Distributions of selenium and related elements in high pyrite and Se-enriched rocks from Ziyang, Central China

Highlights

• Pyrite is capable of accumulating Se 1-2 orders of magnitude higher than host rock.

• Pyrite grains implied two types of oscillatory zones of Se and As.

• Se is likely sourced from hydrothermal fluids during the anoxic sedimentation.

• Se minerals related to elemental exsolution is likely present in high-Se veins.

Abstract

Pyrite-bearing volcanic tuff and carbonaceous slates of the Lower Cambrian Lujiaping Formation have been considered the source of selenium (Se) in high-Se soils in the area of the Naore Village of the Ziyang County, China, where acute Se toxicity occurred in the 1980s. For geochemical exploration, the accumulation and distribution of Se and trace elements within the pyrite grains are of great indicative significance for understanding the possible origin and forming mechanism of the Se enrichment. In this work, 7 rock samples and 89 pyrite grains were collected and analysed to investigate the element micro-distribution within the pyrite grains using EPMA and LA-ICP-MS. The whole-rock analysis shows that the pyrite-bearing Lower Cambrian rocks in Ziyang are commonly enriched in Se, As, Mo, Cd, Cr, Ni, Sb, Cu and Ba. In situ microanalysis indicates that Se (30–1880 μg/g) and As (160–2960 μg/g) concentrations within the pyrite grains are 1–2 orders of magnitude higher than those in the host rocks (Se: 0.56–73.1 μg/g, As: 2.56–195 μg/g) and that the Se contents are negatively correlated with pyrite mineral sizes. Grain-scale element mappings of fresh Se-enriched pyrites reveal two types of Se and As distribution patterns (type-I: high-Se 120–1870 μg/g and low-As <150–1580 μg/g cores, type-II: low-Se 60–680 μg/g and low-As <150–1310 μg/g cores) with alternating Se or As contents during the growth of zoned pyrite. The texture of pyrite element zoning reflects multi-stage hydrothermal pulses with changing trace element concentrations. Type-II pyrite formed later than type-I pyrite. Genetic analysis confirms that Se-enriched strata were strongly affected by hydrothermal submarine deposition and that Se in pyrite was mainly derived from hydrothermal fluids. Secondary hydrothermal erosion led to the formation of high-Se “fractures” (Se content 19.3 wt%). In summary, utilizing pyrite as an indicator for the Lower Cambrian Lujiaping Formation Se-bearing strata extends our understanding of selenium geochemical processes and highlights the mechanistic aspects of Se.

Introduction

Selenium (Se) is an essential trace element for cellular functions of humans and animals, such as improving immunity, delaying senescence, and preventing cancer and cardiovascular disease (Liang et al., 2017). In addition to these biological functions, strong industrial demands are currently increasing to produce lithium‑selenium batteries for energy storage (Eftekhari, 2017; Zhao et al., 2018). Consequently, exploration for Se resources has been intensified. However, Se is toxic to humans at high concentrations and can cause multiple organ toxicity, a condition called selenosis (Cui et al., 2017). Naore village in Ziyang County, southern Shaanxi, is one of the few known Se-enriched areas in China, where cases of human Se poisoning were recorded in the 1980s (Cheng and Mei, 1980). However, the Se enrichment in the soil stimulated agriculture in the Naore area, which gave rise to the name “Chinese Selenium Valley (CSV)” (Tian, 2017). Soil Se contents in Naore village range from 2 to 28 μg/g (Cheng and Mei, 1980), which is 5 to 70 times higher than the global average concentration of Se in soil (0.4 μg/g; e.g., Mayland, 1994). The Se in soil is attributed to pyrite-bearing and Se-enriched tuffs and carbonaceous rocks of the Lower Cambrian Lujiaping Formation (Luo et al., 2004). The maximum Se contents in rocks of the Lujiaping Formation are 128 μg/g in carbonaceous slates, 278 μg/g in siliceous rocks and 303 μg/g in black shales (Feng et al., 2012; Tian et al., 2016a; Long and Luo, 2017). In Naore, pyrite commonly occurring in these Se-enriched rocks has been identified as the main carrier of Se (Tian et al., 2016a, Tian et al., 2016b), which is similar to the situation in the western San Joaquin Valley, California, USA, where cases of Se poisoning of animals have been documented (Wu et al., 2000). These findings reveal that the transport of Se and its subsequent deposition within sulfide systems can constitute a potential resource for Se supplementation and hold great importance for the ecological environment (Fordyce, 2013). Investigating the distribution and accumulation of Se in pyrite is therefore highly important to understand the geochemical migration and cycle of Se among various geological environments (Deditius et al., 2008; Dare et al., 2011; Carbone et al., 2012) and to evaluate the potential of the Se resource and its environmental impact.

