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KIMBERLITE PIPES IN THE KIRKLAND LAKE AREA, ONTARIO
Four kimberlite pipes were investigated in the Kirkland Lake area of Ontario (Fig. 9). Logging was done with the GSC R&D logging system in several company boreholes and three shallow rotosonic boreholes drilled by the GSC (McClenaghan, 1995). The GSC boreholes were drilled to investigate indicator minerals in Quaternary glacial deposits overlying the kimberlite pipes. The geophysical variables measured in the company boreholes included magnetic susceptibility, induced polarization, resistivity, self potential (SP), inductive conductivity, natural gamma-ray spectrometry, spectral gamma gamma (SGG - density and heavy element indicator) and temperature. Borehole 3-component magnetometer surveys were also done in three boreholes at two pipes. The three shallow rotosonic holes intersected only a few metres of kimberlite. These shallow boreholes were logged with density, natural gamma-ray spectrometry, magnetic susceptibility, temperature and inductive conductivity. The data from the overburden boreholes are not included in this paper. Only data from the C14 pipe (Fig. 9) are presented in detail in the following sections.
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Geology and Geophysical Logs at the C14 Pipe
Borehole geophysical logging at the C14 pipe was carried out in drill hole C14N. This hole is approximately 13.0 cm in diameter and approximately 268 m in length. It was drilled through 38.7 m of overburden and intersected kimberlitic material to the bottom of the hole (see Fig. 10). Between 38.7 and 204.5 m the rock is classified as lithic tuffisitic kimberlite breccia (LTKB) with a silicified section between 92.8 m and 94.2 m. The rock contains a heterogeneous mixture of xenoliths that are dominantly limestone. Black shale and siltstone xenoliths are common as well as a variety of volcanic and intrusive rocks. The matrix contains olivine that is usually serpentinized. One major fracture zone occurs between 125 m and 160 m, and several fractures are observed through out the unit. Tuffisitic kimberlite grades into kimberlite between 204.5 and 206.7 m. Two kimberlite (K) zones are separated by tuffisitic kimberlite (TK) and kimberlite breccia (KB); one of these is between 206.7 m and 231.0 m, and the other is between 237.6 m and 253.3 m. The bottom of the hole intersects intercalated tuffisitic kimberlite and kimberlite (253.3 -268.2 m). The drill hole essentially intersects diatreme facies kimberlites (LTKB) and hypabyssal facies kimberlites (TK, KB, and kimberlite) (Mitchell, 1986).
Figure 10 shows the geology and four geophysical logs in hole C14N. The magnetic susceptibility and resistivity data are plotted on logarithmic scales. Since the logging was done in a fairly large diameter borehole and borehole corrections were not applied, the measured values of most variables are approximate. The density data, for instance are underestimated and the apparent resistivity data may also be underestimated. The natural gamma, density, resistivity, and magnetic susceptibility logs clearly delineate the major lithological units identified from drill core logging. The thin, silicified lithic tuffisitic kimberlite breccia between 92.8 m and 94.2 m has an abnormally high gamma-ray activity compared to the rest of the LTKB unit. The increase in gamma-ray activity is probably due to enrichment in uranium along a fracture zone (West and Laughlin, 1976). The density and resistivity are high in the silicified LTKB but the magnetic susceptibility is extremely low. All the measured variables show much higher values in the hypabyssal kimberlite (204.0 -268.0 m) than those in the diatreme facies kimberlite (38.7 - 204.0 m). Kimberlite is composed of massive igneous rock that is less brecciated and less porous than the tuffisitic kimberlite breccia zones. There is more groundmass and phenocrysts content and fewer xenoliths than occurs in the breccia. Therefore, the high resistivity and density are primarily due to a lower porosity. This observation is also confirmed from laboratory rock property measurements (Katsube et al., 1992).
