Geochemical Characteristics And Geological Significance Of Calcite Veins In The Lingshan Island

Lingshan Island,located in the Riqingwei Basin off the coast of eastern Shandong peninsula,is a compressional and tilted island in origin.Structure joints are widely developed within the strata of the island,and many of them were filled by calcite veins as observed in outcrops along the Chuanchang section.This provides a natural laboratory for studying the regional tectonic stress field and the history of fluid activity.After the classification and matching of structural joints in the study area,1200 sets of joints were measured.The combined data of the statistics of the occurrence frequency of tensile joints and the analysis of the isodensity diagram of shear joints show that structural joints in the study area were generated by three stages of regional stress,and the directions of the stresses are:NWW-SEE,NW-SE,and NE-SW.A series of calcite veins were collected and analyzed by optical microscopy,cathodoluminescence,and fluid inclusions.The data show three stages of calcite veins in the Lingshan Island.The first stage of the calcites show small grain size and bright orange luminescence,indicating that the content of Mn²⁺was higher than 0.04%.They were probably formed under reducing environment.The modal homogenization temperature of fluid inclusions in the calcites was 197.3～213.4℃.The second stage veins are composed of horse tooth calcites with brown luminescence,suggesting low Mn²⁺/Fe²⁺value in calcites that formed in an oxidizing condition.The modal homogenization temperature of fluid inclusion was220～261.5℃.The third stage veins were calcites with large grain size and orange red luminescence,indicating Mn²⁺/Fe²⁺value>10 and Mn²⁺content>0.04%.Such veins formed in a weak reducing condition,and the homogenization temperature peak of fluid inclusions was305～410℃.It is possible that the three stages of calcite veins were genetically related to the three stages tectonic activity:the joints formed by NWW-SEE stress resulting in the generation of small grains of calcite,the joints with NW-SE stress resulting in the formation of horse tooth shaped calcite,and the joints with NW-SE stress forming large grain calcite.In order to understand the nature of the fluids responsible for the precipitation of the calcite veins,the veins and surrounding host rocks were measured for their stable carbon and oxygen isotopic compositions.The results show that the fluids precipitating the first stage of the vein have probably been affected by granite-related fluids and are largely formation waters in composition,more specifically a mixture of formation waters and high-temperature waters.In the second stage,the vein-forming fluids were likely sourced from marine carbonate rocks and has been influenced by carbonate dissolution and atmospheric water.The source of vein-forming fluid in the third stage is probably related to granite intrusion,which provided high temperature magmatic hydrothermal solution.