Coupled Sn- and Cu-isotope signatures decipher mechanisms of Sn-Cu ore formation

Abstract The factors leading to the coexistence of tin (Sn) and copper (Cu) mineralization within a single mineral system are ill-constrained. Sn and Cu isotopes can shed new light on metal sources and ore-forming processes, in turn allowing an unravelling of the mechanisms involved in Sn-Cu ore formation. This study utilizes Sn- and Cu-isotope data from the Dulong Sn-Cu-Zn-In deposit, South China, combined with quantitative geochemical modeling, to understand the processes by which Sn-Cu mineralization formed. Results show that the δ124Sn values of cassiterite (relative to National Institute of Standards and Technology Standard Reference Material 3161a) gradually decrease from the skarn and massive sulfide Sn ore with minor Cu to vein Cu-Sn ore (+1.59‰ to −1.95‰), suggesting that Sn-isotope fractionation is redox-controlled during progressive cassiterite precipitation. The δ65Cu values of chalcopyrite decrease from the skarn and massive sulfide ore and are attributed to fluid-rock interaction. The variable and lower Sn-isotope compositions (−1.95‰ to +0.34‰) of cassiterite and elevated δ65Cu values (+0.73‰ to +1.03‰) of chalcopyrite from vein ore are indicative of an additional pulse of magmatic (Cu-rich) fluids. These observations collectively indicate that progressive fluid-rock interaction, involving the leaching of Cu from the country rocks, accounts for the small-scale Cu mineralization found alongside Sn mineralization. In contrast, the addition of a pulse of Cu-rich fluids sourced from oxidized mafic magmas into coeval reduced felsic magmas led to the coexistence of large-scale Cu mineralization and large-scale Sn mineralization (e.g., San Rafael and Gejiu). The two distinct mineralizing episodes could further explain the different scale of Sn-Cu ore formation worldwide.