Exploration Geophysics - Volume 26, Issue 2-3, 1995

The Ernest Henry copper-gold deposit is located approximately 30 km NNE of Cloncurry in northwest Queensland. It contains an indicated resource of 122 Mt at 1.14% copper and 0.55 g/t gold. The deposit is hosted in Proterozoic rocks of the Mt Isa Inlier. The Proterozoic geology is overlain by 40 to 50m of flat lying Phanerozic sediments.

The discovery of the deposit was the result of a geophysically and geologically driven exploration program that was guided by a simple empirical model for the deposit type. Magnetic methods were used to initially focus exploration. Transient electromagnetic (TEM) techniques were used to filter the magnetic target areas. The first drill hole into the deposit intersected economic copper and gold grades.

After the discovery, downhole TEM surveys demonstrated that the initial surface TEM target was due to a supergene layer of mineralisation that in part included a section rich in native copper. It also demonstrated that the bulk of the primary mineralisation did not produce a TEM anomaly. The primary mineralisation is intimately associated with the matrix of a felsic to intermediate volcanic breccia. The matrix to the breccia is rich in magnetite and disseminated sulphide mineralisation. Ground magnetic, gravity and Induced Polarisation methods were used to help guide the delineation drilling of the deposit.

Economically the Outokumpu region in eastern Finland ranks among Finland’s most important metallogenic provinces. Exploration for Outokumpu-type Cu-Co-Zn-deposits has been carried out in the region for more than 80 years. To date several such deposits have been discovered in a zone covering an area about 4500 kmy. Three of the deposits were in production and all of them are now mined out. The last mine, Outokumpu, was closed down in 1989.

A later stage of exploration, started in 1979, led to the discovery of the Kylylahti deposit in 1984. This triggered the latest phase of exploration in 1991. The present paper describes geophysics used in this discovery and later studies of the surrounding area.

The relatively small Kylylahti deposit was successfully detected although at this depth of about 500 m. The most successful methods for locating conductive deposit near surface were Mise-a-la-masse, HLEM and Gefinex 400S (Sampo), an EM frequency sounding system developed by companies in the Outokumpu Group. Even though the Kylylahti deposit is sandwiched between thick, conductive layers of black schists it was eventually intersected with some drill holes guided by the above mentioned geophysical surveys, lithogeochemistry and geological reasoning.

Wannaway, Mariners and Mt. Edwards are three of many type 1A komatiitic peridotite-hosted nickel sulphide deposits clustered in the Widgiemooltha area, approximately 560km east of Perth.

Magnetic susceptibility measurements show that the susceptibility of the ore is generally high but is extremely variable, from 4.5 x 10^-3 SI units to 269 x 10^-3 SI units in se-rpentinised environments, and 14.9 x 10^-3 SI units in talc-carbonate altered environments, hangingwall ultramafic rocks have a generally high but variable susceptibility depending on the style of alteration. Serpentinised ultramafic rocks have a high susceptibility of 70 x 10^-3 SI units, while talc-carbonate altered ultramafic rocks have a low susceptibility of 4.4 x 10^-3SI units due to the respective formation or destruction of magnetite. Footwall basaltic rocks have a consistently low susceptiblity average of 0.8 x 10^-3 SI units.

Susceptibility-temperature data indicate that ores with a high susceptibility contain monoclinic pyrrhotite and magnetite, and ores with a low susceptibility contain only small amounts of monoclinic pyrrhotite, minor magnetite and abundant, nonmagnetic, hexagonal pyrrhotite. Components of remanent magnetisation in the ores and hangingwall ultramafic rocks lie in two dominant orientations. Remanent magnetisations held by pyrrhotite and magnetite at Wannaway and by pyrrhotite at Mt. Edwards are upwardly inclined toward the northwest. Vectors held by pyrrhotite at Mariners and magnetite at Wannaway are upwardly inclined toward the southeast.

Two dimensional magnetic modelling of the Wannaway ore environment indicates that the anomaly over the Wannaway deposit is caused predominantly by the hangingwall ultramafic unit, with a small contribution from the near-surface hangingwall ore. Contact ore makes only a minor contribution to the anomaly due to the depth of the ore below the surface. Although the ore is significantly magnetic, it is difficult to directly detect on magnetic data due to the highly magnetic nature of the surrounding ore environment.

