Introduction Materials and Methods PETROGRAPHY, TRACE
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Pak. J. Sci. Ind. Res. 2004 47(3) 195-204 PETROGRAPHY, TRACE ELEMENT GEOCHEMISTRY, AND DOLOMITIZATION MODEL - JHIMPIR DOLOMITE, SINDH, PAKISTAN Shahid Naseem a*, Shamim Ahmed Sheikh a, M Qadeeruddin b and Khaula Shirin b aDepartment of Geology, University of Karachi, Karachi 75270, Pakistan bPCSIR Laboratories Complex, off University Road, Karachi 75280, Pakistan (Received May 8, 2002; accepted May 24, 2004) Dolomites of Jhimpir area have been examined petrographically and geochemically to observe and characterize the process and extent of dolomitization. These are petrographically classified as calcitic dolomite and dolomitic lime- stones. The texture of dolomite is idiotopic. Most of the dolomite rhombohedrons are fine to medium grained and zoned. The replacement of biomicritic carbonate rocks (Laki Limestone of Early Eocene age) by Mg and Fe-rich solutions is observed in thin section. Major and trace elements have been analyzed in order to assess the compatibility of these elements with dolomite. The average MgO content in the samples close to small fault is between 16 and 17 % and decreases from the fault outwards, down to 5 to 4 %. The Fe, Mn, Na and K contents exhibit a positive correlation with increasing dolomitization, whereas, it correlates inversely with Sr. This paper provides evidence that the dolomites of the Jhimpir area are derived from epigenetic replacement. Key words: Dolomite, Petrography, Geochemistry, Dolomitization model. Introduction Materials and Methods Extensive deposits of dolomite are found in the Upper Indus Three sections namely Quarry, Road and North were sampled Basin of Pakistan (Kazmi and Abbas 2001). In the Lower Indus to evaluate the extent of dolomitization (Table 1). Samples Basin, deposits of dolomite are limited to Jhimpir and the ad- were crushed to 10 mm size using Jaw crusher and pulverized joining areas. Dolomite mineralization is confined to the lower (-200 mesh) in a Tema mill for chemical analysis. One-gram air beds of Laki Limestone Unit, the uppermost member of Laki free sample was treated with hot concentrated hydrochloric Formation of Ypresian age, in the eastern foothills of southern acid. The solutions were analyzed for Mn, Fe, Sr, Na, K and Zn Kirthar Range. using a Hitachi Atomic Absorption Spectrophotometer, model The process of dolomitization creates the scenario for pos- Z-8000, at the Pakistan Council of Scientific and Industrial sible hydrocarbon reservoirs with high porosity and improved Research Laboratories, Karachi. Insoluble residue (IR), com- capillary properties. Dolomite can also serve as host for bined oxides (R2O3) and loss on ignition (LOI) were deter- Mississippi Valley-Type sulphide ores (Beales and Hardy 1980; mined gravimetrically. Calcium and Mg were measured by Buelter and Guillmette 1988). Dolomite itself serves as an EDTA titration. excellent commodity in various applications in metallurgy, Thin sections of dolomite were studied with the help of a refractory material, agriculture and building material. photographic microscope (Laborlux POLS). Photomicro- The origin of dolomite has been a matter of discussion for a graphs of the selected samples were done with the help of long time in sedimentary geology. It is a complex mineral and Photoautomat Wild Leitz (MPS46). difficult to be synthesized in the laboratory. Purser et al (1994), Location and accessibility. The study area is situated at 2 Mresah (1998), Burns et al (2000), Cioppa et al (2000), Teal km west of Jhimpir railway station in the district of Thatta. The et al (2000), Yoo and Shelton (2000), Railsback and Hood area is accessible from Karachi both by Super Highway and (2001) are few examples of recent work on the origins of dolo- National Highway. On Super Highway a link road turns to- mites. The present study is focused on the study the trace wards Jhimpir at 94 km from Karachi. element geochemistry and petrography of the Jhimpir dolo- General geology. The study area lies in the southern ex- mite deposit. Based on these studies and field relationships a tension of the eastern foothills of the Kirthar Range. The area genetic model for dolomitization is suggested. has a low relief, of ~30 m above the local ground. Numerous *Author for correspondence small ridges are scattered in the area. Tertiary sedimentary 195 196 S Naseem, S A Sheikh, M Qadeerudin, K Shirin rocks are exposed in the study area (Table 2). The Ranikot Table 1 Group of Paleocene age is the oldest rock formation of the Location, section, and sample numbers of the study area and occupies the southern part of the study area. The area Laki formation of Ypresian age outcrops mainly in the south- Section Sample No. Location ern part of the Kirthar Province of the Lower Indus Basin. It is Quarry Q1 to Q10 (Bottom to top) N 25° 00’ 36’’ confined to a strip that extends from the Laki and Bhadra Section Q11 to Q16 (West to east) E 67° 59’ 53’’ Ranges in the north to the hills near Thatta in the south. The Nari Formation (Oligocene) is