Wollastonite skarn mineralization and associated plutonic rocks in the Grenvillian Central Metasedimentary Belt, SE Ontario

Potentially economic wollastonite-rich skarns are widespread in the Grenvillian Central Metasedimentary Belt. Their genesis is evaluated through a comparative study of: the Platinova-Cominco (Deloro) deposit (ca. 4 Mt. grading 35% wollastonite), hosted by greenschist facies marbles in the Elzevir terrane; the Hawley (Mountain Grove) deposit (ca. 2.8 Mt.; 35%) in amphibolite facies marbles of the Sharbot Lake terrane: and the St. Lawrence (Seeley's Bay) deposit (ca. 9 Mt.; 40%) in granulite facies metasediments of the Frontenac terrane.;Wollastonite is associated with varying proportions of diopside - hedenbergite - johannsenite and grossular - andradite. In addition, the St. Lawrence skarn contains abundant plagioclase (An<21), microcline, quartz and titanite.;There is no correlation between wollastonite occurrences and regional metamorphic grade, and skarns do not exhibit the mineralogical zonation to be expected in a contact metamorphic environment. Instead, exoskarn and, at Hawley, endoskarn development is ascribed to the incursion of magmatogene, silica-rich, CO2-poor (XCO2 between 0.1 and 0.3) fluids at temperatures of ca. 500-to-650°C, and under pressures ranging from 200 MPa at Platinova-Cominco to 450 MPa at Hawley.;A comprehensive mineralogical study, including petrographic and chemical analyses, of the wollastonite indicates that all three prospects would yield high-purity concentrates with high aspect-ratios, and acceptable Fe and Mn contents for industrial uses.;The Deloro pluton (1241 +/- 2 Ma: U-Pb on zircon) consists predominantly of hypersolvus granite, but gabbro and syenite are exposed at its western margin. The Mountain Grove pluton varies from gabbro to granite and yields a crystallization age of 1153 +/- 2 Ma (U-Pb on zircon). However, 40Ar-39Ar dates for hornblende and biotite from the pluton are 1058 +/- 14 Ma and 1047 +/- 4 Ma, respectively, and phlogopite from the Hawley skarn yields a 1074 +/- 5 Ma date. The amphibole from the Mountain Grove pluton may indicate cooling at a rate of ca. <2°C/m.y. following emplacement. Phlogopite from the skarn yields an age comparable to that of the biotite in the pluton.;Whereas the Platinova-Cominco and Hawley skarns are similar in all salient respects to the Valentine exoskarn in the Adirondack Lowlands, and the St. Lawrence deposit is analogous to the Willsboro district skarns of the Adirondack Highlands, the Ontarian skarns are apparently smaller and/or of lower grade than those in New York State. (Abstract shortened by UMI.).