Non-hydrothermal saddle dolomite in Upper Cambrian dolostones of Tarim Basin: evidence from C-O-Sr isotopic and in-situ trace elemental studies

The Upper Cambrian dolostones in the Tarim Basin are interpreted to be intensely altered by hydrothermal fluids during the burial diagenesis, and the saddle dolomite filled in fractures is commonly considered as hydrothermal origin. In this study, the saddle dolomite samples from drillhole SNP1 and the Quruqtagh area are sampled, and C-O-Sr isotopic compositions and in-situ trace elemental analyses are employed to check their origin. Replaced dolomites with finely crystal size and coarsely crystal size (RD1 and RD2, respectively), and saddle dolomite cements filled in vugs and fractures (VSD and FSD, respectively) are identified in the Well SNP1. Stylolites crosscut RD1 and RD2 dolomites. KZ-SD is the saddle dolomite in hydrothermal dolostone geobodies in the Quruqtagh area, and shows coarsely crystal size up to 1 cm and wavy extinctions. RD1, RD2 and FSD dolomites show similar delta C-13(VPDB) values of - 1.99 to - 0.73 parts per thousand, and similar Sr-87/Sr-86 ratios of 0.70868-0.70905. RD1 and RD2 dolomites show delta O-18(VPDB) values of - 8.74 to - 5.29 parts per thousand and of - 10.22 to - 9.20 parts per thousand, respectively. The delta O-18(VPDB) values of FSD range from - 10.94 to - 10.16 parts per thousand. RD2, VSD and FSD dolomites show Fe concentrations of 154.76-464.65 ppm, Mn concentrations of 15.74-55.93 ppm and REE concentrations of 1.29-4.61 ppm. The REEY profiles of RD2, VSD and FSD dolomites are characterized by flat to convex patterns with slightly MREE enrichment, and no Ce, Eu and Y anomalies. KZ-SD dolomite shows Fe concentrations of 1081.55-1206.35 ppm, Mn concentrations of 427.25-474.38 ppm and REE concentrations of 12.97-23.64 ppm. The C-O-Sr isotopic compositions and REEY features of RD2, FSD and VSD indicate that saddle dolomites filled in vugs and fractures in the Well SNP1 are formed from redistribution of precursor dolostones, and indicate that RD2 may formed by recrystallization during burial diagenesis.

Automated summary

In this study, the origin of saddle dolomite in Upper Cambrian dolostones in the Tarim Basin was examined using isotopic and elemental analyses. The results suggest that the saddle dolomite filled in vugs and fractures is formed by redistribution of precursor dolostones.