A gravity study in the Amisk Lake area, Saskatchewan

A gravity and density study of a Precambrian greenstone belt has been made near Flin Flon in the Amisk Lake area of Saskatchewan. Approximately 800 gravity stations were read in the three hundred square mile area and 1585 density determinations were made on bedrock samples taken from a one hundred square mile portion of the area. The gravity field and rock densities are strongly correlated with the different rocks which underlie the region. The density variations associated with the outcropping rock units need extend to various depths less than five kilometers to account for all the gravity relief (29 milligals) observed in the area. Below these depths lies a fairly uniform, low density rock with composition possibly approaching that of granite. Surface seismic measurements and well log velocity and density measurements also indicate the existence of low density material at depth. Three new interpretation techniques are presented for the quantitative interpretation of gravity anomalies associated with outcropping rock units. The gravity effect of a right rectangular prism is shown to be directly proportional to its width. This fact has been used to develop a set of characteristic curves which permit the interpretation of the depth to the bottom of such a prism whose top is at zero depth. The expression for the gravity effect of the sloping step model has been used to generate characteristic curves for that model whose top is at zero depth. A mathematical expression has been developed for the gravity effect of a gradational density contrast between two semi-infinite slabs. Type curves and interpretation charts are presented for this model. Fortran IV computer programs which were used in the computations are listed in an appendix. Rock densities were found to depend almost entirely on their mineral content. Porosity of these igneous and metamorphic rocks is negligible and, to the depths considered, pressure and temperature effects are also negligible. The lightest rock is the Phantom Lake granite (2.656 ± .017 gm/cc) and the heaviest is the Ruth Lake meta-gabbro (3.042 ± .066 gm/cc). The mean density of all the samples is 2.862 gm/cc. The gravity interpretation shows the Reynard Lake granodioritic pluton to be sheet-like in its southern part, the depth to the bottom being about 1000 meters in some places. The northern part of the Mystic Lake granodioritic pluton may be even shallower. The sheet-like portions of the plutons are underlain by basic volcanic or intrusive rock which is in turn underlain by the low density, possibly granitic, rock at depth. Several large basic and ultrabasic dikes and sills lie in a down buckling of Amisk volcanic rocks east of Amisk Lake. This assemblage extends to a maximum depth of almost five kilometers in the southern part of the area and somewhat lesser depths to the north. Elsewhere, rocks of the Amisk group are about three kilometers deep. Several rock units not recognized in the detailed geological mapping have been indicated by the geophysical work. These include a possible fine grained basic intrusion between Amisk and Mosher Lakes, an acidic intrusion near Denare Beach, a basic intrusion under Comeback Bay and one or more large basic to ultrabasic intrusions extending more than ten miles in the center of Amisk Lake. Some of these interpretations may be interesting from an economic standpoint since both basic intrusions and granodiorite bodies have been thought to be related to ore deposits. In particular, the underside of the sheet-like portions of the granodiorites may be shallow enough to explore with detailed geophysical work and drilling. Also, the existence of previously unrecognized basic intrusions should encourage exploration of these areas with detailed geological and geophysical work and drilling.

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