Regolith mapping in hypersaline environments: a comparison of SAM with helicopter TEM Edward M.G. Stolz 1

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1 Exploration Geophysics (2005) 36, : a comparison of SAM with helicopter TEM Edward M.G. Stolz 1 Key Words: SAM, St Ives, regolith, gold, hypersaline, HoistEM ABSTRACT Direct detection of bedrock structures using conventional electromagnetic and electrical surveys is very difficult in terrains covered by regolith saturated with hypersaline groundwater. The resistivity contrast between the brine-saturated regolith and the fresh, crystalline bedrock is usually much greater than any resistivity contrast within the bedrock itself. Mapping bedrock faults and shears by detecting their expression as changes in depth of weathering at the base of the regolith is considered to be the most effective application of electromagnetic and electrical methods to gold exploration in hypersaline environments. SAM is more effective than airborne electromagnetics (HoistEM) for mapping the regolith expression of structures, because the SAM transmitter dipole enhances current channelling into linear features, and SAM collects much higher resolution data. The two-dimensional current flow from the SAM dipole transmitter produces clearer images of linear, low-resistivity features than does the three-dimensional smoke ring current flow induced by a time-domain electromagnetic loop transmitter. SAM surveys have imaged the base of regolith beneath 100 m of brine-saturated lake sediments, whereas airborne TEM transmitters have not resolved structure deeper than 50 m over the same ground. INTRODUCTION dolerites, and numerous felsic porphyry stocks and dykes. The greenstone sequence is overlain by the Black Flag Group and the late-basin Merougil beds comprised of epiclastic and volcaniclastic sediments. The Archaean rocks are mostly covered by a thick and variable regolith layer that includes palaeochannels and lake sediments under and adjacent to the Lake Lefroy salt lake. Regolith thickness varies from 10 m to 100 m, and comprises in situ weathered material and transported sediments. The regolith has much higher porosity than the underlying fresh rock, and is saturated with highly saline groundwater. Conductivities as high as 20 S/m have been measured from Kambalda mine waters, inferring a salinity of mg/l NaCl, compared with seawater salinity of mg/l (Emerson, 1997). The brine-saturated regolith forms a highly conductive blanket above the generally resistive, crystalline Archaean rocks. Lake sediment clays have measured resistivities as low as 0.1 Ω.m, compared with 14 Ω.m for bedrock sulphidic sediment, and 1000 Ω.m to Ω.m for basalts, dolerites, ultramafic rocks, and porphyries (Emerson, 1997). The highly conductive regolith is a formidable barrier to surface electromagnetic (EM) and induced polarization (IP) methods in the search for bedrock conductors such as nickel sulphides or sulphidic shear zones. Surface and airborne EM and electrical The St Ives gold field is located near Kambalda, 60 km south of Kalgoorlie in the Eastern Goldfields Province of Western Australia (Figure 1). Gold was first mined from the Kambalda and Victory areas at the turn of the 19 th century (Watchorn, 1998). Gold mining ceased during the 1930s, but the area was reinvigorated with the discovery of nickel by Western Mining Corporation (WMC) in the 1960s. Following a rise in the price of gold, WMC recommenced gold exploration in the early 1980s and found substantial new reserves at Victory. More deposits were located and mined over the next two decades, including the Revenge, Junction, and Argo gold mines (Figure 1). St Ives was purchased by Gold Fields Limited in December 2001, and currently produces oz of gold per year from open pit and underground mines. A large exploration effort is ongoing to define additional reserves and ensure the continuing prosperity of the field. The geology of the St Ives gold field comprises a thick Archaean greenstone sequence of basalts and ultramafic rocks, corresponding with the Kalgoorlie Group, folded in a southerly plunging anticline. The sequence is intruded by differentiated 1 Gold Fields St Ives Gold Mining Company PO Box 359 Kambalda WA 6444 Phone: Facsimile: edward.stolz@goldfields.com.au Manuscript received 31 January, Revised manuscript received 13 April, Fig. 1. Location plan showing the major mining areas in the St Ives gold field. 157

