2021 virtual symposium program


Session 4: Case Histories

March 25, 2021, 11:00 AM 12:30 PM (EDT)

Session Chair: Kanita Khaled

Case Studies using Various IP methods

Speaker: Pam Coles, Abitibi Geophysics

SummaryThe Induced Polarization / Resistivity technique is effective in exploration for disseminated and massive polarizable mineral occurrences such as base metal sulphides, barren sulphides associated with gold, porphyry copper systems, graphite, alteration zones and structural features controlling mineral deposition, oxides, and some clay minerals.

For several years Abitibi Geophysics has carefully looked at how to increase the depth of investigation and resolution of induced polarization (IP). The OreVision method has been proven successful on many projects, especially on the Frotet project by Kenorland Minerals Ltd., where the results pointed them towards high grade mineralisation. Researching the sensitivity response of various electrode configurations spawned the design of a 3D configuration which has demonstrated depth, stable inversion characteristics, and is cost-efficient.

Several 3D IP systems are available on the market, and Abitibi Geophysics has a variety of 3D IP options. OreVision 3D incorporates the highest sensitivity electrode arrays to increase exploration effectiveness through conductive overburden with a depth sounding array to help resolve the overburden layer. The quick acquisition allows the client to receive pseudosections as the survey is progressing. Abitibi completed several projects for Probe Mining in Quebec, using the OreVision 3D survey which has allowed them to increase the resolution, the depth of investigation, and better define the anomalies due to more robust inversions.

The second approach we will discuss is the Distributed Array survey using the IRIS full waver system. This method allows us to increase the data density and coverage by using the full waver receivers. It is ideal for large systems such as porphyry or skarn hosted deposits and could also be used as a hybrid solution of ground and borehole IP. Full Waver receivers are installed on the surface, the current is injected at the surface, and the dataset could be complemented with current injection in the available boreholes. It maximizes the depth of investigation by using boreholes, while acquiring a large, systematic coverage of readings in a regular mesh on the surface, resulting in a more reliable 3D inversion.

We will present the results of a hybrid Distributed Array survey completed in Quebec using surface and borehole data to increase the depth of investigation. We will also present another international survey completed for a porphyry system showing a investigation depth of 1000 m.

Major and junior mining companies will always have the challenge to explore deeper, penetrate thick conductive overburden and extract more precise 3D structural information about the mineralization and host geology. In response, Abitibi Geophysics has introduced innovative IP configurations to enhance exploration effectiveness.

Providing insight using multi-geophysics approach at the Filo del Sol high-sulphidation epithermal copper-gold-silver deposit

Speaker: Sarah Devriese, Condor North Consulting ULC

Summary: The Filo del Sol copper-gold-silver deposit straddles the Chilean-Argentinean border along a north-south trend of epithermal and porphyry deposits. Our work focusses on the processing and analysis of 2D and 3D DC resistivity (DC) and induced polarization (IP) data plus ground and drone magnetic data. The property has extensive drilling, from which a 3D geologic model was built. The aim of the geophysical interpretations, along with new drilling in the new year, is to improve the geologic understanding of the deposit.

Filo del Sol is a high-sulphidation epithermal copper-gold-silver deposit associated with a large porphyry copper-gold system. Overlapping mineralizing events, combined with weathering effects and supergene enrichment, have created several different styles of mineralization, including structurally-controlled gold, stratiform high-grade silver (+/- copper), and high-grade supergene-enriched copper within a broader envelope of disseminated and veinlet-hosted sulphide copper and gold mineralization. The mineralized system is associated with Middle Miocene porphyry intrusions and overprinting, telescoped high-sulphidation epithermal alteration within Permo-Triassic rhyolite basement rocks and overlying conglomeratic sediments thought to be either Eocene or Late Oligocene in age. It is located in the Andean Frontal Mountain Range, between the Maricunga gold porphyry trend to the north and the El Indio high-sulphidation epithermal trend to the south.

Standard 2D DCIP surveys were collected in 2006, 2012, and 2016 while a 3D DCIP survey was collected in early 2020. The 2D and 3D DCIP surveys were separately inverted in 3D. The ground magnetic surveys were collected between 2000 and 2008, with a new drone magnetic survey flown winter 2019-2020. The magnetic surveys were inverted in 3D.

The geophysical methods appear to distinguish between the different mapped rock types and alteration in the vicinity of the Cerro Vicuña intrusive body in the Tamberias region. The results indicate that the Filo del Sol silver zone has a distinctive geophysical signature and may extend both to the north and south compared to the current 3D geologic model. The chargeability models suggest that the mid-Miocene intrusive is much more extensive than defined by the current mapping. Our presentation will focus on the geophysical processing and inversions, and the combined geologic and geophysical interpretation.

