Nature and origin of dolomitization associated with sulphide mineralization: new insights from the Tappehsorkh Zn‐Pb (‐Ag‐Ba) deposit, Irankuh Mining District, Iran

Several types of dolomitization are observed in the Lower Cretaceous carbonate sequence of the Tappehsorkh deposit, Irankuh Mining District, Iran: 1) regional and extensive early diagenetic dolomitization (D1), and 2) localized hydrothermal dolomitization (D2 and D3). D1 regional dolomite, which partly to completely replace limestone, is characterized by fine‐grained euhedral to subhedral dolomite rhombs. There is no evidence for Zn‐Pb sulphide mineralization associated with this type of dolomite. Medium to coarse‐grained D2 and D3 hydrothermal dolomites occur along the syn‐sedimentary Gushfil‐Baghabrisham normal fault, about 40 m above the Lower Cretaceous sequence within black siltstone, dolostone, and crystal lithic tuff and lava rocks. Hydrothermal dolomite cross‐cuts and brecciates the host rocks. This dolomite was replaced by quartz and sulphide minerals of the main ore stage. D1 regional dolomite has average values of ‐7.37‰ and 2.20‰ for δ18O and δ13C, respectively. The δ13C values of this dolomite fall well within the range of the Lower Cretaceous carbonates. D2 hydrothermal dolomite has average δ18O and δ13C values of ‐7.92‰ and 2.93‰, respectively. The respective δ18O and δ13C values of D3 hydrothermal dolomite are ‐12.27‰ and 2.39‰. The δ18O values of D2 and D3 dolomites are more negative than those of D1 regional dolomite; this can be due to their hydrothermal origin. Fluid inclusion studies on D3 dolomite and quartz show temperatures of 170‐260 °C. The concentrations of Fe and Mn in D1 regional dolomite are very similar to those of limestone, suggesting that they likely precipitated from the same fluid (seawater), whereas those of D2 and D3 dolomites are relatively high, in agreement with the hydrothermal origin for these dolomites. Precipitation of gypsum, which is ubiquitous in the study area, could have lowered the Ca/Mg ratio of seawater. While the extensive regional dolomitization formed from low‐temperature (evolved) seawater during the early diagenesis, D2 and D3 dolomites associated with the main stage of sulphide mineralization were formed by high‐temperature hydrothermal fluids moving along the Gushfil‐Baghabrisham Fault. Copyright © 2016 John Wiley & Sons, Ltd.

Marine redox variations during the Ediacaran–Cambrian transition on the Yangtze Platform, South China

The widely developed black shales on the Yangtze Platform recorded palaeoceanographic environment information during the Ediacaran–Cambrian transition. This paper describes an integrated geochemical study of rare earth elements (REEs), redox‐sensitive trace elements (RSTEs), and total organic carbon (TOC) contents in Ediacaran–Cambrian black shales at Daotuo, northeastern Guizhou Province, South China. Integrated RSTE data from the Daotuo area, in combination with previously published Fe speciation and Mo‐based proxies from another six sections (Shatan, Jiulongwan, Zhongling, Yangjiaping, Longbizui and Wuhe), suggest three major periods of water euxinia during the Ediacaran–Cambrian transition. Under these conditions, organic matter and RSTEs experienced various levels of enrichment in the black shales, especially in the lower Jiumenchong Formation. Given the patterns of Mo‐U covariations, metal‐oxyhydroxide particulate shuttles may have operated during the black shale deposition of the Doushantuo Formation (Member II) at Daotuo. Conspicuously, the upper slope water was oxic‐dysoxic during the earliest Cambrian, as determined by the REE, RSTE data and sedimentological characteristics of the Liuchapo Formation and the basal Jiumenchong Formation. The generally low RSTE concentrations in the Bianmachong Formation black shales (Cambrian Series 2, end of Stage 3) suggest a persistently oxic water column in upper slope settings. The coincidence between the marine oxygenation and the development of the ecosystem likely indicates the galvanizing effects of enhanced oxygen and biological element content on the fauna during the Ediacaran–Cambrian transition. Copyright © 2016 John Wiley & Sons, Ltd.