Formation of giant gold deposits driven by delamination of carbonated cratonic lithosphere

Abstract

Delamination of the thick continental lithosphere of the craton is an important geological process that accounts for the recycling of continental materials. However, links between such a process and the formation of giant ore deposits are controversial. Although a recent study suggests the base of the cratonic lithospheric mantle at depths of 160–190 km is particularly enriched in sulfur and could be a significant gold reservoir, the mechanisms responsible for gold release are poorly understood. The Jiaodong gold deposits in the eastern North China Craton (NCC) are defined as a giant gold province with approximately 5000 tons of Au. Using new advanced Au analytical technologies, we measured the Au concentrations of 86 samples to determine the Au background values of possible source candidates. The crustal candidates, including Precambrian metamorphic crustal basement (average Au: 0.20 ng/g) and Mesozoic granites (average Au: 0.05 ng/g) from the Jiaodong area, as well as supracrustal rocks from the northern South China Block (SCB) (average Au: 0.12 ng/g), exhibit no discernible Au anomalies. Nevertheless, local mafic dikes, spatiotemporally associated with Au mineralization, exhibit slightly higher Au contents (up to 3.13 ng/g) than other candidates. In-situ laser ablation analyses of S-rich minerals in the mafic dikes reveal heavy sulfur isotopic compositions (δ³⁴S ranging from −3.4 to 17.4 ‰), systematically higher than the mantle value (δ³⁴S = −1.4 ± 0.5 ‰). Since the effects of crustal contamination and sulfide segregation are negligible, the heavy S isotopic compositions of Jiaodong mafic dikes may indicate a potential contribution of heavy sulfur isotopes from marine carbonates or sulfide oxidation in the mantle. The high Fe³⁺/∑Fe values (up to 0.43) of the mafic dikes also indicate derivation from a highly oxidized mantle source. Given that their low δ²⁶Mg values (−0.59 ‰ to −0.24 ‰) have indicated a mantle source partially metasomatized by recycled carbonates, we hypothesize that the reduction of carbonates in deep (>250 km) delaminated cratonic lithosphere may have facilitated the oxidation of sulfides to sulfate, potentially providing a mechanism for the liberation of gold from the mantle.