2 Stolz surveys at St Ives have been unsuccessful at detecting bedrock anomalies because of the conductive overburden. A low-resistivity auriferous shear zone was detected and resolved with a downhole electromagnetic survey (DHEM) at Junction Gold Mine (Stolz, 2003). DHEM places the receiver coil in the drillhole, and measures the secondary transient EM fields beneath the conductive regolith, closer to the target than a surface receiver coil. Surface Sirotem data were collected over Junction in the 1980s using 200 m and 100 m moving loops, but did not detect any anomalies attributed to the shear zone. This paper presents the results from Sub-Audio Magnetic (SAM) surveys at St Ives, and investigates the phenomenon of SAM mapping structure beneath regolith by detecting the preferentially deeper weathering into bedrock shattered by faults and shear zones. A comparison of SAM with helicopter EM (HoistEM) surveys is also included to demonstrate that SAM has advantages over airborne EM methods for defining linear structures beneath the regolith. general rule, linear features oriented within 45 of the transmitter electrode dipole bearing are capable of channelling current and yielding a SAM response. Total-field MMR (TFMMR) and total magnetic intensity (TMI) data are collected along east-west lines spaced 50 m apart, and with a reading interval of 2 m or 5 m, depending upon the transmitter frequency used, allowing excellent spatial resolution of anomalies. The equivalent MMR (EQMMR) data are calculated from the TFMMR data. An image of the EQMMR immediately north of the Delta open cut mining pit is displayed over an aerial photograph of the area in Figure 3. The white zone on the north wall of the pit denotes clay development in the north-west trending Delta Shear, and corresponds with the extrapolation of the strong, linear EQMMR anomaly in the image. The deeper weathering formed along the north-west trending shear is interpreted to have channelled the transmitter current and produced a linear EQMMR anomaly along the base-of-regolith expression of the shear. The majority of SAM anomalies detected at St Ives are correlated with REGOLITH MAPPING Given the failure of high-power, low-resolution EM and electrical methods to map structures beneath the regolith at St Ives, it was proposed that high-resolution electrical methods be used to image the regolith itself. The regolith-fresh rock boundary is the largest resistivity contrast in the ground, and should be readily mappable by EM or electrical methods. Local variations in the regolith thickness are often due to faults and shear zones in the underlying bedrock. For example, in the north wall of the Delta open cut mining pit, the deeper weathering of the shale within the Delta Shear Zone compared with the basalt on either side of the shear is clearly visible as the white clay development and maroon oxidation zones (Figure 2). Preferential weathering of shears and locally deeper regolith over structures has been noted in mines and drillholes throughout the St Ives gold field. Accordingly, mapping structures through their expression in the regolithfresh rock contact is a powerful exploration tool at St Ives. The SAM method was first applied at St Ives in 2000, and uses a grounded electrode dipole source which is excellent for energising the regolith layer with current channelling to produce magnetometric resistivity (MMR) anomalies (Boggs et al., 1998). SAM electrode dipoles at St Ives are orientated either north-south or NNW-SSE, so that transmitter current is flowing along the most important structural trends. As a 158 Fig. 2. Photograph showing the north wall of Delta open cut mining pit. The white and maroon clays delineate the preferential weathering of the Delta Shear Zone, compared with the less weathered brown-green basalt on either side of the shear zone. Fig. 3. SAM EQMMR image overlaid on an aerial photograph of the Delta open cut mining pit. The positive north-west trending SAM anomaly immediately to the north of the pit correlates well with the Delta Shear Zone ( A ) delineated by the white and maroon clay zones on the pit wall.

