Robert J Scott HBSc thesis abstract

The Hemlo gold deposit, Ontario, Canada is located in the Archean Schreiber-Hemlo greenstone belt, Wawa subprovince, of the Superior Province.  The deposit has been subjected to a complex history of deformation and intense hydrothermal alteration.

The host lithology to most of the gold at Hemlo is the Moose Lake Prophyry Complex (MLPC), a felsic orthogneiss, characterized by the occurrence of quartz “eyes”.  These quartz “eyes” are deformed quartz porphyroclasts.  The matrix of the MLPC is mylonitic, composed of quartz, microcline, minor biotite and white mica.  The micas define the weak to strong foliation.  Other minerals present are calcite, plagioclase, and pyrite, with trace gold, titanite and tourmaline.  The porphyroclasts of quartz range in size from 1mm to 5mm, are typically elongated to form the characteristic eye shape and rarely appear together to with feldspar porphyroclasts.

Although common in association with gold deposits, quartz “eyes” are generally rare.  An objective of this thesis is to explain the occurrence of quartz porphyroclasts in a mylonite, quartzo-feldspathic matrix.  Microscopic observations reveal the brittle deformation and alteration of feldspar porphyroclasts in spatial association with ductile deformation of quartz porphyroclasts.  Although debatable, the protolith of MLPC is generally thought to be volcanic.  After detailed examination of samples collected from the MLPC, this study concludes a plutonic origin for the quartz porphyroclasts can not be ruled out.