Iain S. Kelso HBSc thesis abstract

The agate deposit at the TBAM is located 1 km northeast of Thunder Bay, Ontario in ferrodolomite of the Gunflint Formation.  The age of the agate deposit is delimited by the age of The Gunflint Formation and the age of the Logan sills which crown the Gunflint sediments to the west of the deposit.  The agate shows a consistent sequence of mineralization from nucleation against the host-host: dark amorphous silica (opal-A?), light amorphous silica (opal-CT?), translucent chalcedony, a-quartz, and vug-filling graphitic carbon.  The deposition of the agate appears to be related to pervasive, early silicification of the Gunflint and other Lake Superior-type iron formations.  Siliceous solutions from thermal springs located near the centre of the depositional basin were expelled to the margins by compaction.  At the site of agate deposition, the solutions encountered CO₂ from decaying organic matter.  The presence of CO₂ and organic material would create a pocket of decreased pH and temperature.  δ¹³C valves for vug-filling graphitic carbon show it is organic in origin.  Geochemical modeling demonstrates that the sequence of mineralization observed at the TBAM will precipitate from a solution in equilibrium with dolomite and atmospheric CO₂ as it experiences a decline in pH and temperature.