Hydrothermal Dolomitization in Devonian Khyber Limestone Peshawar Basin, Pakistan: Evidence from Outcrop Analogue, Petrography, and Geochemistry.

Abstract

Abstract The prevailing study is concerned with the Devonian succession exposed in the Khyber Agency, where outcrops extend North East-South West from kali shilman to bara fort in the study area and have been altered by hydrothermal fluids and magmatic intrusions. This research aims to understand the origin, diagenetic evolution, and sedimentary features of the dolomites that make up the Khyber limestone Formation. Field relationships, petrographic studies, and geochemical analyses helped in understanding the paragenetic history of carbonate rocks and their relationship to igneous intrusions. The phenomenon of fluid-rock interaction has received significant attention because hot fluids (hydrothermal) alter the composition of the host limestone in contact. A petrographic study demonstrated host limestone alteration, resulting in digenetic alteration and metamorphism. The formation of marble as a result of contact metamorphism has also been observed to act as a barrier against the host limestone being altered by dolomitizing fluids. Field observations, hand specimen examinations, and thin section studies show that the altered carbonate rocks are also extremely finely to moderately to coarsely crystalline. Five different types of dolomite rock textures are identified and categorized based on the distribution of crystal sizes and the shape of the crystal boundaries as "planer" and "nonplaner" dolomite. Planer dolomite crystals have straight boundaries and are further divided into euhedral (Dol-III) and subhedral (Dol-IV) textures, while nonplaner dolomite (Dol-I) crystals have arched, lobate, ridged, obscure, or otherwise irregular intercrystalline boundaries. Planar–euhedral dolomite (Dol-III) were made of fine to coarse dolomite crystals with dense cloudy euhedral rhombs while planner-subhedral dolomite have a unimodal mosaic structure and are subhedral to anhedral in shape with straight compromise boundaries and numerous crystal-face junctions. XRD analyses were done in order to determine the bulk mineralogy. Mostly dolomite and calcite were present along with different type of other minerals like a putoranite, manganocalcite, quartz and copper which can formed when a magmatic activity occur so because of some copper bearing fluids they were deposited. The C and O isotopic variations of ancient ocean water were used to reconstruct the paleotemperature history at the time these rocks were formed and led to the formation of saddle dolomite and considers as high temperature dolomite.