Platinum-Group Minerals in Podiform Chromitites of the Oman Ophiolite

Abstract

Chromitite lenses rich in the platinum-group elements (PGE) have recently been discovered in the deeper part of the mantle section of the northern part of the Oman ophiolite. In terms of stratigraphic level, there are two types of chromitite: Moho transition zone (MTZ) and mantle types. The mantle type can be further classified into two subtypes: PGE-poor and PGE-rich. Platinum-group minerals (PGM) have been mainly observed in the PGE-rich mantle chromitite and, to a lesser extent, in the PGEpoor mantle chromitite, although a few PGM grains have been detected from the MTZ chromitites. The PGM comprise four main categories in the following order of abundance: (1) sulfides [Os-rich laurite, erlichmanite and unnamed (Ir,Rh)(Ni,Cu,Fe) 2 S 3 ], (2) sulfarsenide and arsenide (irarsite-hollingworthite series, palladoarsenide), (3) alloys and native metals [Os-Ir alloy, (Cu-Pt-FeIr-Rh) alloy, Pt-Fe alloy, (Ni-Ru-Os-Ir) alloy, Ir-Pt alloy, Ir-Rh alloy and native Pd metal], and (4) PGE oxide (Ru-Os-Ir oxide). Base-metal sulfides (BMS) and base-metal alloys (BMA) also are found, as inclusions in chromian spinel and interstitial matrix, mainly in the MTZ and PGE-poor mantle chromitites. The common abundance of PGE sulfides, i.e., Os-rich laurite, in the Oman chromitites is ascribed to the sulfide-undersaturated nature of the magma involved in chromitite formation. However, the sulfur fugacity was sufficiently high to produce the Os-rich laurite and, to a lesser extent, erlichmanite and Ru-poor Os-Ir alloy. The sulfur fugacity was higher in the formation of the PGE-poor mantle chromitite relative to the PGE-rich chromitite, as indicated by the presence of the unnamed Ir-rich sulfides and, to a lesser extent, BMS. The presence of PGE oxide commonly associated with fractured chromian spinel indicates that the PGE distribution in rocks can easily be modified at an early stage of serpentinization.Keywords: laurite, chromitite, ophiolite, platinum-group elements, platinum-group minerals, sulfur fugacity, northern Oman. SOMMAIRENous avons récemment découvert des lentilles de chromitite enrichies en éléments du groupe du platine (EGP) dans la partie la plus profonde de la séquence mantellique de la partie nord de l'ophiolite d'Oman. Du point de vue niveau stratigraphique, nous distinguons deux sortes de chromitites, celles qui sont associées à la zone de transition du Moho (MTZ), et celles provenant du manteau sousjacent. Ces dernières comprennent à leur tour deux types, l'un pauvre en EGP et l'autre enrichi en EGP. Les minéraux du groupe du platine se trouvent surtout dans les chromitites mantelliques enrichies en EGP, quoique qu'on en trouve aussi, mais à un degré moindre, dans les chromitites mantelliques à faible teneur en EGP et dans les chromitites de type MTZ. Nous documentons quatre groupes de ces minéraux, présentés ici dans un ordre selon leur abondance: (1) (4) oxyde des EGP (oxyde Ru-Os-Ir). Nous avons aussi trouvé des sulfures et des alliages de métaux de base, en inclusion dans le spinelle chromifère et dans l...

Platinum-group elements distribution and spinel composition in podiform chromitites and associated rocks from the upper mantle section of the Neoproterozoic Bou Azzer ophiolite, Anti-Atlas, Morocco

“…The PGEs, especially the IPGE, tend to increase with an increase in both the volume ratio of chromian spinels and their Cr content (e.g. Crocket, 1979;Page et al, 1982a,b;Economou-Eliopoulos, 1986;Leblanc, 1991;Arai et al, 1999;Proenza et al, 1999;Ahmed and Arai, 2003). With attainment of sulfur saturation under magmatic conditions, the PGE, especially the PPGE, will be collected into the immiscible sulfide droplets that eventually may be concentrated as sulfide deposits at a crustal level.…”

Compositional variations and tectonic settings of podiform chromitites and associated ultramafic rocks of the Neoproterozoic ophiolite at Wadi Al Hwanet, northwestern Saudi Arabia

“…They are important as geochemical monitors of deep-seated mantle processes that provide us with constraints on the physico-chemical conditions and evolution of mantle lithosphere (Lago et al, 1982;Leblanc and Ceuleneer, 1992;Liipo et al, 1995;Ahmed and Arai, 2002;Ahmed et al, 2001Ahmed et al, , 2005Ahmed et al, , 2009. It is now widely accepted that ophiolitic chromitites are dominated by Ir-subgroup (IPGE = Os, Ir, Ru) and depleted in Pd-subgroup (PPGE = Rh, Pt and Pd) of PGE (Page et al, 1982a,b;Page and Talkington, 1984;Talkington et al, 1984;Augé, 1986;Prichard et al, 1996;Zhou et al, 1998;Ahmed and Arai, 2003). However, Pt-and Pd-rich chromitites have been recently found associated with sulfide-bearing pods in some ophiolite complexes (Corrivaux and LaFlamme, 1990;Prichard and Lord, 1993;Pedersen et al, 1993;Economou-Ellopoulos, 1996;Augé et al, 1998;Prichard et al, 2008a,b).…”

Investigation of Platinum-Group Minerals (PGM) from Othrys Chromitites (Greece) Using Superpanning Concentrates

“…Pt and Pd minerals are commonly located at the Moho transition zone, either at the uppermost mantle or in the lower cumulates [27], as melts derived from high degrees of partial melting of a fertile mantle ascend, differentiate and impregnate the uppermost mantle region [5,40]. Therefore, it is concluded that the east Othrys ophiolite was located close to the Moho transition zone, at a higher level relative to the west Othrys.…”

“…Chromitites are one of the major primary host lithologies of platinum-group elements (PGE) and one of the main targets for their exploration [1][2][3][4][5]. Theoretical and experimental results suggest that silicate minerals (e.g., olivine, and pyroxenes) do not significantly contribute to the Ir-and Pd-groups PGE (IPGE: Os, Ir, and Ru; and PPGE: Rh, Pt, and Pd) budget of mantle rocks, despite the fact that they clearly dominate the upper mantle formations (e.g., [6,7]).…”