ASEG Extended Abstracts - Geophysical Signatures of West Australian Mineral Deposits, 1994

The Big Bell gold deposit is hosted by a felsic volcanic sequence of Archaean age. The alteration mineral assemblages within the host- and wallrock units of the deposit produce measurable geophysical anomalies. The deposit and altered wallrocks which contain up to 10 vol. % sulphide are chargeable, producing induced polarisation anomalies in dipole-dipole data of up to 30 mVA/at n=3. In addition, a strong chargeability of up to 40 mVA/ at n=2 is evident in the dipole-dipole data, and corresponds to a graphitic and sulphidic horizon in the immediate footwall to the deposit. This unit has been mapped with the gradient array over the entire extent of the leases, providing a useful marker at the top of the felsic volcanic sequence.

The strong potassic alteration accompanying the gold mineralisation is delineated in downhole spectral radio-metric logs in which highly anomalous potassium values of up to 8 wt % are comparable with those derived by chemical analysis. The downhole logs also indicate that alteration has not enriched or depleted uranium or thorium in the ore zone. There are also ground radiometric potassium anomalies over outcropping lode rocks.

Airborne and ground magnetic anomalies adjacent to the lode are due to sources with very high magnetic susceptibility values, measured in pyrrhotite- and magnetite-altered wallrocks by downhole geophysics. The values measured in the logs are 0.14 SI units in the pyrrhotite-altered zone, and range between 0.025 and 0.5 SI units in the magnetite-altered zone.

Target generation within the remainder of the Big Bell greenstone belt has relied heavily on geophysics, with the highest ranking being given to magnetic, potassium and induced polarisation anomalies within the felsic volcanic sequence.

Magnetic susceptibility measurements of rocks obtained from a number of Archaean mesothermal gold deposits have aided the interpretation of their geological and geophysical settings. Five deposits within the Yilgarn Craton were chosen on the basis of their magnetic host rocks: Youanmi (tholeiitic basalts), Greenfields (layered differentiated gabbro), Mount Martin (strongly sheared komatiitic sequence), Queen Margaret (serpentinised komatiitic peridotite) and Bounty (banded iron-formation). Iron-rich minerals associated with the gold ores include: weakly magnetic pyrrhotite (Greenfields and Bounty), magnetite (Mount Martin and Bounty) and non-magnetic pyrite (Youanmi and Queen Margaret).

The existence of two styles of mineralisation, whose apparent susceptibilities are either less than, or greater than, the host rocks, is a consequence of the geochemical interaction between the hydrothermal fluids and the wallrocks. This can have important implications for exploration since the target magnetic anomalies will be different in each case. Where apparent susceptibility values in the ore are greater than the host rocks, the mineralisation would be expected to be represented by a secondary positive anomaly on the flanks of a larger regional anomaly; where they are less, any local magnetic minima could be highly significant in terms of targeting drillholes. A correlation between gold grades and apparent susceptibility at Bounty and Mount Martin suggests that, in some cases, susceptibility measurements can be used as a guide to ore. In deposits containing komatiitic volcanic rocks, such as Queen Margaret and Mount Martin, the susceptibility meter can be used as an effective mapping tool, particularly where facing evidence is required to interpret complex structures.

Gold deposits at Granny Smith occur along a shear between a granodiorite intrusion and epiclastic sedimentary units. Three ore zones, Goanna, Granny and Windich, were identified by CSR in 1987, during exploration that included ground magnetic and induced polarisation surveys. Magnetic data were useful in helping to map intrusive rocks, banded iron-formations and less magnetic sedimentary horizons, and some faults. Although magnetite is a minor alteration mineral in the granodiorite, the mineralised zones do not give identifiable magnetic responses. Dipole-dipole induced polarisation surveys also gave valuable, and complementary, mapping information. At Granny, carbonaceous shale in the hangingwall and weakly polarisable footwall granodiorite masked any induced polarisation signal that the mineralisation may have produced. Drillhole measurements showed that oxidised mineralisation is not anomalously polarisable, whereas fresh mineralisation is moderately polarisable (some pyrite accompanies gold). Despite the lack of a direct induced-polarisation anomaly from Granny, the IP surveys delineated the deeply weathered mineralised shear north of Granny, which contributed to the discovery of Goanna. Electromagnetic measurements have also been used effectively for resistivity mapping.

