Origin of podiform chromitites

Abstract

Podiform chromitite with a dunite envelope can be interpreted to be a product ofinteraction between mantle harzburgite and an exotic magma, combined with magma mixing. The secondary Si- and Cr-rich melt which is produced by the selective dissolution of orthopyroxene can be mixed with a subsequently supplied primitive melt with the mixed melt oversaturated with spinel to effectively. precipitate chromitite. The dunite envelope is of replacive origin. A moderately refractory harzburgite, which has chromian spinel with an intermediate Cr # (=Cr/[Cr + All atomic ratio), around 0.5, is the most suitable host for large-scale podiform chromitites. Lherzolite wall is too low in Cr # to concentrate chromian spinel; the mixed magma is relatively low in the degree of spinel-oversaturation because of the geometrical constraint from the liquidus phase boundary between olivine and spinel. Highly refractory harzburgite wall is low in the amount of Cr 2O 3 and Al 2O 3, which can be the source of chromian spinel during the interaction. The podiform chromitite can be produced at both oceanic and arc-related settings. The arc-related setting is, however, favorable for most of the podiform chromitites, especially for large-scale economically important deposits. The Cr # of spinel in most of the podiform chromitites, 0.7 to 0.8, cannot be covered with the compositional range of spinel in oceanic rocks, especially in MORB. The Cr # of spinel, 0.7 to 0.8, is comparable with the ratio in primitive arc magmas.