K.A.Pflug, P.G. Killeen and C.J. Mwenifumbo, Acoustic logging signatures of base metal deposits; in Proceedings of the 6th International Symposium of the Minerals and Geotechnical Logging Society; Santa Fe, 22-25 October 1995.
ABSTRACT
The Borehole Geophysics Section of the Geological Survey of Canada (GSC) routinely acquires multiparameter geophysical logs in boreholes in mining districts across Canada to investigate and demonstrate applications of borehole geophysics to mineral exploration. One of the aims of this multiparameter logging is to document the complete geophysical signatures of mineral deposits and their host rocks.
In 1993, the GSC acquired a full waveform acoustic probe to add full waveform acoustic logs to its suite of downhole measurements. Since then, acoustic logs have been acquired in several holes in base metal deposits in Canada and these measurements have been used to document the acoustic signatures of these deposits. The data are also used to calibrate seismic tomography and surface seismic surveys that are currently being conducted by industry and government in Canada to study applications of seismic techniques to mineral exploration in crystalline rocks.
One problem encountered while making these acoustic measurements was noisy waveforms caused by the probe bumping along the borehole wall when logging continuously in angled holes typical of those drilled for mineral exploration. This limits the logging speed and sometimes even requires stationary step-wise measurements in shallow dipping boreholes. Examples of acoustic logs from some base metal deposits in Canada, including the McConnell nickel deposit near Sudbury, Ontario, and the Kidd Creek copper-zinc deposit near Timmins, Ontario, are presented in this paper.
INTRODUCTION
With recent interest in the application of surface and hole-to-hole seismic techniques to base metal mineral exploration and ore delineation, there is a need for in-situ measurements of acoustic properties of base metal minerals and their host rocks. In addition to providing acoustic velocities for interpreting seismic surveys, acoustic logs can aid ore delineation and lithological identification. Combined with density logs, acoustic logs can be used to determine the elastic constants of the rocks, in situ, which is important to mine development.
Since 1993, the GSC has been acquiring acoustic logs in mineral deposits across Canada to demonstrate the applications of acoustic logging to base metal mineral exploration and mining. Examples of acoustic logs from the McConnell nickel deposit and the Kidd Creek copper-zinc mine are discussed below. In all examples, velocity refers to the compressional, or P-wave, velocity.
INSTRUMENTATION AND LOGGING PROCEDURES
Acoustic velocity measurements were made with a full waveform acoustic velocity probe manufactured by Mount Sopris Instrument Company of Colorado. The probe, which is shown in Figure 1, contains a single transmitter and two receivers operating at a center frequency of about 28 kHz. The receiver separation is 30 cm. The probe diameter is 45 mm which permits operation in BQ (60 mm diameter) or larger holes. The measurement process is described in more detail in Pflug et al, 1994.
The examples presented below were acquired in inclined BQ holes. Centralizers were not required on the probe because of the small diameter of the holes. Logs were acquired at a relatively slow logging speed of 3 m/minute to reduce noise produced by the probe bumping along the borehole wall. Measurements were made every second resulting in a sample depth interval of 5 cm.
McCONNELL NICKEL DEPOSIT
The McConnell deposit is located near Sudbury, Ontario, Canada, in the world's most productive nickel mining district (Eckstrand, 1984). The ore deposits in this area consist largely of pyrrhotite, pentlandite and chalcopyrite, which are iron, nickel and copper sulphides, respectively.
As part of a program to document the geophysical signatures of major deposit types in Ontario under the Northern Ontario Development Agreement (NODA), the GSC acquired multiparameter geophysical logs in a fence of five holes intersecting the McConnell deposit (see Figure 2) (Mwenifumbo et al, 1993). In the summer of 1993, acoustic logs were recorded in hole 78930, which intersects the massive sulphides from 136 m to 152 m, to test the GSC's newly acquired acoustic tool and to augment the existing series of measurements with acoustic velocities (Pflug et al, 1994).
The multiparameter geophysical logs from hole 78930 are shown in Figure 3. The massive sulphides are clearly evident on all geophysical logs as having anomalous physical properties. In particular, the velocity log (Vp) shows that the massive sulphides have relatively low P-wave velocities compared to the enclosing rocks.
Average velocities for the different rock types intersected by hole 78930 are shown in Figure 4. The average velocity in the massive sulphide (4.4 km/s) is much lower than the average velocities for the host rocks (5.9 to 6.4 km/s). The amphibolites have the highest average velocity at 6.4 km/s.
A comparison of the velocity distribution in the massive sulphides with the velocity distribution in the rest of the rocks in hole 78930 is shown in Figure 5. These histograms further demonstrate that the massive sulphides can be clearly distinguished from all other rock types in this hole on the basis of their lower P-wave velocities. Also, the distribution of velocities in the massive sulphides is fairly narrow, the average velocity of 4.4 km/s having a standard deviation of only 0.3 km/s. This suggests that the sulphides are distributed fairly homogeneously throughout the massive sulphide zone.
