Induced Polarization Anomalies Research Articles

Self-Potential as a Tool to Monitor Redox Reactions at an Ore Body: A Sandbox Experiment

Ore bodies generate natural electrical fields that are measurable at the ground surface. The ground surface signature of this electrical field is called a self-potential anomaly. We developed a sandbox experiment to monitor the evolution of a self-potential anomaly associated with redox processes mediated by bacterial activity at the surface of a buried metallic object crossing the water table. A Bio-Electrochemical Cell (BEC) is formed by a metal bar connecting the upper, oxygen-rich, part of the tank and an aquifer containing an electron donor in the form of acetate. The self-potential response was observed during a period of 327 days. The tomography of the self-potential signature confirms that self-potential tomography is able to locate the metallic target acting as a BEC. In addition, we performed redox potential, pH, and electrical potential measurements over a vertical cross-section of the tank at several time steps to obtain an idea of where the redox front is located. The distributions of the redox potential and pH further demonstrated the development of the oxidation-reduction chemical processes facilitated by the BEC as bacterial communities developed around the metallic bar. The electrical potential anomaly shows that the bacterial communities followed a short period of exponential growth, then a longer period of a sustained population. These results demonstrate the usefulness of the self-potential method in monitoring redox processes at the surface of a buried ore body. Further works will need to combine such self-potential anomalies with induced polarization anomalies through joint inversion.

A study of time spectrum identification of induced polarization field based on Weibull distribution function

Identification of time spectrum is one of the core issues of the time-domain induced polarization (IP) method, which can be considered as one of the bases for distinguishing various polarized rocks. Fitting the IP data based on the spectrum forward model and obtaining optimal solution of the model parameters is a key step in spectrum identification. However, the suitable forward model should consider the observation conditions, such as the charging time and the time window. The time spectrum identification may be difficult to implement stably due to the lack of objective reference for the optimal solution. Therefore, our purpose is to improve the forward model and implement the spectrum identification for IP data. First, using the Weibull (WB) distribution function as the basis, a time spectrum forward model considering the charging time and observation time window is provided according to the typical measurement mode. Then, based on the WB spectrum model and Barzilai-Borwein gradient optimization, a method for solution of apparent spectral model parameters for spectrum identification is developed. Finally, this method is used to process the IP data from a mine in which the anomalies related to ore-bearing beds are identified based on the processing results. Results obtained demonstrate that the spectrum forward model based on the WB function is feasible in describing the IP data. The limited charging time and a wide observed time window should be considered to realize accurate simulations and description of the time-domain IP data. The essence of the time spectrum identification is to comprehensively reflect the time-varying state of the whole time channel through the spectral model parameters, whereas the decay field ratio of the adjacent time channel can be used as an objective reference. The parameters of the spectrum model characterizing the time-varying state are independent of polarization and resistivity and thus can be directly used for identification of IP anomalies.

Detecting resistivity and induced polarization anomalies of galena veins in the presence of highly chargeable and conductive geological units at Daryan barite deposit in Iran

Assessment of resistivity and induced polarization (IP) data for metallic sulfide ore explorations is considered a routine task in the mining industry. The success in these surveys would be challenging if the sulfide ore lies within conductive and chargeable geological units. In this study, the resistivity and IP data of the Daryan area in northwestern Iran were examined for the detection of high-grade galena veins hosted by the Kahar Formation, a complex geological formation with several conductive and chargeable geological units. To facilitate the geophysical interpretations, a simple approach was adopted to solve the problem, and it was shown that a combination of correct geophysical modeling and approximate clustering of physical properties can help to check and revise (if necessary) the geological model or geophysical interpretations. In the Daryan area, the barite mineralization had high resistivity and low IP anomalies. More importantly, it was found that the highest IP and the lowest resistivity values were not the responses of galena mineralization but the graphite-rich shale. Thus, in complicated situations, the data should be treated and interpreted carefully because the “rules of thumb” may not work. This fact highlighted that prioritization of anomalies based on geophysical models alone is not an appropriate strategy in mineral exploration, and the interpretations must always be evaluated with geology. Moreover, it was discussed that both volume concentration and style of conductive minerals can be vital for creating elusive electrical responses, thus caution should be taken while interpreting electrical models in areas with complicated geology.

