C.J. Mwenifumbo, P.G. Killeen and B.E. ElliottGeological Survey of Canada, 601 Booth St., Ottawa CANADA K1A 0E8
Multiparameter borehole geophysical measurements were conducted at one kimberlite pipe in Saskatchewan and at four pipes in the Kirkland Lake area in Ontario to obtain in situ physical rock property data in kimberlites and their host rocks. The measurements included natural gamma-ray spectrometry, magnetic susceptibility, resistivity/conductivity, induced polarization, spectral gamma gamma (density and heavy element indicator), temperature, borehole 3-component magnetometer and seismic P-wave velocity.
Most of the geophysical measurements show anomalous values within the kimberlite pipes compared to the host rocks. These geophysical parameters, however, vary considerably within individual kimberlite pipes and between the different pipes, primarily due to the different facies and source material of kimberlite intrusions. Density, magnetic susceptibility and P-wave velocity logs indicate higher values in kimberlite compared to the overlying sediments at the Fort à la Corne kimberlite pipe in Saskatchewan.
Crossplots for measurements of conductivity, magnetic susceptibility, gamma-ray activity and spectral gamma gamma ratio (SGGR) in the kimberlites indicate several distinctly different subpopulations of kimberlitic material that represent different eruptive phases of kimberlite in the Fort à la Corne pipe. There are no relationships between the various geophysical parameters except for the magnetic susceptibility and conductivity that are positively correlated. The geophysical signature of the C14 kimberlite pipe in the Kirkland Lake area of Ontario is different from that of the Fort à la Corne pipe that comprises of kimberlite with crater facies pyroclastics. Most of the geophysical variables from the C14 pipe are moderately to highly correlated. The C14 pipe consists of diatreme and hypabyssal facies kimberlites that have distinct geophysical signatures. 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 kernel density distribution analysis reveal several clusters that correspond to the different kimberlite units.
Over the past few years, a wide range of ground and airborne geophysical surveys have been conducted in the search for kimberlite pipes in Canada. Most of these surveys have been mainly airborne and ground magnetics (Brummer et al., 1992, Lehnert-Thiel et al., 1992; Reed and Sinclair, 1991), electromagnetics (Reed and Sinclair, 1991), and to a lesser extent seismic (Richardson et la., 1995) and gravity (Lehnert-Thiel et al, 1992). Although some laboratory physical property data on Canadian kimberlites exist (e.g. Katsube et al., 1992) there is a lack of in situ data to aid in quantitative interpretation of airborne or ground geophysical surveys. Having these data would help to plan, interpret and understand many geophysical surveys.
The Geological Survey of Canada (GSC) is currently making an inventory of in situ physical property data, using borehole geophysics, for a variety of mineral deposit types including kimberlites in Canada. The main objectives are:
A compilation of kimberlite geophysical signatures will provide standards for comparing geophysical data and for evaluating new borehole exploration technology designed to find new kimberlites in Canada. The in situ physical property data will also support the design of new generations of geophysical exploration equipment and survey techniques.
A 242-m deep HQ-size borehole (100 mm diameter) was drilled on pipe 169 at the Fort à la Corne kimberlite field near Smeaton, Saskatchewan, specifically for geological and borehole geophysical investigations by the Geological Survey of Canada. The hole intersected 100 m of Quaternary sediments, 40 m of Cretaceous shales and siltstones interbedded with kimberlite, 80 metres of kimberlite with minor layers of shales and sandstone, 20 m of Mannville sandstone, and 2 m of kimberlite. The kimberlite intersected in this hole belong to the crater facies and consists of fluvial, reworked kimberlite; lapilli tuff-dominated kimberlite; olivine crystal tuff-dominated kimberlite and wave-reworked kimberlite (Kjarsgaard et al., 1995). Kjarsgaard et al. (1995) also identified several kimberlite eruptive phases in this intersection.
Borehole geophysical data were acquired in this hole to determine the geophysical characteristics of the kimberlite and its host rocks. Because the borehole showed signs of collapsing, it was cased with 2-inch plastic pipe which restricted the use of galvanic electrical methods. However, seven different logging tools were run in the hole inside the PVC pipe. Nuclear, electromagnetic and magnetic measurements can all "see" through the plastic pipe. The geophysical logs acquired included natural gamma-ray spectrometry, magnetic susceptibility, inductive conductivity, spectral gamma gamma (density and the spectral gamma-gamma ratio (SGGR) - a heavy element indicator), temperature, three-component magnetometer and sonic P-wave velocity. The sonic P-wave velocity was measured using a surface energy source and downhole recording with an array of twelve hydrophones (Hunter and Burns, 1991).
