the borden gold deposit, northern ontario: contributions ... · the borden gold deposit, northern...

AEGC 2019: From Data to Discovery – Perth, Australia 1

The Borden Gold Deposit, northern Ontario: Contributions of VTEM helicopter time-domain EM and magnetics leading to discovery Marta Orta Jean M. Legault* Karl Kwan Consultant Geotech Ltd Geotech Ltd Vaughan, CANADA Aurora, CANADA Aurora, CANADA [email protected] [email protected] [email protected] Sergio Espinosa Goldcorp Inc. Vancouver, CANADA [email protected]

INTRODUCTION

The Borden Gold deposit, located approximately 10 km east of Chapleau, Ontario (Figure 1), was discovered in 2010 by Probe Mines Limited (Murawhi, 2011). Two VTEMTM (Versatile-time domain electromagnetic; Witherly et al., 2004) helicopter-borne and magnetic surveys were carried out by Geotech Ltd. over the property, in 2010 (Kaminski et al., 2010) and again in 2011 (Fiset et al., 2011). Guided by the results of the first VTEM survey, and the presence of surface gold outcropping coinciding with weak conductors, the discovery was made in the 1st drilling program in summer 2010. Borden Gold deposit contains an indicated resources are 1.6 M oz of gold (9.3 Mt @ 5.3 g/t Au) and remains open over 3.7 km of strike length. Our study focuses on the VTEM and aeromagnetic survey results that contributed to the discovery, along with additional analyses that provide additional information on the geophysics of the Borden Gold deposit. Geology and Mineralization The Borden Gold deposit occurs within the Superior province of northern Ontario. It lies in the southernmost limits of the Kapuskasing Structural Zone (KSZ), at the intersection with the Wawa and the Abitibi sub-provinces (Figure 1, top). The Borden Lake Belt is an east-west trending, supracrustal assemblage and can be traced continuously for 35 km (Figure 1, bottom). High regional metamorphic grade makes this

deposit unique in terms of geologic setting when compared to other Archean-aged mesothermal lode gold occurrences. However, the deposit is paragenetically similar to other structurally controlled deposits of the same type, exhibiting both a high-grade core and a disseminated lower grade alteration halo associated with a ductile shear zone (Dzick, 2014; Murahwi et al., 2012; Murahwi, 2011).

Figure 1. Top: Regional geology and bottom: Local geology of the Borden Gold project (after Dzick, 2014). Gold mineralization occurs as a broad zone with quartz and white mica, biotite and garnet with disseminated and fracture-controlled sulphides (pyrite and pyrrhotite), within a volcano-metasedimentary package of variable composition.

SUMMARY The Borden Gold deposit (9.3 Mt @ 5.3 g/t Au) is a structurally controlled Archean mesothermal lode gold deposit within the Kapuskasing Structural Zone, in northern Ontario. Helicopter VTEM and magnetic surveys were flown over the Borden Gold deposit, helping to design the first drilling program, leading to the discovery in 2010. Initial survey results defined the extension of the near surface low-grade-zone (LGZ). Advanced processing and modelling of VTEM data also appears to map the deeper high-grade-zone (HGZ). Results of subsequent 1D imaging and 2.5D inversions, and airborne inductive induced polarization (AIP) analysis, are also presented. Key words: Archean, lode gold, electromagnetics, modelling, AIP.

HTEM and magnetics of the Borden Gold deposit Orta, Legault, Kwan and Espinosa


Mineralization consists of low to moderate grade gold concentrations, with a higher-grade core that improves toward the southeast. Following the discovery of Borden Gold deposit, two more drilling programs, ending in July 2011 and March 2012, defined the Low-Grade-Zone (LGZ) deposit the north-western portion of Borden Gold deposit (Murawhi, 2012). The results of the 4th drilling program that concluded in November 2012 were significant, resulting in the discovery of the deeper high-grade-zone (HGZ) toward the southeast, and changing the scope and interpreted model of the deposit (Dzick, 2014). In 2014, the Borden Gold deposit contained an indicated resource of 1.6 Moz of gold (9.3 Mt @ 5.3 g/t Au; Dzyck, 2014). Since being acquired from Probe Mines Ltd. by Goldcorp Inc. in March 2015, the Borden Gold mine has been further developed, with commercial production expected to start in 2019 (www.goldcorp.com).

