A classification of gem corundum deposits aimed towards gem exploration

Abstract

This classification of gem corundum (ruby and sapphire) deposits takes into consideration petrographic data, and the mode of genesis of the deposits. It is aimed at being practical and useful in the field, particularly for prospecting purposes. Deposits are classified into primary and secondary deposits. Primary deposits include igneous and metamorphic deposits. Igneous gem corundum deposits are rare and include sapphire-bearing syenites from Kenya. Emphasis is put on metamorphic deposits, that are subdivided into metamorphic s.s., metasomatic, and anatectic deposits. Many gem corundum deposits result from metasomatic processes. Small-scale metasomatism mostly involves desilication reactions between silico-aluminous rocks (pegmatites, gneisses, etc) and silica-poor rocks (ultramafites, meta-carbonates), and leads to the formation of limited-size deposits. Large-scale (diffuse) metasomatism is usually more difficult to characterize, and therefore is not separated from isochemical metamorphism in this classification. In metamorphic deposits, gem corundum results from transformation of an Al-rich and/or Si-poor protolith. Such deposits include ruby-bearing mafic granulites, ruby-bearing meta-limestones, and ruby/sapphire-bearing gneisses and granulites. An intermediate category includes anatectic deposits. Secondary deposits encompass sedimentary and volcanic (xenoclastic) occurrences. In sedimentary deposits, gem corundum occurs as clasts originating from other lithologies. In volcanic deposits, gem corundums are xenocrysts that have a range of origins. The proposed classification outlines geological environments favorable to the crystallization and distribution of gem corundum, thus facilitating prospecting and mining of this gemstone.