Abstract: Thallium is a very dispersed element and it seldom o ccurs in the form of independent minerals and in independent ore deposits. Biogenic loran dite is the major industrial mineral of the Lanmuchang Tl deposit in Xingren Cou nty, Guizhou Province. It was formed as a result of bioenrichment during sedimen tary diagenesis. A large number of micro-paleofossils, especially foramina and zoaria are still observed in the Tl ores. Most of micro-paleofossils have been replaced by Tl-bearing ore fluid, hence forming T1 minerals with bio-pseudomor phs, such as lorandite and christite. 　　Bio-enrichment and hydrothermal reworking are the two important metallogenic fe atures for the Tl deposit studied. Biogenic lorandite provides an evidence for t he ore- formation mechanism and timing. An analysis on the time-bound nature of biocoen oses, sulfur isotopic composition and bio-enrichment metallogenic phenomenon sh ows that the Lanmuchang Tl diposit was originally formed in the Late Permian, an d the hydrothermal reworking took place during the Middle Triassic. No Tl minera ls in the form of bio-pseudomorphs have been identified in the Tl ores of hydro thermal reworking stage, indicating that these minerals have been replaced by ty pical hydrothermal Tl-minerals.