Formation of an Active Giant Nonsulfide Zinc System, Jinding, China: Relationships Among Tectonics, Climate, and Supergene Metal Remobilization

Abstract The oxidized portion of the giant Jinding Zn-Pb deposit, Yunnan, China, accounts for about 40% of the original metal resource, indicating a nonsulfide zinc component of 80 to 90 million tonnes (Mt), making it among the largest nonsulfide zinc concentrations known. The presence of the Jinding deposit within a rugged topographic upland undergoing active weathering and erosion suggests that the nonsulfide zinc supergene system was active at the time of discovery. The ore-bearing strata cropped out at ~2800 m elevation, and strong oxidation effects are present to at least 200 m below the premining surface. The exhumation of the late Oligocene Jinding primary ores and exposure to groundwater modification may have been initiated by late Miocene uplift that coincides with the onset of the Southeast Asia monsoonal climate. The abundance of calcium carbonate in the Jinding host strata and primary ore results in the preserved nonsulfide zinc ores being dominated by smithsonite crusts of a variety of compositions, along with a diverse assemblage of supergene Zn-Pb and other minerals reflecting the primary ore composition. Complex microstratigraphic banding in smithsonite at scales of ≤20 µm is revealed by luminescent microscopy, likely recording groundwater-related annual precipitation events. Jinding smithsonites from the Beichang and Jaiyashan ore zones have δ18OV-SMOW values (in which V-SMOW = Vienna-standard mean ocean water) that vary between 18.6 and 21.5‰, which are unusually low compared with most supergene smithsonites. These values are consistent with the cold conditions of the Jinding area (11°C mean annual temperature) and are compatible with local cold springs and travertines. The δ13CV-PDB values (in which V-PDB = Vienna-Pee Dee Belemnite) of the Jinding smithsonites range between –10.1 and –3.6‰ and are distinct from calcites formed by local hot springs and from low-18O smithsonites from other nonsulfide zinc deposits. The light carbon isotope composition of Jinding smithsonites suggests a major contribution of organic carbon and only minor, if any, contributions from local Triassic marine limestone and atmospheric CO2. The 13C-depleted carbon likely was derived from soils with C3 plant-dominated organic matter, from oxidation of hydrocarbons in the primary ore, or from dissolution of isotopically light sandstone cements or ore calcites. Reaction path models show that the reaction of descending, cool, oxidizing, meteorically derived groundwater with calcite, CO2, and primary sulfide minerals can account for much of the observed supergene mineral assemblage. The Jinding nonsulfide zinc mineralization reflects the interrelationships among tectonic, climatic, and groundwater processes in high elevation and/or latitude settings, notably related to cold groundwater. Although the preservation of thick nonsulfide zinc concentrations was impeded by the dynamic regional tectonic and climatic conditions that resulted in high precipitation and erosion rates, the size of the primary ore zone and focused groundwater transport resulted in the development of a giant nonsulfide zinc mineralization system.