The first occurrence of platinum group minerals (PGM) in a chromite deposit in the Eastern Desert, Egypt

Abstract

The platinum-group mineralogy (PGM) of the chromitite from Gebel Lawi, in the southeastern desert has been investigated. The most abundant base metal sulfides (BMS) associated with the Lawi chromite are pentlandite, millerite and heazlewoodite. The major platinum-group minerals identified were as follows: laurite (IrOsRu)S2, osmian iridium (Oslr), hollingworthite (RhAsS), tellurian arsenopalladinite (PdTeSbAs), potarite (PdHg) besides cuprian palladian gold (CuPdAu), a Pd-Sb-Hg and HgTe phases. Laurite and osmian iridium occur preferentially in chromite. Os-Ir commonly forms composite PGM with laurite. Hollingworthite and tellurian arsenopalladinite are included within serpentine and, close to the base-metal sulfides, the cuprian palladian gold shares boundaries with chromite. Potaritc together with the Pd-Sb-Hg and HgTe phases are embedded in serpentine. Palladium is the most abundant PGE in the Gebel Lawi chromite.A paragenetic sequence of PGM formation is described. Textural evidence indicates that Os-, Ir-and Ru-bearing PGM formed early and were followed by Rh-and Pd-bearing PGM. The concentration ofai1 five PGE could be magmatic, but much of the PGE mineralogy except for laurite and osmian iridium in the center of chromite grains, has been modified by subsequent processes. At later stages, the environment became Te-, Sb-, As-and Hg-rich, which finally led to the formation of low-temperature alteration minerals.The chromite deposits of Egypt are commonly hosted in ophiolitic rocks. The ophiolites of the Eastern Desert form part of an extensive tectonic melange which resulted from the complete dismemberment and total disruption of their original stratigraphy (Petters 1991). The melange is characterized by the presence of a significant proportion of serpentinites either as matrix or as variably sized blocks, in addition to other ophiolitic fragments, deep-sea sediments and calcalkaline volcanics. Other components such as granitic rocks, carbonate rocks, quartzites and mudstone attest to the characteristic heterogeneity of the melange which constitute mappable litho-stratigraphic entities. The melange is commonly formed of thrust sheets or slices which were incorporated within allochtonous belts of metasediments (Hassan and Hashad 1990).The Gebel Lawi (or the Lawi mountain, a geographic name) forms a part of the Ghadir area (Fig. 1). The latter is a large ophiolitic melange with proximal and distal facies. The former consists of rolled and fragmented rock-debris of highly variable sizes in a sheared matrix of scaly and schistose mudstones, abundantly serpentinized peridotite blocks, some of which are surrounded by sheaths of schistose talc-carbonate rock produced by squeezing and rolling of the blocks besides other rock debris including volcanic material, greywackes, quartzites, chert, granite and amphibolites. The distal facies is a low-grade pelitic schist with pockets and lenses of highly schistose talc-carbonate rock.A genetic model (Fig. 2) for the Ghadir ophiolitic melange was given by Elbayoumi (1984) inv...

Platinum group minerals in chromitite bodies of the Santa Elena Nappe, Costa Rica: mineralogical characterization by electron microprobe and Raman-spectroscopy

“…Data are available for the Mesozoic ophiolitic chromitites of the Mediterranean Tethys (Augé and Johan, 1988;McElduff and Stumpfl, 1990;Tarkian et al, 1991;Garuti et al, 1999a;Kapsiotis et al, 2006Kapsiotis et al, , 2009Grammatikopolous et al, 2007;Kocks et al, 2007;Uysal et al, 2007Uysal et al, , 2009aUysal et al, , 2009b, for the Paleozoic of the Urals (Melcher et al, 1997;Garuti et al, 1999b;Zaccarini et al, 2008) and Austrian Alps (Thalhammer et al, 1990) and for the Precambrian of North Africa (Elhaddad, 1996;El Ghorfi et al, 2008), Finland (Liipo, 1990, Mexico (VatinPerignon et al, 2000;Zaccarini et al, 2005), Argentina (Proenza et al, 2008), Central America and Caribbean (Gervilla et al, 2005;Proenza et al, 2007;.…”

Geologic and Metallogenic Framework of North Africa

“…A few small chromitite lenses (up to 3 m  5 m) have been reported within talc-carbonate zones of the serpentinized ultramafic rocks (Ahmed 2013). In addition to chromite, subordinate amounts of PGM, sulpharsenides and arsenides, and base-metal sulphides occur as fine inclusions either within or along cracks of chromian spinel, or as large aggregates within the interstitial silicate matrix among chromian spinel grains, as documented by El Haddad (1996) and Ahmed et al (2006). Geochemically, the Egyptian chromitites show a wide compositional range from high-Al to high-Cr spinels (Ahmed et al 2001, and references therein), with total PGE contents ranging from 80 to 320 ppb (Ahmed 2007(Ahmed , 2013.…”

“…10). These deposits have been extensively investigated by Amstutz et al (1984), Khudeir et al (1992), El Haddad (1996), Ahmed (2007Ahmed ( , 2013, Ahmed et al (2001Ahmed et al ( , 2006Ahmed et al ( , 2012a, Abd El-Rahman et al (2009), El-Taher (2010), and Hamdy and Lebda (2011). Numerous irregularly distributed ophiolites and ophiolitic mélange units have been reported from these areas in both countries, with the most fertile occurrences being located mainly south of latitude 26°N (Ahmed 2007).…”

Osmium isotope systematics of the Proterozoic and Phanerozoic ophiolitic chromitites: In situ ion probe analysis of primary Os-rich PGM

“…Numerous platinum-group minerals (PGM) have been recently described as discrete inclusions within chromian spinel of podiform chromitites both in the Oman ophiolite [16,17] and the Proterozoic ophiolite, Eastern Desert, Egypt [18,19,20]. The two ophiolite suites have different ages; Cretaceous age (∼96 Ma [21]) for the Oman ophiolite and late Precambrian age (∼784 Ma [11]) for the Egyptian ophiolite.…”

“…Among various types of PGM inclusions in chromian spinel [22], Os-rich PGM (mainly laurite) and, to a lesser extent Os-Ir alloy, were used in the present study for in-situ Os-isotope analysis of the Eastern Desert chromitites of Egypt. The SED chromitites of Egypt usually contain Os-Ir alloy PGM, whilst the CED ones occasionally have PGM-sulfides, mainly laurite, as inclusions in spinel [18,19,22].…”