Original paper

Mineralogy and petrology of gneiss hosted corundum deposits from the Day Nui Con Voi metamorphic range, Ailao Shan–Red River shear zone (North Vietnam)

Ngoc Khoi, Nguyen; Hauzenberger, Christoph A.; Anh Tuan, Duong; Häger, Tobias; Van Nam, Nguyen; Thuy Duong, Nguyen

Abstract

Corundum deposits and occurrences are typically associated with marbles in northern Vietnam. Relatively little at- tention has been paid to corundum hosted by partly migmatized gneisses and pegmatoids of the Day Nui Con Voi range, Ailao Shan–Red River shear zone, North Vietnam. The partly migmatized gneisses contain gray, grayish white to bluish, and yellowish gray sapphires (type 1) while dark red to pinkish rubies occur in feldspathic pegmatoid rocks (type II). Corundum crystals from both types are short prismatic with partly hexagonal tabular shape considerably varying in size. Common inclusions are ilmenite, magnetite, rutile, plagioclase, muscovite, biotite, apatite, zircon, boehmite and iron stains. Growth structures and deformed twinning with star effect can be observed as well. Type I corundum is noted for high content of total Fe (5500 to 7133 ppm), low content of Cr and Ti (59 to 65 and 40 to 58 ppm, respectively), whereas, in corundum of type II the Fe and Cr contents are medium (1028 and 940 ppm, respectively), with low content of Ti (42 ppm). Gneisses hosting type I corundum are characterized by three different mineral parageneses (1) sillimanite+corundum+K-feldspar+biotite; (2) sillimanite+garnet+spinel+corundum+biotite+K- feldspar+plagioclase+ilmenite, and (3) sillimanite+garnet+spinel+corundum+clinopyroxene+ilmenite. Type II corundum occurs in feldspathic pegmatoid rocks consisting of a paragenesis garnet+biotite+plagioclase+K-feldspar+corundum. Sapphire in gneisses (type I) crystallized during regional metamorphism of silica undersaturated metapelites and metapsammites at upper amphibolite to lower granulite facies conditions. Ruby found in desilicated feldspathic rocks (type II) probably formed at the contact of migmatitized pegmatoid bodies with either metabasic to metaultrabasic rocks or marbles. The metamorphic P-T condition of corundum formation was estimated with 650 to 760 °C and 5.0 to 7.6 kbar based on thermobarometric calculations.40 K/40 Ar cooling ages obtained from biotite in gneisses indicate that the rocks reached ~300 °C at ~23 Ma.

Keywords

ailao shan–red river shear zonepetrologyday nui con voi rangeparagenesisgenetic aspectscorundum depositgneissmineralogy