3 Stolz local thickness variations in the regolith (see also Whitford, 2004). Sulphide-bearing interflow sedimentary units, such as the Kapai Slate, also cause strong EQMMR anomalies where they subcrop beneath the regolith. These sedimentary units are less than 20 m thick, do not occur often within the St Ives stratigraphy, and hence, are not a common source of SAM anomalies. Fig. 4. South Victory SAM EQMMR image overlain by contours of the depth of base of regolith over fresh bedrock derived from logging of about 500 drillholes. SOUTH VICTORY CASE STUDY MAPPING OF STRUCTURES SAM Data The South Victory SAM survey targeted structures trending south-east from the Victory mining area which lies immediately north-west of the SAM grid (Figure 1). The survey was completed in May 2002 using three 3 km long north-south dipole arrays and 50 m spaced east-west survey lines. The three survey grids were merged, and an image of the EQMMR data is presented in Figure 4. The image is overlain by contours of the depth of regolith over fresh bedrock derived from logging of about five hundred drillholes in the survey area. The SAM image maps a pattern of well-defined linear north- and NNW-trending anomalies. The positive correlation between the SAM EQMMR response and regolith thickness is evident throughout the survey area. SAM provides a more detailed regolith weathering map than the drillholes, which have north-south line spacing of 100 m to 600 m. SAM anomalies do not correlate well with the east-west variations in logged regolith because of the directional bias of current between the transmitter electrodes (Meyers et al., 2004). The high spatial resolution of SAM provides detailed images of the EQMMR response that can be readily filtered and enhanced. The EQMMR data presented in Figure 4 was enhanced by taking a first vertical derivative and applying north-east sun-angle shading (Figure 5). This filter enhances the NNW trending linear anomalies, in particular the subtle features in the western half of the survey area. Two of these features correlate with the position of the Britannia and Sirius auriferous structures, as defined in an open pit mine to the north-west of the survey area. The resolution of SAM surveying allows these structures to be precisely traced from the pit wall across the survey area, and provides quality exploration targets. Comparison with Magnetic and Gravity Data The utility of SAM for regolith and structure mapping at St Ives is demonstrated by Figures 6 and 7, which show SAM Fig. 5. First vertical derivative of the South Victory SAM EQMMR data shown in Figure 4. The image is illuminated by a north-east sunangle. White lines denote the axis of strong linear anomalies, including features correlating with the Sirius ( A ) and Britannia ( B ) gold bearing shears. The position of the Sirius and Britannia open cut mining pits (blue) and mullock dumps (brown) are also shown in the top left of the figure. Fig. 6. Aeromagnetic TMI image of the South Victory area. SAM EQMMR lineaments are shown in white. Two bullseye magnetic anomalies ( C ) are noted adjacent to the Britannia Shear ( B ), and are interpreted as Defiance Dolerite. A good host lithology truncated by an auriferous shear zone constitutes an excellent gold exploration target. 159

4 Stolz lineaments interpreted from Figure 5 overlain on an airborne total magnetic intensity (TMI) image, and gravity image respectively. The aeromagnetic data were collected along 40 m spaced northeast flight lines at an elevation of 50 m, while gravity data were collected on a 100 m by 100 m grid. The majority of the SAM lineaments have no expression in the magnetic image, because the bedrock is non-magnetic basalt, and there are no variations in magnetic susceptibility along the structures. Detailed airborne magnetics is very effective at mapping structures over rocks with magnetic susceptibility contrast, but of limited effectiveness where the rocks are non-magnetic. In these situations, structures can be inferred from geophysical methods that map the regolith thickness (Meyers et al., 2001). Two bulls eye magnetic anomalies were identified adjacent to the Britannia shear zone interpreted from the SAM data. The magnetic anomalies are interpreted as prospective granophyric Defiance Dolerite. The combination of SAM and magnetics has defined a location where a good host lithology is abutting an auriferous structure. This constitutes an excellent gold exploration target. The pattern of anomalies in the first vertical derivative (1VD) gravity image of Figure 7 approximately correlates with the SAM lineaments. Gravity is responding to the lower density of the thicker regolith at the SAM anomaly, and it may be detecting the lower density of the shattered bedrock within the structures. Although the SAM anomaly from the Britannia Shear is subtle compared with the other linear SAM anomalies in Figure 5, it lies along a major contrast in the 1VD gravity image because it forms a boundary between the thick, homogenous Paringa Basalt to the east, and the more variable Devons Consuls Basalt and Defiance