Exploration Geophysics - Volume 32, Issue 3-4, 2001

The Cadia Ridgeway Au-Cu deposit is a deep (> 500 m) gold-rich porphyry copper system located in central NSW. A chargeability anomaly, recognised within a prospective NW-SE mineralised corridor motivated the drilling program which led to the discovery of the deposit.

In-situ and laboratory electrical tests were conducted to characterise the petrophysical properties of the deposit and host sequences. The ore is both chargeable and relatively conductive, although not sufficiently conductive for EM methods.

Significant scale variation of apparent resistivity was observed. In-situ measurements, utilising electrode spacings of one to four m, are up to two orders of magnitude lower than values determined from laboratory measurements.

Forward modelling of surface induced polarisation and apparent resistivity field data was conducted using petrophysical properties determined primarily from in-situ testing. The chargeability anomaly identified over the Ridgeway ore body is due largely to discontinuous halos of pyrite alteration above the economic mineralisation, but a highly chargeable source component associated with the economic mineralisation at Ridgeway is also inferred.

The measured contrast in conductivity between ore and host rocks suggests that Ridgeway-style mineralisation may represent a target for magnetometric resistivity (MMR) surveying. 3D numerical modelling predicts that the MMR response of the Ridgeway deposit would be detectable by both surface and down hole MMR surveys.

The Tritton massive sulphide copper deposit was discovered using systematic moving loop surface TEM surveys. The use of this technique was based on previous experience around the Girilambone copper mine. The deposit consists of an upper and lower lens, both of which are highly conducting. The lower lens was discovered through the use of downhole EM techniques.

The upper lens lies at a depth in excess of 160 m below the surface and the lower lens is at a depth of greater than 400 m. Both these lenses have EM time constants of about 10 ms.

Apart from having a minor associated magnetic anomaly the Tritton deposit has not been detected from the surface by any other geological, geochemical or geophysical technique.

Approximately one kilometre north of Tritton is the uneconomic sulphide deposit, Budgerygar. This has similar geometry to the Tritton deposit but lies at the shallower depth of 60 m. The Budgerygar deposit is readily detectable by a variety of geochemical and geophysical techniques.

The Tritton and Budgerygar deposits are an interesting test site for the development of new exploration technology, due to the similarity in their form but difference in depth and hence detectability. A number of geophysical techniques have been tested over the Tritton deposit since its discovery.

The Copper Hill prospect in NSW is a porphyry copper/gold system. Geochemical and gradient array IP results suggest that grid northwest and grid northeast structural directions control the location of mineralisation. Previous drilling has been oblique to these directions.

The known sulphide rich mineralised zones exhibit moderate to strong IP responses. An IP/resistivity survey was conducted to produce a detailed 3D mineralisation model for Copper Hill. 3D inversion of IP data is considered suitable for mapping the sulphide horizons in detail.

The use of 3D-inversion modeling techniques, accepting a modified pole-dipole IP survey array, allowed an electrode array to be designed for fast acquisition of a large quantity of data.

The use of modified pole-dipole arrays extend the effective depth of investigation of IP surveys as well as improving the across line resolution.

Inversion results delineated the three dimensional foim of sulphides zones at Copper Hill.

A total of twenty line kilometres of IP, DC resistivity and MT data have been acquired using MIMDAS, (MIM's Distributed Acquisition System), over the Cluny prospect, south of Mt Isa. The utilisation of two-dimensional and three-dimensional inversion programmes has provided a significant advantage in the interpretation of these data.

The integration of geology and geophysics through the use of inversion, particularly three-dimensional inversions, has greatly improved the geological understanding of the Cluny region. It has enabled the identification of variations along linear conductors, and anomalous chargeable zones. This has implications for the ability of geophysics to provide discrimination for mineralisation along linearly extensive stratigraphy.

In the Eastern Goldfields of Western Australia, airborne electromagnetic (AEM) data have been acquired over areas prospective for nickeliferous laterite deposits, although their application in exploring for this style of mineralisation is not well understood. In part, this can be attributed to a lack of detail concerning relationships between supergene Ni enrichment and the petrophysical, particularly electrical, properties associated with prospective regolith materials and settings. This issue was addressed using a combination of multi-parameter borehole geophysical techniques and laboratory analyses of mineralogy, geochemistry and soluble salt content for 24 drill holes in a zone of known mineralisation associated with the Cawse lateritic Ni deposits, located in the Eastern Goldfields, W.A.

