YANG Qing¹, CHEN Ai-bing¹, YANG Guang-shu¹, LI Jun¹, QUAN Zhi-fang¹, LI Bo¹, LIU Yan-zhao¹, WANG Hong-yan^1*, XIE En-cai²
(1. Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China;
2. Yunnan Yongchang Lead and Zinc Inc., Baoshan 650217, China)

Abstract: The Mengnuo lead-zinc deposit is located in the east limb of the Mengnuo syncline of the Baoshan-Zhenkang block, and is an important large-scale lead-zinc deposit in the Sanjiang metallogenic belt. The ores are mainly hosted in crystalline limestone in lower member of the Middle Silurian Shangrenheqiao Formation (S21) with the hanging wall and footwall of foliated limestone and mudstone. The crystalline limestone has similar REE patterns to those of other wallrocks in the area with right-declined REE patterns and negative Eu anomalies, indicating they were formed in similar sedimentary environment with similar diagenetic mechanism. At the same time, the Al2O3-SiO2 for hot water diagenetic model diagram, U-Th relationship diagram and Al-Fe-Mn triangle diagram indicate that the wallrocks in the deposit were formed in a normal marine environment rather than in hot water environment. The results of U/Th<0.75, Sr/Cu>5, V/Cr<2, Ce weak positive anomalies and lower Sr contents of wallrock samples indicate that they were deposited in an open oxidized shallow sea environment under the relatively dry climate during sedimentation, with minor influence of terrestrial debris. The ΣREE contents of 57.11×10-6 for crystalline limestone is significantly lower than that of 232.02×10-6 for other wallrocks, and the LaN/YbN ratio of 8.81 for crystalline limestone is lower than that of 11.7 for other wallrocks, indicating that the hydrothermal fluid leaching could have caused the REE loss of crystalline limestone in some extent. According to the comprehensive analysis, the protolith of crystalline limestone could be bioclastic micrite, which had undergone two hydrothermal tectonic movements. The early tectonic movement caused the mainly brittle fracturing of the original rock, mainly brittle deformation. Then the fractured bioclastic micrite was recrystallized to have formed coarse-grained crystalline limestone due to the input of hydrothermal fluid into fractures. The late tectonic movement caused the locally superimposed transformation of the crystalline limestone, mainly the ductile deformation, and then resulted in the mylonitization of crystalline limestone to have formed the unevenly mylonitized limestone. The fractures and fissures of crystalline limestone would be good migration channels and ore-bearing spaces for the ore-bearing hydrothermal fluid under a certain sealing effect by the roof and floor wallrocks.

Keywords: Crystalline limestone; geological and geochemical characteristics; sedimentary environment; the Mengnuo lead-zinc deposit

ACTA MINERALOGICA SINICA Vol. 39, No.5, 2019, page 527-538