Download Project 2003-02A : Gold in high-grade metamorphic rocks There are

 Project 2003-02A : Gold in high-grade metamorphic rocks
There are very few gold deposits in high-grade metamorphic rocks in Quebec. Nevertheless,
these rocks cover a large part of Quebec. This study aims to evaluate the potential of these
terrains for gold mineralization. To this end, the different types of gold mineralization which may
be found in these rocks must be described from known examples around the world.
Hypozonal orogenic gold deposits form under high metamorphic conditions (T> 475 ° C), which
are often in equilibrium with the metamorphic conditions of the host rock or are slightly retrograde.
They are characterized by high temperature alteration minerals (biotite-garnet-pyroxene), by an
assemblage of metallic minerals with pyrrhotite arsenopyrite-loellingite-chalcopyrite, a ductile
structural control and host rocks that are mostly limited to iron-rich lithologies (amphibolites, BIF,
ultramafic rocks, diorites). These ore deposits are formed during high-grade regional
metamorphism of their host rocks. The most important examples outside of Canada are found in
Australia, India, Greenland, Zimbabwe and South Africa.
Mesozonal orogenic gold deposits are clearly retrograde relative to their highly metamorphosed
host rocks. These deposits are characterized by alterations compatible with the greenschist
facies (sericite-ankerite-chlorite-albite) and show a ductile-brittle to brittle structural control. Host
rocks are very varied (orthogneiss, paragneiss and granite). The metallic minerals associated
with the gold are mainly pyrite, arsenopyrite and chalcopyrite. They typically form during the late
stages of orogenic episodes that are characterized by extensional tectonics and the exhumation
of deep rocks. They usually have very similar mineralogical characteristics to mesozonal orogenic
deposits of the Abitibi. The most important examples are located in China, Brazil, Alaska and
France.
There are also some deposits that have very specific and atypical characteristics, or are still
poorly understood. Hemlo, Big Bell, Puffy Lake, and Challenger deposit are all in this category.
An examination of these deposits yields three models that can be used as exploration tools in
Quebec. These models are relevant in the following geological environments:
1. The highly metamorphosed Archean greenstone belts of northern Quebec;
2. The Proterozoic continental magmatic arcs;
3. Regions where major thrust fault fronts superimpose highly metamorphosed rocks over
slightly metamorphosed rocks.