Search

Management guru Peter Drucker is credited with the statement “you can't manage what you can't measure.” In the exploration world the most important outcome of our efforts is the discovery of new ec...

Hans Lundberg was a geoscientist whose career spanned the first half of the 20th century. He was part Indiana Jones, part Howard Hughes and part Jules Verne. He was born in 1893 in Malmo, Sweden an...

Mining geophysicist Richard “Dutch” Van Blaricom passed away in December 2020 in Spokane, Washington, after a brief illness.

• Victor F. Labson, 1952–2020 • Michael John Lewis, 1940–2020

SummaryElectrical methods have been applied to the search for porphyry copper and IOCG deposits since the early 1950s. While there is a generally accepted model of disseminated sulfides giving rise...

Peter Jonathan Hood, 1929–2020

The Nebo Babel Ni-Cu deposit was discovered in 2000 by Western Mining Corporation (WMC). BHP acquired WMC in 2005 and advanced the project. In 2014, BHP optioned the property to Cassini Resources and in 2016 Cassini partnered with Oz Minerals to further advance the project. During the period when WMC and BHP held the project, extensive airborne geophysical investigations were conducted including Geotem, Falcon®, ZTEM, AirMt and Spectrem. Parts of the mineralized system respond well to the various airborne EM techniques but the response overall is complicated by the presence of extensive paleochannel development. A combination of 1D, 2D and 3D processing techniques have been applied to the EM data so as to better understand the overall system responses and where possible, see if it were possible to separate the basement conductive response from that of the overlying Tertiary channels.

Over the period from 1985 to 2005, BHP carried out three major geophysical research projects that were intended to significantly enhance the ability to find new ore deposits. This involved major activities internally as well as external components that involved complex multi-year programs involving large expenditures. In all cases, major efforts were made to deploy the outcomes in BHP’s minerals exploration programs. While the technologies developed could be considered as successful in having met or exceeded the original development goals, in no case did the outcomes of these efforts contribute materially to the discovery of significant new mineral resources. This suggests that the technical objectives for a new technology can be comparatively straight-forward to define, but the subsequent implementation path, once the technological goals are achieved, were poorly conceived. BHP’s experience is much like the exploration industry as a whole over the same period. While much appears to have been developed which has added significantly to the technical capabilities of the industry, it has been less apparent that these developments have been able to contribute significantly to an improved discovery record. Considerable effort is now being directed towards bringing on new geophysical technologies in programs such as Uncover in Australia and CMIC’s Footprint in Canada. Past experience suggests, however, that better technology alone can’t be expected to achieve the sought after goal of improved discovery success.

The Newton Au-Ag deposit is an intermediate sulfidation state epithermal system in British Columbia, Canada. Multiple types of geophysical data are interpreted and evaluated with drillcore petrophysical, geochemical, and geologic observations to better understand the geophysical signature of the Newton epithermal system. Airborne [Formula: see text]-ray data sets indicate elevated emission counts of K, eTh, and eU over the Newton epithermal system, which are caused by hydrothermal alteration. Drillcore [Formula: see text]-ray measurements also indicate high potassium concentrations related to the K-rich phyllosilicates in the form of argillic and quartz-sericite alteration assemblages. Magnetization vector inversion (MVI) is used to recover an unconstrained 3D magnetization vector model of the system on regional and deposit scales. The regional MVI has resolved a deep concentric-shaped low magnetic zone that is interpreted as a porphyry system beneath the epithermal deposit. At the deposit scale, 3D direct current (DC) resistivity and induced polarization (IP) inversion, and unconstrained MVI revealed finer details of the epithermal system architecture. Cooperative DC/IP and magnetic inversion, at the deposit scale, constrained the magnetic susceptibility model and recovered a more precise susceptibility image of the epithermal system that is well-matched with borehole geology. The integrated geophysical interpretation helped to resolve several 3D latent geologic features in places without direct access to drillcore samples. We identified four petrophysical domains based on the three cooperatively inverted physical properties, including electrical resistivity, IP chargeability, and magnetic susceptibility. The combined geophysical models differentiated porphyritic intrusions (chargeability/susceptibility lows), disseminated sulfides (resistivity lows and chargeability highs), a Cu-rich zone in mafic volcanic rocks (susceptibility/chargeability highs and resistivity lows), and an Au-Ag-Cu-rich zone with silicification in felsic volcanic rocks (chargeability/susceptibility lows and resistivity highs). These petrophysical domains also provide useful exploration vectors for identification of similar epithermal systems.

