Early Cambrian high-temperature dolomite of the Rizu Series in the Jalal-Abad iron ore deposit, Central Iran

Abstract

Early Cambrian dolomites associated with siltstone, volcanic, and volcaniclastic rocks are the host rocks of the Jalal-Abad iron deposit. Five types of dolomite are distinguished by petrographic studies; fine to medium crystalline, planar dolomites (Rd1, Rd2), medium to coarse crystalline (Rd3), cloudy, subhedral to euhedral dolomite cements (Cd1), and coarse to very coarse saddle dolomite cements (Cd2). The Rd1-type dolomites formed in replacement stages, while Rd2 and Rd3 dolomites are formed during the early diagenetic stage. Cd1- and Cd2-type dolomites were formed by recrystallization during hydrothermal processes. Cd1 and Cd2 dolomite is mainly associated with different types of iron oxide, quartz, and sulfide minerals. Investigations of fluid inclusions in saddle dolomites show homogenization temperatures from 160 to 260 °C, with a mean of 215 °C, and salinities between 32.1 and 38, with an average of 35.9 wt% NaCl equiv. The δ13C (PDB) compositions of the Rd1, Rd2, and Rd3 dolomites fall in the range of −5.30 to −2.97 and δ18O (PDB) values of dolomite vary between −9.54 and −8.58. The δ13C (PDB) compositions of Cd1 and Cd2 dolomites range from −11.00 to −4.17 and their δ18O (PDB) values from −16.80 to −8.89. The presence of volcanic and volcaniclastic rocks in the dolomitic sequences and crosscutting of dolomite sequences by intrusive bodies indicate magmatic activity as the heat source of high-temperature dolomitization. Based on the compiled data, we argue that high-temperature dolomitization was caused by hydrothermal fluids which migrated through faults and fractures during the Early Cambrian and, probably, later.