Dolerite to the west. Precise resolution of individual structures which lie within the Paringa Basalt, such as the Sirius Shear, has not been achieved with the 100 m by 100 m station spacing of the gravity data. These structures were resolved by gravity read at 10 m station intervals, but gravity data are not usually collected on such dense grids for exploration surveys (Whitford, 2004). The gravity survey defines the general NNW structural orientation in the area, and also maps east-west trending breaks in the structure which were not resolved by the SAM surveys using north-south electrode dipoles, but the 1VD SAM EQMMR image gives much sharper resolution of NNW lineaments, and would be the best image for targeting drillholes to test these structures. Gravity, magnetic, and SAM surveys are complementary tools for geological mapping, each dataset highlighting features which are not evident, or have subtle expression in the other datasets. SAM provides a new channel of information for locating structures beneath the regolith, and can generate exploration targets where magnetics and gravity data show no significant anomalies. Comparison with helicopter TEM (HoistEM) Part of the South Victory area was flown with a HoistEM survey in October 2002 to provide a direct comparison of SAM with airborne TEM data. The specifications of the HoistEM survey are listed in Table 1. The HoistEM system was selected because it TRANSMITTER WAVEFORM PULSE ON-TIME PULSE OFF-TIME PULSE CURRENT SWITCH-ON RAMP SWITCH-OFF RAMP TX LOOP AREA square wave 5 ms 15 ms 320 amperes 1 ms 40 microseconds 375 square metres TX NIA 120,000 A.m 2 TX FREQUENCY 25 Hz RECEIVER A-D CIRCUITRY SAMPLING SAMPLE TIME GEOMETRY EFFECTIVE NA BANDWIDTH 20 bit 128 Linear channels 0 15 ms after switch-off Horizontal In-loop square metres Hz FLYING SPECIFICATIONS SYSTEM TERRAIN CLEARANCE FLIGHT SPEED ALONG LINE SAMPLING LINE SPACING 30 m knots 8 10 m 50 m Fig. 7. First vertical derivative (1VD) Bouguer gravity image of the South Victory area with SAM EQMMR lineaments shown in white. The Britannia Shear ( B ) marks a strong contrast in the gravity image, but other SAM lineaments are poorly resolved by gravity. Table 1. HoistEM survey specifications. 160

5 Stolz has the lowest flying height, smallest transmitter footprint, and closest along-line sampling of currently available airborne TEM systems. HoistEM is considered the best airborne TEM system for matching the high spatial resolution of SAM. In addition, the 30 m HoistEM flying height provides a stronger transmitted primary TEM field into the ground than does the 120 m flying height of fixed-wing AEM systems. A strong primary field is important for penetrating the conductive regolith layer at St Ives. The HoistEM survey was flown along east-west lines spaced 50 m apart to match the survey line spacing of the SAM survey. HoistEM data were processed and presented as conductivitydepth images. These were combined to produce plan images of conductivity-depth slices for a range of depths from 20 m to 150 m. The 30 m conductivity-depth slice image was selected for the comparison with SAM results because it provides the most detail, and is at a depth corresponding with the base of regolith (Figure 8). Contours of the drilled depth of oxidation are overlain on the image, and correlate well with the pattern of HoistEM anomalies. The correlation between HoistEM anomaly and thick regolith in Figure 8 is similar to the correlation between SAM anomaly and thick regolith shown in Figure 4. HoistEM is clearly suitable for detecting zones of locally thicker regolith. The SAM image in Figure 4, however, provides much better resolution of the north-south and NNW-SSE linear features than does the HoistEM image in Figure 8, especially in the western half of the survey area. The good resolution of linear features is attributed to the north-south oriented SAM dipole transmitter driving current channelling along this direction, effectively biasing the survey towards the north-south and NNW-SSE trends. This bias yields no information for east-west trending features, and this is important to remember in exploration areas where the structural trends are unknown (Meyers et al., 2004). Where the main structural trends and controls on mineralisation are understood, it is an advantage to be able to design the survey to enhance the resolution of the most prospective faults and shears. Furthermore, a SAM survey can be repeated over the same area with a different transmitter dipole orientation, and the data later merged to ensure all structural trends are detected. Current channelling from the SAM electrode dipole transmitter can be thought of as flowing linearly from south to north. This twodimensional current flow gives a simpler response to the Earth s resistivity structure than the three-dimensional current smoke rings associated with the diffusing electromagnetic field from the HoistEM transmitter. The TEM smoke rings tend to smear out the resistivity contrasts over complex structures, whereas the linear SAM currents allow good definition of structural patterns, provided