographic units are cumulates formed by fractional crystallization processes The mafic minerals are more magnesian in the core rocks No chromitite or Cu-Ni-sulfide ores or Pt-Fe alloys were found at the present level of erosion All lithologic units are depleted in platinum group elements (< 25 ppb total PGE) The rocks show fractionated chondrite-normalized PGE pattern The Sm-Nd data yield model ages (T DM) of 750 to 796 Ma Geochemical and Sm/Nd isotope data suggest that the intrusion was formed from fairly primitive magma that had experienced sulfide segregation prior to the development of the rocks INTRODUCTION The Eastern Desert of Egypt is covered by a PanAfrican (650-550 Ma) basement complex representing a part of the Arabo-Nubian Shield This part is dissected by deep seated fault zones trending ENE and extend for more than 100 km Many zoned mafic-ultramafic complexes intrude along these fault zones and have many features characteristic of Alaskan-type complexes (Helmy and El Mahallawi, 2003) Unusually, some of these complexes e.g Gabbro Akarem (Helmy and Mogessie, 2001) and Genina Gharbia (Helmy, 2004) host sub-economic Cu-Ni-PGE mineralization Abu Hamamid mafic-ultramafic complex has geological and petrological characteristics typical of Alaskan-type complexes In common with other zoned complexes in the Eastern Desert and worldwide, Abu Hamamid not host sulfide or chromitite mineralization In this contribution we study the Sm/Nd isotope, rare-earth element (REE) and platinum-group element (PGE) geochemistry of this complex to estimate its age and to understand its magmatic history The genetic relationship between the Abu Hamamid intrusion and the surrounding island-arc rocks and other mineralized zoned complexes is discussed on the light of the geochemical and isotopic study GEOLOGICAL SETTING Abu Hamamid Alaskan-type complex is located 100 km to the west of the Red Sea coast at the intersection of two major faults affecting low-grade volcanic rocks (Shadli Metavolcanics: 710 Ma, Stern et al., 1991) This area is a part of the Proterozoic Shield comprising metasedimentary and, metavolcanic rocks, granitoids and ultramafic rocks The AH complex is an elliptical body 1.5 km long and 500 m maximum width (Fig 1) The geologic contacts with the surrounding volcanic rocks are hidden below a thick valley sediments This intrusion comprises a peridotite core enveloped by wehrlite, hornblende clinopyroxenite and hornblende gabbro, at the margin The peridotite core bodies occur as small rounded outcrops aligned in a ENE direction parallel to the direction of elongation of the intrusion Blocks of the hornblende gabbro are commonly observed in the wehrlite The contacts between the rock types are gradational over a short distance 4.1 Figure Geologic map of the Abu Hamamid intrusion MINERALOGY The ultramafic units of AH complex have cumulate textures and the observed crystallization sequence is: olivine (+cotectic spinel)- clinopyroxene (Cpx)hornblende and plagioclase Olivine is present as a major constituent in peridote (60 % modal) and wehrlite (up to 30 %), but less abundant in the hornblende clinopyroxenite (< 10 %) Spinel is more abundant in the peridotite while ilmenite is more common in the hornblende gabbro Augite and hornblende exist in all rock units, hornblende being more abundant than augite in peridotite Detailed microscopic investigations did not locate any platinum-group minerals Mafic minerals from the core of the intrusion are highly magnesian, a consistent decrease in the Mg# of olivine (from 82 to 73), Cpx (from 87 to 73) and hornblende (from 84 to 64) is observed from core to margin rocks Two types of spinel are found; Al-Mgrich and Fe-rich The Al-Mg-rich spinel has a widerange of Al2O3 (16-32 wt.%), Cr2O3 (18 - 31 wt.%) and MgO (4-8 wt.%) A wide range of Fe 3+/Fe2+ (0.3 - 0.7) and Cr# (Cr/Cr+Al, 0.2 - 0.7) ratios is revealed Cr-magnetite contains Cr2O3 within the range 4.7 - 19 wt.