Geological and geochemical constraints on the origin of the giant Lincang coal seam-hosted germanium deposit, Yunnan, SW China: A review

The Lincang germanium deposit, Yunnan, SW China, contains at least 1000 tonnes of Ge at an average grade of ~ 850 ppm Ge, and is one of the largest Ge deposits in the world. The deposit is hosted within coal seams of the Miocene Bangmai Formation, deposited on top of a Ge-rich granite batholith. The Bangmai Formation is divided into eight units among which three are coal-bearing. The Ge-bearing coal seams are inter-layered with siliceous rocks and siliceous limestones in the basal coal-bearing unit. The coal seams of the other two coal-bearing units are not interbedded with siliceous rocks and siliceous limestones, and are also barren with respect to Ge. Equant or elongate Ge-orebodies are generally located at fault intersections. Germanium is mainly associated with organic matter within the coal seams and is concentrated at the top and bottom of the coal seams, where the latter are in contact with the layered siliceous rocks or siliceous limestones. Major and trace element contents, and O- and C-isotopes of the siliceous rocks and siliceous limestones, are similar to those of hydrothermal sediments, indicating formation by hydrothermal sedimentation. Compared with barren coals, Ge-rich coals are notably rich in Nb, Li, Sb, W, Bi and U and show substantial enrichment of HREE which increase together with Ge. Germanium-rich coals contain disseminated pyrite with δ34S ranging from 17.2 to 51.4‰, similar to pyrite in barren coals, and thin vein-like pyrite with δ34S ranging from − 5.4 to 1.9‰, similar to sulfides in granite-related quartz veins. We propose that circulating hydrothermal fluids leached abundant Ge and other elements from Ge-rich granites in the basement, and were then discharged into the basin, mainly along fault intersections, to form layer-like siliceous rocks and siliceous limestones by depositing Si and Ca. The deposit was formed via interaction between Ge in the fluids and organic matter in the coal seams.