Analysing On The Origin Of Cherts

The chert is one of the common components of the carbonate rock, and the forming mechanism of chert is closely related to the sedimentary background and the source of the quartz. This study consists of two reseach areas:Marine carbonate of Middle Permian Qixia Formation in Chaohu, Anhui and Lacustrine carbonate of Lucaogou Formation in Urumchi, Xinjiang. And a primary discussion about the cause of this two kinds of cherts would be made through the analyzing of petrology and geochemistry to provide a study case for the origin of the chert.The Middle Permian Qixia Formation in Chaohu area, Anhui Province, was mainly composed of fine-grained clastic rocks and limestones which were deposited in the coastal marsh and carbonate platform. Most of cherts were axiolitic and moniliform in outcrops. Between some cherts and limestones, there developed transitional belts with0.5cm width around cherts. The feature of petrology suggests that the chert is formed by the replacement of quartz during the early diagenesis stage, this progress involves the dissolution of biological shell and marl under the mildly acidic environment, that is to say the organic acid produced by degradation of the abundant organisms could cause the dissolution of the marl and the calcareous shells and increase the saturation of CaCO3at the same time, which would restrain the depositing of CaCO3and the quartz tends to depositing then fomed the cherts. The Middle Permian Lucaogou Formation in Urumchi,Xinjiang Provience, was mainly consist of conglomeratic sandstones and interbed of black mudstone and limestone which were deposited in the coast and shallow lake or deep to semi-deep lacustrine environments and the particulate chert distributed limitedly in a few layers. The chert deposited radially in the pore which characterized by the recrystallization of the carbonate minerals on its edge. In conclusion, the quartz deposited after the depositing and partial-recrystallization of calcite, the SiO2along with the fluid deposited in the pore and fracture or replaced the sparry calcite prior then formed the particulate and micro-banded cherts.The Major and trace elements as well as Sr-Nd isotope of cherts in Qixia Formation of chaohu, Anhui Provience revealed that the quartz in the chert was continental in origin probably. And the geochemistry of chert in Lucaogou Formation, Urumchi, Xinjiang Provience showed that the quartz in cherts was from the deep substance probably.