“Is it pyrite, or a shed?”: intricacies of induced polarisation surveying near grounded metallic infrastructure

SummaryMetallic grounded infrastructure commonly found in built-up areas such as fences, sheds and various dwellings, can produce induced polarisation anomalies often indistinguishable from bedrock responses typically targeted by base and precious metals explorers. Unintended presence of measurements that have been affected by such cultural causative sources in input datasets for inverse modelling can lead to misleading resulting models and compromised exploration performance. Circumvention of anticipated cultural interference during data acquisition cannot always be guaranteed in built-up areas due to the often-inevitable density of grounded infrastructure and limited land access. Instead, survey design should be optimised to deliberately collect data affected by cultural contribution in order to facilitate accurate interpretation of anthropogenic responses in resulting inverted models.After demonstrating the threat of unmindful induced polarisation surveying near grounded metallic infrastructure, we present the results from an unprecedented optimised time-domain 3DIP dataset using trilinear multi-dipole arrays, which is trusted to enable the confident discrimination of undesirable surficial anomalies from features deemed worthy of follow-up investigations despite patchy land access.

Comparing induced polarization responses from airborne inductive and galvanic ground systems: Tasmania

We have first analyzed the ability of polarizable and superparamagnetic thin sheets in the near surface to fit airborne electromagnetic (AEM) data using data from western Tasmania. Then we analyzed the results of such fitting in the context of geologic mapping and available ground induced polarization (IP) data. Small to large IP effects were found to considerably improve the fit to the observed AEM data, and the overall fitted IP parameters were spatially consistent. However, the locations of anomalous IP parameters were quite distinct from anomalies in other geophysical data. The airborne chargeability highs were adjacent to or surrounded the ground chargeability highs in the five cases analyzed from Tasmanian data. Modeling using the established Cole-Cole physical property values for sulfides predicts that an inductive airborne system is insensitive to many conventional IP targets, unless the mineral grain size is substantially less than 1 mm. In the cases in which airborne IP responses were adjacent to ground IP targets, we hypothesized that the airborne IP may be finer grained minerals in an alteration halo surrounding the sulfide sources of the large ground IP anomalies. Surficial clays encountered in drillholes did not have significant ground or airborne IP responses. A companion paper comes to a similar conclusion using ground and airborne data from a copper deposit in New South Wales.

Influence of non-aqueous phase liquid configuration on induced polarization parameters: Conceptual models applied to a time-domain field case study

Resistivity and induced polarization (IP) measurements on soil contaminated with non-aqueous phase liquids (NAPLs) show a great variety in results in previous research. Several laboratory studies have suggested that the presence of NAPLs in soil samples generally decrease the magnitude of the IP-effect, while others have indicated the opposite. A number of conceptual models have been proposed suggesting that NAPLs can alter the pore space in different ways, e.g. by coating the grain surfaces and thus inhibiting grain polarization, or by changing the pore throat size and thus affecting the membrane polarization mechanism. The main aim of this paper is to review previously published conceptual models and to introduce some new concepts of possible residual NAPL configurations in the pore space. Time domain induced polarization measurements were performed at a NAPL contaminated field site, and the data were inverted using the Constant Phase Angle (CPA) model and the Cole–Cole model respectively. No significant phase anomalies were observed in the source area of the contamination when the CPA inverted profiles were compared with soil sampling results of free-phase contaminant concentrations. However, relatively strong phase and normalized phase anomalies appeared next to the source area, where residual free-phase presence could be expected according to the chemical data. We conclude that depending on the NAPL configuration, different spectral IP responses can be expected. In previous research, the NAPL configurations in different samples or field sites are often unknown, and this may to some extent explain why different results have been achieved by different authors. In our field case, we believe that the NAPL forms a more or less continuous phase in the pore space of the source zone leading to an absence of IP anomalies. The increase in phase and normalized phase angle observed next to the source zone is interpreted as a degradation zone. The ongoing biodegradation may have led to a fractionation of the continuous NAPL in the outer part of the original source zone, leading to residual presence of isolated NAPL droplets in the soil pores. With such NAPL configurations, an increased polarization can be expected according to the electrochemical- and membrane polarization mechanisms. More research is needed to confirm the effects of different NAPL configuration on spectral IP parameters.