The 2019 edition of ETD (Mineral Exploration Trends and Developments) tracks the recent developments in exploration technology around the world.ETD has been published since 1965, and in that time h...

SUMMARY The minerals system approach is based on concepts that have been largely germinating for the past 20 years but are now becoming a major component of efforts to rejuvenate minerals exploration into the next decade. Geophysical mapping will take on a far more critical role in this effort as it offers very often the most cost effective means to explore the subsurface, especially when the overlying rocks are unrelated to the targets at depth. This review will examine what might be called the ‘stateof-play’ in how geophysics is currently being used to start to build an understanding of what mineral systems look like in the real world. While much remains to be done, there is encouragement in what has been achieved to date, although most of it typically without the benefit that the overarching concept of developing a mineral system model.

SUMMARY MT and ZTEM data were inverted with a number of 2D and 3D algorithms to recover the subsurface conductivity structure of an area of interest. A 2D inversion algorithm was used to model the magnetotelluric TM and TE mode impedances and the ZTEM tipper data, separately. The derived conductivity-depth sections don’t show much agreement, possibly indicating the conductivity structure of the area to be highly three-dimensional. A 3D inversion algorithm was used to invert the MT and ZTEM data, separately and jointly. Overall, there is good agreement between the derived conductivity structures. This suggests that a joint inversion can extract successfully the combined subsurface conductivity information from the two data sets.

SUMMARY The Cannington deposit is a high grade Ag-Zn-Pb deposit found in 1990 by BHP Minerals drilling an isolated 1,000 nT aeromagnetic feature. Following the discovery of Cannington, numerous airborne, ground and borehole surveys have been carried out which overall, provided some assistance at better defining the ore system but did not lead to the discovery of new major deposit in the area. While Cannington possessed a clear magnetic response, the presence of a thick conductive cover made the use of EM and electrical techniques challenging. BHP used Cannington as a test ground for a variety of new techniques including a ground SQUID EM sensor, modified airborne EM technology (higher power and lower base frequency) and over 10 years after discovery, the first ever Falcon airborne gravity gradiometer survey in Australia.

ZTEM data acquired across the Humble magnetic anomaly of almost 30,000 nT were analyzed for the presence of a magnetic gradient response and the effects from elevated magnetic susceptibilities. The response of moving the receiver coil through the magnetic-field gradient peaks at 0.01 Hz and drops off strongly with frequency. Lacking information about the field strength at the base station precludes the comparison of amplitudes between computed gradient responses and the survey data, but the comparison of response shapes suggests that the gradient responses are too small to have a noticeable effect on the survey data. The 3D inversion of the magnetic survey data indicates magnetic susceptibility values as high as 2.0 (SI). Forward-modeling the ZTEM response for these Ƙ–values combined with resistive half-spaces indicates that the response amplitudes and shapes strongly depend on the background resistivities. Ignoring the elevated Ƙ–values during an inversion can result in the underestimation of conductivities and other artifacts, such as the mapping of patterns that resemble crop circles. For an environment such as Humble, with deep-seated zones of elevated Ƙ–values, the shallow inverted conductivity structure appears to be reliable, but the deeper structure should be interpreted with caution.

ZTEM data acquired across the Humble magnetic anomaly of almost 30,000 nT were analysed for the presence of a magnetic gradient response and the effects from elevated magnetic susceptibilities. Mag3D inversion of the magnetic data indicates magnetic susceptibility values as high as 2.0 (SI). The response of moving the receiver coil through the magnetic-field gradient peaks at 0.01 Hz and drops off strongly with frequency. Lacking information about the field strength at the base station precludes the comparison of amplitudes between computed gradient responses and the survey data, but the comparison of response shapes suggests that the gradient responses are too small to have a noticeable effect on the survey data. ZTEM responses were forward modeled with a UBC-GIF 3D algorithm that takes into account electric conductivities  and magnetic susceptibilities , in order to assess the impact of the elevated values derived from the Mag3D inversion. Computing the ZTEM response for these values combined with resistive halfspaces indicates that the response amplitudes and shapes strongly depend on the half-space resistivities. Ignoring the elevated values during an inversion can result in patterns that resemble crop circles. A comparison of the synthetic data with the Humble survey data suggests that the observed responses are more affected by the subsurface conductivities than by the magnetic susceptibilities. The approximate conductivity structure of the survey area was derived with a UBC-GIF 3D ZTEM inversion, which models . Forward-model results of these conductivities combined with the elevated values derived from the Mag3D inversion indicate that the conductivities are underestimated with the  assumption. For an environment such as Humble, with deep-seated zones of elevated values, the shallow inverted conductivity structure appears to be reliable, but the deeper structure should be interpreted with caution.

