Geochemical constraints on the origin and environment of Lower Cambrian, selenium-rich siliceous sedimentary rocks in the Ziyang area, Daba region, central China

Abstract

The Ziyang area is one of the two major regions of central China subjected to selenium (Se) poisoning. Systematic studies of Se contents of different lithologies from this area indicate that Lower Cambrian, carbonaceous, and siliceous strata host the highest Se contents (with Se contents of up to 278 ppm). We have investigated their geochemical characteristics (major and trace elements, and Si and O isotopes), origin, and sedimentary environment of formation. The siliceous rocks are characterized by a wide range in major elements, and are enriched in Se, Ba, Cu, Ni, V, As, Sb, and U relative to average continental crust. They are also enriched in light rare earth elements relative to heavy rare earth elements (La N /Yb N = 1.64-35.7) and show weak or moderate negative Ce anomalies and strong positive Eu anomalies. δ 30 Si NBS-28 and δ 18 O V-SMOW values range from -0.3‰ to 0.6‰ and 16.1‰ to 21.7‰, respectively. The homogenization temperatures of inclusions within the studied samples range from 113 • C to 319 • C, and their salinities from 1.2 to 13.7 wt.% NaCl equivalent. Our results suggest that the studied siliceous rocks resulted from hydrothermal sedimentation in a relatively anoxic semi-deep sea sedimentary environment. The hydrothermal fluid responsible for Se-mineralization involved the mixing of low-temperature high-salinity fluid, low-temperature low-salinity fluid, and a high-temperature low-salinity basinal fluid in the NaCl-(KCl)-H 2 O system.

Submarine silica-rich hydrothermal activity during the earliest Cambrian in the Tarim Basin, Northwest China

“…However, the mechanisms responsible for such a high silica flux and extensive submarine silica precipitation are debated. Hydrothermal activity Yu et al 2009;Feng et al 2012;Wang et al 2012a;Fan et al 2013) as well as biogenic (Guo et al 2007) and hydrogenic origins (Chang et al 2009(Chang et al , 2012Ramseyer et al 2013) have been proposed as potential causes. In recent years, increasing pieces of sedimentological and geochemical evidence, particularly from the Yangtze block, South China, have suggested that silica fluxes from extensive submarine hydrothermal activities played an important role in the unusual silica deposition (Lott et al 1999;Chen et al 2009;Feng et al 2012;Wang et al 2012a;Fan et al 2013).…”

“…In the E-C boundary successions, siliceous sedimentary rocks (bedded chert and phosphatic chert) are widely distributed around the world, including those in the Yangtze block, south China (Guo et al 2007;Chang et al 2009Chang et al , 2012Chen et al 2009;Feng et al 2012;Wang et al 2012a); the Tarim block, Northwest China (Yang et al 2006;Yu et al 2009); Lesser Himalaya, India (Mazumdar et al 1999); Oman (Ramseyer et al 2013); Lesser Karatau, Kazakhstan (Litvinova 2007); and Taoudeni cratonic basin, West Africa (Flicoteaux and Trompette 1998). These sediments were deposited in a short time interval (about 550-520 Ma) in peritidal to basinal environments; the widespread spatial distribution of these primary silica precipitates points to abnormally silica-rich seawater ), likely due to excessive silica fluxes into the ocean (Yool and Tyrrell 2005).…”

Mineralogy and Selenium Speciation Analysis of Early Cambrian Selenium-Rich Black Shale in Southern Shaanxi Province, China

“…(1) the enrichment of polymetallic elements of the Early Cambrian Niutitang Formation in Zunyi, Guizhou, southern China [19,21], and Xiangxi, western Hunan, China [22];…”

“…(2) the selenium enrichment of the Lower Cambrian Lujiaping Formation in Shuang'an, Ziyang, the southern Qinling Mountains, China [23,24]; (3) the Larma Se-Au deposit in the Cambrian Taiyangding Group [25]; and (4) the independent selenium deposit in Yutangba of Enshi Shuanghe, western Hubei, in the Permian Maokou Formation [20,26,27].…”

“…Previous studies on mineralogy and selenium in southern Qinling found the following: (1) The black rock series in the Qinling Mountains is made up of carbonaceous argillite, carbonaceous siliceous argillite, marl, stone coal, and carbonaceous limestone; additionally, phosphatic nodules, pyrite, and barite are always found in these rocks [50]. (2) Systematic studies on the Se content of different lithologies from this area have indicated that the Lower Cambrian, carbonaceous, and siliceous strata host the highest levels of Se [24]. (3) By combining studies on ore characteristics, performing analyses of mineralization elements, and conducting electron probe analyses, the contents of Fe, Cu, Zn, and the other metallic elements in carbonaceous shale were determined [51].…”