Various geophysical techniques were applied experimentally to exploration for epithermal gold deposits. The Hishikari gold-silver deposit, located in northeastern Kagoshima, Kyushu, Japan, was selected for this study. This paper presents the results from these case studies and discusses the applicability of each method in several stages of the exploration; the reconnaissance stage, the detailed follow-up stage and the final stage where the surveys are directed at detecting veins. The results are summarized as follows;

1. A high-gravity anomaly was interpreted as the swollen basement. This subsurface structure was considered to be local doming of basement associated with volcanic activity. Local gravity highs are worthy of notice because the doming of basement might cause fractures;
2. A resistivity cross-section was characterized by three layers; an uppermost layer with high resistivity (100-1000 Ωm) corresponding to unaltered volcanic rocks, an intermediate layer with low resistivity (3-8 Ωm) corresponding to altered volcanic rocks and a bottom layer with relatively high resistivity (80-200 Ωm) corresponding to the basement. Resistivity mapping and sounding are useful to identify lithologies and structures that may control gold mineralization;
3. IP anomalies (high chargeability) within the area of low resistivity are the most important indicators of vein zones. For this reason, in the detailed follow-up stage and the final stage of exploration, IP surveys play the most important role.

The Mt Leyshon magnetic anomaly is an intense magnetic low located to the south of the Mt Leyshon Gold Mine. Recent investigations including deep drilling, detailed core logging, petrophysical measurements and magnetic modelling indicate the source of the anomaly to be biotite-magnetite alteration. Geological relationships suggest that this alteration is an early hydrothermal phase of the breccia complex whereas the gold mineralisation is interpreted as post dating emplacement of the breccia complex.

Reconnaissance gravity and high-resolution aeromagnetic data, acquired in 1993 as part of the Tasmanian Government NETGOLD project, were used to investigate the regional structural setting of the area and the distribution of gold mineralisation. Gold mineralisation in northeast Tasmania occurs both close to, and remote from, outcropping granitoids. The total recorded gold production is in excess of 50 tonnes. In the Alberton-Mangana Goldfield the distribution of mineralised sites defines a prominent NNW trending regional lineament which passes through the corridor of Mathinna Group rocks separating the Scottsdale and Blue Tier Batholiths. Mineralisation occurs primarily within discrete quartz-sulphide veins hosted by the metasediments.

The Mathinna Group rocks are variably magnetic and the majority of the granitoids are effectively non-magnetic. NE-trending magnetic lineaments mark faults which truncate subtle NNW-trending anomalies attributed to lithological variations in the Mathinna Group. There is no consistent relationship between NE-trending faults and mineralised sites but dextral movement on these structures may have been important in dilation and mineralisation of pre-existing ENE-trend-ing fractures. Broad ovoid magnetic anomalies in areas of Mathinna Group outcrop result from large scale alteration systems probably related to underlying granodioritic intru-sives. The economic potential of these systems has not been tested.

The residual Bouguer gravity field is characterised by strongly negative anomalies in areas of granite outcrop. Modelling suggests that the entire Alberton-Mangana area is underlain by low density granite with the thickness of Mathinna Group rocks increasing from less than 1 km at Alberton in the north to in excess of 3 km in the south near Mangana.

The Batman gold deposit at the Mt Todd Project is situated within Early Proterozoic metasediments of the Pine Creek Geosyncline, Northern Territory. It was discovered by the Mt Todd Joint Venture (Billiton Australia and Zapopan) in 1988, following regional geochemical reconnaissance surveys.

The gold mineralisation is contained in a sheeted and stock-work quartz-sulphide vein system hosted by massive grey-wackes and siltstones of the Burrell Creek Formation.

Although the sulphides (mainly pyrite and pyrrhotite) are less than 5% in volume, the network of quartz-sulphide veins means that the deposit is an excellent conductor and this results in very strong EM and IP responses against a resistive background.

The total volume of pyrrhotite also leads to a moderate (140nT) aeromagnetic anomaly which is distinctive and isolated against a generally low regional magnetic field.

Although any substantial amount of known gold mineralisation in the area has a geophysical response, it also appears that some anomalies (such as to the north of the Batman deposit) reflect sulphide mineralisation which has little or no associated gold. Thus geophysics proves to be a useful, but not totally conclusive, tool for detecting gold mineralisation in this part of the Pine Creek Geosyncline.