Figure 6 shows the density, P-wave velocity, acoustic impedance and full waveform logs for hole 78930. The acoustic impedance is the product of the density and velocity, and it is an important parameter in seismic exploration techniques because seismic energy is reflected from boundaries between rocks with different acoustic impedances. The massive sulphide zone from 136 to 152 m (Fig. 6) would be easily detected by surface and hole-to-hole seismic techniques because the acoustic impedance in this zone (14 km/s g/cm3) is lower than that observed in the rocks immediately above and below it (about 16.5 km/s g/cm3). The amphibolites from 50-63 m and from 167-176 m would also be detectable by seismic techniques because their acoustic impedance is higher than the acoustic impedance of the surrounding rocks.
The most notable feature in the full waveform log (Fig. 6) is the relatively low amplitudes in the massive sulphides compared to the rest of the hole. Except for an apparent fracture zone between 24 m and 29 m, the amplitudes are lowest in the massive sulphides. Acoustic energy is more strongly attenuated in the massive sulphides than in the other rock types.
KIDD CREEK COPPER-ZINC MINE
The second test of the acoustic probe in a base metal deposit was conducted at the Kidd Creek copper-zinc mine near Timmins, Ontario, Canada. This is one of the world's largest volcanogenic massive sulphide deposits. Multiparameter geophysical logs were acquired in several holes at the Kidd Creek mine as part of the borehole geophysical signatures (NODA) project. Two holes were logged with the acoustic probe in the North Rhyolite Zone, holes 4509 and 4741. Hole 4509 intersects semi-massive to massive pyrite from about 505 m to 515 m and hole 4741 intersects semi-massive to massive pyrite between 480 m and 490 m. Neither hole intersects any significant economic mineralization.
Figures 7 and 8 show the multiparameter geophysical logs recorded by the GSC in holes 4509 and 4741, respectively, plotted along with the pyrite content. The average velocities for the different rock types intersected by these holes are shown in Figure 9. Figures 7 and 8 show that the velocity correlates well with lithology in these two holes. The velocities in the semi-massive to massive pyrite, however, are not particularly distinctive compared to the other rock types. These observations are summarized in Figure 9 which shows that the average velocities for the rock types intersected by holes 4509 and 4741 ranges from 5.3 km/s to 6.8 km/s. The average velocity for the semi-massive to massive pyrite is about mid-range at 6.0 km/s.
The velocity distribution in the semi-massive to massive pyrite zones is shown in Figure 10. This distribution is multimodal which probably reflects the heterogeneous distribution of the pyrite in these zones. The standard deviation of the velocity distribution is 0.5 km/s. Although the pyrite content logs in Figures 7 and 8 only show the average pyrite content for the semi-massive to massive pyrite zones, these zones are described in the geological logs as containing pyrite pods in a rhyolite matrix. In Figure 11, the velocity and SGG ratio in these zones have been plotted for both holes. This figure shows the heterogeneous nature of these zones and that the higher SGG ratios, which indicate higher pyrite content (Killeen et al, 1989), correlate with higher velocities. These higher velocities are between 6.0 and 7.0 km/s.
The density, velocity and acoustic impedance logs for holes 4509 and 4741 are plotted along with the pyrite content in Figures 12 and 13, respectively. These figures show that the semi-massive to massive pyrite zones would be detectable by seismic techniques because the average acoustic impedance in these zones (18 km/s g/cm3 in hole 4509 and 18.7 km/s g/cm3 in hole 4741) is higher than the acoustic impedance of the enclosing rock (about 16 km/s g/cm3).
CONCLUSION
In summary, velocity is a better indication of lithology at the Kidd Creek mine than at the McConnell deposit. Excluding the massive sulphides at the McConnell deposit, the average velocities of the Kidd Creek lithologies cover a larger range, 5.3 to 6.8 km/s, than the average velocities of the McConnell lithologies, 5.9 to 6.4 km/s.
The massive sulphides (pyrrhotite/pentlandite/chalcopyrite) at the McConnell deposit are unique in that they have an extremely low average velocity (4.4 km/s) compared to all other lithologies encountered at both sites. In contrast, the average velocity of the semi-massive to massive pyrite at Kidd Creek, 6.0 km/s, is not anomalous. The velocity in the massive sulphides at the McConnell deposit shows a narrow, approximately normal distribution (the standard deviation is 0.3 km/s) while the velocity distribution in the semi-massive to massive pyrite at Kidd Creek is wider (standard deviation is 0.5 km/s) and multimodal. These distributions indicate that the sulphides are more homogeneous in the sulphide zone at the McConnell deposit than is the pyrite in the pyrite-rich zones at Kidd Creek. At Kidd Creek, higher velocities, from 6.0 to 7.0 km/s, correlate with higher SGG ratios and, therefore, higher pyrite content (Killeen et al, 1989).
The sulphide zones at both sites would be detectable by surface and hole-to-hole seismic techniques. At the McConnell deposit, the massive sulphide zone has a low acoustic impedance relative to the surrounding rocks, while the semi-massive to massive pyrite zones at the Kidd Creek mine have high impedances relative to the surrounding rocks.
