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Abstract
The Taharoa prospect is one of the many beach and dune deposits of titanomagnetite sands ("ironsands") along the west coast of the North Island of New Zealand. Various geophysical methods were used to determine which of these is suitable for delineating titanomagnetite concentration patterns in the deposit, which covers an area of c. 15 km2, and whether a reliable estimate of the total mass of titanomagnetite ore could be obtained.
Seismic refraction studies provided a detailed structural model of the deposit. Interpretation of these data, controlled in part by a few deep exploratory holes, defined an upper horizon of unconsolidated, titanomagnetite‐rich sands (Nukumiti and Paparoa Sands Members) reaching a maximum thickness of 60 m. It is underlain by a weakly cemented, saturated sand layer (Te Akeake Sands Member) up to 100 m thick, with an average titanomagnetite concentration of about 18%. The two horizons are separated by a 42 ka old layer of tephras and paleosols up to 15 m thick. The compressional‐wave seismic velocity of the upper, dry, enriched sands is very low, and increases linearly with depth from about 0.24 km/s to a maximum of 0.65 km/s, with a rate of increase between 18 and 32/s. The velocity of the lower horizon is almost constant at 1.7 ± 0.1 km/s. Mesozoic greywacke basement with a velocity of 4.1 ± 0.7 km/s underlies the deposit.
The concentration of titanomagnetite within the seismic model was determined by detailed interpretation of airborne magnetic surveys flown at altitudes of 185 m and 370 m above sea level. Residual anomalies were obtained after reducing the magnetic effect of the entire deposit, assuming a homogeneous induced magnetisation of 6.7 A/m (equivalent to that of sand with 18% by weight of titanomagnetite). The residuals were interpreted in terms of an array of vertical prisms representing the enriched Nukumiti and Paparoa sands. Magnetic susceptibilities of up to 0.75 were interpreted, corresponding to a titanomagnetite concentration of 54% by weight.
Although titanomagnetite concentrations in the sands can produce a gravitational effect of up to 10 um/s2, residual gravity anomalies could not be used to assess concentration patterns independently because the anomalies are disturbed by larger effects associated with the irregular greywacke basement. However, gravity interpretation models were used to define the depth to basement rocks in areas not covered by the seismic surveys. Tests showed that titanomagnetite sands at Taharoa are nonconductive; electrical resistivity, IP, and EM methods could not be used to define ore concentrations.
The total mass of titanomagnetite in the Taharoa deposit was obtained by combining the magnetic and seismic models. The Nukumiti and Paparoa Sands Members contain a total of 205 ± 30 million tonnes of titanomagnetite ore within sands with an average concentration of 38% (by weight). An additional 360 ± 75 million tonnes of titanomagnetite are contained in the Te Akeake Sands Member, but at a lower concentration of about 18% by weight.
