GRAVITATIONAL AND MAGNETOMETRIC SURVEYS OF THE ONAKAWANA LIGNITE AND GRAND RAPIDS SIDERITE DEPOSITS

Abstract

A traverse about three miles in length was made with a torsion balance over the lignite deposit at Onakawana in northern Ontario, where it has been outlined by extensive drilling over an area of a few square miles. The drilling shows that it is overlain with 50 to 100 ft. of boulder clay and is imbedded in Cretaceous clay of considerably greater density than the coal. The traverse crossed the seam in three places. At each crossing a definite lowering in the value of gravity was observed. The boundary of the deposit was shown along two parts of the traverse by the large and definite effect on the gradient of gravity. Owing to the fact that as a rule the stations were as far as 100 metres apart and that the drill holes were usually 1000 ft. or more from one another, uncertainty exists at some stations regarding the respective gravitational effects of variations of the coal seam, on the one hand, and possible irregularities in the boulder clay, on the other. The results indicate that the torsion balance may be of use in outlining such deposits as the Onakawana lignite. A better estimate of the value of the instrument could be made if it had been possible to make a more definite test of the effect of irregularities in the boulder clay.Numerous magnetometer readings over and in the vicinity of the coal seam showed so little variation that there appears to be little hope that this instrument would be of use in prospecting for the coal, at least in so far as the immediate boundaries are concerned.A preliminary survey of the Grand Rapids siderite deposit on the Mattagami river was made with vertical and horizontal magnetometers. The iron occurs in limestone which, except at the river, is buried beneath 50 ft. or more of drift. The anomaly produced by the iron does not apparently exceed 100 γ and is complicated by a larger and more regional anomaly which makes definite interpretation of the results difficult. A more detailed survey might be productive of useful results. Magnetometric traverses were made along the rivers from Grand Rapids to James Bay and from Williams Island in the Abitibi to its mouth. A similar magnetic anomaly to that at Grand Rapids was observed at Williams Island. There is quite a marked magnetic anomaly at Moose Factory.