Baruch Spiro, Natural History Museum (United Kingdom)
Xingchun Zhang, Institute of Geochemistry (China)
Chris Halls, Natural History Museum (United Kingdom)
Richard Herrington, Natural History Museum (United Kingdom)
John Spratt, Natural History Museum (United Kingdom)
The Lanping Basin, inYunnan Province, SW China is 400 km long and 25 km wide, lying between the Tibet-Yunnan and Yangtze plates. It is the setting for a variety of sediment-hosted ore deposits containing Pb-Zn-Ag, Cu ±Co, Hg and Se mineralization. The sediments range in age from Triassic to Eocene and fission track dating indicates that mineralization in the basin took place between 52 and 19Ma. Jinding lies S of the County town Lanping (99° 25' E Long, 26° 24' N Lat.). The local basin fill consists of Eocene sandstones, shales and carbonaceous limestones with associated bitumen, the Eocene Yunlong Formation. This hosts the Jinding sediment-hosted Pb-Zn deposit with a reported resource of 130 Mt @ 8% Zn, 1.4% Pb and 10 g/t Ag. It is composed of a stratigraphically upper concordant sandstone-hosted mineralization (Eyc) and stratigraphically lower discordant mineralization in a sequence of sandstone breccias containing olistoliths related to thrust fault growth (Eyb). The lower sequence contains limestone blocks from <1m to several hundreds of metres in size. The olistolith-hosted ores are mined in the open cast Jiayashan mine and the sandstone-hosted ores are mined underground in the Nanchang mine (Beichang Block). Both the sandstone-hosted and carbonate-hosted mineralizations contain Cd, and this has implications for the processing of the ore and the disposal of the waste.
The mineralization in the fluviatile-deltaic quartz sandstones in the Nanchang Mine follows the bedding and fills the interstices between quartz grains, partly replacing the carbonate cement and also the detrital quartz. Spheroids of barite, dolomite and marcasite/pyrite are common and seem to have nucleated and grown in the sediment at an early stage of diagenesis. Interstitial filling by galena, sphalerite and cerussite formed in the latest stage. Barite is the major gangue mineral and contains varying amounts of Sr.
The carbonatic ore in Jiayashan is vuggy and contains aggregates of pyrite/marcasite with zones enriched in As. It contains relics of the sandstone ore but consists mainly of aggregates of galena and cerussite surrounded by botryoidal overgrowths of schalenblende (sphalerite), smithsonite, and cerussite with associated euhedral quartz. A bright yellow surface coating in the vugs has been identified as dundasite PbAl2(CO3)2(OH)4.H2O. The black bituminous limestone contains veinlets of sphalerite and the carbonate ore contains discrete veinlets and vugs filled with bitumen.
Cadmium is present in the structure of the sphalerite in the sulphidic ore (Cd/Zn∼ 0.01wt/wt) but larger amounts are contained in the sphalerite in the carbonatic ore (Cd/Zn∼0.02). Here the concentration of Cd increases systematically towards the edge of each layer in the botryoidal aggregate and in the encrusting dundasite (Cd/Zn∼0.05wt/wt).
The Cd/Zn ratio shows a general increase in the later stages of the paragenesis. The distribution indicates that the phases containing most Cd have formed mainly in the carbonatic ores, reaching a maximum in the latest generation of smithonite and dundasite. Knowledge of this pattern in the chemical and paragenetic evolution of the ores should be useful in determining mining and environmental strategy.