Assessment of rock quality variability due to smectitic alteration in basalt using X-ray diffraction analysis

The quality of basalt rock, in particular the distribution of deleterious smectitic alteration, at Blakebrook Quarry, northeast New South Wales, Australia, varies considerably and accurate resource assessment requires detailed mineralogical investigation. X-ray diffraction analysis was used to assess the primary and alteration mineralogy and was found to provide a rapid and cost-effective tool for basalt exploration and resource assessment. A smectitic alteration index (SAI) relating smectite content to primary mineral crystallinity was devised as a basis of rock quality comparison. Samples from quarry faces and sub-surface samples from core and percussion drilling were analysed leading to delineation of reserves on the basis of quality. Rock quality was found to relate closely to the succession of horizontal flows comprising layers of premium-grade smectite-free basalt, second-grade slightly altered smectitic basalt intercalated with layers of third-grade moderately to highly altered smectitic basalts.