Genesis of Dolomitization in the Jutana Formation (Cambrian) Salt Range (Pakistan): Based on Field Studies, Petrographic Observations, Geochemical Signatures and Isotopic Analysis

Abstract

In Salt Range and Trans Indus Range (Pakistan), field observations in Jutana Formation (Cambrian) exhibits two main sub-divisions which includes; (i) the lower oolitic-pisolitic unit and (ii) upper massive dolomite unit. In the lower unit, dolostones containing ooids/pisoids and faunal assemblages are interlayered with whitish grey sandstone with distinct depositional sedimentary features (i.e., ripple marks, trough-, herring bone- and hummocky-cross bedding). The upper dolostone unit consists of thick bedded to massive dolostone. These two units are separated by shale beds. Detail microscopic studies identified three distinct diagenetic phases containing; (i) fine crystalline dolomite (Dol. I), (ii) medium-coarse crystalline dolomite (Dol. II) and, (iii) fracture associated, coarse crystalline dolomite (Dol. III). Mineralogical analysis, which include X-ray powder diffraction (XRD) analysis identified the concentration of dolomite and quartz mainly in the studied formation. Based on stoichiometric calculations, dolomites of Jutana Formation are classified into two groups; fine crystalline dolomite (Dol. I), exhibits non-stoichiometric low ordered dolomites, while medium-coarse crystalline dolomite (Dol. II) and, (iii) fracture associated, coarse crystalline dolomite (Dol. III) are identified as stoichiometric ordered dolomites. Major and trace elemental studies exhibits high concentrations of Sr and Na with comparatively low values of Mn and Fe for Dol. I, whereas Dol. II and Dol. III represents high Mn and Fe contents with low Na and Sr concentration. Stable isotopes ( 18O) signatures denotes low values in fine crystalline dolomite (Dol. I). Furthermore, medium-coarse crystalline dolomite (Dol. II) and, (iii) fracture associated, coarse crystalline dolomite (Dol. III) represents high and more depleted values respectively. In addition, 13C values of fine crystalline dolomite (Dol. I), mediumcoarse crystalline dolomite (Dol. II) and fracture associated, coarse crystalline dolomite (Dol. III) lie in the range of marine signatures. Results of strontium isotopes lies above its original marine signatures indicating interaction with radiogenic lithologies. Magnesium 25 26Mg) result elucidates altered marine (mixing zone) and hydrothermal dolomitization origin for these dolomites. In conclusion, above mentioned studies demonstrated that dolomites of Jutana Formation are formed in multi stages i.e. initially in mixing zone near surface shallow condition followed by hydrothermal dolomitization in intermediate to deep burial environmental setting in later stages.