Consistent trace element distribution and mercury isotopic signature between a shallow buried volcanic-hosted epithermal gold deposit and its weathered horizon

Highlights

• Paired rocks and soils display congruent patterns for immobile elements.

• Soils display congruent Hg isotopic signals to the underlying rocks.

• Mineralized rocks and soils showed common enrichment of elemental suite As–Ag–Sb–Hg.

• Limited Hg isotope fractionation occurs during the release of Hg from ore deposit to soils via weathering.

Abstract

Trace elements and Hg isotopic composition were investigated in mineralized rocks, barren rocks, and mineral soils in the Xianfeng prospect, a shallow buried epithermal gold deposit in northeastern China, to understand whether this deposit has left a diagnostic geochemical fingerprint to its weathered horizon. All the rocks and soils display congruent patterns for immobile elements (large ion lithophile elements, high field strength elements, and rare earth elements), which reflect the subduction-related tectonic setting. Both mineralized rocks and soils showed common enrichment of elemental suite As–Ag–Sb–Hg, suggesting that the Xianfeng gold deposit has released these elements into its weathered horizon. Similar mercury isotopic composition was observed between mineralized rocks (δ²⁰²Hg: −0.21 ± 0.70‰; Δ¹⁹⁹Hg: −0.02 ± 0.12‰; 2SD) and barren rocks (δ²⁰²Hg: −0.46 ± 0.48‰; Δ¹⁹⁹Hg: 0.00 ± 0.10‰; 2SD), suggesting that mercury in the Xianfeng deposit is mainly derived from the magmatic rocks. Mineralized soils (δ²⁰²Hg: −0.44 ± 0.60‰; −0.03 ± 0.14‰; 2SD) and barren soils (δ²⁰²Hg: −0.54 ± 0.68‰; Δ¹⁹⁹Hg: −0.05 ± 0.14‰; 2SD) displayed congruent Hg isotopic signals to the underlying rocks, suggesting limited Hg isotope fractionation during the release of Hg from ore deposit to soils via weathering. This study reveals evidence of a simple and direct geochemical link between this shallow buried hydrothermal deposit and its weathered horizon, and highlights that the weathering of shallow-buried hydrothermal gold deposits can release a substantial amount of heavy metals (e.g. Hg, As and Sb) to surface soil.

Introduction

Due to its chalcophilic nature, mercury (Hg) is found in abundance in sulfide minerals in hydrothermal deposits, making them the most important pools of Hg in the Earth’s crust (Rytuba, 2003). Hydrothermal gold (Au) deposits have common enrichments of Hg and other chalcophile elements such as arsenic (As)-antimony (Sb) (Hedenquist et al., 2000; Saunders et al., 2014). These metals are also known as heavy metals, which are of great environmental concern. The weathering of shallow buried gold deposits may leave high anomalies of Au–As–Sb–Hg in the weathered soil horizon, although these anomalies may be distorted by leaching, seepage, and solid physical processes. Meanwhile, the weathering of shallow buried gold deposits represents as an important source of heavy metals to the weathered soil horizon. Systematic analysis of the Au–As–Sb–Hg anomalies in the weathered horizon, where the soil is residual and mineralization is not covered by younger rocks or transported overburden, represents a straightforward method of prospecting buried orebodies (Bradshaw et al., 1979; Freyssinet et al., 1997).

The measurements of mercury isotopes and their geochemistry have become a new discipline for understanding the sources and fates of Hg in hydrothermal ore deposits (Hintelmann and Lu, 2003; Smith et al., 2005; Smith et al., 2008; Sonke et al., 2010; Yin et al., 2016a; Tang et al., 2017; Xu et al., 2017). Natural stable isotopes of mercury (¹⁹⁶Hg, ¹⁹⁸Hg, ¹⁹⁹Hg, ²⁰⁰Hg, ²⁰¹Hg, ²⁰²Hg, and ²⁰⁴Hg) can undergo both mass-dependent fractionation (MDF, defined as δ²⁰²Hg) and mass-independent fractionation (MIF, defined as Δ¹⁹⁹Hg). The MDF of Hg isotopes is ubiquitous and occurs during a variety of biogeochemical processes in the mercury cycle (Bergquist and Blum, 2009). The MIF of Hg isotopes is widely observed in Earth’s surface pools (e.g. soil, sediment, water, atmosphere, and biota), and is mainly generated during Hg photochemical reactions in the aquatic environment (Blum et al., 2014; Sonke, 2011). This makes Hg-MIF a powerful tracer for distinguishing between syngenetic and epigenetic Hg in hydrothermal deposits (Xu et al., 2017; Yin et al., 2019). Syngenetic Hg in magmatic/mantle materials is characterized by the absence of MIF (Δ¹⁹⁹Hg ∼ 0), and epigenetic Hg in sedimentary rocks has significant MIF signals (Δ¹⁹⁹Hg ≠ 0) (Yin et al. (2016a) and references therein).

In northeastern (NE) China, a large number of hydrothermal gold deposits have been discovered in the last decades (Zhou et al., 2002; Han et al., 2013; Zhai et al., 2015). Most of these deposits are associated with extensive forest cover and poor rock exposure, and exploratory trenching has been employed as a conventional method for prospection. With the increased preservation of forests and ecosystems in recent years, extensive exploratory trenches have been forbidden by the Chinese Forestry Administration. The application of soil sampling for geochemical exploration, which has low environmental and practical costs, should, therefore, be considered. To date, however, it is still unclear whether the isotopic composition of Hg in weathered horizon can be used for prospecting shallowly buried orebodies, because soil not only receives Hg from the weathered rocks but also via atmospheric deposition, and the two Hg sources have distinct isotope signatures (Zhang et al., 2013).

Through a recent project, we systemically collected mineralized rocks, barren rocks, and their overlying mineral soils from the Xianfeng gold prospect, a shallow buried epithermal gold in northeastern China. In our recent study about the Xianfeng deposit (Yin et al., 2019), we observed anomalously high Hg levels in mineralized rocks (geometric mean: 0.68 μg g⁻¹; n = 33) compared to barren rocks (geometric mean: 0.02 μg g⁻¹; n = 25). The isotopic compositions of Hg in rock samples were also studied, which displayed consistent Hg isotopic signals between mineralized rocks (δ²⁰²Hg: −0.23 ± 0.72‰; Δ¹⁹⁹Hg: −0.02 ± 0.12‰; 2SD, n = 33) and barren rocks (δ²⁰²Hg: −0.45 ± 0.44‰; Δ¹⁹⁹Hg: −0.01 ± 0.12‰; 2SD, n = 13). The absence of Hg-MIF in these rocks suggested that Hg in the Xianfeng gold deposit is of syngenetic origin (Yin et al., 2019). We further hypothesized that hydrothermal deposits may leave a diagnostic elemental and Hg isotopic signal in their weathered horizon. In this study, we conducted analysis of the concentrations and isotopic compositions of Hg in soil samples. The trace elements in both rock and soil samples were also measured. The aims of this study are to test: (1) whether the Xianfeng deposit left a diagnostic elemental distribution pattern in the weathered horizon, (2) whether the weathering of shallow buried hydrothermal deposits can release heavy metals to surface soil, and (3) whether Hg isotope compositions are similar between orebodies and weathered horizon in the Xianfeng deposit.