Platinum-group minerals in the middle group of chromitite layers at Marikana, western Bushveld Complex: indications for collection mechanisms and postmagmatic modification
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Abstract:
The platinum-group minerals in a drill core taken through the middle group of chromitite layers in the Critical Zone at Marikana in the western Bushveld Complex were found to consist mainly of laurite as inclusions in chromite grains. The platinum-group minerals containing Pt, Pd, and Rh are concentrated in the intercumulus silicates and frequently associated with base-metal sulphides. Up to about 20% of all platinum-group minerals in the investigated chromitite layers contain sub stantial amounts of As. The base-metal sulphides are strongly modified in the postmagmatic stage, which led to a significant loss of Fe and S, in this way concentrating Cu, Ni, and the platinum-group elements by factors of up to 10. Interaction between chromite and base-metal sulphides cannot account for all the Fe lost in chromite-poor samples, and the importance of additional processes is indicated. Inclusions in chromite and orthopyroxene indicate the formation of discrete platinum-group minerals and As-rich phases before the formation of an immiscible sulphide melt. Resorption of earlier formed platinum-group minerals into the immiscible sulphide melt and postmagmatic sulphidation destroyed most of the evidence of the early formed platinum-group minerals.Keywords:
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The Khoy ophiolites are one of the largest Iranian ophiolitic complexes and are characterized by a very complicate geology, consisting of several ophiolites with different ages and geodynamic settings of formation. Few small lenticular and irregular bodies of podiform and schlieren type chromitites have been recognized in the Khoy ophiolites, occurring in the serpentinized mantle peridotites. They have massive and nodular textures. According to their chromite composition, the Khoy chromitites can be classified as Al-rich and fall clearly in the field of podiform chromitites. The magmatic chromite composition of the podiform chromitites of the Khoy ophiolites suggests that they crystallized from a MORB type melt in an extensional regime. The Khoy chromitites were altered during ocean floor metamorphisms and no magmatic silicates were preserved. The composition of chlorite occurring in the silicate matrix or in contact with chromite has been used as a geothermometer. The estimated temperatures possibly reflect the effects of low-grade metamorphism. The Khoy chromitites have very low total platinum group elements (PGE) concentration with a predominance of Os + Ir + Ru over Rh + Pt + Pd. Consistently with the geochemical data, they contain few platinum group minerals (PGM) of Ru, Ir and Os. Laurite is the most common PGM along with a minor Ir-Cu sulfide, possibly cuproiridsite, osmium and erlichmanite. Most of the PGM are magmatic in origin. They crystallized in a narrow range of temperature, around 1000 o C, at relatively low sulfur fugacity and in the absence of fluids. This observation is consistent with formation of the chromitites in a mid-oceanic ridge (MOR) rather than in a supra-subduction zone (SSZ).
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The platinum-group minerals in a drill core taken through the middle group of chromitite layers in the Critical Zone at Marikana in the western Bushveld Complex were found to consist mainly of laurite as inclusions in chromite grains. The platinum-group minerals containing Pt, Pd, and Rh are concentrated in the intercumulus silicates and frequently associated with base-metal sulphides. Up to about 20% of all platinum-group minerals in the investigated chromitite layers contain sub stantial amounts of As. The base-metal sulphides are strongly modified in the postmagmatic stage, which led to a significant loss of Fe and S, in this way concentrating Cu, Ni, and the platinum-group elements by factors of up to 10. Interaction between chromite and base-metal sulphides cannot account for all the Fe lost in chromite-poor samples, and the importance of additional processes is indicated. Inclusions in chromite and orthopyroxene indicate the formation of discrete platinum-group minerals and As-rich phases before the formation of an immiscible sulphide melt. Resorption of earlier formed platinum-group minerals into the immiscible sulphide melt and postmagmatic sulphidation destroyed most of the evidence of the early formed platinum-group minerals.
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