LATE GRENVILLE METAMORPHIC AND SHOSHONITE-RELATED CALCITE-APATITE VEIN FORMATION NEAR ESSONVILLE AND LAKE CLEAR, ONTARIO, CANADA. ANALOGUES FOR LARGE-SCALE CALCITE-APATITE VEIN-DYKE FORMATION IN THE BANCROFT TERRANE OF THE GRENVILLE PROVINCE?

Recent fieldwork along Highway 4 near Essonville, Haliburton County and Opeongo Road, south of Lake Clear, Renfrew County provides evidence for both hydrothermal/metasomatic and synmagmatic origins of late Grenville-aged calcite-apatite veins. The petrogenesis of large-scale, calcite-apatite vein-dyke systems in the Bancroft Terrane (e.g. Bear Lake Diggings, Silver Crater Mine, and Smart Mine) has been inferred to be either carbonatitic in origin (cf. Moecher, et al, 1997; Lumbers and Vertolli, 1998; Mills, et al., 2003) or of a hydrothermal/metasomatic origin (cf. Lentz, 1998). At Essonville and Opeongo Road, metasomatized marble units grade into irregular pegmatitic carbonate veins ranging in size from a few centimeters to a meter in width consisting of pink to orange calcite. Idioblastic green apatite crystals up to four centimeters in length are commonly found within the calcite veins. Other minerals associated with the calcite veins are dark-green pyroxene, perthitic feldspar, +/-biotite, and pyrite. Near Essonville, the calcite veins appear to be related to hydrothermal segregation of adjacent metasomatized marbles with some of the calcite being injected into 10-15 centimeter, sub-vertical tension gashes in adjacent gneisses. At Opeongo Road, calcite veins are related to both metasomatism of marble similar to the Essonville Road outcrop and to shoshonitic dike intrusion. Childe, et al., 2000 infer that the origin of some of the carbonate veins is synmagmatic with a ca. 1156 Ma shoshonitic magmatic event in the Lake Clear region. Within the sub-meter wide shoshonitic dikes, synmagmatic calcite veins are present ranging from grey at the outer contact to deep orange in the vein interior. The synmagmatic veins contain euhedral apatite, pyroxene, perthitic feldspar, and pyrite. While potassic magmatism may be responsible for calcite-apatite vein-dykes such as those at the Smart Mine north of Lake Clear, the absence of shoshonitic magmatism in the western Bancroft terrane implies that the formation of large-scale calcite-apatite vein-dykes is most likely related to either late Grenville hydrothermal/metasomatic activity or intrusion of carbonatitic magma. Forthcoming geochemical data from Essonville Road and Opeongo Road outcrops will be used to test these petrogenetic models.