Brian W. Nelson HBSc thesis abstract

The Big Duck Lake metavolcanic belt is an east - west trending, steeply north-dipping sequence of mafic and felsic intrusive and extrusive rocks.  Field mapping has revealed two distinct porphyritic felsic rock types.  They are pyroclastic porphyritic tuffaceous rocks and quartz feldspar porphyritic intrusive rocks.  The intrusive felsic rocks in the Big Duck Lake area occur as sill-like bodies and are generally massive except for foliated contact zones.  The extrusive units are comprised of laterally extensive fragment and crystal-rich beds, which have been selectively altered by hydrothermal solutions.  Field relationships, petrography and chemistry were used to compare the two felsic porphyries.  These studies indicate that the extrusive and intrusive phases are distinct.  On the microscopic scale the intrusive rocks exhibit primary crystallization textures displayed by gradational quartz and feldspar phenocryst reduction into the fine-grained groundmass.  The extrusive rocks display distinctive bimodal phenocryst and groundmass phases and are dominated by secondary metamorphic textures.  Whole rock and trace element analysis indicate that the intrusive rocks have more Sr and Al and less Zr than the extrusive rocks.  Zinc is preferentially concentrated in the tuffaceous rocks while higher values of Cu and Au are associated with the main intrusive sill.

However, similar concentrations of Pb, Ni, Co, Cr, Ti and P imply that both units are derived from the same parent magma.  A model based on intracontinental rifting is presented to show a possible origin for the bimodal volcanic activity.  This model also proposes that the synvolcanic intrusives are feeders for the extrusive felsic crystal tuffs.