ACKNOWLEDGEMENTS
The authors wish to thank INCO Exploration and Technical Services and Falconbridge Limited for their assistance in selecting boreholes at the McConnell deposit (INCO) and at Kidd Creek (Falconbridge), for logistical support, for providing geological logs and for granting permission to publish the data. Financial support for this work was provided by the Canada-Ontario Subsidiary Agreement on Northern Ontario Development (1991-1995), under the Canada-Ontario Economic and Regional Development Agreement.
REFERENCES
Eckstrand, O.R.
1984: Gabbroid-associated nickel, copper, platinum group elements;
in Canadian Mineral Deposit Types: A Geological Synopsis,
Geological Survey of Canada Economic Geology Report 36, O.R.
Eckstrand (ed), p.41-42.
Killeen, P.G., Schock, L.D. and Elliott, B.E.
1989: A slim hole assaying technique for base metals and heavy
elements based on spectral gamma-gamma logging; in Proceedings
of the 3rd International Symposium on Borehole Geophysics for
Minerals, Geotechnical and Groundwater Applications, 2-5 October,
1989, Las Vegas, Nevada, p.435-454.
Mwenifumbo, C.J., Killeen, P.G., Elliott, B.E. and Pflug, K.A.
1993: The borehole geophysical signature of the McConnell nickel
deposit, Sudbury area; in Proceedings of the 5th International
Symposium on Geophysics for Minerals, Geotechnical and Environmental
Applications, 24-28, October, 1993, Tulsa, Oklahoma, paper I, 8 pp.
Pflug, K.A., Killeen, P.G. and Mwenifumbo, C.J.
1994: Acoustic velocity logging at the McConnell nickel deposit,
Sudbury area, Ontario: preliminary in situ measurements; in
Current Research 1994-C; Geological Survey of Canada, p.279-286.
Figure 1: Mount Sopris Instrument Company Inc., model CLP-4681 full waveform acoustic velocity probe.
Figure 2: Vertical section through the McConnell
Nickel Deposit showing the five holes logged by the GSC. Acoustic
logs were acquired in hole 78930.
Figure 3: Multiparameter geophysical logs in hole
78930 at the McConnell Deposit. Vp = P-wave velocity, SGG ratio =
spectral gamma-gamma ratio (heavy element indicator), IP = induced
polarization, MS = magnetic susceptibility (in milli SI units) and TG
= temperature gradient (in milli Kelvin/metre). The IP and MS logs
have been truncated above at 100 mV/V and 15 mSI, respectively. Incl.
Quartz Diorite Dyke = sulphide inclusions in quartz diorite dyke, and
Incl. Massive Sulphide = quartz diorite inclusions in massive
sulphide.
Figure 4: Average P-wave velocities for the
different rock types intersected in hole 78930 at the McConnell
deposit. Incl. Quartz Diorite Dyke = sulphide inclusions in quartz
diorite dyke.
Figure 5: Distribution of P-wave velocities in
the massive sulphides compared to the distribution of P-wave
velocities in all other rocks in hole 78930 at the McConnell
deposit.
Figure 6: Density, P-wave velocity (Vp), acoustic impedance and full waveform acoustic logs in hole 78930 at the McConnell deposit. Incl. Quartz Diorite Dyke = sulphide inclusions in quartz diorite dyke, and Incl. Massive Sulphide = quartz diorite inclusions in massive sulphide.
Figure 7: Multiparameter geophysical logs and pyrite content in hole 4509 at the North Rhyolite Zone, Kidd Creek mine. Vp = P-wave velocity, SGG ratio = spectral gamma-gamma ratio (heavy element indicator), IP = induced polarization, SP = self potential, MS = magnetic susceptibility (in milli SI units) and TG = temperature gradient (in milli Kelvin/metre).
Figure 8: Multiparameter geophysical logs and pyrite content in hole 4741 at the North Rhyolite Zone, Kidd Creek mine. Vp = P-wave velocity, SGG ratio = spectral gamma-gamma ratio (heavy element indicator), IP = induced polarization, SP = self potential, MS = magnetic susceptibility (in milli SI units) and TG = temperature gradient (in milli Kelvin/metre).
Figure 9: Average P-wave velocities in the different rock types intersected by holes 4509 and 4741 at the Kidd Creek mine.
Figure 10: P-wave velocity distribution in the semi-massive to massive pyrite intersected by holes 4509 and 4741 at the Kidd Creek mine.
Figure 11: P-wave velocity (Vp)and SGG ratio for holes 4509 and 4741 at the Kidd Creek mine. The velocity and SGG ratio correlate well in the semi-massive to massive pyrite zones and reflect the inhomogeneous distribution of pyrite in these two zones.
Figure 12: Density, P-wave velocity (Vp) and acoustic impedance logs plotted with the pyrite content for hole 4509 at the Kidd Creek mine.
Figure 13: Density, P-wave velocity (Vp) and acoustic impedance logs plotted with the pyrite content for hole 4741 at the Kidd Creek mine.