The regolith associated with supergene Ni enrichment exhibited a complex vertical conductivity structure. In places, elevated conductivities were coincident with Ni-Mn-Co mineralisation. These correlations were noted to occur at hydromorphic barriers associated with structural discontinuities, or in places where regolith textural changes were marked. A better understanding of the complex spatial patterns associated with the electrical character of the regolith at Cawse, was achieved by examining the weathered profile as a set of hydrostratigraphic units, namely the transported cover, zone of illuviation and in situ regolith. This framework allows the observed regolith electrical structure to be explained in terms of contemporary groundwater processes and the accumulation of soluble salts.

The observed petrophysical (electrical) response of the regolith at Cawse suggests that AEM has a role in locating impermeable barriers and faults which may, respectively, represent sites for shallow, high grade Mn-Co-Ni and garnierite mineralisation, particularly where they are coincident with local accumulations of soluble salt and moisture.

Petrophysical data are important for constraining the geophysical signatures of the Endeavour Cu-Au deposits within the Goonumbla Volcanic Complex (GVC). Susceptibilities vary systematically with lithology and, particularly, with alteration. Remanence tends to be subordinate to induced magnetisation in the GVC. Densities are predictably related to composition, Alteration effects on densities are generally minor, although rocks with particularly strong development of secondary magnetite, hematite or sulphides have higher densities.

Modelling of magnetic and gravity profiles over the Goonumbla volcanic belt and the GVC, constrained by geological information and petrophysical properties, suggests that the GVC is underlain by a large zoned intrusion, representing the parent magma chamber, which has a substantial low density, weakly to moderately magnetic, core of alkali feldspar granite to monzonite composition, enclosed by marginal mafic monzonite and monzodiorite phases. The mafic roof zone and marginal phases of the GVC have high susceptibilities (> 0.08 SI, > 2.5 vol % magnetite).

A prominent ridge of low density material occurs along the eastern margin of the mother intrusion. A zone of lower susceptibility occurs above the felsic ridge, probably representing magnetite-destructive alteration due to fluids emanating from the inferred underlying felsic intrusion. This zone appears to be related to the Endeavour lineament, which is thought to control the emplacement of many of the mineralising intrusions in the GVC. Magnetic signatures of deposits tend to be obscured by the heterogeneous magnetic environment, but reflect variably developed halos of enhanced magnetite content, associated with early potassic alteration, surrounding a core of reduced magnetite content, which represents the combined effect of felsic mineralising intrusives, mineralising phase (K-feldspar dominated) alteration and phyllic overprinting. Different signatures can be expected for lava-dominated wall rock sequences (weak to moderate annular high with well-developed central low) and volcaniclastic-dominated sequences (unimodal weak to moderate high).

Numerical modelling was used to investigate the results from a downhole transient electromagnetic (DHTEM) test survey, which was performed over the Cadjebut lead-zinc ore body in 1987. Unexpectedly, the response due to the inductively well-coupled near-loop contained no signature of the known ore-body, whilst the response due to the offset loop contained a prominent negative intersection anomaly.

A dual-slab block model representing the known mineralisation was constructed using a 3D integral equation program MARCO. This algorithm was used because it permitted modelling of both inductive and galvanic effects. The response generated from this model for both the near and far loops contained a weak and strong single-signed galvanic anomaly, respectively. Current channelling was recognised as the primary excitation mechanism after consideration of the anomaly sign and verification with decay analysis. Specifically, decay analysis of the anomaly within the offset loop data yielded decay indices α < -3.27 and α < -3.47 for the field and model data, respectively. These results are in agreement with the decay index identifiable with current channelling, namely α < -3.5.

The modelling has demonstrated the importance of offset loops in the production of detectable current channelling effects, which in turn enables the detection of weakly conducting targets which may otherwise be invisible to traditional inductive exploration methods.

The Trilogy deposit, located near Ravensthorpe in southern Western Australia, is a polymetallic massive sulphide deposit hosted within the conductive graphitic phyllites of the Proterozoic Mount Barren metasediments. There are two different styles of mineralisation, Pb-Zn massive-style and Cu-Au stringer-style mineralisation, contained within a silicified envelope that hosts the mineralisation.

Petrophysical samples were taken and geophysical surveys conducted to characterise the response of the massive sulphide system and to help define an exploration model.

