Resistivity and IP over the McDermott Deposit

Reference, location and geology

Data for this example were extracted from Hallof and Yamashita, 1990, page 227⁽¹⁾. Resistivity and IP pseudosections are presented from the book, followed by results of inverting these data with the UBC-GIF DCIP2D code. The location of the survey is near Matheson, Ontario, Canada. Geology can be summarized as follows:

• The main ore zone was located under a glacial-like clay belt. This is a common situation that makes resistivity IP surveys difficult to interpret with conventional pseudosections because the conductive overburden tends to mask useful features below.
• There was sulphide mineralization associated with host volcanics and metasediments, and higher gold values were directly related to the sulphide mineralization.

The survey involved a conventional dipole-dipole array using a = 40 metres, and n = 1 to 4. Phase IP data were gathered with the resistivity information.

Original survey results

Data were gathered along a set of parallel lines, but only results gathered along line 1200E are presented here. Raw data were presented in pseudosection contour plots and interpreted directly. Resistivity and chargeability (phase data gathered at 1.0 Hertz) pseudosections are shown here, with the geologic cross-section interpreted from borehole logs presented under the data. The chargeable sulphide mineralization is clearly evident but these plots do not provide quantititative information about depth to top, depth extent, or size and shape of the target rocks. The resistivity pseudosection suggests that overburden is relatively conductive, and that underlying rocks are generally of at least two types - moderately conductive to the left of station 40N, and moderately resistive to the right.
 

DC resistivity inversion results

Conductivity structure recovered by inversion shows surface features, and suggestions of lateral extent of overburden. However, overburden thickness cannot be accurately estimated.

2D conductivity structure recovered by inversion. Geology overlaid is interpeted from boreholes; see the original figures above. Location AND depth of overburden is accurately imaged. Scale is in units of conductivity, mS/m.

Top: Observed data. Bottom: Predicted data based on recovered model (scale in mS/m).

IP inversion results

Chargeability structure is recovered by inversion of IP data. There is clearly a chargeable high associated with the sulphide mineralization.

Chargeability structure recovered by inversion.  Depth and lateral position of mineralization are clearly located. The smoothness is an effect of the choice of model objective function. Chargeability appears to fade with depth, but is more likely due to reduced information content in data caused by finite survey geometry.

Top: Observed data. Bottom: Predicted data based on recovered model (scale in mrad).

⁽¹⁾ Fink, J.B. ... (et al.) editors, 1990, Induced Polarization: applications and case histories. Society of Exploration
Geophysicists, Tulsa, Ok.