Systematics of chromitite occurrences in Central Palawan,Philippines |
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| Authors: | D Rammlmair H Raschka L Steiner |
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| Institution: | (1) Bundesanstalt für Geowissenschaften und Rohstoffe, P.O. Box 510153, D-3000 Hannover 51, Germany |
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| Abstract: | The chromitite occurrences in the Central Palawan ophiolite can be classified into four groups based on their chemistry and geological relationships. Group-I chromites from a deep tectonite level of the ophiolite complex show Cr/(Cr+Al) values of between 0.78 and 0.90, whereas group II from a shallower level gives values of 0.64–0.78, and group III from the immediate gabbro lower contact (dunite: cumulates and diapirs) show values of 0.5–0.64. Group-IV chromites from the gabbro zone have ratios around 0.38–0.5. Larger orebodies (type A), representing a first stage of partial melting of a primordial mantle, show Cr/Fe ratios between 2.5 and 4.5, whilst those of small chromitite bodies with a thin dunite envelope (type B) lie below 2.5. These small bodies belong to a second stage of magma generation. For both types the Cr/Fe ratio increases with the chromite/silicate ratio within individual occurrences. An additional method which may help to subdivide these chromitites involves their silicate and ore inclusions. The platinum-group minerals, for example, occur mainly in chromites of group II, type A. The intensive Tertiary tectonic activity to which the ophiolite was subjected during and after emplacement disturbs the regular pattern of the ophiolite stratigraphy and its chromitite occurrences. |
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