Gravity, ultra-detailed airborne magnetic and radiometric, airborne time domain electromagnetic (EM) and ground time domain EM surveys have all been conducted over the deposit. The gravity survey outlined the position of the massive Pb-Zn mineralisation. A radiometric low around the surface projection of the silicified zone was evident in the airborne radiometric data. There was no response from the deposit recorded in the airborne magnetic survey. There is a subtle airborne EM anomaly over the known mineralisation, but the ground EM better defines its location and EM response. The airborne EM anomaly produced by the deposit was small compared with the response of the conductive phyllites elsewhere in the region.

Geophysical exploration may aid in the search for additional sulphide mineralisation in the area. The combination of airborne EM with followup detailed gravity and ground EM has proven to be the most effective geophysical strategy in locating Trilogy style mineralisation in the Mount Barren basin.

The Hamersley Basin of Western Australia holds significant tonnages of detrital iron deposits. Formed by a cyclic weathering and eroding sequence of nominally barren banded iron formation, they may form rich proximal accumulations of iron cemented gravels distal from any obvious hard-rock iron mineralisation. While generally small, they nonetheless represent excellent economic targets as they are near surface and easy to mine. The relatively high density of iron detritals, when compared with their sedimentary hosts, makes the gravity method the primary method of exploring for these blind deposits. Identification of possible trap sites is used as a precursor to gravity surveying, with magnetics used as an ancillary method prior to drilling.

Application of the gravity method in this terrain is not straightforward. Detrital trap sites are best developed next to the spectacular cliffs that form the range fronts to the Hamersley Ranges. Precise terrain corrections and use of first vertical derivatives are routinely applied to gravity data in an effort to map every possible gravity high that may reflect the presence of a detrital accumulation. Extensions to Hamersley Iron Pty. Limited’s BS2D deposit were discovered by the routine application of gravity. This methodology has been used to create a model by which exploration for detrital deposits continue to the present day.

Conductivity values derived from TEMPEST AEM data were compared with conductivity values derived from ground EM data and borehole induction logs. The high degree of correlation at scales of around 100 m laterally and less than 10 m vertically allow the conductivity data derived from AEM data to be interpreted with confidence.

An extensive drilling database was used to interpret the conductivity distribution. This showed that the conductivity variations at Grant’s Patch are principally associated with the regolith, specifically with in situ saprolite. The fresh bedrock in this area is uniformly resistive in comparison with the regolith, which attains a maximum conductance of around 50 S. The regolith conductivity variations can be used to infer basement lithology because the regolith is largely in-situ and the thickness and conductivity of the regolith materials are demonstrably litho-dependent.

Information derived from the AEM data has lead to a significantly improved understanding of the geology (lithology, structure, alteration and weathering) in the Grant’s Patch Area.

The cost and risk associated with mineral exploration in Australia increases significantly as companies move into deeper regolith-covered terrain. The ability to map the bedrock and the depth of weathering within an area has the potential to decrease this risk and increase the effectiveness of exploration programs.

This paper is the second in a trilogy concerning the Grant’s Patch area of the Eastern Goldfields. The recent development of the VPmg potential field inversion program in conjunction with the acquisition of high-resolution gravity data over an area with extensive drilling provided an opportunity to evaluate threedimensional gravity inversion as a bedrock and regolith mapping tool.

An apparent density model of the study area was constructed, with the ground represented as adjoining 200 m by 200 m vertical rectangular prisms. During inversion VPmg incrementally adjusted the density of each prism until the free-air gravity response of the model replicated the observed data. For the Grant’s Patch study area, this image of the apparent density values proved easier to interpret than the Bouguer gravity image.

A regolith layer was introduced into the model and realistic fresh-rock densities assigned to each basement prism according to its interpreted lithology. With the basement and regolith densities fixed, the VPmg inversion algorithm adjusted the depth to fresh basement until the misfit between the calculated and observed gravity response was minimised. The resulting geometry of the bedrock/regolith contact largely replicated the base of weathering indicated by drilling with predicted depth of weathering values from gravity inversion typically within 15% of those logged during RAB and RC drilling.

Grant’s Patch is a semi-mature lode gold corridor located 45 km NW of Kalgoorlie in Western Australia. Gold mineralisation at Grant’s Patch sits within geological structures less than 20 m wide (lithological contacts, ductile shears and brittle cross-faults). These structures are masked by 10 to 100 m of regolith cover and are often impossible to correlate between exploration drillholes.

The southern part of this corridor is covered by high-resolution aeromagnetic, airborne electromagnetic and ground gravity data. This provides dense, regularly spaced data coverage to image patterns reflecting the sub-surface geometry of the regolith and bedrock geology. There is also uniform exploration drillhole coverage to non-weathered bedrock and very little surface disturbance from mining infrastructure.