Environmental applications of the IP method: Surveys of subsurface waste

Induced-polarization (IP) effects in electrical geophysical surveys have been used in mineral-exploration projects for many decades, but only in the past 15 years or so has IP in environmental geophysics become a more common, commercially applicable method. This is primarily because of improvements in the economics of the technique and in data quality resulting from equipment advances. One field of environmental applications in which the IP method has been particularly useful is the study of buried waste. In the past, IP data acquisition was relatively expensive, and as a result, many landfill IP surveys were limited in scope for budgetary reasons. With the improving economics of the method, however, extensive amounts of data have been acquired at more than 50 landfills. At one site alone, 95 lines of IP and resistivity data were acquired, and at another, more than 14,000 transmitter-receiver pairs were used. Examination of this large, diverse data set allows several interesting general conclusions to be drawn that were not evident from the more limited surveys in the past. For example, early interpretations of IP results over landfills assumed that the IP effect was primarily the result of metallic debris in the waste. However, from the large database over many landfills, it appears that multiple sources of the IP effect are associated with the waste, and IP anomalies are clearly evident over landfills that contain little or no metal. It is also clear that IP surveys are substantially more reliable than other more traditional waste-mapping methods such as resistivity (or its inverse, conductivity) and magnetics. In addition, comparisons of data sets over time suggest that the IP method might be a useful tool in monitoring the degradation of subsurface waste.

Types and genesis of induced polarization anomalies in the water-rich structures of the Taian karst area

From December 2010 to April 2011, agricultural production in North China and Huang-Huai basin experienced difficulties because of severe drought. Based on samples from the development of deep karst groundwater resources, a set of laws has been established to reflect the types and genesis of induced polarization (IP) anomalies in a range of water-rich karst structures in Taian area. The curves of IP anomalies in the water-rich structures can be classified into ramps, slow downs, low-resistance platforms and low-resistance V patterns. IP anomalies observed in apparent resistivities contours can be classified as low-resistance steps, low-resistance gradients and low-resistance U patterns. In water-rich areas hosted mainly in caves, IP anomalies include low-apparent resistivity, low-visible polarization, low half decay time, low attenuation and low multi-parameter synchroneity. These water-rich areas mainly consist of fault structures, where the IP anomalies have a low-apparent resistivity and low-vision polarization, and exhibit relatively long half decay time, maximum attenuation value exceptions, apparent resistivities and multi-parameter “reverse phase” anomalies.

Modelling induced polarization effects due to pyrite in geochemical alteration zones above hydrocarbon accumulations

Geoelectrical methods measure resistivity and induced polarization (IP) effects in the subsurface. The differentially normalized electromagnetic method (DNME) detects geochemical alteration zones due to anomalous electrical responses which are often located above a hydrocarbon accumulation. Leakage above a non-perfect top seal is postulated to change the pH in the overlying sediments where a shallower mobility barrier is encountered. Epigenetic pyrite mineral growth is stimulated when iron and sulfur ions are available. Leakage and mineral growth are modelled by mathematical formulae assuming diffusion migration through the overburden. Effects of time and tectonic faulting are examined. Pyrite is highly polarizable and easily detected by an electrical IP survey in the field, whereby a current is introduced into the ground and subsequently turned off. Decay of the potential difference over the receiver electrodes is monitored in time. Special parameters facilitate detection of IP anomalies related to the presence of hydrocarbons. An inverted electrical depth model is computed using the Cole–Cole formula. The impact of geochemical modelling and predicted epigenetic mineral growth is demonstrated for the Severo-Gulyaevskaya dataset (Siberia). The overall DNME track record shows a reduced risk attached to hydrocarbon prospects, with a more reliable ranking at reasonable costs.

Application of Electrical Resistivity and Chargeability Data on a GIS Platform in Delineating Auriferous Structures in a Deeply Weathered Lateritic Terrain, Eastern Cameroon

Exploration for primary gold in tropical settings is often problematic because of deep weathering and the development of a thick soil cover. In this paper we present the results of both chargeability and resistivity surveys carried out over the Belikombone hill gold prospect (14?00' - 14?25'E, 5?25' - 6?00'N) in the Betare Oya area (eastern Cameroon), where previous soil sampling had identified gold anomalies. The geophysical data were obtained using Syscal Junior 48 resistivity meter and the Schlumberger configuration array for both the vertical electrical soundings (VES) and horizontal profiling. These data were further built into a GIS framework and the continuity of favourable gold-bearing structures at depth modeled using WINSEV, RED2INV and SURFER extensions softwares. IP (Induced Polarization)-chargeability and resistivity data combined, have identified irregular anomalous zones trending NE-SW. This trend is consistent with the attitude of most auriferous quartz veins exposed in artisanal pits and parallel to the regional shear zone system and foliations. The high resistivity anomalies correspond to quartz veins while the relatively high IP anomalies correspond to low sulphide ± gold concentrations in the quartz veins. Modeling IP-chargeability and resistivity data prepared as contours and 3D maps, culminated to the development of an inferred, irregular and discontinuous mineralized body at depths of up to 95 m. The size and shape of this mineralized body can only later be tested by drilling to ascertain the resource.