Magnetite-rich banded iron-formations (BIFs) exhibit characteristic magnetic properties, including strong anisotropy. Interpretation of the geological structure of BIF units from the associated magnetic anomalies is complicated by anisotropy of susceptibility and, frequently, by remanent magnetisation. The timing of remanence acquisition relative to folding exerts a crucial influence on the form of the anomalies. Neglect of susceptibility anisotropy and remanence can lead to large errors in interpreted dips and thicknesses of BIF units. The effective susceptibility of BIFs parallel to bedding exceeds the susceptibility normal to bedding, typically by a factor of 2 to 4. Bedding-parallel susceptibilities of magnetite-rich BIFs are typically 0.5 to 2.0 SI (0.05-0.16 G/Oe). Remanence directions in BIFs usually lie close to the bedding plane. Koenigsberger ratios (Q) of BIFs vary widely, but characteristic values can often be determined for individual units. Q values in the range 1 to 2 are common. The magnetisations of the haematite-rich supergene-enrichment iron ores are much lower than those of their BIF precursors.

Magnetic properties of outcropping BIFs are usually greatly modified by weathering, which substantially decreases the bulk susceptibility, the degree of anisotropy and the remanence intensity. Deeper and more intense weathering of BIFs is encouraged by faulting and can be associated with reduced magnetic response over intensely faulted zones.

The remanence of BIFs from the Hamersley Basin is carried by late diagenetic to low-grade metamorphic magnetite after primary haematite. At Wittenoom and Paraburdoo, the remanent magnetisation of Brockman Formation BIFs is pre-folding, whereas in an area of higher metamorphic grade, the Turner Syncline, the remanence of the BIFs is probably post-folding. Aeromagnetic signatures over the Turner Syncline clearly reflect anisotropy and remanence.

Anisotropy and remanence effects are also evident in observed magnetic signatures over Archaean BIFs in the Yilgarn Block. Magnetic property measurements on samples from the Mount Magnet area and elsewhere confirm the high anisotropy and strong remanent magnetisations of these rocks.

Aeromagnetic and downhole logging data have been acquired for iron ore deposits at Jimblebar and Shay Gap-Yarrie in the Pilbara region of Western Australia. The Jimblebar deposits are from the Archaean-Early Proterozoic Hamersley Group and the Shay Gap-Yarrie deposits are from the Archaean Gorge Creek Megasequence.

Aeromagnetic data are used to assist in regional mapping and generation of exploration targets. In structurally complex areas, a very close line-spacing may be necessary to provide data of sufficient resolution. Careful processing is necessary to reduce the large dynamic range of the data, caused by highly magnetic banded iron-formation (BIF), so that subtle features may be seen. Deposits display both structural and stratigraphic controls which may be evident in aeromagnetic data. In addition, the iron enrichment process alters magnetite within the parent BIF to haematite, which may give rise to subdued responses in aeromagnetic data. The application of the aeromagnetic technique to exploration at Yarrie was an integral part of the discovery of the Y2 deposit.

Downhole natural gamma logging is used as an in-hole stratigraphic mapping tool. In the Jimblebar area, the stratigraphy, comprising interbedded oxide BIF and silicate iron formation (shale) macrobands, is very regular. As a result, it is generally possible for gamma ray logging to identify the strata intersected in drillholes to within a several metres, even where they are complexly deformed. Hence natural gamma logging can play an important role in resolving complex structural problems. At Shay Gap-Yarrie, gamma logging does not show the stratigraphic discrimination seen in the Jimblebar area, because the stratigraphy in the Shay Gap region is not as laterally consistent. However, gamma logging is still useful for general delineation of rock types.

Density logging is used for a variety of applications, including confirmation of ore grades, bulk density estimates for resource calculation, and geotechnical studies. At Shay Gap-Yarrie, back-scattered gamma density logging is used downhole to determine the density of iron ore, an important parameter in resource calculations. Frequent calibration of the probe with known reference samples is critical.