Aeromagnetics identifies magnetic greenstone units, such as mafic to ultramafic sills and dolerite horizons, faults as disruptions crossing proterozoic dykes, and paleochannels containing ferruginous gravel. Patterns in the filtered gravity data reflect subtle density differences between rock units, regolith features, and weathered fault zones that cannot be detected in the aeromagnetic data. The airborne electromagnetic (AEM) response is dominated by conductive clays, predominately in situ as saprolite, and saline groundwaters in the regolith that follow lithodependant weathering of bedrock units. Cross-faults are mapped as disruptions of the conductive units and may be either conductive or resistive.

Geophysical ‘pattern mapping’ from such surveying is relatively inexpensive when compared to drilling. It increases success rates by identifying mineralised structures and fluid pathways at the early stages of an exploration program and should also be routinely applied in ‘brownfields’ project areas.

High-resolution aeromagnetic surveys were acquired for the Albuquerque basin in the central Rio Grande rift, a basin filled with poorly consolidated sediments. The surveys proved successful in efficiently and economically mapping previously unknown hydrogeologic features of the shallow subsurface. This success suggests that aeromagnetic methods may be useful in hydrogeologic studies of other sediment-filled basins.

The aeromagnetic surveys were used primarily to delineate buried igneous rocks and to locate faults within the basin fill, both important for understanding the subsurface hydrogeology. Buried igneous rocks were recognized from their high-frequency, high-amplitude magnetic responses and characteristic map patterns. The horizontal-gradient and local wavenumber methods were used to obtain estimates of their source depths.

The aeromagnetic surveys were also successfully used to locate faults within the basin fill. Magnetic signatures associated with faults are produced by the juxtaposition of sediments having differing magnetic properties rather than the products of secondary processes. Expression of faults is abundant throughout the basin, revealing patterns that cannot be mapped at the surface due to widespread cover.

A fault signature recognized in the high-resolution data that has multiple inflection points is best explained by a fault with a thin magnetic layer on the upthrown block and thick magnetic layer on the downthrown block, called the thin-thick layers model. Geologically, this signature indicates erosion of the upthrown block or a growth-faulting scenario: fault-controlled sedimentation for faults that offset sediments, and successive accumulation of basalt on the downthrown block for faults that offset volcanic rocks.

High-resolution aeromagnetic surveys were acquired for the Albuquerque basin in the central Rio Grande rift, a basin filled with poorly consolidated sediments. The surveys proved successful in efficiently and economically mapping previously unknown hydrogeologic features of the shallow subsurface. This success suggests that aeromagnetic methods may be useful in hydrogeologic studies of other sediment-filled basins.

The aeromagnetic surveys were used primarily to delineate buried igneous rocks and to locate faults within the basin fill, both important for understanding the subsurface hydrogeology. Buried igneous rocks were recognized from their high-frequency, high-amplitude magnetic responses and characteristic map patterns. The horizontal-gradient and local wavenumber methods were used to obtain estimates of their source depths.

The aeromagnetic surveys were also successfully used to locate faults within the basin fill. Magnetic signatures associated with faults are produced by the juxtaposition of sediments having differing magnetic properties rather than the products of secondary processes. Expression of faults is abundant throughout the basin, revealing patterns that cannot be mapped at the surface due to widespread cover.

A fault signature recognized in the high-resolution data that has multiple inflection points is best explained by a fault with a thin magnetic layer on the upthrown block and thick magnetic layer on the downthrown block, called the thin-thick layers model. Geologically, this signature indicates erosion of the upthrown block or a growth-faulting scenario: fault-controlled sedimentation for faults that offset sediments, and successive accumulation of basalt on the downthrown block for faults that offset volcanic rocks.

An integrated geophysical study of salinisation at Cape Portland, NE Tasmania, has mapped the distribution of saline areas, identified constrictions in the hydrogeologic basement and a possible source, store and transport mechanism for the salt.

EM-31 data not only clearly delineates the extent of salt scalds, but also highlight areas of elevated conductivity not visibly affected by salt. EM-31 data maps the distribution of the salt at a much higher resolution than was previously possible using shallow drilling.

Results from time-domain electromagnetic surveys confirm the responses seen in the EM-31 data and provide additional information about the subsurface distribution of conductive material. Conductivity depth pseudosections and layered earth inversions indicate depressions of up to 100 m in the resistive basement that are infilled with more conductive material.

