IngentaConnect Chromite and PGE in the Logar Ophiolite Complex, Afghanistan

Authors: Benham, A.J.¹; Kováč, P.²; Petterson, M.G.³; Rojkovic, I.⁴; Styles, M.T.³; Gunn, A.G.³; McKervey, J.A.³; Wasy, A.⁵

Source: Applied Earth Science : IMM Transactions section B, Volume 118, Number 2, June 2009 , pp. 45-58(14)

Publisher: Maney Publishing

Abstract:

The Logar Ophiolite Complex (LOC) is located 30 km south of Kabul, Afghanistan, and extends over approximately 2000 km². It comprises a lower lherzolitic-dunitic-harzburgitic-gabbro ultramafic-mafic unit that passes upwards into a dolerite dyke complex, basaltic pillow lavas and an uppermost sequence of volcaniclastic- and terrigenous-dominated sedimentary units. The ophiolite represents an obducted remnant of intra-Tethyan basin oceanic crust, thrust onto a platform-style cover component of the Kabul Terrane during the Himalayan orogeny. Platinum-group minerals have been detected for the first time in chromitites and ultramafic units from the LOC. Two distinct types of chromitites and ultramafic lithologies with different origins have also been identified in this study. The first type is a low Cr, PGE-poor chromitite interpreted to have been produced in a mid ocean ridge (MOR) environment. The second type is a high Cr, relatively PGE-rich dunite and peridotite from a boninitic magma produced in a supra-subduction zone (SSZ) setting. Platinum group element (PGE) abundances in these chromitites average 12 ppb and 26 ppb for Pt+Pd+Rh for the dunite and peridotite. Chondrite-normalised PGE patterns have two distinct trends: (a) the MOR rocks have a positive Ru anomaly with a negative Pt anomaly and a generally negative slope; and (b) the SSZ rocks show weak positive Ru and Pt anomalies and a positive slope. It is concluded that the negative sloping pattern is typical of PGE in most ophiolites elsewhere. In contrast, the positively sloping pattern is more unusual and may indicate PGE remobilisation and enrichment.

Keywords: CHROMITE; PGE; PLATINUM; OPHIOLITE; AFGHANISTAN

Document Type: Research article

DOI: 10.1179/174327509X434957

Affiliations: 1: British Geological Survey, Keyworth, Nottingham NG 12 5GG, UK;, Email: abenham@bgs.ac.uk 2: GET Limited, Partizanska cesta 3, 974 01 Banska Bystrica, Slovakia 3: British Geological Survey, Keyworth, Nottingham NG 12 5GG, UK 4: Geological Institute Slovak Academy of Sciences, Dúbravska cesta 9, 840 06 Bratislava, Slovakia 5: Afghanistan Geological Survey, Kabul, Afghanistan

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