Exploration Geophysics - Volume 15, Issue 1, 1984

The major hardrock tin deposits in south-east Australia have a characteristic magnetic signature in the vicinity of each deposit. This magnetic signature is due to the high temperatures of intrusion, being close to the Curie point of magnetite, imposing a remanent effect within the meta-morphic aureole.

The tin deposits are intimately related to the ilmenite series granitoids of Ishihara (1978) which have been linked with the S-type granitoids of Chappell and White (1974). These granites are usually non-magnetic and circumscribed by magnetic aureoles in the country rock. These magnetic parameters can be explained by the temperature - oxygen fugacity characteristics of the parent melt.

The non-tin-mineralized granitoid suite, labelled magnetite series by Ishihara and l-type by Chappell and White, are moderately to strongly magnetic in south-east Australia. Non-magnetic l-type granitoids are common elsewhere, and can be caused by the replacement of Ti-magnetite by secondary sphene. The l-type granitoids (magnetite + sphene series) have a Cu - Mo - W mineral association.

The relationship of tin mineralization to the S-type granitoids is that the deposits are found peripheral to the granitoids, in close association with their late and fine grained derivatives. The S-type granitoids may themselves be quite barren (Strong 1981). The mineralized late phase intrusions of north-west Tasmania exhibit a similar magnetic aureole to that surrounding the granitoids, as evidenced near the carbonate-replacement deposits at Mt Lindsay, Cleveland and Severn at Zeehan.

Examples are given from north-west Tasmania to illustrate the signature of the Renison-Bell, Cleveland, Mt Lindsay and Severn deposits. Regional and local geophysical data from central New South Wales are presented to further illustrate the model for granites and mineralization at Ardlethan, Kikoira and Tallebung.

Some of the results of an electromagnetic test survey conducted over the Flying Fox body at Forrestania using the Geonics EM-37 equipment are presented. The results are dominated by a strong overburden response and the presence of a confined conductor is illustrated by the presentation of the response in contour form. The conductor response is further highlighted by the removal of the apparent half-space response. The residual response, following the removal of a homogeneous half-space, is compared to a free-air plate model which supports the interpreted coincidence of an electromagnetic conductor with the Flying Fox massive mineralization.

Formulae for the transient electromagnetic response of a layered half-space for ramp function current turn-offs are derived for coincident loop, vertical dipole and horizontal dipole receiver configurations. For the case of a uniform half-space, these take the form of an easily summed series. The errors involved in assuming the ramp function to be a step function are illustrated for four different earth models. An apparent resistivity algorithm for the coincident loop configuration is also presented.

Instruments designed to measure the one-dimensional surface impedance of the earth using distant VLF navigation transmitters have been commercially available for some years. The effect of an anisotropic earth on the results obtained from such an instrument are discussed and a simple technique is proposed to determine the degree and direction of the anisotropy.

A mathematical expression for the potential of a varying anisotropic earth with an inhomogeneous overburden has been derived for a point current source. The apparent resistivity for Wenner and Schlumberger configurations have been calculated from the derived potential. Sounding curves for these two configurations have been computed by using digital filter methods.

Barometers are frequently used to provide elevation control data for gravity surveys. Provision of reliable elevations by this means is not straightforward and depends on an appreciation of physical reduction factors, such as temperature or elevation itself, field technique and selective rejection of some observations. Data redundancy is essential.

The theoretical and practical issues which affect interpretation of barometric data are briefly reviewed. Most errors can be traced to instrumental variability, observer inconsistency, inadequate controls or atmospheric abnormalities. An elevation accuracy of 1 -1.5 m is possible given care in survey design and practice.

Barometric elevation determinations in variable terrain must be based on careful, multiple and controlled observations. Pressure distortions within the atmosphere or around topographic features may invalidate many field techniques including those using base barometers. Considerable manual involvement is advised throughout the reduction process and unfortunately no simple universal sequence can be recommended. Local conditions must be uniquely assessed.