The ZTEM airborne EM system was introduced by Geotech Ltd. into commercial service in 2008. Based on measuring the tipper ratios of AFMAG frequencies, ZTEM provides conductivity information about the earth's near surface that can assist in the exploration for a variety of mineral deposit styles. The results of a surveys over three deposits will be examined; a skarn-style or carbonate replacement deposit in Mexico, a porphyry copper deposit in British Columbia and an epithermal precious-base metals deposit in the Yukon. For each survey, an helicopter time domain EM (HTEM) data set is available to compare with the ZTEM results. Overall, while similarities were noted between the ZTEM and HTEM results, there were also some significant differences that are not totally understood based on available geological information.

The ZTEM airborne EM system was introduced by Geotech Ltd. into commercial service in 2008. Based on measuring the tipper ratios of AFMAG frequencies, ZTEM provides conductivity information about the earth's near surface that can assist in the exploration for a variety of mineral deposit styles. The results of a surveys over three deposits will be examined; a skarn-style or carbonate replacement deposit in Mexico, a porphyry copper deposit in British Columbia and an epithermal precious-base metals deposit in the Yukon. For each study, both 2D and 3D processing has been undertaken and the outcomes evaluated in light of known geology. A simple technique will be discussed which has been developed to assess data sets to establish whether or not 3D processing (which requires more time and hence more costly) is required.

A 2D Occam inversion algorithm for modeling ZTEM data is described that takes into account topography and EM receiver terrain clearance. The responses of hills and depressions are shown for synthetic models. These results show that hills and depressions produce responses that could be confused with the responses of conductors and resistors, respectively. The analysis of ZTEM data acquired at different flight-line directions at the Forrestania test site, WA, shows great consistency and repeatability of the ZTEM data. 2D inversion results indicate that the derivation of pseudo tipper profiles, perpendicular to the survey flight-line direction, from across-line data contain valuable information, especially where the local strike is not perpendicular to the flight-line direction.

The ZTEM airborne EM system was introduced into commercial service by Geotech Ltd. in late 2007. ZTEM is unlike any other commercial EM system in that it relies on the measurement of natural occurring EM fields in the Afmag frequency range of 25-720 Hz. As a result of using natural EM fields that pass through the earth as plane waves, the ZTEM system response shares similarities and important differences to traditional inductive source EM systems such as VTEM or MegaTEM, used extensively by the minerals industry to explore for targets of high conductance. While ZTEM can detect discrete conductors like inductive systems, it also responds to bulk changes in resistivity and conductivity gradients that often characterize geological contacts or structures. One important deposit style that typically lacks a discrete conductivity response are porphyry copper-gold deposits, such as commonly found in many locations around the circum Pacific region. The present study will examine the ZTEM responses for several porphyry deposits in light of other exploration data including drilling, mapped geology and other forms of airborne and ground geophysics

The primary mission of the minerals industry is to profitably identify and develop a steady supply of minerals at reasonable cost to support the economies of the world. All projections on consumption show across-the-board increases, driven largely by the huge demand for metals to feed the growing Chinese economy. For copper alone, forecasts are that more of this metal will have to be produced in the next 25 years than has been used since industrialization began roughly 200 years ago.

Memorials for: George Podolsky 1932–2010 Ed Stewart 1931–2010

A VTEM airborne EM and magnetic survey was completed over an area in southwest Western Australia in 2004. The purpose of the program was to aid in the identification of additional base and precious mineralization as had previously been encountered in follow-up drilling to a Tempest airborne EM and magnetics survey. As part of the assessment of the VTEM results, layered earth inversion and parametric modeling of the EM responses was undertaken. The outcomes of this work showed the two approaches were complimentary and appear to highlight different aspects of the mineralized system.

In the last five years the minerals exploration industry has placed greater emphasis on the mapping of targets of high conductance. In response to this end user requirement, the VTEM system has been modified to allow the recovery of an enhanced conductance measurement. Field results show this new parameter to have improved sensitivity to targets of high conductance, approximately doubling the performance over the previously available results.

