Geophysical exploration of Quaternary ironsand deposits at Taharoa, Waikato North Head and Raglan, west coast, North Island, New Zealand

Abstract

Extensive beach and dune deposits of titanomagnetite sands occur along the west coast of the North Island of New Zealand. Some of the larger deposits cover an area of greater than 10 km2, with a thickness greater than 50 metres. Deposits at Taharoa, Waikato North Head and Raglan were investigated to determine whether geophysical methods can be used to delineate titanomagnetite concentration patterns within the deposits and also to assess the total reserves of titanomagnetite in each deposit. Laboratory measurements showed that the magnetic susceptibility of the sands increases monotonically with the volume concentration of magnetite CV; the observed variation of magnetic susceptibility as a function of CV can be explained by changes in the resultant magnetic permeability of a binary mixture of magnetic and non-magnetic grains. The density of the sand increases linearly with CV; the particle density of titanomagnetite is 4.7 x 103 kg m-3. The natural remanence of magnetite sands is small (Koenigsberger ratio Q<0.2). No significant induced polarization response could be observed even for mixtures of almost pure magnetite. When placed in an electric field, the magnetite sands were found to be non-conductive. Field measurements showed that total magnetic force anomalies with peak values of up to 1600 nT and 800 nT could be observed over the deposits during aeromagnetic surveys at elevations of 183 m and 366 m respectively. Topographic anomalies were subtracted from the observed data, assuming an average concentration of 18% magnetite for sands in the deposits dawn to an arbitrary datum level. The resulting residual anomalies are caused entirely by sand which is enriched in magnetite (commonly up to 45% to 50% by weight). At Taharoa, the enriched sands were modelled by a 33 x 13 array of vertical prisms (200 m x 200 m dimension), retaining the length and magnetization of each prism as parameter. At Waikato North Head and Raglan, the enriched sands were modelled by three-dimensional polygonal bodies. Ground magnetic studies are suitable for outlining particular magnetite concentration patterns, such as in streams or on beaches. Magnetite concentration patterns Rave gravitational effects of up to ±1 mgal, which are distorted by interfering gravitational effects from basement structures at Taharoa and by a strong regional gradient at Waikato Heads. However, the gravity data at Taharoa could be interpreted in terms of basement depths, whereas at Waikato North Head, gravity anomalies outline magnetite-enriched sub-deposits which occur below sea level. Concentration patterns above sea level at Waikato North Head could not be accurately defined from the gravity data. Seismic refraction studies showed that the Taharoa deposit is up t o 200 m thick. The seismic velocity in the drifting surface sands increases linearly with depth, from a surface velocity of 0.24 km sec-1 to a maximum of 0.65 km sec-1, with a rate of increase varying from 19 to 30 sec-1. These sands are underlain by a weakly cemented, homogeneous sand unit which has a velocity of 1.7 ± 0.1 km sec-1 and has a titanomagnetite concentration of less than 18% by weight. The two sand members are separated by a sequence of tephras and paleosols. Values of DC-resistivity observed over the deposits exceed 104 ohm-m in dry sand and are as low as 102 ohm-m in saturated clay- bound sands. Small frequency effects of up to 14% were observed in clean sands at Taharoa. Magnetite sand deposits can be successfully prospected by geophysical methods. Airborne magnetic surveys are most applicable; a flightline spacing of 0.5 km allows magnetite concentration patterns with wavelengths greater than 200 m to be resolved. The seismic refraction method delineates the vertical extent of the deposits. Gravity, ground magnetic and electrical methods can be used to test specific objectives. Electromagnetic methods were found to be unsuitable for the prospecting of ironsand deposits. Ore reserves of magnetite calculated from the interpretation of the geophysical data were found to be significantly greater than previous estimates. At Taharoa, a total mass of 545 x 106 tonnes of enriched sand with an average magnetite concentration of 38% was calculated from geophysical data. Three major and three minor sub-deposits of enriched sands, containing a total of 580 x 106 tonnes with an average concentration of 46% magnetite are indicated from similar data at Waikato North Head; of these, two major sub-deposits, which occur below Sea level and which contain 230 x 106 tonnes of sand (114 x 106 tonnes of magnetite) were not previously known. The ore reserves at Raglan amount to 56 x 106 tonnes with an average concentration of 36% titanomagnetite. As well as the enriched sands, all three deposits also contain large reserves of low-grade magnetite sand. It is postulated that a large proportion of the magnetite sands which have accumulated along the west coast of the North Island may have been derived from the erosion of extensive Quaternary tephra sheets which originated from numerous centres in the Central Volcanic Zone.