%, whereas the Al2O3 and MgO contents vary from 0.3 to 4.4 wt.% and from 0.6 to 1.9 wt.%, respectively The Fe3+/Fe2+ and Cr# ratios in the Cr-magnetite are within the range 1.4 - 1.9 and 0.4 - 0.9, respectively The MnO contents in both types of spinel are similar, normally < 0.8 wt.% Detailed mineralogy of the intrusion will be published elsewhere (Helmy and Farahat, in prep.) GEOCHEMISTRY Major elements Hornblende gabbro samples are clearly distinguished from the ultramafic units by their high Al2O3 (average 23 wt.%) and Na2O (average 0.9 wt %) and low MgO (average 7.7 wt.%) contents Relatively low Cr contents are found in the hornblende gabbro (350 ppm) The wehrlite rock samples show a narrow range of chemical compositions (average 10.0 wt.% MgO and 6.0 wt.% FeO) Al2O3 (18.0 – 19.5 wt.%), CaO (11.0-16.0 wt.%) and total alkali (Na 2O+K2O) (1.01.5 wt.%) contents are higher than those in peridotite samples Low Cr contents are found in wehrlite (500 ppm) The peridotite samples contain 17-24 wt.% MgO and 10.2-12.9 wt% total FeO The relatively high CaO (average 9.2 wt.%) and Al2O3 (average 9.2 wt %) contents are related to the abundance of amphiboles, augite and plagiocalse Relatively low Cr contents are found in peridotite (2250 ppm) 4.2 Rare-earth elements The chondrite-normalized REE patterns of the AH rocks are shown in Figure The AH rocks have depleted LREE patterns (La/Yb)n = 0.5-0.9) The slight LREE depletion relative to HREE indicate that the AH rocks represent residue after different degrees of melt extraction Small positive Eu anomaly is characteristic of the various rock units The Eu-anomaly indicates the role of plagioclase fractionation Figure Chondrite-normalized REE pattern of Abu Hamamid rock units 4.3 Platinum group elements Ten samples were analysed for platinum group elements and base metals The Ir and Os contents in wehrlite and hornblende gabbro are close ppb, only one hornblende gabbro sample (AH1) contain 3ppb Ru The Rh contents are close to or below the detection limit (I ppb) in all rock units Low Pd and Pt contents are recorded in all rock units (less than and ppb, respectively) A narrow range of variation in Pt and Pd contents characterizes Abu Hamamid rocks (3 – and - ppb, respectively) The Pt:Pd ratio in samples in which both elements were detected varies from 3:1 to 1:1, only one hornblende clinopyroexnite sample (AH28) contain lower Pt than Pd (Pt:Pd = 1:2) The sample in which all PGE were detected is a Cu-bearing (84 ppm) and Cr-rich (2300 ppm) peridotite The total IPGE (Os + Ru + Ir) content in this sample is 14 ppb while total PPGE is ppb The IPGE/PPGE ratio in this sample is >1 which is not similar to other lithologic units Chondritenormalized PGE pattern of Abu Hamamid rocks is illustrated in Figure Fractionated PGE pattern is shown by wehrlite, hornblende clinopyroxenite and hornblende gabbro Figure Sm – Nd isochron diagram for Abu Hamamid rocks Figure Chondrite-normalized patterns of different lithologic units, Abu Hamamid Chondrite (C1) value from Naldrett and Duke (1980) NEODYMIUM ISOTOPIC COMPOSITION Neodymium isotopic data for three samples of peridotite, wehrlite and hornblende gabbro are plotted on Figure Epsilon-Nd at 773 Ma ranges from +6.9 to + 7.7 These data are similar to ca 710 Ma island arc rocks hosting the AH intrusion (Shadli Volcanics, Stern et al., 1991) The depleted mantle of Nelson and Depaolo (1985) predicts that the depleted mantle had an Epsilon-Nd of + 7.7 at 770Ma, the data for AH are very close to this value The Epsilon Nd for AH falls in the range for depleted mantle about 770 Ma ago DISCUSSION Abu Hamamid complex show geological and petrological characteristics typical of Alaskan-type complexes as given by Irvine (1974) The only difference, is that this complex not host chromitite mineralization, at least at the present level of erosion Also, this complex is different from other concentrically zoned complexes from the Eastern Desert of Egypt where some of these