ABSTRACT Till sampling near the Lac des Iles Pd mine north of Thunder Bay, Ontario, Canada has led to the identification of heavy mineral dispersal trains in till as unique as the local Pd deposits. The till at Lac des Iles was deposited by ice actively flowing in a 240° direction. It is commonly thin and ideal for exploration sampling.
Several dispersal trains of chrome-andradite garnet and chromite occur within the area surrounding the Mine Block Intrusion (MBI). Cr-andradite garnet is normally a hybrid, assimilation-type indicator mineral; however, here the Cr-andradite is of a low-temperature, cryptocrystalline form, suggesting that hydrothermal alteration, possibly associated with the Lac des Iles late-stage mineralizing event, may have played a significant role in its formation. A major Cr-andradite + chromite dispersal train occurs over the MBI. It is a narrow, ribbon-shaped, composite train that trends 240°, is c. 600 m in width and exceeds 5 km in length. Its farthest up-ice source appears to be located to the NW of the margin of the MBI, in rocks of the Northern Lac des Iles Intrusion (NLDI-I). The distal part of the dispersal train SW of the MBI may contain additional indicator mineral grains from a septa of ultramafic rocks associated with the North Lac des Iles intrusion that lies west of the MBI. The accompanying chromite dispersal trains are shorter and appear to be derived from similar source areas as well as an area in the vicinity of the Baker zone.
The indicator mineral contribution of the main Roby (ore) zone is difficult to assess due to a sampling gap but appears to be minimal, probably because this zone occurs in the interior of the intrusion and is hosted mainly by Cr2O3-poor gabbro. The only PGM are rare grains of Pt and Pd arsenides, which are uncommon in the mineralized zones but are stable in near-surface till.
- Received April 15, 2009.
- Accepted July 21, 2009.
- © 2010 AAG/Geological Society of London