SURVEY RESULTS HTEM and Magnetic Surveys Two helicopter-borne VTEM and combined magnetic surveys, flown in NS orientation (with perpendicular EW tie-lines), were carried out by Geotech Ltd. over the Borden Lake property. Both surveys employed a VTEM Plus system with receiver and transmitter coils in concentric-coplanar and Z-direction oriented configuration. The receiver system also included a coincident-coaxial X-direction coil to measure the in-line dB/dt and calculate B-field responses (Kaminski, et al., 2010; Fiset, et al., 2011). Figure 2 shows the flight path of two surveys over the bedrock geology (OGS, 2011). The first survey flown in 2010 (blue flight path) employed a single magnetic sensor and was flown with 100 m NS line spacings with 1 km EW tie-lines. The area was extended during the second survey in 2011 (black), that was also flown at 100 m line-spacings but offset 50 m eastward with the original in order to provide improved coverage of the Borden Gold area at 50 metres line-spacing. The second VTEM survey employed a horizontal magnetic gradiometer system, with the purpose of 1) identifying small and shallow magnetic anomalies located in between lines that might not have associated conductive response, and 2) increasing the structural knowledge of the area. Also shown in Figure 2 is the layout of the mineralized zones at Borden Gold area (LGZ in green, HGZ in red), and the most relevant drill holes of the 2010-2014 programs (blue dots). The VTEM and magnetic surveys identified several conductors of variable geometry cross-cutting magnetic lineaments trending in varied directions. A number of anomalies, including a 700 m long conductor, are located just south of the gold mineralization identified in outcrop. Figure 3 shows the image of Time-constant (Tau calculated from dBz/dt data) and contours of the calculated vertical magnetic gradient (CVG 1st order) with the purpose of verifying magnetic association. The response observed toward the west half of the deposit (location of the LGZ), exhibits time-constant ranging between 0.2-2.5 milliseconds, indicating that conductors are induced by low conductive targets. The contours of the calculated vertical magnetic gradient confirm the complex structural character of the mineralized zone; structurally controlled by a ductile shear zone.

VTEM results helped define the northwest and southeast original extension of the Borden Lake deposit over a strike length exceeding 2 km (D. Palmer, pers. comm., 2015).

Figure 2. Flight path of the two VTEM and Magnetic surveys (2010 in blue, 2011 in black) over the bedrock geology (OGS, 2011), highlighting the mineralized zone at Borden Lake property (LGZ in green, HGZ in red), with relevant DHs (blue dots).

Figure 3: Image of time-constant (Tau dBz/dt) and CVG contours, highlighting the mineralized zone at Borden Lake property (LGZ in green, HGZ in red), with relevant DHs (blue dots). During the survey of 2010, two targets were subject of Maxwell (EMIT Technology, Midland, WA) EM plate modelling (Kaminski et al., 2010), located south of where the first gold occurrences were found. These targets represent sub-vertical conductors (double-peak anomalies). Maxwell results of L1321 are presented in Figure 4 (left), interpreted to have dip orientation toward the northeast and conductance of 223 and 100 Siemens respectively (Figure 4, Figure 4 top-right). The results are attributed to the presence of disseminated sulphides containing minor (2%) pyrite and pyrrhotite. The drillhole (DH) plan of the first drilling program of summer 2010 was designed according to the gold outcrop occurrences coinciding with weak airborne conductors, following Maxwell results drill planer (Figure 4, bottom right). Some DHs were planned starting at the same location on surface with varied dip orientation.