Geophysical investigation of a mineral groundwater resource in Turkey

The hydrogeological conditions in Uludag (Nilufer River catchment, Bursa, Turkey) were assessed, using time-domain electromagnetic soundings, electrical resistivity and induced polarisation tomography, to detect the most promising zones for new water-well siting, in order to increase the quantity of water for bottling. The hydrogeological model is quite complex: deep mineral and thermal water rises from a main vertical fault which separates two lithological complexes. The highly mineralised (deep) water is naturally mixed with low mineralised water at a shallow depth, 30–40 m; the mixed mineral water is found in some surface springs and shallow wells, while the highly mineralised water is found at depth in some unused deep wells located close to the main fault. All the water points (springs and wells) are located inside a “mineral water belt” on the north side of the Nilufer River. The geophysical survey confirmed the hydrogeological model and highlighted four promising zones for well siting (zones with very low electrical resistivity and high induced polarisation anomalies, corresponding to the main water-bearing faults). One of the geophysical anomalies, the furthest from the exploited sources, was verified by means of a test well; the drilling results have confirmed the water mixing model.

Electrical resistivity and induced polarization tomography in identifying the plume of chlorinated hydrocarbons in sedimentary formation: a case study in Rho (Milan — Italy)

Resistivity and induced polarization surveying were originally developed for mineral exploration but are now finding new applications in the field of environmental and engineering geophysics. The present article reports the results of a geophysical survey performed with the aim of identifying a plume of chlorinated hydrocarbons in sedimentary formations of the Pandania plain. The tested site is characterized by three sand and gravel aquifers containing a quantity of clay particles which influence the overall bulk resistivity and chargeability. According to data obtained using shallow boreholes, mainly dense non-aqueous phase liquids were found as contaminants in the first and second aquifer. The aforementioned geo-electrical methods were applied in both two- and three-dimensional approaches. Steel and copper electrodes were used in the process of field data acquisition and the results of the survey were compared. The geophysical survey revealed some anomalies that could be explained by the presence of dense non-aqueous phase liquids in the soil medium. The concept of normalized chargeability facilitates the interpretation of detected induced polarization anomalies. The shape of the plume was inferred from maps of resistivity and chargeability to a depth of 25 m below the surface of the ground.