The sensitivity of RESOLVE data to highly conductive targets is examined. Modelling results from synthetic data across horizontal plates suggest that RESOLVE data detect highly conductive structures, but don?t resolve their conductances and time-constants if those values exceed 500 S and 8 ms, respectively. A RESOLVE data set flown initially for kimberlite exploration has been re-analysed for nickel sulphides. Since the latter are highly conductive, the standard processing products such as apparent resistivity grids, deemed useful for mapping kimberlites, did not provide the optimum resolution for data analysis. The survey area is characterized by strong magnetic and EM responses due to the presence of banded-iron formations and pyrrhotite-rich massive sulphides. In order to determine the strongest conductors, time-constants and apparent conductivities were derived from the RESOLVE survey data. Due to the strong magnetic response, correction of the RESOLVE data for magnetic permeability resulted in better-resolved time-constants. This was achieved by inverting the data for the conductivity s and magnetic permeability mr of a layered-earth, followed by the forward modelling of the layer conductivities with mr=1. A comparison of independently derived time-constants and apparent conductances suggests that reliable values do not exceed 0.5 ms and 200 S, respectively. Conductors with values above 200 S / 0.5 ms are detectable, but not resolvable.

The Athabasca Basin, Saskatchewan Canada is the world?s premier location of high grade uranium deposits. Most occurrences show a close spatial association with graphite shear-fault zones in the basement rocks (overlain by resistive sandstone) and EM techniques have been used for over 30 years to help map these conductive features. While exploration initially focused on shallower parts of the Basin, current exploration is requiring investigation through thicknesses of sandstone well in excess of 500 m depth. With drilling costs typically now approaching CAN$0.5 million per hole in the deeper parts of the Basin, considerable efforts are being expended to define basement targets with as much spatial resolution as possible. Consequently, most companies are employing some form of ground geophysical surveys to try and sharpen the target focus prior to drilling. We have had the opportunity to compare airborne and ground surveys in a number of locations and have found that there can be considerable disagreement between the conductivity models derived from airborne surveys and those produced from ground EM (active and natural field) or DC resistivity surveys. A number of these examples are presented and discussed so as to better understand what are the likely sources of error and how best to manage the risk of multiple but non-conforming outcomes.

The Geotech Ltd. VTEM heli-time domain EM system entered into commercial service in late 2002 after a development program that started in early 2001. Since then, the system has performed commercial surveys in North and South America, Australia, Africa, Europe and Asia. The VTEM system has been used to explore for a wide variety of deposit styles in a diverse range of settings. The results of four surveys are presented that demonstrate the performance of the VTEM compared with other airborne and ground EM geophysical outcomes.

The Geotech Ltd. VTEM heli-time domain EM system entered into commercial service in late 2002 after a development program that started in early 2001. Since then, the system has performed surveys in Canada, the USA, Brazil and southern Africa for a range of mineral deposit styles in a variety of geological settings. The VTEM system bears similarity to many of the past and currently operating heli-time domain system designs but has focused more than other developments on operational simplicity and flexibility in how the system is configured. A very high dipole (coupled with very low system noise levels) is deemed a major differentiating factor of the system compared with others in current operation. In terms of applications, VTEM is seen as an effective bridge between existing airborne electromagnetics technologies, combining the high spatial resolution associated with helicopter systems but with a depth of investigation similar to what fixed wing time domain systems were traditionally required to achieve.

The airborne electromagnetics (AEM) technique has been used very successfully for over 50 years to locate conductive ore bodies. For most users of the technique, the primary purpose of AEM has been to provide a rapid and inexpensive means to locate targets, whereupon the best features were then followed up with ground geophysical techniques prior to possible drill testing. In this role as a first-pass detection tool, AEM equipment development has focused primarily on spatial resolution, depth of detection and rejection of conductive cover. Traditionally little emphasis was placed upon the need to provide quantitative information about a target?s conductance. This situation has been changing in the last 10 years, driven in large part from concerns expressed from those groups exploring for magmatic nickel deposits. Drawing upon both petrophysical studies and field observations, a case is developing that magmatic nickel deposits have conductances that place them far outside the conductance bandwidth of effectively all AEM systems. This being the case, then the use of AEM as a primary target identification tool for new nickel deposits is a seriously flawed strategy. So as to better understand this issue, AEM data sets acquired over the Voisey?s Bay deposits, Labrador, Canada have been analysed using standard commercial processing techniques such as CDIs and time constant analysis. Additionally, a series of numerical models have been generated to try to better explain the observed field results, as well as simulate the response that other AEM systems would produce over the deposit. While the Voisey?s Bay deposit undoubtedly contains mineralisation of very high conductivity, the processing and modelling shows the AEM technique to be an effective means to locate and discriminate targets of high conductance. Although such conductance discrimination may only be achievable on a relative scale, it is seen as generally adequate for most exploration situations.