Resistivity-depth imaging (RDI) of the VTEM results is generated using the transformation scheme by Meju (1998). A voxel is then produced from the georeferenced RDI database, mapping both the near-surface LGZ and also the deeper high-grade-zone (HGZ), as shown in Figure 5 (bottom). The RDI section of L21630 (Figure 5, top) shows a low resistivity < 440 ohm.m below 150 m depth, with stronger response at depth below 300 m. In the RDI voxel (bottom), toward the east part of the mineralized zone, an area of low-resistivity < 70 ohm.m is observed near-surface that could be attributed to high concentration of clays.

Figure 4. Maxwell results of modelling two plates on L1321; left) profiles of X and Z components, CVG and plates layout; top-right) parameters of the modelled plates; bottom right) Tau image with CVG contours of the mineralized zone, showing position of drill holes of the summer 2010 program. For the purpose of this study, additional Maxwell modelling was performed over two lines located more to the east of the outcrop occurrences. Location of these lines is shown in Figure 6 (top) over the image of Tau and CVG contours.

Figure 5. RDI results showing both the LGZ and the HGZ; top) RDI of VTEM line L21630 intersecting the deeper HGZ; bottom) RDI voxel of the mineralized zone. Model M3 over L21565 is compared with the results of DH BL10-10 which represents the first discovery. As seen in Figure 6 (bottom left), the response is observed from near-surface and extending at depth. The discovery at DH BL10-010 contains a 41m intercept averaging 3.3 g/t gold within a broader lower-grade mineralized zone grading 1.1 g/t gold along 182 m (19 m – 201m), confirming VTEM Maxwell results.

Figure 6. Maxwell results over known mineralization; top) Tau image and CVG contours with flight path of two lines; bottom left) Model M3 on line L21565 over the LGZ; bottom right) Model M4 on line L21630 over the HGZ. The conductance of 2 Siemens observed in the M3 plate, represents well the content of sulphide mineralization of the LGZ, compared with 30 Siemens resulting in the M4 plate, attributed to its proximity to higher content of mineralization in the HGZ. Model M4 over L21630 represents a deeper conductor. Depth to the top of the body estimated by the Maxwell is of 219 m (Figure 6, bottom right). An Airborne Inductive Induced Polarization (AIIP) mapping tool, based on Airbeo (CSIRO/AMIRA; Raiche, 1998)), has been implemented for the in-loop VTEM system (Kwan et al., 2015). Negative transients observed in the mid-late times, while positive in early-time data are attributed to AIIP effects. Extracting the chargeability from VTEM data using the Cole-Cole formulation (Kratzer and Macnae, 2012) reveals new and potentially valuable information about the sub-surface geology. The method was applied over the mineralized zone using off-time dBz/dt data from channels 0.096 to 7.036 milliseconds. AIIP apparent chargeability and resistivity are recovered, highlighting two low-resistivity and chargeable zones at both



ends, attributed to clay deposition in the bottom of the lakes (Figure 7, left). The response observed in the apparent resistivity image (top left) associated to weak conductors drilled during the program of summer 2010, indicate less chargeable material (bottom left). The more prominent results observed toward the east are interpreted as a response from near-surface clay concentration, spatially consistent with deeper disseminated sulphides (pyrite and pyrrhotite).

Figure 7. Results of AIIP analysis; top-left) recovered apparent resistivity; bottom left) recovered apparent chargeability; right) Cole-Cole parameters derived from forward modelling of a VTEM decay with c=0.7; observed VTEM EM decay (black), Cole-Cole modelled VTEM IP decay (red) and inductive VTEM EM decay (m=0, green). In the Cole-Cole formulation, the frequency factor (c) defines the size distribution of the polarizable elements. For this dataset, a frequency factor c=0.7 was chosen (Figure 7, right), which is more likely related to the membrane-polarization type, such as clays. But it does not preclude the presence finely disseminated sulphides that also exhibit similar Cole-Cole parameters.