The geological setting of volcanogenic sulphide orebodies in Albanian ophiolites

Cu–Fe sulphides with minor Zn, Au and Ag occur as massive, disseminated and vein mineralizations in volcanosedimentary rocks, volcanics and occasionally associated intrusions of a sheeted dyke complex in ophiolites in Albania. The ophiolites form two sub-parallel units, the Western Ophiolite and Eastern Ophiolite Belts, stretching across the country from north-west to south-east, and have mid-ocean ridge and island arc (boninite supra-subduction type) affinities, respectively. The ores have been extensively prospected and were mined until the 1990s.The Western Ophiolite Belt comprises a volcanosedimentary series, pillow basalts, ultramafic rocks, gabbros and plagiogranites. The most significant orebodies are in volcanosedimentary rocks at Rubik, Palaj-Karme and Porave. The orebodies with massive chalcopyrite–pyrite ores vary in shape occurring between hematised radiolarian chert and tholeiitic basalt pillow lavas. Mapping the shape of the main orebody at Rubik shows a series of mounds comparable to the collapse of 'black smoker' chimneys. Massive and disseminated sulphides are hosted by MORB affinity basalts at Kachinar, Derven and Rehova. A geophysical induced polarisation survey has indicated further resources at depth at West Kachinar.The Eastern Ophiolite Belt consists of a basalt–andesite–rhyolite series cut by a sheeted dyke complex and underlain by quartz diorite–plagiogranite, gabbro, gabbronorite and harzburgite. It includes a 25-km long belt of a significant longitudinal continuity of mineralization between Chafe Mal and Reps. Massive sulphide ores underlain by disseminated veins and stockworks are hosted by a Middle–Upper Jurassic lower basalt–andesite succession at Reps, Spach, Maja e Made, Tuch, Lak Rosch, Paluce and Fushe Arres. Alteration includes chloritization, epidotization, silicification, argillisation, zeolitization and carbonatisation. The Upper Jurassic, upper dacite volcanoclastic, andesite–dacite, boninite succession hosts massive pyrite–chalcopyrite–sphalerite ore underlain by few or no disseminated veins or stockworks at Gurth Spach, Munelle and Chafe Bar. At Perlat, massive ores are in an andesite pillow lava. Induced polarisation anomalies indicate further orebodies at depth in this highly thrusted area. Sulphide veins and other quartz–lode sulphides occur in some gabbros. An unusual orebody at Gjegjan occurs in a tectonic mélange of volcanics, greenschists and amphibolites.Similarities between Albania and other ophiolitehosted volcanogenic deposits at Cyprus and Oman include a clustering of orebodies along the general strike of the ophiolite, the occurrence of massive ores at successive horizons, and the presences of vein, stockwork and disseminated mineralization surrounding and beneath the massive sulphide zones; but, in Albania, ochres and umbers are not found, the age is older and the age-span greater. The Albanian orebodies range in size from 0·3–10·0 Mt. Those in the Western Ophiolite Belt are comparable in size to modern seafloor deposits at ocean ridges. Eastern Ophiolite Belt orebodies are similar in size to others interpreted as having formed in a suprasubduction environment. The Eastern Ophiolite Belt offers the greatest potential for locating future orebodies, possibly beneath thrust planes.

Induced Polarization—A Tool for Mapping Titanium-Bearing Placers, Hidden Metallic Objects, and Urban Waste on and Beneath the Seafloor

Recent experiments with a new marine induced polarization (IP) streamer system show that the system is capable of identifying and mapping placer concentrations of titanium-rich minerals on and beneath the seafloor, as well as facies changes in the substrate beneath sandy shoals. Examination of profile data acquired in the Mississippi Sound and Cape Fear shows that the system can also detect ferrous∕non-ferrous metallic objects buried beneath the seafloor which are not discernible by seismic or other geophysical means. The streamer system was originally designed to detect and map polarizeable titanium-bearing placer deposits to at least [Formula: see text] below the sediment-water interface. Recent data show broad phase-shift (a measure of the IP effect) anomalies from titanium sands along the east side of Cat Island (off the coast of Biloxi, Mississippi), and possibly over the now-submerged portion of the PeeDee river on the Atlantic Continental shelf southwest of Cape Fear (North Carolina-South Carolina border). Interspersed in these data are a number of narrow resistivity and coincident resistivity-IP spikes. Both kinds of anomalies show pronounced drops in the already-low resistivity expected for seawater-saturated unconsolidated sediments, but a number of the anomalies also have coincident phase-shift (induced polarization) excursions. These range from less than 5 to over [Formula: see text] across (half-amplitude measurement), but the IP anomalies can be seen [Formula: see text] laterally from their centers. The sources of these anomalies are apparently man-made objects, buried under a veneer of modern sediments. We speculate that the coincident resistivity-IP spikes are caused by shipwrecks, sunken buoys or parts thereof, or other man-made metallic objects lost and later buried beneath inert, modern sediments. The data imply that this marine IP system should be immediately applicable to finding metallic (ferrous and non-ferrous) unexploded ordnance, as well as buried sub-sea cables and pipelines.

