PROTEROZOIC AND PHANEROZOIC GEOLOGIC HISTORY OF THE COBDEN AREA, GRENVILLE OROGEN, ONTARIO, BASED ON NEW GEOLOGICAL AND GEOPHYSICAL DATA

The Cobden area lies approximately 80 km NNW of Ottawa and straddles the Central Metasedimentary Belt tectonic zone (CMBtz) of the Grenville Orogen in Ontario. New 1:50,000 scale mapping and new high-resolution (200-m line spacing) aeromagnetic data shown in this presentation aid in mineral resource assessment and indicate a more complex geological history than previously thought. Although the presence of several northwest-trending subvertical faults that result in the local preservation of middle Ordovician limestone in the area was previously known, a major north-trending fault, the Ross fault, was previously unrecognized. The Ross fault divides the Cobden area into 2 roughly equal-sized domains: 1) an eastern domain underlain by lower amphibolite facies calcite and dolomite marbles with relic primary features cut by the ca. 1231 Ma Chenaux gabbro; and 2) a western domain underlain by a complex of marble tectonic breccia and ca. 1030 Ma syenite intrusions and metasomatic rocks, that abut and intrude thin-layered, highly strained upper amphibolite gneisses of the CMBtz. Older low-resolution (800-m line spacing) aeromagnetic data suggested that the Ross fault was near vertical; however, the high-resolution data show that the Ross fault is a shallow, east-dipping thrust fault. The Ottawa River is also oriented N-S rather than WNW in the Cobden area, but does not directly follow any known faults. The new aeromagnetic and geological data also indicate that upper amphibolite facies gneisses along the Ottawa River, including the host rocks to the former Calumet Zn (Pb-Ag-Au) mine, are in thrust contact with the underlying eastern domain marbles. The new aeromagnetic data also reveal the presence of 3 main west-trending zones, spaced at roughly 10 km intervals, of Grenville swarm (ca. 590 Ma) diabase dikes. Most of these dikes occur beneath middle Ordovician limestone. Implications for mineral exploration are a) exploration for high-quality marble is restricted to the eastern domain, b) exploration for U-Th-REE mineralization is restricted to the western domain, and c) host rocks for the former Calumet mine have limited extent in Ontario, although the presence of cordierite in these rocks suggests that the deposit may result from hydrothermal alteration in a VMS-environment, rather than a SEDEX-system.