Figure 1 shows five of the borehole geophysical variables measured; natural gamma-ray, magnetic susceptibility, electrical conductivity, density and acoustic P-wave velocity. Density, magnetic susceptibility and seismic P-wave velocity show distinctly higher values in kimberlite than in the Quaternary or Cretaceous sedimentary units. This suggests that gravity, magnetic and seismic methods can be successfully applied in exploration for this type of kimberlite pipe (Lehnert-Thiel et al., 1992, Richardson et al., 1995). Although the conductivity of the kimberlite is generally high, that of the overlying Cretaceous shales and siltstones is equally high, making this type of kimberlite a difficult target to find using surface and airborne electrical prospecting techniques. The gamma-ray signature is highly variable in the kimberlite and is similar to that observed in overlying and interbedded Cretaceous shales, siltstones and sandstones. The gamma-ray data alone, therefore, do not characterize this kimberlite.
Figure 2 shows a brief statistical summary of the distribution of four geophysical variables in the form of box and whisker plots for the four main Cretaceous lithological units intersected in the Smeaton borehole. The boxes are bounded by the 25th (Lower Hinge) and 75th (Upper Hinge) percentiles, i.e. 50% of the data have values in the boxes. The notch locates the median (50th percentile) and its 95% confidence bounds. The whiskers, lines drawn from the lower and upper hinges, represent data within 1.5*IQR from the hinges , (i.e. between - 1.5*IQR and + 1.5*IQR, for the lower and upper hinges, respectively) where IQR is the interquartile range or box length. The statistical summary of the data is also given in Table 1.
The distribution of magnetic susceptibility (Fig. 2A) shows that more than 50 percent of the kimberlite data are distinctly higher (>103µSI) than those observed in the three sedimentary rock units. The magnetic susceptibilities of the siltstone, shale and sandstone are low and fall mostly between 102 and 103 µSI, with the sandstone having the lowest values. The density of the kimberlite (Fig. 2B) is also distinctly higher than that of the Cretaceous sedimentary rocks. The siltstones have the lowest densities and are distinct from the shale and sandstone whose density distributions overlap and lie between the siltstones and kimberlites. The natural gamma-ray data (Fig. 2C) show the distribution of kimberlite overlapping that of the shale and sandstone which indicates that these three rock types cannot be readily differentiated based solely on gamma-ray data. The gamma-ray activity in the siltstone is much higher than in the other three rock types. Kimberlite has a wide range of conductivities (Fig. 2D and table 1), varying from 2.5 to >600 milliSiemen/metre. The median values for the shale and siltstone that overlies the kimberlite are higher than that of the kimberlite.
|
Variable |
Lithology |
Min |
Max |
Mean |
|
|
|
|
Magnetic Susceptibility (µSI) |
|||||||
|
Shale |
224 |
1126 |
588 |
447 |
563 |
631 |
|
|
Siltstone |
93 |
2787 |
547 |
200 |
324 |
646 |
|
|
Sandstone |
112 |
722 |
305 |
200 |
251 |
355 |
|
|
Kimberlite |
500 |
83176 |
18653 |
1995 |
6031 |
31622 |
|
|
Density (g/cm") |
|||||||
|
Shale |
1.69 |
2.57 |
2.09 |
1.95 |
2.05 |
2.20 |
|
|
Siltstone |
1.20 |
2.19 |
1.65 |
1.53 |
1.60 |
1.80 |
|
|
Sandstone |
1.37 |
2.80 |
2.19 |
2.00 |
2.10 |
2.30 |
|
|
Kimberlite |
2.21 |
2.81 |
2.52 |
2.40 |
2.54 |
2.59 |
|
|
Natural Gamma-Ray (cps) |
|||||||
|
Shale |
46 |
117 |
82.3 |
73 |
83 |
92 |
|
|
Siltstone |
97 |
204 |
158.9 |
145 |
165 |
178 |
|
|
Sandstone |
22 |
120 |
68.7 |
49 |
72 |
84 |
|
|
Kimberlite |
53 |
181 |
86.7 |
53 |
99 |
115 |
|
|
Conductivity (milliSiemen/m) |
|||||||
|
Shale |
66.9 |
311.7 |
204.2 |
77.6 |
144.5 |
223.9 |
|
|
Siltstone |
156.6 |
240.2 |
194.0 |
173.8 |
190.5 |
218.8 |
|
|
Sandstone |
46.5 |
115.7 |
79.4 |
69.2 |
79.4 |
83.2 |
|
|
Kimberlite |
2.5 |
624.2 |
87.4 |
25.1 |
39.8 |
125.9 |
The box and whisker plots presented above provide the summary statistics of the distribution of the data sets but do not show the modality of the data. Histograms, however, provide better distribution statistics than the box-and-whisker plots. Histograms in figure 3 show the distribution of natural gamma-ray, electrical conductivity, magnetic susceptibility and density in the kimberlite. Also included in the figures are the box-and-whiskers plots. The gamma-ray (Fig. 3A) and electrical conductivity (Fig. 3C) data show a bimodal distribution. This bimodal distribution indicates that the central tendency and variance of these data is poorly represented in the summary statistics. The data should be treated as having two distinct populations. The magnetic susceptibility data (Fig. 3B) show a multimodal distribution with a wide range of values (see Table 1). The density data (Fig. 3D) show a slightly skewed, unimodal distribution with a narrow range indicating a fairly homogeneous density distribution.