Figure 11 shows the geophysical interpretation of the geology from drill hole C14N. The geophysical logs reveal four distinct signatures within the lithic tuffisitic kimberlite breccia (LTKB in Fig. 10). These four units are grouped into two major subdivisions based on the gamma-ray data; the upper unit between 28.4 m and 125.2 m is a low gamma-ray, lithic tuffisitic kimberlite breccia (LTKB), and the lower unit between 125.2 m and 195 m is a higher gamma-ray, tuffisitic kimberlite breccia (TKB). The LTKB is further subdivided into three units; the upper (LTKB1) and lower (LTKB2) units separated by the silicified LTKB. The silicified LTKB unit is characterized by high density and resistivity, and low magnetic susceptibility. A closer examination of the gamma-ray activity and density data within the TKB unit (Fig. 11) indicates that this unit can be further subdivided into two additional units; the upper unit is less radioactive than the lower unit.
The geophysical variables also show that the two kimberlite units at the bottom of the borehole can be further subdivided. The upper kimberlite, for instance can be divided into two subunits based on the magnetic susceptibility response; an upper subunit (200-214 m) with higher magnetic susceptibility and a lower subunit (214-227 m) with lower magnetic susceptibility. The lower kimberlite which has higher gamma-ray activity, magnetic susceptibility and density also shows some variations in these parameters that could be subdivided. Several tuffisitic kimberlite and kimberlite breccia units can be identified in the lower kimberlite unit (between 237.6 and 253.3 m). These are lower in magnetic susceptibility, gamma-ray activity and density.
Data Summaries and Univariate Distributions
Figure 12 shows a brief statistical summary of the distribution of the four geophysical variables in hole C14N in the form of box and whisker plots. There are three main kimberlite units intersected: (i) lithic tuffisitic kimberlite breccia (LTKB), (ii) tuffisitic kimberlite breccia (TKB) and (iii) kimberlite which includes TK/KB and TK/kimberlite sections (see Fig. 11).
The distribution of magnetic susceptibility (Fig. 12A) in the kimberlite shows that there is a general increase in magnetic susceptibilities from the upper LTKB1 unit to the kimberlite at the bottom of the hole except for the silicified LTKB which shows lower magnetic susceptibilities. The low magnetic susceptibility for the silicified unit is due to alteration. All the five units are, however, distinctly different. The distributions of density (Fig. 12B) in the five units also show a similar trend to that of the magnetic susceptibility except for the silicified LTKB unit. The density is higher in this unit compared to the overlying and underlying LTKB units. The density of the silicified LTKB is slightly higher than that of the tuffisitic kimberlite breccia. The increase in density is a result of a reduction in porosity due to silicification. The densities are characteristically different for the five units. The resistivity data (Fig 12C) show increasing resistivities with depth corresponding to the changes in kimberlite from lithic tuffisitic kimberlite breccia to kimberlite except for the silicified LTKB which shows higher resistivities compared to both the lower and upper lithic tuffisitic kimberlite breccia. The natural gamma-ray activity (Fig. 12D) indicates that the silicified LTKB has the same distribution as the lithic tuffisitic kimberlite breccias above (LTKB1) and below (LTKB2). This suggests that silicification did not change the distribution of the radioelement concentrations. These three units are the same in terms of gamma-ray signature. The tuffisitic kimberlite breccia and the kimberlite are different in that the latter has the highest gamma-ray activity.
In summary, the distributions of magnetic susceptibility, density, resistivity and gamma-ray activity indicate distinctively different units. Higher values are characteristic of the kimberlite that belongs to the hypabyssal facies and lower values to the diatreme facies kimberlite.
Two Dimensional Distributions
Density versus Magnetic Susceptibility. Figure 13 shows a scatter plot of density plotted against magnetic susceptibility for data from drill hole C14N. The data are grouped into subsets according to lithology interpreted from the drill core logging and geophysical logs. The density and susceptibility correlate well except for the data from the silicified lithic tuffisitic kimberlite breccia unit that cluster in the low magnetic susceptibility and relatively higher density field. The high density in this unit is due to a decrease in porosity because of silicification. The density and magnetic susceptibility increase as a function of depth from LTKB1 to kimberlite. This increase also reflects the change from upper diatreme facies kimberlite to the hypabyssal kimberlite facies. The two-dimensional kernel density distributions (Fig. 14) clearly identify the silicified LTKB and the four other kimberlite units. The two lithic TKBs (LTKB1 and LTKB2) are classified as two distinct units; LTKB1 has a lower density and magnetic susceptibility than LTKB2. Most of the data are from the tuffisitic kimberlite breccia.
