This is a reprint of a paper presented at:"The role of in-mine geophysics in resource evaluation" workshop at the 1996 Society of Exploration Geophysicists International Exposition and Sixty-Sixth Annual Meeting, Denver, Colorado.
Drilling 50m diameter holes with geophysics
- A case study of downhole EM at Kambalda, Western Australia
G Turner*, R Ramsay, A Wellington, S Fogarty, G King: WMC Resources Ltd
Introduction
The Kambalda nickel sulphide orebodies occur at or near the basal contact of a komatiitic (ultramafic) volcanic rock sequence with underlying basaltic volcanic rocks. The ore occurs as thin (1-3m), narrow (10-600m) elongate (up to 3km) ribbon-like zones which are generally located in linear embayments within the underlying basalt contact. Typically the ore profile consists of a zone of massive sulphides at the base overlain by matrix sulphides and disseminated sulphides. Subsequent to deposition these zones have been folded, faulted and intruded to create a large number of smaller bodies. Economic pods can be as small as 10x10x1m.
Standard NQ (76mm diameter) and BQ (46mm diameter) holes can only sample a very small portion of the prospective basalt/ultramafic contact. Figure 1 illustrates two examples of how orebodies could be missed by drilling alone. Often there can be little geological or geochemical evidence that an ore pod is nearby. In order to ensure intersecting all 10m x 10m ore pods a 10m x 10m drill grid is required. This is very expensive, particularly when the orebodies can be in excess of 1 km deep.
Figure 1
Cross-sections showing how ore pods may be missed by drilling aloneDownhole electromagnetics (DHEM) techniques are now being routinely used at Kambalda to expand the effective search radius of surface and underground drillholes by 3 orders of magnitude. This paper presents a case history of the use of DHEM which played an important role in establishing the acceptance of the value in the mine environment at Kambalda. The case history is from the Otter-Juan Mine.
Historical exploration of the 31C surface at Otter-Juan
The Juan orebody essentially consists of two subparallel NNW trending ore shoots, one high tenor and one lower tenor. These shoots are broken into a number of surfaces by steep westerly dipping reverse faults. The 31C surface is a postulated high tenor surface occurring approximately 500 metres below the surface parallel to the low tenor 32C surface and on the same basalt wedge (the Deeps F wedge)(see Figure 2). It is thought to be the up-plunge continuation of the 10C surface which terminates at the leading edge of a large west-dipping reverse fault to the north, and the downplunge continuation of the 19C surface.
Figure 2
Plan view of interpreted 19C and 31C ore surface trends showing major intersections and proposed drill drive positions prior to DHEM surveyingA number of encouraging drill intersections had been obtained with this surface over several years (eg 22m @ 4.8% Ni along contact in JS 12-97) (see Figure 2). Despite the number of holes that had been drilled (see Figure 3) insufficient data had been obtained to prove up an ore reserve. The cross-section in Figure 4 graphically shows the difficulties associated with detecting the sulphide lenses by drilling. Detailed drilling from the surface 500m above is prohibitively expensive and underground drilling can only be oriented subparallel to the contact. The lenses may have thicknesses of only 1-2m. It is therefore statistically unlikely to intersect them end on from a distance 100-200m with a 46mm diameter hole. Consequently, given the successful intercepts that had been obtained and the relationships with the 10C, 19C and 32C surfaces, the surface was still very prospective.
Figure 3
Plan view showing drillholes through the 19C and 31C surfaces between -150m RL and
Figure 4
Typical cross-section through the 19C and 31C surfaces showing the difficulties associated with drilling from the available developmentTo continue exploration in the historically conventional manner, two new hanging wall drives were proposed to be developed to obtain suitable angles to test the surface by drilling. The first drive involved 185m of development from the 1319 decline to test the southern section of the surface. The second drive involved 140m of development from 12-23 cross-cut (see Figure 4). The cost of these drives including subsequent drilling to confirm the existence of a mineable surface was estimated at about A$0.5 million.