Crossplots of conductivity, susceptibility, gamma-ray and spectral gamma-gamma ratio (SGGR) measurements in the kimberlite are presented in figures 4 to 8 as scatter plots (A) and two-dimensional density distribution plots (contour map (B) and perspective view (C) of the kernel density estimate). The kernel density estimate shows areas where data are most concentrated on these bivariate plots and provides a way of recognizing subpopulations in a given data set (Mwenifumbo, 1993). The crossplots of these data were made to explore trends, clusters and patterns in the data that are of interest in interpreting compositional and physical property changes in the kimberlite.
Gamma-Ray versus Magnetic Susceptibility. The scatter plot (Fig 4A) shows four fuzzy clusters or clumps of data :- (i) high gamma, high susceptibility; (ii) high gamma, low susceptibility; (iii) low gamma, high susceptibility; and (vi) low gamma, low susceptibility. The contour map and perspective view of the kernel density estimate, however, show these clusters to be relatively distinct. Within the low gamma, low susceptibility cluster, there are two distinct minor clusters. Most of the data are, however, concentrated in the high gamma, low susceptibility cluster. Since changes in magnetic susceptibility relate to variations in ferromagnetic minerals such as magnetite and ilmenite, and gamma-ray activity to variations in concentrations of radioelements such as potassium, the observed clusters may correlate to compositional changes in the kimberlite. These clusters may indicate different phases of kimberlite intrusions/eruptions.
Gamma-Ray versus Conductivity. The contour map and perspective view of the kernel density estimate (Fig. 5B, C) show five distinct clusters:- ( i) high gamma, high conductivity; (ii) high gamma, low conductivity; (iii) low gamma, high conductivity; (vi) medium gamma, low conductivity and (v) low gamma, low conductivity. This two-dimensional density distribution is fairly similar to that observed for the gamma-ray versus magnetic susceptibility plots because conductivity and magnetic susceptibility are highly correlated (see crossplot in Fig.6).
Magnetic Susceptibility versus Conductivity. The perspective view and contour map of the kernel density estimate show three clusters (Fig 6B, C). The data are plotted on a log-log scale. There is a strong, positive, correlation between magnetic susceptibility and conductivity (correlation coefficient, r=0.74) and the clusters lie approximately along the regression line (Fig 6B). This relationship is unusual for most kimberlite pipes. Generally high conductivities are associated with low susceptibilities as a result of alteration of magnetic minerals to non-magnetic minerals. The conductive, altered or weathered kimberlite, consists of abundant clays known as yellow ground (Macnae, 1979). Pipe 169, at the Fort à la Corne kimberlite field, is believed to be a reworked and altered pipe consisting of crater facies (eruptive) kimberlite (Kjarsgaard et al., 1995) where higher susceptibilities are a result of secondary magnetite enrichment.
SGGR versus Magnetic Susceptibility. From the scatter plot (Fig 7A) there is no apparent relationship between the two data sets and any clustering of the data is unclear. The kernel density estimate (Fig 7B, C), however, clearly shows three distinct clusters. The SGGR maps variations in the effective atomic number of the intersected lithology and hence reflects changes in the whole rock chemistry. The two-dimensional density distribution indicates three mineralogically different groups of kimberlite. The high magnetic susceptibility, low SGGR cluster is fairly tight whereas the low magnetic susceptibility, high SGGR is more dispersed along the magnetic susceptibility dimension. This broad, diffuse cluster represents two subpopulations that were identified on the magnetic susceptibility versus gamma-ray crossplots (Fig. 4).
SGGR versus Conductivity. There is again no apparent no relationship between SGGR and conductivity (Fig 8A). Three distinct clusters are, however, recognized on the kernel density distribution. The similarity between the cross plots in Figure 7 and 8 is due to the high, positive correlation between magnetic susceptibility and conductivity.
Both gamma-ray and electrical conductivity data show bimodal distribution in the kimberlite which indicates two distinct populations. The magnetic susceptibility values are fairly heterogeneous and show a multimodal distribution. However, when these data sets are crossplotted as discussed above (see Fig. 5) five distinct clusters emerge: (i) low gamma, low conductivity; (ii) medium gamma, low conductivity; (iii) low gamma, high conductivity; (vi) high gamma, high conductivity; and (v) high gamma, low conductivity, which likely represent different phases of the kimberlite eruptions. The five units identified from the kernel density estimate (KDE) are indicated in figure 1. Most of these units correspond to the kimberlite eruptions identified by Kjarsgaard et al. (1995). It should be noted that the identification of these five units by borehole geophysical methods was done before the geological examination of the drill core.
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.
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.
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.
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