REFINING THE HISTORY OF THE MATACHEWAN LIP EVENT AND SUBSEQUENT PALEOPROTEROZOIC ALTERATION EVENTS WITH HIGH-PRECISION U-PB GEOCHRONOLOGY
In spring 2015, International Montoro Resources Inc. drilled 2 deep (PDH-1: 996m and PDH-2: 1317m) drill holes through the lower Huronian Supergroup into a 3.5km x 1.5km aeromagnetic high (Pecors Magnetic anomaly) located ~15km east of Elliot Lake. Both drill holes intersected a medium-grained, homogeneous, non-layered gabbro intrusion > 680m thick. The entire intrusion is altered to an albite-epidote-chlorite rock with large (~1 mm) titanite grains. Macroscopic examination of the drill core was unable to relate the gabbro to any Matachewan LIP rock or Nipissing intrusion (ca. 2.2 Ga). Small (<30µm) baddeleyite grains rimmed by zircon found in samples from the upper 50m of drill hole PDH-1 were selected for U-Pb analysis. All 7 analyzed baddeleyite grains define an age of 2475±14 Ma, with the 4 most concordant data giving an age of 2480±5 Ma. The data indicate a significant Pb loss event after ca. 1.8 Ga; perhaps as young as ca. 1 Ga. These data indicate that the gabbro is part of the Matachewan LIP, and geochemical analyses of the gabbro indicate a near identical chemistry to the Matachewan dikes, not the East Bull Lake intrusive suite.
To better constrain the timing of the alteration event that affected the gabbro, an albitite dike that cuts the Matinenda Formation (Huronian Supergroup) above the roof of the intrusion, was also studied. Zircons from the dike are ca. 2660 Ma, consistent with inheritance from the host Matinenda Formation. A near-concordant monazite grain is 1966±18 Ma, with 3 other grains defining a Pb loss line towards 0 Ma. The 1.966 Ga age does not correspond to any known regional event, whereas the circa 1.8 Ga time of Pb loss in the gabbro is coincident with the peak of the Penokean orogeny (ca. 1.835 Ga) and the Sudbury impact event (1.85 Ga). The U-Pb and geochemical data indicate that the gabbro is a subsurface magma chamber for part of the Matachewan dike swarm. More work is needed to better constrain the post-emplacement alteration history of the intrusion and adjacent rocks.