CALCITE VEIN-DYKE COMPLEXES IN THE BANCROFT, ONTARIO REGION: EVIDENCE FOR A 930 MA CARBONATITIC INTRUSIVE EVENT IN THE GRENVILLE PROVINCE, ONTARIO

Geochemical analyses of several calcite vein-dyke complexes in the Bancroft, Ontario region, indicate that the complexes are compositionally similar. Samples were collected from the Bear Lake Diggings, Gibson Road Occurrence, Richardson-Fission Mine, Dwyer Occurrence, Silver Crater Mine, Fluor-richterite Occurrence, Davis Hill and the Smart Mine at Lake Clear, Renfrew County. Chondrite normalized LREE enrichments of 1000 - 3000 are similar to normal sovite enrichments (cf. Bell, 1989). The La/Lu ratios however range only from 10 to 20 due to the enrichment of the HREE. In addition, all samples exhibit moderate Eu anomalies. The strong enrichment of trace elements such as Ba, Th, and Zr normally found in sovites is not present in the sampled vein-dykes (< 10 ppm).

Mineralogically the vein-dykes are cored by coarsely crystalline pink calcite and abundant euhedral red to green apatite crystals up to 40 cm in length. The margins of the vein-dykes may contain large eudhedral crystals of biotite, edenite, titanite, and perthite. Numerous trace phases such as monazite may also occur. The Richardson-Fission Mine and Dwyer Occurrence are distinctive due uranium mineralization and ubiquitous massive purple fluorite stringers (cf. Lentz, 1998). The vein-dykes commonly intrude amphibolite gneisses and cross-cut the gneissic foliation. The Davis Hill vein-dyke complex intrudes and cross-cuts poorly foliated nepheline syenite.

New ⁴⁰Ar/³⁹Ar plateau age dates on biotite from the Bear Lake Diggings and a similar vein-dyke complex 13 km northeast indicate that these vein-dykes formed 929 (± 7) Ma. These dates document a previously unrecorded alkaline intrusive event in the Grenville Province. Formation of these calcite vein-dyke complexes may be associated with extension at the end of the Grenville orogeny.