Hill 50 Gold Mine is located at Mt Magnet, 560 km NNE of Perth, in the Murchison Province of the Yilgarn Craton. The Mt Magnet greenstone belt consists of a sequence of mafic, ultramafic and felsic volcanic rocks, with interbedded vol-canogenic sediments, predominantly banded iron formations (BIFs). The belt has been deformed into a principal structure, known as the Boogardie Synform. Major NNE - NE striking faults, superimposed on the synform, are referred to as Boogardie Breaks and exhibit important control over many of the ore deposits. Historically, most of the gold production in the Mt Magnet district has come from BIF - hosted deposits (e.g. Hill 50 and Boomer - styles).

Hill 50 - style deposits are hosted by BIF "bars" of the Sirdar Formation, having a width generally greater than 20 m, and being structurally controlled by the Boogardie Breaks. Gold mineralisation is closely associated with pyrrhotite and to a lesser degree, pyrite. Hillcrest South and Brown Hill West prospects are currently being explored using a Hill 50 model. Petrophysical measurements carried out on samples from these areas demonstrate that pyrrhotite - rich BIFs are characterised by lower magnetic susceptibilities and higher Koenigsberger Ratios, conductivities and densities. Use of downhole EM in locating the pyrrhotite - rich BIF has been highly successful.

Boomer - style deposits are hosted by multiple NW - trending BIF units, and breccias, and are distinguished by the close association of gold mineralisation with pyrite. Petrophysical testing on samples from the Boomer pit showed that the pyrite - rich rocks are characterised by higher densities and magnetic susceptibilities than their pyrite - poor counterparts. Although the conductivities are also slightly higher, Boomer -style deposits are not detectable using downhole EM.

The differing conductivities observed in unmineralised BIFs and BIFs with significant pyrite and/or pyrrhotite can be explained by varying textural and mineralogical characteristics of the metallic lustre minerals involved.

The White Devil gold deposit is developed within a steeply-dipping altered shear zone (predominantly chlorite schists containing stringers of magnetite) which is hosted by inter-bedded turbiditic greywackes and siltstones of the Protero-zoic Warramunga Group.

Ground magnetic, gravity, and triaxial drillhole magnetic surveys have been undertaken at the deposit to assist in the mapping of the size and extent of magnetite accumulations, ranging from magnetite stringers (2-5% magnetite) to massive magnetite ironstones (>60% magnetite), which occur within the steeply-dipping shear zone.

The extensive database of three component magnetic data (based on triaxial drillhole magnetometer readings from over 200 drillholes) has enabled a three dimensional view of the magnetic field within a complex, highly magnetic ironstone system.

Basic geophysical techniques were used to characterize two gold deposits that lie within tropical-to-subtropical climates. These two deposits are the Posse deposit, which lies within the Mara Rosa Greenstone Belt of central Brazil and the Syama Mine which lies within the Bago, Greenstone Belt of southeastern Mali. Both deposits were discovered by geo-chemical techniques but subsequent magnetic and induced polarization (IP) surveys were used to identify rock types and basic structures and contacts while VLF-EM was used to define more detailed structures associated with both ore deposits. The radiometric method was used at Syama to map the surface expression of sericitic alteration. The success of geophysics in both areas led to an extension of those surveys along strike. The results were used to assist in the ranking of geochemical anomalies.

The desire for additional oxide reserves at Syama and the success of the ground geophysical surveys led to an airborne magnetic and radiometric survey over the mine. The results confirmed the usefulness of the two methods for mapping geology and structure in greenstone belts of West Africa. Additionally, the results were used to identify and rank prospective oxide reserve targets. Initial follow-up soil auguring was successful in identifying significant gold reserves but the success rate declined as the lesser-ranked targets were tested. Potassium concentrations unrelated to the subsurface geology in areas of transported laterite limited the success of the venture.

Although both deposits are covered by a thick mantle of laterite, in neither case did this impede the ability of basic geophysical techniques to provide very useful information relating to lithologies, structure and alteration.

Geophysical techniques have been shown to be effective in diamond exploration. Airborne magnetics and EM can in most cases accurately detect and map most kimberlite and lamproite pipes. The variations in observed response are large and controlled by both geological and physical factors. The controls on EM for example are multiple and include geological fades: whether diatreme or crater; degree of brec-ciation and porosity, depth of weathering and groundwater salinity.

There is strong evidence that the source of unweathered kimberlite and lamproite magnetic anomalies is predominantly remanent. Viscous magnetisation may become important in weathered rocks. Observed local anomalies may thus be of normal or reversed polarity compared to a non-magnetic background.

