Multiple metal sources of coupled Cu-Sn deposits: Insights from the Tongshanling polymetallic deposit in the Nanling Range, South China

Abstract

Formation of hydrothermal Cu and Sn deposits is thought to be related to magmas of differing oxygen fugacities, so the existence of mixed Cu-Sn hydrothermal ores poses a challenge to our understanding of the controls on their genesis. The Tongshanling polymetallic deposit in western Nanling Range (South China) is exceptional for its multiple mineralization types, which include skarn Cu-Pb-Zn and quartz-vein Sn-W-Pb-Zn mineralization, providing an excellent opportunity to reveal the genesis of coupled Cu-Sn deposits. An integrated study was carried out from petrological, mineralogical, geochronological, isotopic, and geochemical perspectives to determine the genesis of the Tongshanling polymetallic deposit and obtain a better understanding of coupled Cu-Sn mineralization processes. The granodiorite in the Tongshanling ore field is classified as oxidized I-type granitoids and emplaced at ∼ 167 Ma based on zircon U-Pb dating, and it is derived from partial melting of amphibolite-facies crustal rocks. In sharp contrast, the quartz porphyry and granite porphyry in the Tongshanling ore field were emplaced at ∼ 171 Ma and ∼ 162 Ma, respectively, both of which are highly evolved, reductive, and volatile-enriched A-type partial melts produced by remelting of granulite-facies crustal rocks. The close spatial relationship between the granodiorite and the skarn orebody is due to a ∼ 167 Ma skarn Cu-Pb-Zn mineralization event. In comparison, cassiterites from the quartz-vein ores define a later ore-forming event at ∼ 162 Ma, consistent with the intrusive age of the granite porphyry. Together with the crosscutting relationship of the quartz vein and skarn mineralization-related rocks (e.g., granodiorite, skarn, and sulfide ores), a ∼ 162 Ma Sn-W polymetallic mineralization event in the Tongshanling deposit may have been associated with a concealed, highly evolved granitic magmatism. Oxygen fugacity of the I-type granodiorite (median ΔFMQ = −1.47) is higher than those of the A-type quartz porphyry and granite porphyry (median ΔFMQ = −1.65 and −2.66, respectively), which may have been a key factor leading to multiple mineralization in the Tongshanling ore field. The chemical compositions of cassiterites indicate that the ore-forming fluid of the quartz-vein stage was reductive, volatile-enriched, and of magmatic origin. In addition, its temperature was lower than that commonly associated with greisen or pegmatite mineralization. These observations suggest that the Cu-Sn metallogeny of the Tongshanling polymetallic deposit was spatially-coupled rather than temporally-coupled, and that the genesis of this deposit can be explained with a two-stage metallogenic model. A comparative study of worldwide Cu-Sn deposits indicates that, in most cases, the Cu and Sn metals do not come from a single source, and that Cu-Sn deposits are a product of spatial coupling, i.e., sequential mineralization events within a single ore field.