A gravity study in the Amisk Lake area, Saskatchewan
Date
1968
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Degree Level
Doctoral
Abstract
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.
Description
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Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Geological Sciences
Program
Geological Sciences