Post by
diabase1 on Sep 08, 2023 7:08am
Kendrey Colquhoun 2
The colour filled contours of the residual magnetics (Map 82813), reveals the characteristics of a 'carbonatite-like' magnetic anomaly quite clearly. It exhibits a magnetic intensity, from low to high, of approximately 2400 nT. This compares favourably with other interpreted carbonatites (Map P.0452). Towards the east, there also exists a 2 bulls-eye north-south trending magnetic high, which may also be of interest. It is not as strong as the Kendrey-Colquhoun anomaly, but could be related to either a feldspar porphyry or a gabbro.
In referring to Map 82824, the Shaded colour image of the second vertical derivative of the residual magnetic field and Keating coefficients, I can see an oval-shaped east-west structure that is at least 10 km long, with the Kendrey-Colquhoun anomaly being located at the extreme west end. There is an obvious lower magnetic gap in the middle, that I attribute to a downward block fault (graben). This may have something to do with the previously mentioned Lake Timiskaming Rift Valley, where block faulting is known to be pervasive along its belt. The eastern edge of the Kendrey-Colquhoun magnetic anomaly also appears to be cut off, indicating a NNW/SSE fault zone. The eastern portion of this large 10 km structure remains intact.
The spectrometer data (Map 82835) clearly shows an anomaly that closely correlates with the Kendrey-Colquhoun magnetic anomaly. It shows a radiometric anomaly that appears to have influenced, or may have offset, a north-south linear trending potassium anomaly. Could an east-west fault, with a left-handed movement, be the cause? With respect to the radiometric anomaly itself, there appears to be a potassium (fenitic?) rim, along with a thorium/uranium core. The latter would seem to be some of the characteristics of a rare earth carbonatite (niobium?).
I noticed that the 2 magnetic highs to the east, that were mentioned earlier, does not have any radiometric anomalous signatures.
Since there is a lack of penetration with the spectrometer equipment, it is puzzling that there is an apparent radiometric anomaly over the Kendrey-Colquhoun magnetic anomaly, especially when there is a 70 foot cover of overburden ( sand and gravel). One explanation could be that the radioactive ions (thorium, uranium and potassium) have migrated vertically, sufficiently enough to reach surface. If that was the case, then the spectrometer equipment may be able to detect the accumulated radioactive ions on surface. If this is the case, then a MMI soil geochemical survey (Mobile Metal Ion) over the large magnetic anomaly may be useful.
Gravity would be a good geophysical tool to assist with the identification of a carbonatite. Knowing what the differences are in the density of the various rock types can be very helpful in the interpretation. In general, carbonatites have distinctive gravity, magnetic, and radiometric signatures because these deposits are relatively dense, have primary magnetite, and are enriched in uranium and thorium. I have no knowledge of any gravity surveys having been carried out over the Kendrey-Colquhoun magnetic anomaly, by either the Ontario Geological Survey (Map 2598), or by any company.