In China, black shale series exposed in several regions are extremely enriched in Se, such as the Ziyang-Langao area of southern Shaanxi, the Enshi area of Hubei, the Laerma area of north-western Sichuan, and the Zunyi area of Guizhou (Fig. 1A), most of which belong to Early Cambrian strata with a few Permian ages (Fan et al., 2011; Liu et al., 2000; Wen et al., 2006; Wen and Qiu, 2002; Zhu and Zheng, 2001). Although many investigations have been performed on these specific strata, including stratigraphic, petrographic and geochemical analyses (Steiner et al., 2001; Wen and Qiu, 2002; Luo et al., 2004; Jiang et al., 2007; Lehmann et al., 2007; Feng et al., 2010, Feng et al., 2012; Fan et al., 2011; Long and Luo, 2017), the Se sources and processes that have led to Se enrichment are still under debate. The theories of Se enrichment in Lower Cambrian sediments of the Yangtze Platform and southern Qinling include submarine-hydrothermal origin (Steiner et al., 2001; Wen and Qiu, 2002; Jiang et al., 2007; Han et al., 2017), seawater scavenging (Mao et al., 2002; Lehmann et al., 2007; Yin et al., 2017) and multiple sources (Kříbek et al., 2007; Feng et al., 2010; Fan et al., 2011). Wen and Qiu (2002) made a summary of the geological characteristics of five typical selenium-bearing formations in China, involving the information of tectonic settings, strata, rock assemblage, element assemblage, Se content, genesis of cherts, etc. They suggested that a Se-bearing formation is a suite of rocks with anomalies of Se (usually >5 mg/kg) and of many other elements, and the proposed genesis of cherts in these Se-bearing formations were all hydrothermal sedimentation. One distinction between Ziyang and the other Se-enriched areas is that the rock assemblage in Ziyang is dominated by siliceous, carbonaceous slate with intercalated carbonaceous chert, while other formations mainly comprise of carbonaceous chert, black mudstone, shale and phosphorite (Luo et al., 2004; Wen et al., 2006; Jiang et al., 2007). Furthermore, although all of these sediments were enriched in organic matters and pyrites, both of which were abundant of Se, among which the highest sulfur content was measured in the sediments from the Ziyang area, eastern Qinling Mountains region (Wen and Qiu, 2002).

To date, only a few studies have been conducted aiming to reveal the source(s) of Se and its enrichment in the Lujiaping Formation. Feng et al. (2012) suggested a hydrothermal origin by investigating the geochemical characteristics, isotopes and inclusions of the strata. Similarly, Long and Luo (2017) concluded, based on geochemical whole-rock data, that the main Se sources for the Lujiaping Formation were hydrothermal fluids.

As a ubiquitous sulfide, pyrite and its geochemical information are adopted to speculate about the genesis, migration and transformation of associated minerals; this approach is one of the important directions for mineralogical study (Keith et al., 2016). Moreover, pyrite has been reported to be one of the main carriers of Se, for example, in the Lower Cambrian black shale series of South China (Fan et al., 2011; Tian et al., 2016b). For geochemical exploration, the accumulation and distribution of Se and other trace elements within pyrite, controlled by the formation environment and processes, are strongly indicative for understanding the possible origin and formation mechanism of Se enrichment in the Lujiaping Formation. Therefore, a micro-scale study of pyrites combined with whole-rock analysis was conducted to further delineate the possible mechanism of Se enrichment in pyrite-bearing rocks. In this study, 7 rocks containing 89 pyrite grains were collected from the Naore village area to evaluate the element micro-distribution and its geochemical implications. Whole-rock analysis was conducted by X-ray fluorescence (XRF) and inductively coupled plasma optical emission spectroscopy (ICP-OES) to obtain the contents of major and trace elements (Se, As, etc.). Quantitative in situ microanalysis and element distribution analysis were performed on single liberated pyrite crystals (fresh and altered) as well as on pyrites still occurring in their textural rock context. In addition, sulfur isotopes of pyrite were measured to constrain the sources of S and Se. The results are utilized not only to better understand the enrichment mechanism of Se in the Lower Cambrian pyrite-bearing strata but also to provide a comprehensive dataset for further research on the geochemical Se cycle and the sulfide systems.