Mineralogical, Textural And Weathering Impact On The Physico-Mechanical Properties Of Selected Plutonic Igneous Rocks From North-West, Pakistan

Abstract

The mineralogy, texture, and weathering grades of the plutonic igneous rocks from the Salarzai region (Bajaur), N-W Pakistan, as well as their effects on physico-mechanical attributes, are investigated. Field relationships together with petrographic investigations lead to the distinction of Salarzai plutonic igneous rocks into (i) fine-grained gabbronorites (F-GN), (ii) medium-grained gabbronorites (M-GN), (iii) coarse-grained quartz diorites (C-QD) and (iv) coarse-grained anorthosites (C-AN). Essential minerals including Plagioclase, quartz, pyroxene and olivine with minor to accessary amount of biotite amphibole, quartz, epidote, chlorite, garnet and sphene were found. In contrast to acidic rocks, intermediate and basic rocks has lower percent abundances of felsic minerals and greater specific gravity (S.G), ultrasonic pulse velocity (U.P.V), strength includes unconfined compressive strength (U.C.S) and Schmidt rebound (R-value) values. Likewise, sample having irregular grain geometry, uneven surfaces and fine medium grained (F-GN, M-GN) shows higher strength with unconfined compressive strengths values of 115 and 86 MPa and R-values 59 and 51 correspondingly. The physical parameters of the studied samples are in good accordance with the weathering grade given to the sample, like fresh (W.G-I), slightly weathered (W.G-II), and severely weathered (W.G-III). As the grade of weathering increases from W.G-I to W.G-III, the values of porosity (P) as well as water absorption (W.A) increases from 0.264 to 0.642% and 0.208 to 0.552% correspondingly, whereas the specific gravity and ultrasonic pulse velocity (U.P.V) decreases from 2.934 to 2.755, 6597 to 2660 respectively. In comparison to coarse-grained anorthosite the coarse-grained quartz diorite variety have greater values of specific gravity, ultrasonic pulse velocity, unconfined compressive strength and Schmidt hammer rebound and reduced values of porosity and water absorption even though the quartz diorites are highly weathered. It is due to the fact, quartz diorite variety have comparatively more amount of mafic minerals and recrystallized quartz which are responsible for slightly greater values of specific gravity, ultrasonic pulse velocity, unconfined compressive strength and schmidt hammer rebound and lower values of porosity and water absorption.