There are many sources of confined geophysical anomalies with lateral extent around the nominal 300m minimum diameter of economic diamondiferous pipes These include amphibolites, collapse breccia pipes, massive sulphides, skarns, inhomogeneous regolith, culture, alkaline diatremes, confined mafic and ultramafic bodies, local sediment patches and magnetite concentrations in granite. Some simple geophysical tests can help discriminate these possible sources in many cases.

Diamond grades within economic pipes are known to be strongly zoned, grade also varies greatly between separate intrusions within one pipe. Detailed ground geophysics can usually map out the surface extent of many diatreme and crater facies kimberlites or lamproites, and at times clearly identify separate intrusions within one kimberlite or lamproite. Such detailed geophysics is likely to be far more cost-effective than the alternatives such as grid drilling or pitting.

Numerical EM 3D modelling and inversion are computationally challenging problems that can be solved by traditional methods only for models with a rather limited number of parameters. This severely restricts their application to realistic geophysical problems. In an effort to ease this restriction a number of levels of approximate forward modelling have been implemented including the 1D approximation, the 3D Born approximation, the finite element method, and the direct hybrid solution. A comparison of these modelling methods for the simulation of airborne EM data over a 3D conductivity earth model shows that the 1D approximation is very fast and gives a reasonable qualitative response, the Born approximation is moderately rapid but gives an inaccurate response, and the finite element and direct hybrid methods give similar responses with the finite element method requiring significantly less computer resources. The corresponding inverse problem is more challenging than the forward problem because each method of solution for the forward problem may require a different optimisation method for the inverse problem. This occurs because of the trade-off between the CPU time and accuracy of the forward modelling and the CPU time and number of forward modellings required by the optimisation method. Inversion tests based on the same airborne EM simulation show that for moderately large models the preferred optimisation method is quasi-Newton however for smaller models or when the finite element or hybrid modelling methods are used then the preferred method is Gauss-Newton.

Magnetotelluric (MT) impedances are estimated from electric and magnetic field data using a higher order spectral (HOS) method. This technique rejects Gaussian noise contaminating the MT signal because of the insensitivity of the third order cumulant to Gaussian signals. The non-Gaussian characteristics of a representative set of MT signals are demonstrated.

Artificial non-Gaussian datasets contaminated with varying levels of coherent and incoherent Gaussian noise are analysed using this method and three existing methods: the robust remote reference (RRRMT) method, the time-frequency (TF) method and a variant of the conventional (CONV) method. The resistivity and phase estimates obtained by the HOS method and CONV method are more consistent than those of the TF and the RRRMT methods, and in general the resistivity and phase estimates from the HOS method are subject to the least bias.

RIM surveys in underground potash mines in Saskatchewan, Canada, have been undertaken for several years with generally positive results. The methodology is similar to surveys carried out in underground coal mines in the U.S. and Australia, where TEM-mode magnetic-field amplitudes are measured and subsequently processed to produce absorption-coefficient images of the seam being exploited. The primary differences between the coal and potash situations are the great transmission range possible in potash, which exceeds 3000 m, and the nature of the target anomaly, which for potash is always the anomalous presence of brine.

The RIM method responds to zones of anomalous conductivity in a manner that does not well match the commonly-applied ray assumption. There is a difference between how two rays respond to the same anomaly where one of the rays has both end points outside the anomaly and the other has either the beginning or end of the ray within the anomaly. In each case grossly different measurements are made of essentially similar volumes of earth which, if unrecognised, can lead to serious interpretational pitfalls. These effects have been recognised in field data and turned to advantage: if both ray "modes" traverse the same anomaly the thickness and conductivity of the anomalous material can be derived by a curve-matching method.

The relationship between the secondary field due to a TEM source and a dipolar potential field has been documented (Dyck and West, 1984; Grant and West, 1965). In light of this relationship it is possible to utilise potential field interpretation aids when dealing with TEM data. Where only the axial component of the anomalous field is available, in the case of downhole TEM data, the implementation of these techniques is very restricted and may yield ambiguous model parameters, hence alternative techniques must be considered.

Euler deconvolution may be used to obtain the approximate coordinates of the source even in the presence of an overburden response. A neural network system has been trained to output the dip and strike direction of a plate model using scaled field data as input. Utilising both the neural network and Euler deconvolution techniques, approximate model parameters may be computed directly from the field data.