Integration of DHEM surveying
Before commiting this expenditure we decided to perform DHEM surveys in a number of existing underground holes which passed close to, or through, the prospective basalt/komatiite contact. Whilst it had been unlikely to intersect any ore pods with many 46mm diameter boreholes it was thought that with a search diameter of the order of 50m, DHEM would enable large tracts of the prospective contact to be explored from a single borehole.
The Kambalda nickel ore provides an excellent target for electromagnetic geophysical techniques. The host basalt and ultramafic rocks have resistivities in excess of 1 000 W m and massive sulphide ore has conductivities generally in excess of 10 000 S/m (Figure 5 shows an example of a conductivity profile through an example ore sequence). The effectiveness of surface based surveys can be limited by the deep (20-100m) conductive (1-100 W m) weathered layer. This can be partially overcome by placing the transmitter or receiver (or both) below the weathered layer.
s
(S/m)
Figure 5
An example of the conductivity profile through a Kambalda ore profile based on galvanic measurements on laboratory samplesIn the space of a couple of weeks, 7 holes were surveyed over a 250m plunge extent. The cost of these surveys was estimated to be about $10 000. All holes were logged with an MCI DHR-1S axial component EM probe using the same 500 m x 500 m surface loop. The holes all dipped slightly below horizontal and varied in length from 130 - 270 m. Strong anomalies were detected in all the holes surveyed at the south end of the surface (550820N-550960N). The anomalies occur at a position which is consistent with a continuation of the 19C surface rather than the 31C surface. Figure 6 shows one of the anomalies. The anomaly is centred on 80m downhole and has a negative and a positive peak. This is characteristic of the response from a conductor subparallel to the hole. The crossover occurs approximately where the drillhole passes the centre of the conductor. The assymetry of the anomaly suggests that the conductor dips slightly away from the hole and is closest at the edge nearest the collar. The polarity of the anomaly indicates that the conductor is beneath the hole. To the north no significant anomalies were detected in a fan of holes at 551040N. An anomaly was detected in hole JS 12-120 which intersected 14cm of massive ore but the short wavelength of the anomaly indicated that the massive ore does not extend far from the hole.
Figure 6
DHEM data from JS13-113 showing anomaly from a conductor sub-parallel to the holeAs a result of the surveys a decision was made to focus the next phase of exploration on the southern section and not proceed with the northern drill drive. The southern drill drive has been repositioned slightly further east to allow drilling of the 19 surface (Figure 7). Each anomaly provides a good indication of the Easting and RL of the ore surface and therefore provides a focus for drilling from the drive. Furthermore the presence of conductors along the extension of the 19C surface and not the 31C surface shifted the drilling emphasis and should result in fewer barren holes and less chance of missed ore pods.
The first section of the southern drill drive has now been completed and the initial phase of drilling completed. The conductor identified by JS12-120 is the only conductor that has been targeted to date. Drillhole JS13-137 intersected disseminated and stringer sulphides at the target location and returned 1m @ 2% Ni. This intersection ties in well with the time constant of about 1-2ms which is too small to indicate any large concentrations of 30 000 S massive ore. The EM log Figure 8 in this hole shows a positive "in-hole" anomaly which indicates that the conductor was intersected.
Figure 7
Plan view of interpreted 19C and 31C ore surface trends showing DHEM anomaly locations and revised drill drive position.
Figure 8
DHEM data from JS13-137 showing anomaly from intersected ore zoneConclusions
The savings achieved by detecting the lack of mineralisation by the program of geophysical surveying, as opposed to the construction of a drill drive and drilling is approximately $230 000. In addition to this, locational information provided by the geophysical data will allow more efficient drilling through better drill drive positioning and more informed targeting, and the earlier indications of ore potential aid mine planning.
The ability to expand drillhole search areas by 3 orders of magnitude has ensured that DHEM is now in regular use in Kambalda’s nickel mines. DHEM is now an integrated part of mine exploration programs with drillholes planned on the basis of the information they will provide both from core and downhole surveying. The successes of the technology have won over many who were previously skeptical regarding the use of geophysical technologies in a mine environment and are now paving the way for the introduction of other imaging technologies which will enable a smarter more efficient mining environment.
Acknowledgements
The work is the result of the effort of many people involved in the mine geophysics program at Kambalda over several years.