Mercury isotope evidence for basement as the metal source of mantle plume rock–hosted Carlin-type gold deposits in South China

Abstract Accurately constraining the metal sources feeding Au mineralization provinces is crucial for understanding ore genesis but remains highly challenging. One of the best examples is the world’s second largest Carlin-type gold (Au) province in the Youjiang basin of South China, where various candidates ranging from shallow crust to deep mantle end members have been proposed as the source of Au. Some important Carlin-type Au deposits in the basin, such as the Pingle and Shijia deposits, are hosted by mantle plume–derived mafic rocks largely free from the diagenetic effects experienced by sedimentary rocks, making them ideal opportunities to put the debate to rest. Mercury (Hg) and Au are cogenetic metals within the same source of the Carlin-type Au deposits. Mass-independent fractionation of Hg isotopes (MIF-Hg, reported as Δ199Hg) results in reservoir-specific Δ199Hg values. Magmatic and hydrothermal processes do not generate MIF-Hg, with Δ199Hg being directly inherited from source regions. These characteristics make MIF-Hg a reliable diagnostic tracer to address the link between Carlin-type Au mineralization and the assumed reservoirs. Here, ore-associated sulfides from the Pingle and Shijia deposits display negative Δ199Hg (−0.27‰ to −0.03‰) values, which are distinct from the near-zero Δ199Hg (−0.06‰ to +0.04‰) values of mantle plume–related mafic hosts, the positive Δ199Hg (0.0‰ to 0.40‰) values of metasomatized subcontinental lithospheric mantle–derived ultramafic rocks, and the positive Δ199Hg (0.04‰ to 0.22‰) values of sedimentary rocks. Thus, the deep mantle beneath the Youjiang basin, mantle plume, and shallow sedimentary strata did not feed Hg, and by inference Au, for Au mineralization in the Pingle and Shijia deposits. In contrast, the negative Δ199Hg values of ore-associated sulfides are consistent with the Proterozoic basement (−0.21‰ to −0.04‰), suggesting basement as the primary metal source for the Pingle and Shijia Au deposits. Integration of the reported multiple isotopic and experimental data for other Carlin-type Au deposits in the basin shows that ore-forming metals in the Precambrian basement were predominantly leached by magmatic fluids and deep crust–derived fluids. Our study agrees with previous studies showing that there is no genetic linkage between mantle plume and Carlin-type Au mineralization in the Youjiang basin.