CONCLUSIONS The Borden Gold deposit (9.3 Mt @ 5.3 g/t Au) is a 3.7 km long x 120 m wide x up to 650 m depth, structurally controlled, Archean mesothermal lode gold deposit within the Kapuskasing Structural Zone (KSZ). Prior to drilling, a helicopter VTEM-Magnetic survey identified a number of conductors, interesting structures and magnetic anomalies across the property in 2010. A second VTEM survey was flown in 2011 to provide infill follow-up. Initially targeting a surface gold occurrence coincident with a weak VTEM conductor and magnetic lineament, the first phase of drilling led to the Borden discovery, due to the presence of minor (2%) pyrite and pyrrhotite in the gold horizon. VTEM-Magnetic survey results helped guide drilling and extend the northwest and southeast extension of the Borden Lake deposit along a 2.2 km strike length, consisting of a low-grade-zone of mineralization known as the LGZ. Additional processing of VTEM data appear to image both the near-surface LGZ and also the deeper high-grade-zone (HGZ), extending to 500 m+ depths. Extracting the chargeability from VTEM data using the Cole-Cole formulation has added additional information about the sub-surface geology.

These results showcase the importance of advanced interpretation applied to VTEM data for improved success in exploration for orogenic type disseminated and structural-controlled py-po sulphide-hosted gold deposits.

ACKNOWLEDGEMENTS The authors are thankful to Goldcorp Inc. and Geotech Ltd. for allowing presenting this case study. We are also grateful to David Palmer, of Probe Metals Inc., for his contributions to this study.

REFERENCES Dzick, W., 2014, Mineral Resources Estimate Update, Borden Gold Project: NI 43-101 report for Probe Mines Ltd. by Snowden Mining Industry Consultants, project No. V1393, 179 p. Fiset, N., Kumar, H., Prikhodko, A., 2011, Report on a helicopter-borne VTEM Plus and Horizontal Magnetic Gradiometer geophysical survey, Borden Lake project & White Owl project, Chapleau, Ontario: Internal report, Geotech Ltd., 84 pp Kaminski, V., Legault, J., Smith, G., Venter, N., 2010, Report on a helicopter-borne VTEM Plus and aeromagnetic geophysical survey, Borden Lake block, Chapleau, Ontario: Internal document, Geotech Ltd., 46 pp Kratzer, T., Macnae, J., 2012, Induced polarization in airborne EM: Geophysics 77, E317–E327. Kwan, K., Prikhodko, A., Legault, J.M., Plastow, G., Xie, J., Fisk, K., 2015, Airborne Inductive Induced Polarization Chargeability Mapping of VTEM Data: ASEG-PESA 24th International Geophysical Conference and Exhibition, Extended Abstracts, 4 pp Meju, M.A., 1998, A simple method of transient electromagnetic data analysis: Geophysics, 63, 405-410. Murahwi, C, 2011, Technical report on the initial mineral resources estimate for the Borden Lake Gold Deposit, northern Ontario, Canada: NI 43-101 report for Probe Mines Limited by. Micon International Limited, 142 pp. Murahwi, C, Gowans, R., and San Martin, A.J., 2012, Technical report on the updated mineral resources estimate for the Borden Lake Gold Deposit, Borden Lake property, northern Ontario, Canada: NI 43-101 report for Probe Mines Limited by Micon International Limited, 188 pp. Ontario Geological Survey 2011. 1:250 000 scale Bedrock Geology of Ontario; OGS, Miscellaneous Release-Data 126 - Revision 1. Raiche, A., 1998, Modelling the time-domain response of AEM systems: Exploration Geophysics, 29, 103–106, Witherly, K., R. Irvine, and E. B. Morrison, 2004, The Geotech VTEM time domain electromagnetic system: 74th Annual International Meeting, SEG, Expanded Abstracts, 1217–1221.

the borden gold deposit, northern ontario: contributions ... · the borden gold deposit, northern...

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