Resistivity versus Magnetic Susceptibility.
The plot of resistivity versus magnetic susceptibility (Fig 15) shows the same trend as that of density versus magnetic susceptibility (Fig 13). The silicified LTKB unit is anomalous, showing up as a cluster in the low magnetic susceptibility-high resistivity region. The data are slightly spread out along the resistivity variable for the LTKBs, TKB and kimberlite. This may reflect porosity variations within the different units. The two-dimensional kernel density distributions (Fig. 16) identify all the different kimberlite units as clusters.
Resistivity versus Density. The plot of resistivity versus density (Fig. 17) shows a positive correlation between the two variables. There is a slight spread in the data with some anomalous values within the tuffisitic kimberlite breccia (TKB); spreading towards higher density values. Only three clear clusters emerge from these data sets. The TKB, LTKB2 and the silicified LTKB are lumped together as one cluster whereas the LTKB1 unit and kimberlite appear as distinct units.
Gamma-Ray Activity versus Magnetic Susceptibility, Density and Resistivity. The plots of gamma-ray against magnetic susceptibility (Fig. 18 and 19) indicate that these variables are moderately correlated. These data show four clusters of kimberlite; two clusters correspond to the upper and lower kimberlite units (Fig. 10) and the intercalated kimberlite/kimberlite breccia layers within. The tuffisitic kimberlite breccia is clearly defined but the LTKBs are, however, poorly defined because of the non characteristic gamma-ray response within these units. The plots of gamma-ray against density (Fig. 20 and 21) show similar characteristics as observed in figures 18 and 19.
However, the TKB unit is not as clearly defined. The plots of gamma-ray activity versus resistivity (Fig. 22 and 23) show a slight positive correlation. Several clusters can, however, be identified from the two-dimensional kernel density distributions. These clusters correspond to lithological variations as well as reflecting porosity variations in the different kimberlite units.
CONCLUSION
The geophysical data from the kimberlites investigated indicate that the physical properties are variable in a kimberlite pipe and also between different pipes. Although there is a high degree of variability of the physical properties within the kimberlite, most geophysical measurements show anomalous values which are characteristic of the kimberlites compared to the surrounding sediments. Density, magnetic susceptibility and sonic P-wave velocity logs for example, show distinctly higher values within kimberlites compared to the host rocks. The geophysical data can also be used to classify the different facies and source material of kimberlites. Five different kimberlite units were readily identified from borehole geophysical measurements at one Fort à la Corne kimberlite pipe. These different units correspond to several kimberlite eruptions that have also been observed from the drill core analysis.
The heterogeneous nature of the kimberlite is primarily due to the source material, and the amount and type of host rock ingested during the emplacement process. The conductivity-magnetic susceptibility relationship within pipe 169 kimberlite at Fort à la Corne confirms the geological interpretation that it is a reworked and altered kimberlite that belongs to the crater facies.
The geophysical signature in the C14 kimberlite pipe near Kirkland Lake, Ontario, is different from the Fort à la Corne pipe. Magnetic susceptibility and resistivity (the reciprocal of conductivity) shows an inverse relationship to that observed at the Fort à la Corne pipe. Several geophysical variables show distinct responses within the different kimberlite units. The C14 pipe consists of diatreme and hypabyssal facies kimberlites. The hypabyssal, igneous kimberlite zones are distinct from the breccias and tuffisitic kimberlite since the former are characterized by their high natural gamma-ray activity, density, resistivity and magnetic susceptibility. The diatreme facies kimberlites are characterized by lower density, resistivity, gamma-ray activity and magnetic susceptibility than the hypabyssal facies kimberlites. Crossplots and two-dimensional density distribution analysis revealed several clusters corresponding to the different kimberlite units.