complexes contain sub-economic Cu-Ni-PGE mineralization (e.g Gabbro Akarem; Helmy and Mogessie, 2001, Genina Gharbia; Helmy 2004) Relative to Gabbro Akarem (Mg# 66 of parent melt) and Genina Gharbia (Mg# 61), Abu Hamamid is made of more evolved magma (Mg# 59, Helmy and Farahat, in prep.) of depleted mantle source Gabbro Akarem and Genina Gharbia are older in age (973 Ma, 945 Ma TDM, respectively, Helmy, un-published data) Although depleted in PGE, AH Hamamid show a chondrite-normalized PGE pattern similar to Alaskan-type complexes (e.g Alto Condoto Complex, Tistl, 1994) and other concentrically zoned complexes from the Eastern Desert It is widely accepted that PGE are commonly hosted in platinum group minerals (PGM) or incorporated in the structure of some sulfides Chromite-rich rocks of zoned ultramafic complexes contain the highest PGE contents (Crocket, 1981) This is supported by our geochemical data where the highest PGE content is recorded in the sample with the highest Cr (2300 ppm) contents The various lithologic units at Abu Hamamid contain low Cr contents which should be a primary magmatic feature Magma generation is probably the stage at which the low PGE content is established Either partial melting or the nature of source region are possible fundamental causes of PGE depletion Low degrees of partial melting (< 10 %) would produce melts depleted in PGE Although the low Cr contents may support low degrees of partial melting as a cause of PGE depletion, the very low sulfide content is problematic Low degrees of partial melting will produce a melt rich in sulfides than that of Abu Hamamid The rocks could have been formed after early sulfide segregation process Sulfide-rich rocks could be met with at lower stratigraphic level of the intrusion The other alternative, the source region was depleted in PGE, may be supported by the Sm/Nd isotope data and other trace- and REE geochemistry A complex scenario of partial melting and pre-melting processes in the mantle is likely REFERENCES Crocket, J.H 1981, Geochemistry of the platinum group elements: CIM Special Volume 23, p 391402 Helmy, H.M 2004 Cu-Ni-PGE mineralization in the Genina Gharbia mafic-ultramafic intrusion, Eastern Desert, Egypt Canadian Mineralogist, 42, 351-370 Helmy, H.M & El-Mahallawi, M.M 2003 Gabbro Akarem mafic-ultramafic complex, Eastern Desert, Egypt: a Late Precambrian analogue of Alaskan-type complexes Mineralogy Petrology, 77, 85-108 Helmy, H.M & Mogessie, A 2001 Gabbro Akarem, Eastern Desert, Egypt: Cu-Ni-PGE mineralization in a concentrically zoned mafic-ultramafic complex Mineralium Deposita ,36, 58-71 Irvine, T.N 1974 Petrology of the Duke Island ultramafic complex, southeastern Alaska Geological Society America Mem, 138, 240p Naldrett, A.J & Duke, J.M 1980 Platinum metals in magmatic sulfide ores Science, 208, 14171424 Nelson, B.K & DePaolo, D.J 1985 Rapid production of continental crust 1.7-1.9 b.y ago: Nd and Sr isotopic evidence from the basement of the North American mid-continent Geological Society of America Bulletin 96, 746-754 Stern, R.J., Kröner A & Rashwan A.A 1991 A late Precambrian (~710 Ma) high volcanicity rift in the Southern Eastern Desert of Egypt Geolgisch Rundschau, 80, 155-170 Tistl, M 1994 Geochemistry of platinum-group elements of the zoned ultramafic Alto Condoto complex, Northwest Colombia Economic Geology, 89, 158-167 ... mineralization in the Genina Gharbia mafic-ultramafic intrusion, Eastern Desert, Egypt Canadian Mineralogist, 42, 351-370 Helmy, H.M & El-Mahallawi, M.M 2003 Gabbro Akarem mafic-ultramafic complex, ... (e.g Gabbro Akarem; Helmy and Mogessie, 2001, Genina Gharbia; Helmy 2004) Relative to Gabbro Akarem (Mg# 66 of parent melt) and Genina Gharbia (Mg# 61), Abu Hamamid is made of more evolved magma... wt% total FeO The relatively high CaO (average 9.2 wt.%) and Al2O3 (average 9.2 wt %) contents are related to the abundance of amphiboles, augite and plagiocalse Relatively low Cr contents are found