Formation and implication of a Cr-bearing spinel seam by interaction between alkali basaltic melt and lherzolite

Abstract

It has long been known that an indisputable fact that chromite/spinel deposits are genetically related exclusively with komatiitic, tholeiitic or boninitic magmas derived from high-degree mantle melting. Here we report formation of a seam of Cr-bearing spinel through interaction between an alkali basaltic magma and fertile lherzolite. The thin seam occurrence of spinel resembles concordant podiform chromitites in ophiolites and stratiform chromitites in layered intrusions. We show that the Cr-bearing spinel seam formed by melt-rock reaction that observed in ophiolitic peridotites. The most significant difference is that the Cr-bearing spinel produced from the alkali basaltic melt-lherzolite interaction is extremely rich in Al with Cr# values of 3–6. We consequently re-classify chromitite into four types based on their Cr# values namely: high-Cr (chromite Cr# > 70), intermediate (Cr# = 70–50), high-Al (Cr# = 50–40), and extremely high-Al (Cr# < 10) with a Cr# gap between 40 and 10. The chromite/spinel compositions correspond closely to diagnostic parental magmas and the lithologies of the host peridotites.