Quality assessment, reserve estimation and economic analysis of roofing slate in the west central Lesser Himalaya

Abstract

Quality assessment, reserve estimation and economic analysis of roofing slate can be carried out at Tharpu of Tanahun District which lies in the Nawakot complex of the Lesser Himalaya. It represents a part of northern limb of the Mahabharat Synclonorium. Petrological study (pressure and temperature of metamorphism from mineral assemblage in thin section) and physio-chemical test (flexure testing, water absorption, weathering resistance, abration resistance, sulphuric acid immersion test, wetting and drying test) have been done in the laboratory for quality assessment. Geological mapping and preparation of columnar sections have been done in the field for Reserve Calculation. The total reserve of an area is determined by dividing the tonnage with its tonnage factor. The volume is calculated by multiplying the total cross-section area by the perpendicular distance between each cross-section. Cost Benefit Analysis was applied for cost and benefit of slate mining to evaluate the viability of the slate business. The major slate deposits of the study belong to the Benighat Slate and Nourpul Formation of the Lesser Himalaya. The pressure and temperature of the metamorphism on the basis of b₀-spacing and IC methods are 4.23 kbar and 380°C for Benighat Slate and 5.10 kbar and 375°C for Nourpul Formation roofing slate. Flexure strength of the slate along grain ranges from 26. 26 to 50.57 MPa with average 36.24 MPa and standard deviation (SD) of 9.28 MPa. While, the same property across grain ranges from 36.37 to 59.78 MPa with average value 43.1 MPa and SD of 9.59 MPa. Similalry, the elasticity of the tested sample of slate ranges from 1055.4 to 2974 MPa having mean value of 1774 MPa and SD of 740 MPa. Water absorption by weight is 0.789 to 1.473 having mean value 1.02 and SD 0.3. While, the weather resistance of the slate lies within 0.31 mm to 0.55 mm with average value of 0.41 mm and SD is 0.1. Abrasion by weight has a range from 14.3 to 20.4 with average value 16.22 and SD 2.73. The permeability, sulphuric acid immersion, and wetting and drying tests give excellent results to the slate. It was observed that from the field study, there is finegrained with a fairly perfect natural cleavage, readily splitable into thin and smooth sheets of slate at Seratar (3000 m northwest from Tharpu Bazaar) and Otandi (1000 m west from Tharpu Bazaar). Due to this thin splitting properties, most slate are used for roofing purpose. On the basis of physiochemical testing and Petrological study, the slate of Nourpul Formation at Seratar and Benighat slate at Otandi are best for roofing as well as construction purpose even though inferior to the ASTM standard. The total probable reserve of the slate calculated by the cross-sectional method is to be 52.9 million m³ at Otandi. Mining method appropriate for the slate deposit is open pit mining. As cost benefit analysis show that B/Q = 1.23, the mining of slate is profitable. For profitable business, the benefits and cost ratio should always be greater than one. Journal of Nepal Geological Society, 2007, Vol. 36 (Sp. Issue)p.17