Characteristics and Origin Analysis of Lacustrine Dolomites in the Lower Ganchaigou Formation, Western Qaidam Basin

Key words: 
• western Qaidam Basin /
• lacustrine dolomites /
• seepage-reflux dolomitization

Abstract: The rock fabric,mineral composition,main trace-element geochemistry and carbon and oxygen isotopic characteristics of lacustrine dolomite in the Lower Ganchaigou Formation of the Qaidam Basin were studied,and the origin of the dolomite was analyzed. The sedimentary environment of dolomite in the study area was shallow water and a relatively closed lacustrine environment. The dolomite is predominantly micritic,and occurs in layers and blocks. It is seen as algal,gypsiferous,and grain dolomite. The degree of ordering is generally low. The ratio of Mg²⁺ to Ca²⁺ is low. The Mn,Fe,Na and Sr contents are all high in the major elements,indicating that the water body was highly sa-line and that it was relatively closed. Light rare earth elements among the trace elements are relatively enriched,with negative europium anomalies,reflecting a low-temperature diagenetic environment and shallow burial. Oxygen iso-topes have a low negative value,indicative of salinized water overall. Carbon isotopes are low-positive or negative, mainly due to atmospheric water and biological effects. The study suggests that the dolomite was formed penecontem-poraneously in a relatively closed saline water environment. The Mg²⁺ in the dolomitic fluid is mainly derived from leaching by atmospheric water and deep brine,with a small amount from algal decomposition. The Mg²⁺ was enriched by evaporation and salt mineral precipitation,and the enriched fluid was dolomitized by seepage-reflux infiltration of calcite.