Cores from the Ordovician Yingshan Formation limestone in the of Well Shunnan 4 of the Tarim Basin show remarkable signs of hydrothermal alteration and metasomatism of silicon.Dissolved fracture-cavity

loose siliceous limestone/siliceous rock

hydrothermal veins and large quartz crystal cluster were abnormally developed.Observation of geological characteristics of the samples and analyses of fluid inclusion/rare earth elements and trace element

all suggested a mixed origin of the thermal fluids.It is believed that igneous activities provided energy and volcanic thermal fluid together with formation water formed a mixture of low-to-medium temperature and high salinity silicified hydrothermal fluid.REE analyses reveal that δEu values range from 0.040 to 0.118 and average 0.069-negative anomaly that indicates a reductive environments; Sm/Nd ratios range between 0.169 and 0.256 and average 0.194

indicating a crustal diagenetic fluids.The high similarity between the average REE distribution curve of samples and those of earth's crust suggests the origin of heat from magma

and to be more specific

the upper crust as indicated by a lack of Mg

2+

in the samples.Ba/Sr values range from 0.43 to 19.76 and average 6.70.Such a wide variation means the relative complexity of the fluid.U/Th ratios range from 0.35 to 4.76 and average 2.14.Th/U ratios range from 0.21 to 2.88 and average 1.03.And Th/U-Y/Ho correlations all indicate that the fluid is of low-temperature type.