Copper occurrences in the Heathcote Greenstone belt

has been known in the Heathcote Greenstone Belt since the 1880s when fencing contractors discovered the Mount Camel Copper The Copper Show had native copper, cuprite and copper carbonates that were exposed in costeans in 1906. The first systematic exploration occurred when Western Mining Corporation Ltd explored for copper between 1967 and 1972, conducting extensive soil and induced polarization surveying. The Western Mining Corporation Ltd soil grid shows anomalous copper values along the entire northern segment of the Heathcote Greenstone Belt, but the mineralisation was regarded as essentially pyrite and pyrrhotite with minor chalcopyrite. The Toolleen Group explored the Mount Camel Copper Show area as an aside from their exploration for gold in the greenstone belt between 1984 and 1990, after discovering numerous gossans. Gossans have also been discovered near Mount Ida and near Cornella. Geological understanding of the Heathcote Greenstone Belt has progressed since the last exploration for copper. The belt is thought to consist of two genetically different parts, in three segments. The northern and southern segments are ophiolite sequences with a simple structural style. In the aeromagnetic data these are seen as extensive (tens of kilometres) linear magnetic and nonmagnetic bands due to banded iron formation, cherts and jasper. The central segment, host to the Mount Camel Copper Show and a small copper occurrence west of Mount Ida, is thought to be of island-arc origin, and has a complicated structure, with faulted slices of Ordovician sediments within the Cambrian volcanics. Induced polarization anomalies are attributed to these Ordovician graphitic shales. The central segment has discrete, high magnetic responses, inferring that the magnetic units have short strike length, unlike the magnetic units in the northern segment. The geophysical surveys to date do not appear to have detected any mineralisation. Induced polarization and spontaneous potential anomalies are probably due to Ordovician graphitic shales approximately 20 m west of the Mount Camel Copper Show.

Porphyry-type copper deposits, eastern Victoria

occurrences of porphyry-type copper mineralisation in Victoria are within the Lachlan Fold Belt in eastern Victoria. Chalcopyrite, chalcocite and pyrite are the main ore minerals. Molybdenum is sometimes present. The mineralised intrusions include the Bete Belong Suite (Double Bull Creek, Sunday Creek, Stringer Knob and Mount Buck prospects), the Kaerwut Trondhjemite (Dogwood Prospect) and the Nunniong Granite (Tiger Creek Prospect). These mineralised, Silurian-Devonian intrusions are either side of the Buchan Rift. Most of the mineralised intrusions are intermediate in composition and tend to be magnetic. The Bete Belong Suite granites lie east of the Buchan Rift and form a series of magnetic ''highs''. The Kaerwut Trondhjemite is a much smaller intrusion and is associated with a more subdued magnetic response. The Nunniong Granite is less easily recognised. In most cases a magnetic survey would find the host intrusion but not the mineralisation. The induced polarization method is successful in locating the sulphides. However, most induced polarization anomalies are probably caused by pyrite, which is present in larger concentrations and is more widespread than the economic sulphides. Supergene chalcocite also contributes to induced polarization anomalies. The resolution of the gravity data are too low resolution to permit the correlation of gravity signatures with any of the intrusions. All known copper porphyry prospects in Victoria are uneconomic.

The Benambra volcanic hosted massive sulphide deposits

Benambra volcanic hosted massive sulphide (VHMS) zinc-copper deposits, Wilga and Currawong, lie in the Limestone Creek Graben in northeastern Victoria. Early stream sediment sampling indicated the prospectivity of the region for base metals. Detailed soil geochemistry helped establish several prospects. However drillholes based on geochemical and induced polarization anomalies intersected disseminated pyrite and, in some cases, stringer chalcopyrite, but no massive These disappointing results with induced polarization prompted the use of time-domain electromagnetics (TEM). A 50 m moving loop MPPO-1 TEM survey revealed an elevated value at the last station on the last line. The survey was extended, revealing a strong anomaly, which was subsequently drilled. The discovery drillhole, BEND17, drilled 25.5 m of massive sulphides (4.10% Cu, 0.46% Pb, 7.28% Zn, 31.5 g/t Ag). A 100 m loop MPPO-1 TEM survey of Currawong Prospect detected southern and northern anomalies. BEND24, drilled into the northern TEM anomaly, intersected massive pyrite-sphalerite-chalcopyrite between 126.48 and 140.68 m (1.98% Cu, 0.50% Pb, 3.06% Znand 42 g/t Ag). Gold values within the massive sulphides ranged from 0.05 g/t to 0.40 g/t. A second lens was intersected when the southern anomaly was drilled. The Wilga deposit is a single ellipsoidal lens with a strike length of 300 m, extending 240 m down dip, and with a maximum thickness of 37 m, giving a reserve of nearly 4 Mt grading 3% copper, 6.2% zinc and 23 glt silver. The Currawong deposit, about 4 km to the northeast, consists of several lenses giving a resource of 9.5 Mt grading 1.7% copper, 4.3% zinc, 0.9% lead, 38 g/t silver and 1.3 g/t gold. The two deposits are both pyrite-sphalerite-chalcopyrite massive They occur as concordant to subconcordant bodies in interbedded sediments and dacitic-andesitic volcanics of the Middle to Upper Silurian Enano Group. The volcanics and sediments have undergone substantial folding and faulting, giving steep to moderate west-dips, and northeast-strikes. Petrophysical measurements on barren and mineralised core indicate that the host volcanic and sedimentary rocks are resistive and weakly to moderately magnetic, the disseminated sulphides are polarizable but resistive, and the massive sulphides are conductive and moderately to intensely magnetic. At both Wilga and Currawong, the massive sulphides are associated with conductivity anomalies (from the TEM surveys) and with discrete magnetic responses (evident in aeromagnetic data). In 1986 the deposits were sold to Macquarie Resources who went into a joint venture partnership with Denehurst Limited in 1992. Mining at Wilga commenced in September 1992, and to June 1995, 727 000 tonnes of copper-rich ore and 237 765 tonnes of zinc-rich ore was mined. Both Wilga and Currawong are genuine geophysical discoveries. The prospective area was defined by geochemistry but both deposits were found by following up TEM anomalies after numerous drillholes based on geochemical and induced polarization anomalies failed to intersect massive sulphides.