Kintyre is an unconformity-related, vein-style uranium deposit estimated to contain 36,000 t of U3O8. The deposit, located 70 km south of Telfer, was discovered during heliborne follow-up of ²¹⁴Bi channel anomalies detected by an airborne magnetic and radiometric survey. Ground inspection of the strongest anomaly identified outcropping secondary uranium-silicate mineralisation. Drilling beneath the mineralised outcrop intersected the Kintyre ore lens, with the best hole containing 71 m at 5.94 kg/t U3O8. Since then, six additional ore lenses have been discovered and these make up the Kintyre deposit.

A wide range of airborne, ground and borehole geophysical techniques has been applied to the evaluation of the deposit in an attempt to locate additional ore lenses and to determine a geophysical signature for use in regional exploration. Two types of geophysical signature have been determined for the deposit; that of the mineralised zones and that of the host unit.

The deposit has an anomalous ²¹⁴Bi channel radiometric response coupled with elevated counts in the potassium channel. Induced polarisation surveys have shown that a distinct, high apparent resistivity and high chargeability response coincides with the Kintyre mineralisation.

The ore is hosted by a lithological package which contains variable amounts of magnetite, leading to a moderate- to high-amplitude, inhomogeneous magnetic response. A density contrast detectable by gravity surveying has been noted between the host sequence and surrounding rocks. Electrical surveys have shown that the host unit is resistive relative to the rest of the host sequence and other rocks in the area.

The Balla Balla titaniferous magnetite deposit is situated about 120 km southwest of Port Hedland, in the Archaean Pilbara Craton of Western Australia. The titaniferous magnetite, and associated vanadium, occurs as layers within a mafic intrusion. A detailed gravity survey was conducted over the deposit. After band-pass filtering, residual gravity highs (about 10 gu) associated with mineralisation were defined. A ground magnetic survey was also conducted over the deposit. Positive magnetic anomalies (about 4000 nT) were interpreted to be due to more massive areas of mineralisation. Offsets between these anomalies allowed faults to be mapped. Elementary modelling of the magnetic data indicates that remanent magnetisation is responsible for a significant part of the observed magnetic anomalies. Ground electromagnetic data (VLF) and resistivity data were moderately successful in mapping the contact between the mafic intrusion and underlying granitic rocks, and confirmed the location of faults inferred from the magnetic data. Induced polarisation data show that the mineralisation is chargeable.

The Argyle lamproite diatreme is located close to the eastern margin of the Halls Creek Mobile Zone, in the East Kimberley, 120 km south of Kununurra. The pipe was discovered by the Ashton Joint Venture during reconnaissance gravel sampling of the East Kimberley in late 1979. The Argyle diatreme is intruded into Revolver Creek Formation and Can Boyd Group Proterozoic sedimentary rocks. The diatreme is an elongate body 2 km long and oriented approximately north-south, with widths varying from 150 to 500 m. It is composed dominantly of pyroclastic rocks.

A range of geophysical techniques have been used over the Argyle pipe, partly to assist in prospect evaluation but mainly to test their suitability for locating lamproite diatremes in adjacent areas. Airborne and ground magnetic and electromagnetic methods were used with varying degrees of success and a limited borehole logging programme carried out. None of the methods produced a definitive response over the Argyle pipe. Results indicate that the Argyle pipe is, at best, weakly magnetic, mainly in the northern bowl area, and weakly conductive, making it a very difficult target to locate using geophysical exploration techniques. Severe topographic problems which affected both airborne and ground survey results compounded the inherent problem of locating a subtle geophysical response.

The Ellendale lamproite diatremes are located in the West Kimberley 125 km east-southeast of Derby. They are of Miocene age and occur in Lennard Shelf sedimentary rocks which overlie the King Leopold intracratonic Mobile Zone. The Ellendale province was discovered by the Ashton Joint Venture during stream sampling of the West Kimberley in 1976. Follow-up of indicator minerals led to the discovery of the diamondiferous vent, Ellendale 4, and nearby vents were then rapidly delineated by an aeromagnetic survey. Two pipes, Ellendale 4 and Ellendale 9, have significant diamond content but are subeconomic at present diamond prices.

The Ellendale province contains 48 lamproite intrusions in an elongate cluster, 40 km long by 10 km wide, oriented west-northwest, parallel to the major faults in the area. The lamproites are intruded into flat-lying Permian sandstones and Devonian to Carboniferous shales and limestones. The terrain is fairly flat with low hills bordering some vents.

