Elemental and mineralogical anomalies in the coal-hosted Ge ore deposit of Lincang, Yunnan, southwestern China: Key role of N₂-CO₂-mixed hydrothermal solutions

The Lincang Neogene high-Ge coal deposit in Yunnan, southwestern China, is one of the major coal-hosted Ge deposits in the world. This study reports new data on the petrological, mineralogical, and geochemical compositions of 57 samples (including coal bench samples, roofs, floors, partings, and batholith granite) of three high-Ge coal seams (S3, Z2, and X1) from the Dazhai Mine, Lincang Ge ore deposit, and provides new insights into the origin and modes of occurrence of the minerals and elements present.The coals have huminite random reflectances in the 0.33-0.48% range. On a mineral-free basis, the coal samples are dominated by huminite-group macerals, all having more than 88.5% total huminite. Ulminite and attrinite generally dominate the huminite macerals. Structured inertinite is rare, with funginite being the most abundant inertinite form. The minerals in the coals are mainly composed of quartz, and, to a lesser extent, kaolinite, illite, and mica. A hydrous beryllium sulfate phase (BeSO₄·4H₂O) is present in the low temperature ashes of several coal samples.Compared to average values for world low-rank coals, beryllium (up to 2000 μg/g and 343. μg/g on average), Ge (up to 2176 μg/g and 1590 μg/g on average), and W (up to 339 μg/g and 170. μg/g on average) are unusually enriched in the Lincang coals, with a concentration coefficient >. 100 (CC = ratio of element concentration in investigated coals vs. world low-rank coals); elements As (156. μg/g on average), Sb (38. μg/g), Cs (25.2 μg/g), and U (52.5 μg/g) are significantly enriched (10 < CC < 100); niobium (28.2. μg/g) is enriched (CC = 8.55); zinc, Rb, Y, Cd, Sn, Er, Yb, Lu, Hg, Tl, and Pb are slightly enriched (2 < CC < 5).The biotite- and two-mica granites, which served as both the basement for the coal-bearing sequence and as a source of sediment input, were also either hydrothermally-altered or -argillized. The alteration appears to have taken place during or shortly after deposition of the coal-bearing sequence. Two types of metasomatites of hydrothermal origin, including quartz-carbonate and carbonate, were identified, which occur as partings and as roof and floor strata. These metasomatites were formed at the syngenetic or early diagenetic stages of coal deposition. The rare earth elements in these hydrothermal rocks are characterized by a heavy REE enrichment type and by distinct positive Eu anomalies, compared to the upper continental crust.Hydrothermal solutions have played a significant role in producing the elemental and mineralogical anomalies in the Lincang Ge ore deposit. The hydrothermal solutions leaching the batholith granite were a mixture of alkaline N₂-bearing and volcanogenic CO₂-bearing fluids, which led to the enrichment of trace elements, not only including assemblages of Ge-W and Be-Nb-U (both leached from granite and the deposited in the peat), but also As-Sb (from volcanogenic solution), as well as the alteration and argillization of the batholith granites, and the formation of carbonate and quartz-carbonate metasomatites.