Chanelle Boucher HBSc thesis abstract

The Coldwell Alkali Complex is a Proterozoic intrusion emplaced within the Archean Schreiber-Hemlo Greenstone belt in northern Ontario.  The rocks that comprise the greenstone belt include metavolcanic and metasedimentary rocks, as well as granitic intrusions.  Along the northwestern contact of the complex, the Archean host rocks have been altered in a manner not normally seen throughout the complex.  This study analyzed sandstones, felsic pyroclastic rocks, iron formation, lamprophyres and a gabbroic intrusion in order to characterize the mode of occurrence of alteration present and determine a potential origin.  The alteration types include silicification, sericitization, tourmaline alteration and hematite alteration.

Trenches were created in order to produce exposure of the units that host the alteration.  Hematite alteration has created a distinct reddening of the felsic pyroclastic units present, and displays great variability in its mode of occurrence and alteration intensity.  Although hematite alteration is most intense in the felsic units within the trench area, the alteration occurs throughout the area and affects all intrusive units present.  Distal sandstone samples taken as control units did not exhibit pervasive alteration.  The mode of occurrence of the hematite is consistently pervasive into the crystalline lattice of quartz and feldspar grains, and it is concentrated along quartz veins, within fractures, and forms as replacement of Fe-sulphides.  There is a weak correlation to iron concentration corresponding to areas of more intense hematite alteration.

The dominant lithology is a felsic tuff unit consisting of a quartz and feldspar dominated groundmass with varying intensity of pervasive hematite alteration.  There is a high incorporation of volcanic ash within the iron formation unit, and thus it has acquired the geochemical signature of the felsic tuff unit.  This felsic tuff is defined by U-Pb zircon geochronology that represents a Neoarchean zircon population that indicates an age of roughly 2708 ± 23Ma.