EVIDENCE FOR THE PETROGENESIS OF A CALCITE VEIN-DYKE COMPLEX, BEAR LAKE DIGGINGS, MONMOUTH TOWNSHIP, ONTARIO, CANADA

Northwest-trending calcite vein-dykes near Bear Lake, Ontario are up to 1.5 meters wide and 120 meters long and are cored by pegmatitic, pink calcite with 1-5% sub-parallel green to red euhedral apatite crystals up to 0.5 meters long. Rare instances of monazite on apatite have been reported. The vein-dyke selvages are composed of pegmatitic bytownite, edenite, biotite, titanite and apatite. Occurrences of edenite and biotite vary across as well as along strike in several of the veins. Isolated crystals of the silicate minerals can also be found within the calcite cores. The vein-dykes, intruded into an amphibolite gneiss, crosscut local gneissic foliation.

A primary mantle origin for the vein-dykes is unlikely due to their lack of enrichment in Nb, Ti, Ba, Th and Zr, and, only moderate LREE enrichment ((La/Lu)cn=11-15)(cf. Bell, 1989). New major and trace element data collected during this study from metamorphosed carbonatite outcrops sampled by Moecher et al. (1997) can be used to distinguish the Bear Lake calcite vein-dykes from the meta-carbonatites. LREE enrichment is similar for both units; however, the Bear Lake vein-dykes exhibit a distinct negative Europium anomaly as well as HREE enrichment.

Calcite vein-dykes exposed in a road cut along Highway 121, 13 km east of Bear Lake, are mineralogically and chemically identical to the vein-dykes at Bear Lake. This occurrence significantly enlarges the outcrop area of the vein-dyke complex. The vein-dykes in the Highway 121 road cut have intruded across local siliciclastic and carbonate metasedimentary units and younger granitic pegmatites. The calcite vein-dykes are in intrusive contact with, and have metasomatized one of three metasedimentary marble units based on major, trace and REE data. The other two marble units in the road cut have not been metasomatized, are geochemically similar to other regional marbles, and are compositionally distinct from the vein-dykes. The data from this study preclude the petrogenesis of the vein-dykes by simple anatexis of regional marbles. The data, however, are consistent with a model of volatile fluxing and metasediment syntexis as proposed by Lentz (1998, 1999) for other calcite vein-dyke complexes in the Central Metasedimentary Belt of Ontario.