Tracing fault-related fluids using in situ trace elements: A case study in Upper Cambrian dolostone in Gucheng area, Tarim Basin, NW China

摘要

The Gucheng area, located in the east of the Tarim Basin, is one of the important gas exploration targets, and the Upper Cambrian dolostone is the main pay layer. The dolostone is considered to be altered by hydrothermal fluids, but the hydrothermal altering degrees are still unclear. In this study we report the petrography and in situ trace elemental data, and try to discuss the hydrothermal fluids and their impact on the Upper Cambrian dolostones. Based on the petrography and CL properties, five types of dolomites are identified. MD1 and MD2 are the matrix dolomites. The crystal size of MD1 range from approximately 30 to 100 mu m, while the MD2 is larger than 200 mu m. Cement dolomite (CD) occurring only in vugs and fractures is subdivided into three types depending on the CL properties. CD-zone1 is the core of CD with dull CL colours. CD-zone2 comprises a bright red CL colour zone and a dull CL colour zone, and CD-zone3 is the rim of CD dolomite with bright red CL colours. MD2 and CD-zone1 show light rare earth elements (LREE) depletion, middle rare earth elements (MREE) enrichment and negative Eu anomalies. The REEY patterns of CD-zone3 dolomite are marked by LREE enrichment and positive Eu anomalies. CD-zone2 is the transitional mineral phase that shows slightly LREE depletion and negative Eu anomalies. The REEY patterns of CD-zone1 suggest that CD-zone1 is likely formed by local redistribution of older dolomites in a closed system. The REEY patterns of CD-zone3 may indicate a magmatic associated hydrothermal fluid. Based on the petrography and in situ trace elemental data, the Permian magmatic fluid, which is recorded in CD-zone2 and CD-zone3 dolomites, may be only limited within fractures, while unlikely significantly altered the matrix dolomite in the Upper Cambrian.