Case study from a Western Australia Graphite Exploration Survey with 12.5 Hz AEM Data

Speaker: Adam Smiarowski, CGG

Summary: Exploration for targets at depth or targets obscured by conductive overburden have historically been a challenge with airborne EM methods. Although modern systems have been improved with greater primary transmitter moments, noise levels from receiver coil motion in the Earth’s ambient field has limited the detection of secondary target signals, especially at late times, and has limited the use of lower base frequencies. The new Helitem² system uses a patented low-noise receiver, a 50% duty cycle square pulse transmitter waveform, and low Tx base frequency, to achieve increased signal detectability for deep and covered targets.

Modeling using a thin-plate showed that a low base frequency square pulse will have a significantly larger response than a significantly higher moment (>3x) half sine pulse at standard 30 Hz base frequency for a wide range of target conductances. At early times, the sharper (quicker) turn off of the square wave results in much more high-frequency energy, and therefore better signal, for weakly conductive targets, and better near-surface resolution. At the other extreme, the response from very conductive targets is determined by the area under the transmitter curve, so the low frequency square waves with 16 and 33 ms widths produces more than twice the signal as the half sine. A demonstration survey line comparing narrow and wide pulses over a known 400 m deep target confirmed the expectations from modelling. The combination of pulse width, power, and low noise enabled the Helitem2 system to be effective at low base frequencies, where very late time data is beneficial for detecting strong and deep targets.

We show an example from the very conductive Forrestania deposit in Western Australian, illustrating data improvements of Helitem2 operating at 12.5Hz, over a previous survey at 25Hz. We then show a case study from a graphite project. Here, a ground slingram survey was unable to map conductive targets. Subsequently, a small ground TDEM survey was conducted to show feasibility of the method. The ground survey showed a clear anomaly over a known target. We show 12.5 Hz Helitem2 data, comparing airborne and ground survey data with Maxwell models and drill results.

Case History of Epithermal Gold Exploration using the Wireless Alpha IP System, Jersey Valley, Nevada

Speakers: Greg Hollyer, Simcoe Geoscience

Summary: Epithermal Gold deposits (including other minerals, such as Ag, Pb, Zn and Tl variably) are an important class of Gold exploration target globally. The model for epithermal deposits is a multi-stage process of mixed magmatic and meteoric fluids and boiling to initiate mineral precipitation within host rocks and along structural access points. In this paper, we review three Epithermal gold deposits as case histories with examples of high-resolution, wireless Alpha IP system results to depth. Examples are taken from the Dunwell property in the Golden Triangle, BC; an epithermal gold project in Nevada; and an existing mine in the African area.

Case studies are shown from the high resolution Alpha IP system that was initially conceptualized and is also now operated globally by Simcoe Geoscience. The Alpha system is a wireless-receiver based IP system with highly variable capability in its ability to prospect for numerous deposit types, ranging from Epithermal Gold to Carlin Gold to Bushveld Intrusive PGEs and more. Experience has shown that Epithermal Gold deposits comprising vein and breccia ores are excellent targets for deep prospection with Alpha IP.

Alpha IP represents a modern IP and Resistivity surface prospecting system that operates all configurations from Dipole-Dipole to Pole-Dipole and other proprietary set ups. The system uses advanced wireless communications to record receiver data, combined with an active current electrode moving along the survey lines. All receivers are positioned by GPS for elimination of errors in modeling, inversions and final targeting plots.

In the case histories that are presented; several are grassroots exploration examples, and the third example is a full mine survey with results presented in 3D. Overall, it is evident that results from all surveys produced high-value drill targets with subsequent Au and Ag intersections (with other minerals in each case history). The mine site survey revealed hidden depths of mineralization to 700+ m which were drilled successfully. This latter depth figure represents a mid-range operating depth for the Alpha IP system with deeper exploration to 1500 m practicable.

In summary, the case histories show full resolution of ore bodies and their characteristics, including structural controls, host rock environments and mineralized Gold targets. As well, the paper illustrates selected details of the innovative Alpha IP system – including novel features, such as the elimination of the “n” electrode barrier; high volume sampling for increased resolution to depth; and full resolution of Gold-bearing Epithermal targets to depth.

Putting AIP into perspective

Speakers: Andrea Viezzoli, Aarhus Geo

Summary: The exploration industry recently stepped up its interest in Induced Polarization effects in AEM data (AIP). I present a series of case studies revolving around AIP from different continents, for different applications, from different helicopter TDEM systems. The results are compared against geological information and ground IP data. In all cases AIP modelling (layered earth) provide a significant improvement with respect to conductivity depth images and previous modelling without AIP. Laterally continuous conductors are better resolved, as well as the sharp resistivity contrast associated with mapped faults. The depth of investigation in the AIP modelled areas increases markedly. IP-stripped voltages are recreated, providing a useful complementary dataset to be used with alternative type of modelling/interpretation that rely on voltages unaffected by IP.

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