Synthetic examples are presented, showing that these techniques are applicable even in the presence of noise and conductive overburden. A field example from the Flying Doctor prospect is also presented. The computed model parameters were found to be consistent with previously published data (Cull, 1993).

The analytical solutions for the TFMMR parameter of the Sub-Audio Magnetics technique are important as a tool in indirect interpretation of the field data, in assessing which subsurfact structures will produce measurable signals, and for designing optimal field survey configurations.

In this paper, we investigate the theoretical TFMMR anomalies that would be observed over simple model structures such as an homogeneous isotropic and anisotropic earth, vertical contacts or faults, inclined contacts, thick dikes and outcropping hemispherical-depression models. For each model, comparisons are made with the MMR method.

In general, the magnitude and shape of the TFMMR signal varies with changes in the geomagnetic field. For 2-D structures, in particular, such as faults and dikes, the TFMMR and MMR methods produce signals of similar quality. In cases where subsurface conductive bodies are striking para-lel to the line joining the electrodes, our results show that the MMR method produces relatively small signals whereas the SAM signal is still quite measurable. We have also shown that the TFMMR type responses is sensitive to the geomagnetic field rather than geological strike directions as MMR is. TFMMR measurements have higher resolution, higher data acquisition rate, lower field costs and higher amplitudes for certain subsurface geometries than MMR.

The process of converting a set of airborne and ground EM data into valid geoelectrical sections remains a difficult problem. Although significant advances have been made in 3-D inversion technology, coupled with remarkable increase in desktop computing power, it will be some time before full 3-D inversion becomes a practical tool for routine EM interpretation.

One of the first approaches was to approximate 3-D inversion as a series of 1 -D layered Earth inversions, but a number of papers in the literature have demonstrated the deficiencies of this method. More recently, several approximate 3-D conductivity depth section methods have come into use. These include Spiker, Conductivity Depth Imaging, and various filament inversion schemes. Their use has been justified on the fact that time domain EM is a low resolution diffusive process and that they are fast and easy to use.

How valid are they? In this paper we generate a number of simple 3-D models including simple blocks, interacting dykes, and heterogeneous targets overlain by irregular overburden. This can be done for both ground and airborne surveys. We apply the various imaging algorithms to each of these and examine how close the image was to the original conductivity section model as a function of model parameters.

We use this information to answer the question posed by the title: How necessary is full 3-D EM inversion for useful EM interpretation?

The Osorezan geothermal area is located in the centre of the Shimokita Peninsula, the northernmost tip of the island of Honshu, Japan. Osorezan is a long-lived composite volcano having a caldera lake. There are many active geothermal manifestations at the north shore of the lake. High-grade gold mineralisation is found in the hot-spring sediments at concentrations as high as 400 g/t (6,500 g/t max.).

Controlled-source audiofrequency magnetotelluric (CSAMT) surveys were carried out at 374 sites to investigate the detailed resistivity structure of the active gold depositing zone. The apparent resistivity contour maps show that resistivity anomalies correspond to the geothermal manifestations. Two-dimensional models analysed along west-east profiles indicate that most of the sites are underlain by a conductive layer. An extremely low-resistivity anomaly in the central part of the area is probably due to high-salinity hot water and intense argillic alteration. A high-resistivity anomaly in the western part of the area is interpreted as a silicified zone or a vapor-dominated zone.

In order to investigate deep resistivity structure, magnetotelluric (MT) surveys were carried out at 46 sites in and around the Osorezan geothermal area. A two-dimensional model analysed along a west-northwest-east-southeast profile indicates that the area basically has a two-layer structure; an upper conductive layer and a lower resistive layer. The lower resistive layer correlates well with the basement interpreted from gravity, which reflects pre-Tertiary basement rocks or intrusive rocks. However, the basement beneath the Osorezan geothermal area itself is relatively conductive. This may be due to fractures containing hot water and associated hydrothermal alteration. It seems likely that fracture systems forming circulation paths of deep geothermal fluids are developed in the basement beneath this area.

A conductivity-depth transform (CDT) of airborne transient electromagnetic (EM) data generates approximate sections and maps of the subsurface electrical conductivity variations. The resulting products have instrumentation parameters such as the transmitter waveform removed and can be directly integrated with other information from the survey area. Although approximate, the conductivity depth transform has a number of advantages over an exact inversion method. These are illustrated with two examples: a groundwater salinity study in Australia and a survey over the Bushman mineral deposit in Botswana.