Constraints on dolomite formation in a Late Palaeozoic saline alkaline lake deposit, Junggar Basin, north-west China

Alkaline lakes (pH > 9) are among the few modern sedimentary environments that are hydrochemically favourable for low-temperature dolomite formation. While Mg-clays and Mg-evaporites also form more easily in alkaline environments, few studies have focused on how the kinetically inhibited dolomite wins the competition for Mg2+. Here, a basin-wide survey of the distribution, paragenesis and stable C, O and Mg isotopes of main Mg-rich minerals in the Late Palaeozoic saline alkaline lake deposit of the north-west Junggar Basin, north-west China, is conducted to study the influence of the formation and diagenesis of eitelite, northupite and Mg-clays on dolomite formation. Large, isolated dolomite crystals (20 to 70 & mu;m in diameter), show positive & delta;C-13 values (ranging from +1 to +7 & PTSTHOUSND;) and a restricted distribution in the mudstones of the lake-transitional zone. These crystals have been interpreted as organogenic dolomite driven by methanogenesis via fermentation of organic substrates. The & delta;O-18 values of dolomitic mudstones (from -7.4 to +3.4 & PTSTHOUSND;), calcitic mudstones (from -15.1 to -3.3 & PTSTHOUSND;) and bedded Na-carbonate evaporites (from +0.08 to +3.7 & PTSTHOUSND;), together with their Mg isotopic compositions, suggest that dolomite was not enriched in the most concentrated environments or during stages with most Mg sources, but in the organic-rich deposits containing few other authigenic Mg-rich minerals. Dolomite is at a competitive disadvantage for Mg2+ ions compared to Mg-evaporite and Mg-clay minerals due to its slow crystallization rates and the deficiency of micritic calcium carbonate precursors. However, it can nucleate and progressively grow into large crystals (>20 & mu;m) if bacterial methanogenesis could effectively lower porewater pH (<8.5) and induce the dissolution of generated eitelite, northupite or Mg-clays. These findings suggest that high salinity and/or high alkalinity are not always favourable conditions for dolomite formation and highlight the active role of pH fluctuations in inducing low-temperature dolomite formation.