Chromite alteration in the Panton Sill, East Kimberley Region, Western Australia

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Abstract
Summary: In addition to the normal type of chromite alteration, an antipathetic form of alteration has been observed in the ultramafic zone of the Panton Sill, Western Australia. The chromites display margins of lower reflectivity related to a marked enrichment in MgO and Al2O3and depletion in Cr2O3, Fe2O3, and FeO, such that a spinellid of composition lying between the end members MgAl2O4and FeAlaO4is formed. The alteration is related to the formation of secondary spinel in the ground-mass during the hornblendization of primary silicates under metasomatic conditions accompanied by low oxygen activity.