they are roughly aligned with the dipole orientation. SAM is thus very well suited to gold exploration scenarios where a particular structural trend is being targeted. Fig. 8. HoistEM 30 m conductivity-depth slice image for the South Victory area overlain by the depth of base of regolith (compare to the SAM EQMMR image in Figure 4). Fig. 9. Intrepide SAM EQMMR image for the Intrepide area of Lake Lefroy, overlain by the depth of base of regolith contours. INTREPIDE CASE STUDY PENETRATION OF LAKE SEDIMENTS The HoistEM SAM comparison was repeated at the Intrepide prospect on Lake Lefroy, where highly conductive lake sediment cover is up to 100 m thick (Figure 1). The EQMMR SAM and 50 m depth slice HoistEM images are presented in Figures 9 and 10, respectively. Both images are overlain by contours of the drilled base of the regolith (including lake sediments) over fresh bedrock. The SAM image matches the pattern of the drilled depth of regolith contours reasonably well, indicating that current from the grounded electrodes is penetrating to the base of the brinesaturated lake sediments and oxidised basement. The Intrepide SAM image does not provide the same definition of structures as noted in the South Victory image, and it is suggested that the Fig. 10. HoistEM 50 m conductivity-depth slice image for the Intrepide area of Lake Lefroy, overlain by contours of the depth of base of regolith derived from drilling information (compare to the SAM EQMMR image in Figure 9). 161

6 Stolz resolution of the SAM measurement decreases with increasing depth of target. The HoistEM image in Figure 10 does not correlate with the depth of regolith, in particular, in the northern section of the image. Analysis of the full suite of conductivity depth images generated from the HoistEM survey indicates that the electromagnetic field of the TEM transmitter loop has not penetrated deeper than about 50 m into the lake sediments. In addition, the amplitude of the TEM responses are closely correlated with the radar-altimeter height, indicating that a strong near-surface response from brines beneath the salt lake crust is dominating the signal. HoistEM is not considered to be an effective method for mapping the base of regolith over Lake Lefroy due to poor penetration in this highly conductive environment. CONCLUSIONS SAM is a highly effective geophysical method for mapping the base of regolith in terrains with saline ground water and salt lake deposits. SAM surveying results can define subsurface bedrock structures by detecting their weathered expression at the regolithfresh rock contact. Faults and shears interpreted from SAM images are complementary to structural information from magnetic and gravity images. SAM is considered more useful for mapping structures than airborne TEM, because the linear current flow of the SAM transmitter dipole drives current channelling into structural trends in the orientation of the transmitter dipole. The two-dimensional dipole current flow provides clearer images of narrow, linear features, whereas the three-dimensional smoke ring currents from a TEM transmitter can only detect broad features. SAM is more effective for gold exploration than airborne TEM, because the SAM transmitter dipoles can be placed in the orientation of known mineralised structures to enhance detection of features that follow this trend. Therefore, SAM is an excellent geophysical tool for gathering high-resolution, subsurface information over a prospect scale area where the trend of the main structures is predicted. REFERENCES Boggs, D.B., Stanley, J.M., and Cattach, M.K., 1998, Feasibility studies of TFMMIP and TFEM surveying with sub-audio magnetics: Exploration Geophysics, 29, Emerson, D.W., 1997, The galvanic electrical resistivities of Archaean country rocks in the Kambalda region: unpublished Systems Exploration Pty Ltd report to WMC Resources Limited, 7pp. Meyers, J.B., Worrall, L., Lane, R., and Bell, B., 2001, Exploring through cover the integrated interpretation of high resolution aeromagnetic, airborne electromagnetic and ground gravity data from the Grant s Patch area, Eastern Goldfields Province, Archaean Yilgarn Craton Part C: Combining geophysical methods for a holistic exploration model: Exploration Geophysics, 32, Meyers, J. B., Cantwell, N., Nguyen, P., and Donaldson, M., 2004, Sub-audio magnetic survey experiments for high-resolution surface mapping of regolith and mineralisation over a blind gold discovery near Agnew in Western Australia: 17 th Geophysical Conference and Exhibition, Australian Society of Exploration Geophysics, Expanded Abstracts, 4pp. Stolz, E.M., 2003, Direct detection of gold bearing structures at St Ives, WA DHEM vs DHMMR: Exploration Geophysics, 34, Watchorn, R. B., 1998, Kambalda-St Ives gold deposits: in Berkman, D.A., and Mackenzie, D.H., (eds), Geology of Australian and Papua New Guinean mineral deposits: AUSIMM Monograph 22, Whitford, M.D., 2004, Geophysical properties of the regolith near the Victory Gold Mine at Kambalda, Western Australia: B.Sc.(Honours) thesis, Curtin University of Technology. 162