GEOPHYSICAL IMAGING OF A KAOLINITE
DEPOSIT AT SYLVAN, MANITOBA, CANADA
Ian J. FERGUSON, Johannes
P. RISTAU, Virginia G. MARIS and Ifti HOSAIN
A geophysical survey was performed at Sylvan, Manitoba,
Canada (51o5' N, 97o22' W) to investigate a Lower Cretaceous kaolinite
deposit. The deposit consists of zones of kaolinite, silica sand, and lignitic
clay located in a series of channels formed during karsting of the underlying
Palaeozoic bedrock and is covered by 3 to 5 m of glacial drift. The aim
of the study was to identify cost-efficient electrical and electromagnetic
(EM) geophysical methods for locating, mapping, and assessing this target.
Methods applied included terrain conductivity (EM31), VLF-EM, time-domain
electromagnetics (TEM), DC-resistivity, and shallow seismic refraction.
The survey showed that EM methods offer a viable alternative to more expensive
seismic reflection surveys in the investigation of small industrial mineral
deposits. Comparison of the geophysical survey results with those of a
drilling program indicated that VLF-EM and TEM were the best methods for
delineating the kaolinite deposit. VLF-EM was the most cost-efficient method
for delineating the kaolinite deposit over a ca. 10 Ha area and for exploring
for further deposits within several kilometres of the main site. Joint
interpretation of the in-phase and quadrature response is required for
increased reliability in identifying the major kaolinite-filled channels.
The TEM method provided more detailed resolution of the deposit than VLF-EM
and was the optimal method for assessing its thickness. However, TEM data
acquisition is too slow and inefficient for reconnaissance mapping of 10
Ha sites. EM31 surveying is useful for defining the palaeokarst surface
and overburden thickness in area surrounding the deposit but cannot be
used reliably for mapping the kaolinite deposit itself. The combined geophysical
survey results show the kaolinite deposit at Sylvan to be located in a
channel which is 100 m wide and about 25 m deep. The deposit has a bulk
electrical conductivity between 13 mS/m and 25 mS/m consistent with low
cation exchange capacity values measured for the kaolinite. The palaeokarst
surface surrounding the deposit contains a series of sinuous, discontinuous,
depressions with a depth variations of several metres.