Ground magnetic data show the distribution of shallow Jurassic dolerite basement features and define a major negative anomaly that transects the study area. This feature is coincident with depressions identified from the time-domain electromagnetics and with a negative Bouguer gravity anomaly.

The electromagnetic and potential field interpretations are consistent and indicate the presence of a major palaeochannel infilled with conductive material that cross-cuts the study area. Seismic refraction surveys and shallow auger holes in this zone provide additional support for the presence of a palaeochannel. This feature is inferred to be a major control on salinisation at Cape Portland.

Over highly-conductive Earths, the maximum effective depth of investigation of the EM-31 terrain conductivity meter is reduced from its nominal value of 6 m due to the effects of skin depth attenuation. In extreme cases, depths of investigation may be as small as ~2.5 m and ~4 m for the vertical coplanar and horizontal coplanar coil configurations respectively. Sensitivity of the instrument to variations in the parameters of layered earth models is highly dependent on the geoelectric structure. For high values of ground conductivity the in-phase component of the response depends strongly on the subsurface electrical conductivity, and for some layered earth models may offer an increased depth of investigation in comparison with apparent conductivity (quadrature) data. The restricted measuring range of the EM-31 instrument is however a limitation on the usefulness of in-phase measurements in such cases.

An airborne electromagnetic (AEM) survey was flown with the TEMPEST AEM System over the Kamarooka study area north of Bendigo.

Conductivity values derived from AEM measurements were compared with borehole conductivity values obtained with an EM39 induction probe, indicating an approximately 1:1 relationship between the two quantities when taking into account the vertical (several metres) and horizontal (100 m) resolution of the AEM values.

The thickness of Tertiary and Quaternary cover overlying Paleozoic basement was mapped by resolving the transition from moderately conductive cover to resistive basement. Palaeodrainage lines were mapped in the lower parts of the survey area using the topography of this interface, and in the upper portions of these buried valleys by tracing low amplitude magnetic trends associated with the valley infill material.

The lateral extent and depth to a shallow (0 to 15 m), highly conductive layer was mapped. This layer was interpreted to correspond to high salt concentrations at the groundwater table. Areas where this layer intersects the land surface correlate with mapped areas of saline discharge.

The conductivity values derived from TEMPEST AEM data provided a regularly sampled 3D framework into which surface observations and sparsely distributed sub-surface observations were incorporated as part of an ongoing integrated hydrogeological investigation.

Magnetic inversion programs such as mag3d from the University of British Columbia’s Geophysical Inversion Facility (UBC-GIF) have proven to be very useful for generating realistic 3D susceptibility models from surface Total Magnetic Intensity (TMI) data.

These programs do not perform well when the observed data includes the response of bodies which are strongly remanently magnetised. This failure occurs because the forward model algorithm used in the inversion only generates the induced response, so the remanent component in the TMI has to be modeled using the induced response for an unrealistic distribution of susceptibility.

In this paper we introduce two transforms: the analytic signal of the vertical integral (ASVI) and the vertical integral of the analytic signal (VIAS). When applied to TMI data, these transforms produce data which is qualitatively similar to a purely induced TMI response for a vertical magnetic field. We investigate the effectiveness of using mag3d to invert the ASVI for a synthetic dataset and the VIAS for a real TMI dataset. For both datasets we find that the inversion of the transformed data produces a model which is much more realistic than that obtained by inverting the TMI data.

BHP Billiton commenced exploration surveying with the world’s first fully operational, airborne gravity gradiometer in October 1999. This gradiometer (called Einstein), together with a later one called Newton, was developed in conjunction with Lockheed Martin by BHP Billiton’s FALCON project. FALCON data are acquired by Sander Geophysics Ltd., flying a Cessna Grand Caravan to survey specifications typical of aeromagnetic surveys.

The first FALCON survey was flown over a portion of the Bathurst mining camp in New Brunswick, Canada in order to compare system performance with existing extensive and detailed ground-gravity data.

The ground-gravity data, supplied courtesy of Noranda Minerals Exploration Ltd., were upward continued to the flying height and vertically differentiated to provide vertical gravity gradient data suitable for comparison with the airborne data.

The two data sets compare very well and the results demonstrate that the FALCON airborne gravity gradiometer is capable of detecting sources with a vertical gravity gradient signal of greater than 10 Eö and a full-width at half-maximum amplitude of 500 m.