Characteristics of chemical and mineral composition of marbles in relation to the

corundum and spinel formation in Khoan Thống - An Phú area, Lục Yên, Yên Bái provinces

Nguyễn Văn Nam, Nguyễn Ngọc Khôi, Nguyễn Thị Huyền, Quách Đức Tín, Công Thị Diệp,

Nguyễn Đức Chính, Bùi Thế Anh, Christoph Hauzenberger, Chakkaphan Sutthirat

Gem deposits in Khoan Thống - An Phú area, Lục Yên district, Yên Bái province have been known worldwide as a crucial source of various gemstones, such as corundum (ruby, sapphire), spinel, etc. Many Vietnamese and foreign geologists considered that the formation of gemstone in the study area related to marbles of the Neoproterozoic - Cambrian An Phu Formation (NP-€1ap). Based on analytical results of petrography of the marbles and chemical composition of calcite, spinel and corundum obtained by EPMA, Raman, ICP-MS methods, the marbles were divided into three different types, namely: (ruby, sapphire) corundum-bearing marble type, spinel- bearing marble type and corundum and spinel-free marble type. In general, these marbles are medium to coarse grained rocks having major rock-forming mineral of calcite. Additionally, the corundum and spinel- bearing marbles also contain phlogopite, margarite, pargasite, clinohumite, pyrite, graphite, sometimes include apatite, chlorite and titanite. The P- T-X formation conditions of the marble in the study area is about 550-750oC.

Keyworrds: ruby, sapphire, spinel, Lục Yên, Yên Bái