also exposed as small outcrops. Road R1 to R9 (Bottom to top) N 25° 01’ 87’’ Section E 68° 00’ 01’’ The Laki Formation consists mainly of limestone, but marl, calcareous shale, sandstone and lateritic clay are locally sig- North N1 to N5 (Bottom to top) N 25° 02’ 68’’ nificant constituents of the formation. At the type locality of Section E 68° 00’ 05’’ this formation, consists predominantly of white argillaceous and nodular limestones. The basal sequence of the Laki For- MgO content in the calcite-dolomite system, being a rock- mation, named Sohnari Member is a ferruginous assemblage. forming mineral of magnesian limestones, dolomitic lime- The main components of this member are lateritic clays and stones, calcitic dolomites and dolomites with increased Mg variegated shales, colored in yellow, white, orange, red and content. There are certain drawbacks in this sketch. First, the brown. Variegated sandstones are also common as lenticular carbonate may not form a 100 % of the rock composition. beds. The sandstones are soft, sugary, friable and generally Second the percentage of dolomite is not the reciprocal of ferruginous. Some of the clays, lignite, white sandstones and the calcite content. Fairbridge (1970) presented a schematic ochre of this formation have some commercial value. The Laki classification for the Ca-Mg carbonate rock system (Fig 1). In Formation is 250 m thick in the type locality (Mari Stream) and this scheme the terms are directly applied to the carbonate attains more than 600 m in thickness near Thatta. The forma- fraction only and the analysis are plotted in that diagram, in tion contains rich fossil assemblages including foraminifers, order to illustrate the possible classification of the studied gastropods, bivalves, echinoids and algae (Shah 1987). samples. The upper part (Chat Member) constitutes the major portion The plots of all samples, collected close to fault plane fall into of the formation. It consists of hard limestones with soft shales the “Calcitic dolomite” field on the Fairbridge (1970) diagram, and marls. The limestones are thickly bedded, massive and while the samples from the fold limb fall into the “Dolomitic rich in large foraminifers. The Chat Member is divided into limestone” field (samples R4, N1, Q11, Q14, Q15 and Q16) three units (Table 3) and the dolomite beds are found in the except for the sample Q2. Two samples (R1 and N3) are ploted basal part of the uppermost Laki Limestone Unit. into the “Limestone” field of the diagram. The latter were in- cluded to study the process and extent of dolomitization to- Structurally, the area consists of several small asymmetrical, wards the basal part of the Laki Limestone Unit of the Laki gently-dipping plunging folds, which are displaced by faults Formation. running NE-SW. Regionally, this region is the part of the Sonda- Jhimpir monoclinal structure, dipping westward. The dolo- Iron can substitute Mg in dolomite to give ferroan dolomite mite-bearing structure is an asymmetrical anticline. The west- (<2 mole % FeCO3), and the mineralogical term ankerite is used ° ° ° ° ern limb dips 10 -12 whereas, the eastern limb dips 12 -22 . for Fe-rich compositions about CaMg 0.5 Fe0.5 (CO3)2. The axis plunges 3° towards south. The western limb is Petrography. Twenty selected samples were analyzed pet- affected by a NE-SW running fault with a maximum displace- rographically. The rocks are composed of zoned crystals of ment of 7 m (Hussain 1976). dolomite and ferroan dolomite (Fig 2a-d) with a 30 % of the area of the thin section composed by dolomicrite. A few Results and Discussion allochem ghosts (forams) are also seen bounded in dolomicrite Classification of dolomites. Dolomite is a mineral that (Fig. 2b) indicating an epigenetic replacement. The size of belongs to the mineralogical class of carbonates. Chemically, dolomite rhombohedrons has a bimodal distribution and is it contains 45.65 weight % MgCO3 and 54.35 weight % CaCO3. classified as fine-grained (0.125- 0.05 mm) and medium grained Structurally dolomite is a rhombohedral carbonate consist- (0.3-0.25 mm). The cores of crystal have higher concentra- ing of alternating layers of carbonate anionic group and the tions of cations with higher atomic number (Fe) and are cloudy, equivalent number of cations of Ca and Mg. Stoichiometri- whereas, the rims have higher concentrations of cations with cally, dolomite has an ideal CaCO3/MgCO3 molar ratio of 50:50 lower atomic number (Mg) and are relatively clear. About 80 (Tucker 1990). Dolomite can be classified on the basis of its to 90 % of the dolomite crystals are zoned. The variable con- Petrography, Trace Element Geochemisty in Sindh 197 Table 2 Lithostratigraphy of the study area (after Shah 1987) Era Cenozoic Period Tertiary Epoch Paleocene Eocene Oligocene Group Ranikot Group Formation Khadro Bara Lakhra Laki Kirthar Nari Sandstone with Sandstone with Limestone and Mainly limestone Limestone and Sandstone and shale, minor lime- minor shale and subordinate sand- subordinate marl, shale with minor shale with subor- stone and basaltic volcanic debris.