The petrology of the metavolcanic rocks of the Rusty Lake area, Manitoba

Thumbnail Image
Steeves, Michael Albert
Journal Title
Journal ISSN
Volume Title
A study of the mineralogical and chemical properties of the metavolcanic rocks of the Rusty Lake greenstone belt was accomplished by means of sampling on a 1000 foot grid. In total, this greenstone belt is exposed over more than 250 square miles. Metavolcanic rocks constitute about 40 per cent of this belt with the remainder being made up of pyroclastic rocks, volcanic wackes and volcanic siltstones. Of the samples collected, 44 were submitted for chemical (major element) analyses. Based on field observations and the mineralogical and chemical composition of these metavolcanic rocks, the following conclusions were drawn: 1) The order of decreasing abundance in the volcanic pile, which consists of two separate volcanic sequences, is balsalt - andesite - picrite - dacite - rhyodacite - rhyolite. 2) There is no chemical difference between the two separate volcanic sequences except that there are no acid volcanic rocks associated with the upper volcanic sequence. 3) The metavolcanic rocks are chemically similar across the whole of the Rusty Lake greenstone belt and, in general, exhibit a high-alumina to tholeiitic character. 4) Field observations of preserved primary structures suggest that the major part of the volcanic pile was extruded in a shallow marine or subaerial environment. 5) The metavolcanic rocks of the Rusty Lake greenstone belt show six distinctive chemical characteristics: (a) a high and extremely variable Al2O3 content, (b) a low TiO2 content, (c) high FeO/Fe2O3 ratios, (d) a high CaO content with a corresponding low MgO content, (e) a low alkali content, especially K2O and (f) a high volatile (CO2 and H2O) content. 6) The intrusive rocks in the Rusty Lake area show only minor differences in chemical and mineralogical composition to their respective extrusive equivalents.