DIAGENETIC ALTERATION RESULTING FROM DOLERITIC INTRUSIONS IN THE LIMESTONE OF ATTOCK-CHERAT RANGES AND GANDGHAR RANGES, PAKISTAN

Abstract

Utch Khattak limestone (late-precambrian) is exposed in Southern Gandgher Range, in district Haripur having East-West extension. It is intruded and altered by dolerite dykes that are present throughout the formation in the study area. These igneous intrusions have mostly occluded the pore spaces and fractures in the host limestone. Fluids interaction with host rock change the mineralogy of the host rock and this phenomenon is least studied. Current study focuses on the diagenetic alteration in the host lithology due to igneous intrusion using field observations, petrography, mineralogical studies and stable isotopes analysis. These studies helped to generate a detailed paragenetic history of the altered carbonates and its relationship with the igneous intrusions. Field observations show numerous dikes (having basic composition) and thickness generally 2m are present at regular interval throughout the Utch Khattak limestone. These intrusions have altered the carbonates in the study area result in contact metamorphism (resulted in marble), recrystallized limestone and dolomites. Fracturing due to intrusions is apparent in the vicinity of dikes. Brecciation has occurred in the host lithology due to cataclastic deformation. From petrographic study (Cross-cutting relationship) it can be revealed that the dolomites are formed after the host lithology intruded by the dikes. The alteration in the host limestone is also clear from the petrographic study. Hydrothermal fluids coming along with dikes have invaded the country rocks through fractures and joints present in the strata and has caused alterations. Marble in the contact zone has partially acted as a barrier for the hydrothermal fluids to invade the country rocks. This is apparent in the field, where this marble cover is missing or crosscut by fractures, dolomitization has occurred in those area in the host strata. XRD analysis of the dike sample shows the mineralogy and alteration products of the dolerite dikes minerology. Altered carbonates mineralogy show dolomite along with clays minerals and some alteration products of dolerite dikes. This shows that the alteration in the host lithology occurred due to dolerite intrusions. These alteration products have affected the porosity of the host carbonates. In addition, fractures due to cataclastic deformation have also affected the porosity. Results from different studies helped to conclude that the interaction of hydrothermal fluids with the rock resulted in the substitution of different minerals in the host rock and the formation of different types of rocks. Such alteration shows the effects on the porosity and permeability of the host rock and affecting its reservoir properties.