Geophysics and exploration of the volcanic hosted massive sulphide prospect near Wickliffe, western Victoria

The Wickliffe Prospect (also known as Victor 2 Prospect) is located on the eastern edge of the Cambrian Mount Stavely Volcanic Complex near the town of Wickliffe in western Victoria. The Mount Stavely Volcanic Complex comprises volcanic breccia/ sandstones thought to be time-equivalent to the Mt Read Volcanics in western Tasmania.North Ltd regarded the prospect as an alteration system to a lead-poor, copper-zinc rich volcanic hosted massive sulphide system. Disseminated and stringer sulphide mineralisation and alteration has been discovered in the volcanic units of the Towanway Tuff, towards its stratigraphic top. This horizon has been sheared off in the Wickliffe Prospect area, but may still be present in unexplored areas to the south.Regional geophysical datasets, including airborne time domain electromagnetics, magnetics and radiometrics, together with detailed ground magnetic and gravity surveys, have been used to define lithological variations and structure but do not directly detect the mineralisation.Western Mining Corporation Ltd, Pennzoil of Australia Ltd, CRA Exploration Pty Ltd and North Ltd, have explored the prospect area. In 1977, Pennzoil discovered the stringer-type pyrite-chalcopyrite, which led to definition of the prospect. Best drilling intersections were 7 m @ 0.13 ppm gold, 1 m @ 0.93% copper and 24 m @ 0.29% copper.The mineralisation has coincident soil geochemistry copper, lead and zinc highs and geophysical responses. Magnetic induced polarization anomalies of -3 to -2 mV/V chargeability and 80–110% HN normalised horizontal field conductivity are coincident with the mineralisation. A gradient-array induced polarization chargeability anomaly of 15 mV/V was recorded, and a rapid reconnaissance magnetic induced polarization broad chargeable zone and relative phase shift anomaly of 0.3° to 0.4° are also coincident with the mineralisation.

Rapid methods for interpretation of induced polarization anomalies

The induced polarization (IP) method is used at various stages of prospecting in poorly accessible regions that have complex structure. For such conditions we suggest approximate procedures for acquiring information about the location of the structures in question. Correlation between the observed fields and the heights of observation points is used for reducing the effect of terrain relief. Simultaneously, the problem of revealing an anomalous body is qualitatively solved. The qualitative construction of pseudosections of apparent polarizability η a and semi-quantitative determination of the depth of a polarized bed using vertical sounding allow us to obtain information about the ore potential of a region as a whole and the location of ore deposits specifically. We demonstrate common aspects of typical models (thin and thick beds, horizontal circular cylinder) with models used in magnetic prospecting. On this basis it is suggested that one should apply quantitative methods of magnetic anomaly interpretation in the cases of rugged topography, oblique magnetization (polarization) and unknown levels of the normal field for rapid inversion of η a anomalies. These rapid methods (improved modifications of characteristic point and tangent methods) are successfully tested both on models and in actual prospecting and investigation of a karst terrain.

GOLD: Geophysics of the Big Bell Gold Deposit, Western Australia

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 mV/V at n53. In addition, a strong chargeability of up to 40 mV/V at n52 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 radiometric 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.