Ore geology, mineralogy and geochemistry of a fault-controlled hydrothermal clay-Li deposit hosted by Precambrian metasedimentary rocks in south China

The Jinyinshan-Huangdi'nao Li deposit (12,000 t Li₂O@0.60%) was recently discovered in southern Hubei Province, South China. This deposit is divided into two ore sections, namely, Jinyinshan (0.24-1.32 wt.% Li₂O) and Huangdi'nao (0.20-0.47 wt.% Li₂O). The dominant Li-bearing phase and mechanism for Li enrichment remain unclear. Herein, a comprehensive study of ore geology, mineralogy and geochemistry is conducted. Field and petrographic investigations revealed that Li mineralization in the deposit was fault-controlled and that the altered metasedimentary rocks and hydrothermal veins with intensive Li mineralization contained high abundances of Li-rich clay minerals. Whole-rock XRD and in situ analyses of SEM-EDS, EMPA and LA-ICP-MS of clay minerals reveal that cookeite (0.99-2.80 wt.% Li₂O) is the dominant Li-bearing phase, with subordinate illite (0.02-0.57 wt.% Li₂O). The widespread replacement of Li-enriched illite by cookeite combined with the compositional continuum suggests that cookeite was likely formed by the hydrothermal replacement of illite at a temperature of 240-270℃, as constrained by chlorite geothermometry (with average temperatures of 254 ± 2℃ in Jinyinshan and 259 ± 2℃ in Huangdi'nao). Since metasedimentary rocks of the Neoproterozoic Lengjiaxi Group in the deposit with variable Li anomalies host abundant Li-rich illite, Li mineralization was inferred to have occurred via hydrothermal metasomatism of these clay-rich clastic rocks. The hydrothermal fluids may have been driven by a deep magmatic heat source, as evidenced by previously reported U-Pb dating of apatite from the clay-Li ore, similar to the age of the Mufushan granitic batholith in the south, both of which are Early Cretaceous. The deep-sourced hydrothermal fluids caused the mobilization, migration and reprecipitation of Li as Li-rich clays along the fault zones. This mechanism of Li mineralization is different from existing models for clay-Li deposits worldwide, and this deposit can be classified as a new type, namely, fault-controlled hydrothermal metasomatic clay-Li deposit. Similar deposits are highly prospective both regionally and worldwide.