Summary The Ring of Fire is an intrusive complexion composed of mafic and ultramafic rocks hosted in the Archean age McFauld’s greenstone belt located in James Bay lowlands of northern Ontario. Due to low topographic relief and an extensive cover of Paleozoic platform carbonate rocks, the area remained largely under explored until kimberlites were found in 1988. This lead to the development of the Victor diamond mine in 2006. Subsequent exploration for kimberlites resulted in the serendipitous discovery of the McFaulds VMS deposits in 2002 (ref Mugall 2010). With this discovery came the recognition that there was a greenstone belt present in the area and it could host economic deposits. A semi-regional Geotem survey was flown in 2003. Exploration in the area was complicated as a number of junior companies had positions in the area and while they would share the costs of expensive surveys, they were competing for what was deemed the best land positions. Ground surveys were conducted based on the Geotem results, with the targeting model being either kimberlite or VMS. This work eventuated in the discovery of a major deposit of chromite and a number of significant nickel sulfide deposits in 2007-2008. During this time, numerous airborne and ground surveys were carried out including a regional Falcon AGG and mag survey in early 2011. While various technical and commercial presentations have been made on the Ring of Fire geophysical work, due to the complicated claim ownership most of these have tended to focus on the results controlled by one group. This review is intended to look at the overall area which hosts at three significant deposit styles; VMS, magmatic nickel and chromite.

Airborne electromagnetics (AEM) surveying has been by far the most successful mining geophysical technique ever developed. Since the start of commercial use in early 1951, AEM can be credited with directly aiding in the discovery of more than 80 deposits, with a gross value of more than $100 billion.

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.

Synthetic ZTEM responses computed with 2D and 3D algorithms are compared. Excellent agreement is observed between 2D and 3D responses for structures with long strike lengths. Using the 2D inversion algorithm on synthetic 3D responses indicates artifacts being introduced when limited strike length is present: the conductivity of structures such as resistive hills and conductive structures is underestimated.

Summary The Falcon airborne gravity gradiometer system was flown over two areas in the Athabasca Basin, Saskatchewan to aid in the discovery of unconformity style uranium deposits. The Falcon outcomes along with other geoscience data, including airborne EM and magnetic data were evaluated. The assessment of these data suggests that the Falcon survey is showing responses believed to be derived from both the overlying Athabasca sandstone and likely, the Archean basement both which could be related to oreforming mineral processes

A transient audio magnetotelluric (TAMT) survey was carried out in the Athabasca Basin of northern Saskatchewan as part of a uranium exploration program at Pasfield Lake. Inspection of the TAMT data revealed a complex structural setting near the line extremities with a large conductive uplifted block in central portions of the lines. Two-dimensional (2D) inversion was helpful but failed to fit the data satisfactorily in the structurally more complex areas. The results of 2.5D inversion (three-dimensional inversion on a single 2D profile) will be presented and compared to the original 2D inversions and known drilling. The 2.5D inversions show a much better fit to the measured data, returning models in good agreement with drill core defined lithology. Additionally, three-dimensional (3D) inversion of airborne total field magnetic and airborne gravity gradient data was performed and clearly corroborates the presence of an uplifted basement under Pasfield Lake. It would seem that the most likely source mechanisms to explain the Pasfield geophysical data and drill core to date are an explosive event in the form of a meteorite impact or a volcanic intrusive.

The Athabasca Basin area of northern Saskatchewan provides approximately 20% of the world’s uranium production; the majority of this coming from what is termed unconformity style deposits located at the base of the overlying Athabasca sandstone and underlying Archean rocks. Triex Minerals Corporation (now Canterra Minerals) has been conducting exploration in the Pasfield Lake area in the eastern part of the Athabasca Basin since 2005. In the course of this program, a series of aerial and ground geophysical surveys as well as core drilling has been undertaken. This work has to date failed to have revealed the presence of any significant unconformity style uranium mineralization but has shown that there is a major uplift of the basement in Pasfield Lake. The potential field results show concentrically zoned features within the lake and based on the geophysical character of other known astroblemes, there appears to be a reasonable case that Pasfield Lake is located over top of a meteor impact site.

As part of SEG’s 1998 Annual Meeting in New Orleans, the Mining and Geothermal Subcommittee organized a workshop on aviation safety titled “Getting the Best Data and Getting Back Alive.” This workshop was intended to provide a forum in which airborne geophysical survey contractors, their clients, and other interested parties would learn more about a subject that has become considerably more important in the last five years.