The lithic tuffisitic kimberlite breccia identified as one unit from drill core geological analysis was readily subdivided into three units with distinct geophysical characteristics. The upper part of this unit was interpreted to be highly altered based on the geophysical signature.
ACKNOWLEDGEMENTS
This geophysical study of Canadian kimberlites is a contribution to the Canada-Saskatchewan Partnership Agreement on Mineral Development (1992-1997) and the Northern Ontario Development Agreement (1993-1996). Work on the Fort à la Corne, Saskatchewan, kimberlite was done with the co-operation of Uranerz Exploration and Mining Limited, and work on the C14 pipe in Kirkland Lake area with the permission of Regal Goldfields Ltd., and J.E. Tilsley and associates.
REFERENCES
Brummer, J.J., MacFadyen, D.A., and Pegg, C.C.1992: Discovery of kimberlites in the Kirkland Lake area, Northern Ontario, Part I: Early surveys and surficial geology; Exploration and Mining Geology, v1, p.339-350.
1992: Discovery of kimberlites in the Kirkland Lake area, Northern Ontario, Part II: Kimberlite discoveries, sampling, diamond content, ages and emplacement; Exploration and Mining Geology, v1, p.351-370
Hunter, J.A., and Burns, R.A.1991: Determination of overburden P-wave velocities with a downhole 12-channel eel. in Current Research, Part C, Geological Survey of Canada, Paper 91-C, pp. 61-65.
Katsube, T.J., Scromeda, N., Bernius, G. and Kjarsgaard, B.A.1992: Laboratory physical property measurements of kimberlites: in Current Research Part E, Geological Survey of Canada, Paper 92-1E, 357-364.
Kjarsgaard, B.A., Leckie, D.A., McIntyre, D.J., NcNeil, D.H., Haggart, J.M., Stasiuk, L. and Bloch, J.1995: Smeaton kimberlite drill core, Fort à la Corne field, Saskatchewan, Geological Survey of Canada Open File 3170.
Lehnert-Thiel, K., Loewer, R., Orr, R.G. and Robertshaw, P.1992: Diamond-bearing kimberlites in Saskatchewan, Canada: The Fort a la Corne case history, in Exploration and Mining Geology Vol. 1, no.4, p.391-403.
Macnae, J.1979: Kimberlite and exploration geophysics. Geophysics, vol.44, no. 8, p.1395-1416.
McClenaghan, M.B.1995: Kimberlite glacial dispersal studies, Kirkland Lake. in Summary Report 1994-1995, Northern Development Agreement, Natural Resources Canada-Ministry of Northern Development and mines, p.74-77.
Mitchell, R.H.1986. Kimberlites: Mineralogy, Geochemistry, and Petrology. Plenum Press, New York.
Mwenifumbo, C.J.1993: Kernel density estimation in the analysis and presentation of borehole geophysical data; In The Log Analyst, vol. 34, no. 5, pp.34 - 45.
Reed, L.E. and Sinclair, 19911991: The search for kimberlite in the James Bay Lowlands of Ontario. CIM Bulletin, vol. 84, No. 947, p.132 - 139.
Richardson, K.A., Katsube, T.J., Mwenifumbo, C.J., Killeen, P.G. and Hunter, J.A.M., Genzwill, D.J. and Matieshin, S.D.1995: Geophysical studies of kimberlite in Saskatchewan: in Investigations Completed by the Saskatchewan Geological Survey and the Geological Survey of Canada Under the Geoscience Program of the Canada-Saskatchewan Partnership Agreement on Mineral Development (1990-1995), Geological Survey of Canada Open file 3119, (ed. D.G. Richardson) p.197-205.
West, F.G and Laughlin, A.W.1976: Spectral gamma logging in crystalline basement rocks. Geology, v.4, pp. 617-618.
Orr, Pauline M.Sc. Spring (1998)
Geochemistry and petrology of the Yamba Lake kimberlites, central Slave province, Northwest Territories.
Supervisor: R. Luth.
By the by, thanks Don, long time no chat.
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