Shirinab Formation of Jurassic age exposed in Mor range and Piaro ridge of Lasbela -Khuzdar area was mapped and sampled to study the environment of deposition, to reconstruct the paleoenvironment with the help of mineralogical, textural and chemical characteristics of the rocks. The associasted sulphate and sulphate deposits in the Formation were also investigated.
The detailed study of the Formation revealed fourteen (14) lithofacies and forty four(44) microlithofacies. The lithofacies established in the lowermost Spingwar, the Loralai and the uppermost Anjira members are: Ferro -Silicic clastics, Ferro Calc. Silicic clastics, Mudstone/Micrite, Calc -Silicic clastics, Calcareous shale, Bio -calc -Silicic clastics, Wackstones/Sparse biomicrite with molluscan and brachiopods shell fragments,Packstone -Grainstones/Packed bio -oomicrite, Mudstone/Micrite with burrows indications, Packstone/Packed biomicrite with radiolarian molds, Claystones/argillites, Packs tones/Packed biomicrite with thin filsmentous shells, Ferro –argillaceous Mudstone/Micrite, Wackstones/Sparse biomicrite with radiolarian molds/and or thin filamentous shells and Packstone/Packed biomicrite with molluscan and brachiopods shells fragments.
Out of the forty four (44) microlithofacies of the Formation ten (10) were established in the lowermost Spingwar, sixteen (16) in the middle Loralai and eight (8) in the uppermost Anjira member. The identification of the microlithofacies were based on the microscopic details of minerals, textures/structures and the diagenetic fabrics of the rocks in the three members of the Formation (details in Figure No.- 7).
The quantitative percentages of the insoluble residues estimated in the rocks of the Formation have been concluded to be a good source for the evaluation of turbidity in the basin water, tectonic pulses, influx pf sediments and the degree of calmness in the depositional basin.
The sulphate and sulpahate mineralization found in the members of the Formation are mainly stratiform, stratabound replacement type prominently noted in coarsed textured allochemical limestones of middle Loralai and the lowermost Spingwar members. The fissure filling mineralization were noted mainly in clastic rocks of Spingwar member.
The major sulphides observed in their order of abundance were sphalerite, pyrite/marcasite and galena. Barite occurs mainly at the top and at the periphery of the sulphide deposits.
The geochemical investigation of the selected rocks of Shirinab Formation revealed higher anpmalies than the recommended background values for the metllic ions Fe, Mn, Zn, Pb. Cu and Hg. Same is true for barium also. The concentrations of Calcium and Magnesium in the rocks have been satisfactorily used as tools for seprating the rocks of relatively deep water and shallow water marine conditions, on the assumption that the content of Magnesium shall be higher in shallower marine water and reverse shall be true in case of Calcium.
On the bases of evidences gathered from petrological, Insoluble residues and geochemical investigations it is concluded that the rocks of the Shirinab Formation were deposited in an intracratonic basin related to the rifting of the Gondwanaland fragments that initiated during Triassic time. The sulphide deposits associated with the rocks of the Formation were formed at or near the sea floor during submarine venting/oozing out of argillaceous constituents bearing hydrothermal fluids rich in Zn, Pb, Ba, Cu etc in a third order basin of Large Classification (1980, 1981 & 1983). The carbonate hosted sulphide and sulphate deposits in the area under study are comparable in lithology to Sedex deposits of Ireland but differ in age.
The rocks of this Formation have been concluded to be of high economic potential with respect to huge resources of limestones suitable for cement industry, chemical industries and also for decorative stones in addition to the zinc -lead sulphide deposits and the sulphate of barium.