Tracing concealed gold deposits using mercury isotopes in surface soils: A study from the Shuiyindong gold deposit, Guizhou province, China

Abstract

Surface soil sampling has been proved to be an effective and low-cost mineral exploration method in covered terrains. The critical question is to determine whether the surface anomalies originated from the mineralization at depth. Mercury isotope signatures have recently been employed as a tool for understanding the sources of Hg anomalies. Meanwhile, Hg is a traditional pathfinder for Au exploration. Can Hg isotopes be applied to trace the sources of Au anomalies in topsoils in covered areas? This is the first study that employed Hg isotopes as an exploration proxy for Au mineralization at depth. Surface soil samples over the concealed Shuiyindong Au deposit, China, were collected and their total Au (TAu), total Hg (THg) contents, and Hg isotope compositions were analyzed. Gold and Hg surface positive anomalies display high correlation indexes along the sampled traverse above the Shuiyindong Au deposit. Moreover, the Hg anomaly is significantly stronger than that of Au. Soils from deep ores-related areas show clearly positive δ202Hg values (−0.78‰ to −0.23‰, averaging −0.46 ± 0.19‰, n = 10, 1SD) compared with those from the background areas (−1.67‰ to −0.66‰, averaging −1.07 ± 0.28‰, n = 10, 1SD). δ202Hg and 1/THg in deep ores-related soils have been altered by the ascending geogas from the mineralized rocks of the Longtan Fm. Soil positive Au anomalies detected in the study area are sourced at depth in the Shuiyindong Au deposit. However, it is not feasible to apply Δ199Hg to trace the anomalous Au sources in topsoils, because Hg(II) photoreduction can imprint the Δ199Hg signature, leading to negative Δ199Hg values in the soils. Semi-volatile Hg and ultra-fine Au particles are mobile and could continuously migrate from the depth to the surface through macro-structures and micro-fractures with the ascending geogas, then most of them are absorbed by the surface soils. Overall, this study successfully traced the Au deposit at >650 m depth using δ202Hg, which is significant for mineral exploration of deep-seated Au deposits, especially in unknown covered areas.