DETRITAL ZIRCON GEOCHRONOLOGY OF THE URANIFEROUS PALEOPROTEROZOIC MATINENDA FORMATION (HURONIAN SUPERGROUP), ELLIOT LAKE, ONTARIO

From 1956 to 1996, 12 uranium mines operated at Elliot Lake. The ore was hosted in quartz pebble conglomerate beds in the Matinenda Fm near the base of the Huronian Sgp. The Matinenda Fm is composed almost entirely of trough and planar cross-stratified coarse-grained sandstones deposited in a braded fluvial system. Many have suggested a paleoplacer origin for the mineralization but the location of the source region has been problematic. To understand its provenance, 4 Matinenda sandstones were collected from a 180 m thick section of the formation exposed west of Pecors Lake on the south limb of the Quirke Lake syncline. Two samples are from the Ryan Mbr 5 m below and 7 m above the mineralized Main Conglomerate Bed (MCB). One is from the lower Stinson Mbr ~75 m above the MCB. The fourth is from the uppermost Stinson Mbr ~10 m below the contact with the McKim Fm. U-Pb isotopic analyses were carried out by laser ablation multi-collector inductively coupled plasma mass spectrometry at the University of Alberta.

The Ryan Mbr samples give near identical weighted average ²⁰⁷Pb/²⁰⁶Pb ages of 2648.9±4.3 (MSWD=2.1, n=100) and 2648.5±6.3 (MSWD=2.2, n=118), respectively. The youngest concordant grains are 2620 and 2634; the oldest are 2776 and 2684 Ma, respectively. The lowermost sample contained 4 concordant grains (2702 to 2776 Ma). The sample overlying the MCB contained many discordant grains, perhaps due to metamict effects from the mineralized MCB. The lower Stinston Mbr sample has a weighted average ²⁰⁷Pb/²⁰⁶Pb age of 2638.5±5.2 (MSWD=0.55, n=72), but with younger grains (2546 Ma) and 3 grains with an upper intercept age of 2836±31 Ma. The stratigraphically highest sample has peaks at 2657 and 2708, suggesting a more diverse provenance. These results are consistent with ages of 2687±2 Ma age for felsic metavolcanic rocks at Elliot Lake and ages of 2620 to 2670 Ma from Archean granitic rocks to the NE. Geochemistry of the sandstones also suggests a local, felsic dominated source. The ages are younger than those typical of the Abitibi or Wawa greenstone belts (>2690 Ma), some 400 km distant, and contrast with those from other Huronian Sgp sandstones which have a provenance of mixed <2700 Ma and >2700 Ma populations. Local sourcing of the Matinenda means that the character of the adjacent Archean basement is relevant to exploration.