A range of airborne and ground geophysical techniques has been used over the Ellendale lamproites.

Aeromagnetic, helicopter magnetic and ground magnetic surveys proved to be very effective, since the response of the weakly magnetic lamproites is quite clear against a background devoid of other shallow magnetic features. Airborne radiometric surveys proved effective in mapping areas covered by black soils and gave clear responses over a number of diatremes. Electromagnetic surveys were used to explore for lamproites which may not have a clear magnetic response. INPUT, DIGHEM, Turam and SIROTEM produced good responses in the more resistive areas but pipe responses were ambiguous in areas covered by conductive black soil.

The precise geometric definition of the Tertiary palaeochannels in the Yilgarn Block of Western Australia is important in the exploration for uranium and for secondary and placer gold which occur in the channel sediments. The positions of the palaeochannels normally show considerable displacement from the positions of the present-day drainages. Physical property contrasts which exist between the channel sediments and the underlying Archaean bedrock can be differentiated by geophysical methods to locate the best parts of the channels for follow-up drilling. The low densities of the channel-fill sediments in many areas give rise to 1 to 2 mGal gravity lows and the gravity technique can be used to define the broad shape and depth of the channel. The deepest parts of the channel, which correspond to the zones of highest salinity, can then be delineated using time-domain electromagnetics. Downhole gamma logs are useful in identifying and quantifying prospective uranium zones.

Magnetic petrophysical studies of ilmenite concentrates from the Eneabba area have shown that ilmenite-rich placer deposits can be detected with high-resolution magnetic surveys and that some of them can be remanently magnetised. Airborne magnetic surveys have been of limited use as the magnetic response of widespread surface laterite and cultural sources such as roads and buildings can mask the weak target anomalies. However, routine use of low-cost, high-resolution ground magnetics on east-west survey lines of up to 20 km length and spaced 1 km apart has proved successful in detecting the very weak anomalies which are typically less than 10 nT in amplitude. The results have been used to target exploratory drilling.

Airborne radiometrics has failed to detect buried thorium-rich monazite placers except where exposed by mining. Although induced polarisation and resistivity surveys have been used to detect ilmenite elsewhere, relatively high costs have precluded their use at Eneabba. Ground-penetrating radar failed to detect the buried mineralised placers.

Rutile-, zircon- and monazite-rich placers, which have minor concentrations of ilmenite, do not produce an observable magnetic response so extensive drilling is a necessary exploration technique. Because drilling is necessary and inexpensive, the routine application of geophysical techniques is restricted to ground magnetics, which is also inexpensive and able to quickly locate ilmenite-rich placers.

The use of remote sensing in mineral exploration has evolved from basic photo-geology to interpretation of more sophisticated satellite and airborne multi-spectral data sets. Although the mineral mapping capabilities of Geoscan airborne multi-spectral scanners have been demonstrated for well-exposed and arid terrains, the question remains as to their effectiveness in deeply weathered regimes such as Western Australia.

Geoscan airborne multi-spectral data have been collected for a number of Western Australian mineral deposits and prospects. The applicability of the data to exploration for similar deposits, based upon a series of simple image-processing treatments derived according to the known reflectance spectra of associated alteration minerals, has been assessed.

Spectral characteristics of ultramafic rock such as kimberlite or lamproite are recognisable at Blue Well, but not over the Aries kimberlite pipe which has a spectral response dominated by the reworking and removal of desert varnish from detritus derived from its surrounding sandstone host. Shear-zone-hosted gold mineralisation in the deeply weathered Yilgarn Block is characterised by spectral signatures related to the presence of sericite, silica and iron oxides.The same minerals and spectral variations are also apparent in Carlin-style mineralisation at Kazput Pool.

In each example studied, simple image-processing treatments are used to enhance the spectral characteristics of the deposit. These signatures are dominated by the present-day surface mineralogy of the deposit and are the result of the interaction between hydrothermal and supergene alteration. In an exploration mode, a method has been successfully developed and cost-effectively applied for first-pass testing for the basic spectral criteria of each geological model, followed by more detailed spectral discrimination of key mineral assemblages and early geochemical follow-up.