Cenozoic age detrital sequences in the Pilbara region of Western Australia are becoming a focus for engineering geological and hydrogeological investigations with an increasing number of final open-pit walls developed in these materials for iron ore mining. Historically, detrital sequences were classified chronostratigraphically. However, within each chronostratigraphic unit exist sub-units of variable engineering geological and hydrogeological character. As the majority of drill-hole data from Pilbara iron ore mines is derived from reverse circulation techniques, a methodology to identify the engineering geological units through downhole geophysics and geochemical assays was required to progress model development to the level of detail required for geotechnical and hydrogeological studies. The methodology entails a review of cored hole data and use of twin holes to assess the typical geochemical and geophysical signatures of units identified. Improved interpretation of reverse circulation drill-holes has resulted in the development of detailed 3D engineering geological models, which have improved the understanding of geological variability and engineering properties for geotechnical and hydrogeological studies.

This paper presents a case study on the proposed Kempfield Silver Project where geochemistry was used to define sub-surface fault structure orientation. The Kempfield site is located within the Hill End Trough along the eastern margin of the Lachlan Fold Belt. Surface outcrop is sparse with mapping mostly limited to lithology. However, similarities in the alteration of the lithologies limited their use in correlating faults. Geochemical assays of chip samples were made for over 35 minerals/elements, including aluminium, titanium, potassium, calcium and iron. Using Vulcan to view boreholes in 3D, the amounts of each element was assessed for correlations that may define changes in lithology, alteration or the presence of structures. Iron grades (Fe) proved the most favourable for the identification of structures at depth, with a distinct correlation of lithology with >5% Fe on the hanging wall of faults and <5% on the footwalls. The fault dips indicated by the geochemical assessment were commensurate with fault orientations from oriented core. This information was then factored into the geotechnical model and used as part of the slope stability analysis. An understanding of the orientation of major geological structures is fundamental to a geotechnical model. This paper presents a case study from the proposed Kempfield Silver Project where non-traditional techniques were used to increase confidence in the geotechnical model for the site. The oriented core data available suggested the site was structurally complex however the benign topography of gently rolling hills limited the ability to gather surface mapping data. Airborne geophysics supported the surface understanding, though could provide no further insight as to the dip angle or dip direction. Faults were recognised in the core but uncertainties remained in extrapolating structures to depth. The site however had significant non-cored drilling data including geochemical assays of over 35 elements. This data was used to assist in delineating the major structures. The hypotheses behind using this data and the results are summarised. Interest has been expressed in the Kempfield area since the early 1900s, first as a source of alluvial gold then later as a silver and barite mine. From 1971 to 1975 the Geological Survey of NSW undertook detailed mapping and geochemical assessment to assess the extent of mineralised zones. At the same time, International Nickel Australia (Inco) commenced exploration for polymetallic base-metal deposits associated with volcanic rocks, drilling within a single stratigraphic horizon, known as the Mather Zone. From 1979 to 2007, various companies including the Shell Company of Australia, Jones Mining, Plutonic https://papers.acg.uwa.edu.au/p/1308_12_Rutherford/ Geochemistry review of the geotechnical models-a case study from the proposed H. Baxter et al.