Abstract:

A series of Devonian SEDEX deposits occur at the Xicheng ore field in east section of the Western Qinling orogenic belt, and the Luoba Pb-Zn ore deposit in Hui County of Gansu Province is a typical one. The Luoba deposit is hosted in the upper section of the Jiaogou Layer in the Anjiacha Formation (Middle Devonian period) and controlled by Luoba anticline and fractures developing in its north and south wings. The ore bodies are accordant with the hosting strata in occurrence. The predominately ore structures include lamellar, layered, disseminated and massive structures. Channel and quenching structures, commonly related to SEDEX, are identified by the analysis of microstructure and SEM. Ba, As, Sb, B, Ag and Hg contents in the ore and wall-rock are relatively high and LREE are enriched. Obvious positive δEu and medium positive δCe imply that the REE in the ores inherited from the wall-rocks (chert). Fluid inclusions studies show that the mineralizing temperature ranged from 168 ℃ to 350 ℃. The δ³⁴S values of the early sulfides (averaged at 9.8‰) are significantly higher than those of the late ones (averaged at 5.6‰), which indicates that the strata sulfur was involved in the late reconstruction processes. Authors suggest that the Luoba Pb-Zn ore deposit belong to the SEDEX type, and be reformed by Late Hercynian-Indosinian metamorphism. It was derived from the hydrothermal fluid at the later Qinling orogenic intraplate extension stage. A metallogenic model for the Luoba SEDEX lead-zinc deposit is also given in this paper.

Key words: Luoba lead-zinc deposits, sedimentary exhalative, microstructure, geochemistry, metallogenic model