Abstract:
The Tongchanghe native copper-chalcocite deposit at Ninglang occurs in low-Ti basalts of western Yunnan, and the mode of fault-filling & metasomatism metallogenesis indicates that this deposit is of late-stage hydrothermal origin. This makes it more complicated to define the source of ore-forming materials. This paper introduces the Pb isotope data of Himalayan alkali-rich porphyries, regional Early-Middle Proterozoic metamorphic rock basement and various types of rocks of the mining district in western Yunnan with an attempt to constrain the origin of the Tongchanghe native copper-chalcocite deposit at Ninglang. The results showed that the ores are relatively homogeneous in Pb isotopic composition, implying a simple ore-forming material source. The three sets of Pb isotopic ratios in the Himalayan alkali-rich porphyries are all higher than those of the ores; the regional basement metamorphic rocks show a wide range of variations in Pb isotopic ratio, quite different from the isotopic composition of ore lead; the Pb isotopic composition of the Triassic sedimentary rocks and mudstone and siltstone interbeds in the Late Permian Heinishao Formation (corresponding to the forth cycle of basaltic eruption) in the mining district has the characteristics of radiogenic lead and is significantly different from the isotopic composition of ore lead; like the ores, the Emeishan basalts in the mining district and those regionally distributed possess the same Pb isotopic composition, showing a complete overlap with respect to their distribution range. From the above, the possibilities can be ruled out that the ore-forming materials of the Tongchanghe deposit were derived from the basement, a variety of Himalayan magmatic activities, etc. It is thereby defined that the ore-forming materials were derived largely from the Emeishan basalts. From the data available it is deduced that the native cupper-chalcocite-type metallogenesis that occurred in the Emeishan basalt-distributed area has the same metal source as the Tongchanghe deposit.
