Rio Tinto operates several channel iron mines, the main orebodies of which can generally be divided into an upper unit and lower unit. From the operational perspective, where predicting product (ie lump and fines) deportment and anticipating handling/processing risks is critical, the key difference between the upper and lower units is textural. The upper unit is generally competent, strongly-cemented and consistently high-grade, whilst the lower unit is typically more variable in grade and texture, with a higher proportion of fine materials including ochreous goethite and clay. Grade is a secondary characteristic for mine geologists, with identical cut-offs applied to both units.Resource modelling geologists generally recognise the importance of the physical differences between these units; however, reliance on a high proportion of limited, historical drilling at one deposit seems to have resulted in a strong reliance on grade to define this key contact, where they have effectively been modelled as higher and lower grade.After repeatedly encountering the lower unit contact at a higher RL than predicted by the resource model, an investigation confirmed significant challenges to accurately defining it using traditional methods and data sources. As a result, supplementary downhole data was interrogated to determine whether additional differentiating factors could be used.Downhole wireline logging density data was found to be available for approximately one third of drilled metres and distributed almost uniformly across the deposit. While it has historically been used only to estimate bulk density for tonnage calculations, it was found to be a potentially effective indicator of the contact. Density shells created using implicit modelling software identified a prominent domain boundary within the upper unit of the resource model, concurring with observations from site. This density-based boundary provides a complementary method of identifying a critical contact, which could allow resource modellers and mine geologists to interpret and model it with greater consistency going forward, both at this deposit and at nearby, analogous ones. CITATION:Cross, T, Wlasenko, M and Mackenzie, P, 2019. Using wireline logged density to help define an elusive contact in a historical channel iron deposit, in Proceedings Iron Ore 2019, pp 280290 (The Australasian Institute of Mining and Metallurgy: Melbourne).