Daniel LaFontaine MSc thesis abstract

Thesis Title: 
Structural and Metamorphic Control on the Borden Gold Deposit, Chapleau, Ontario

The Borden gold property is a multi-million-ounce deposit that is located 10 km east of Chapleau and 160 km southwest of Timmins.  It occurs within the Wawa Subprovince of the Archean Superior Province.  What is atypical about the Borden gold deposit is its location near the southern margin of the Kapuskasing Structural Zone (KSZ), a structurally controlled region of granulite and upper amphibolite facies metamorphic rock.  The deposit occurs within the Borden Lake Belt, an east- striking lithological assemblage, consisting of metasedimentary, felsic and mafic gneisses.  Gold mineralization is hosted by garnet-biotite gneiss (±sillimanite), amphibolite (±garnet) and deformed quartz veins.  Lithons of granulite facies rock are surrounded by foliated amphibolite facies gneisses and schists.  Structure and microstructure indicate polymetamorphism with retrograde amphibolite facies metamorphism after granulite facies metamorphism.  Garnet-biotite geothermometry based on the composition of unzoned almandine garnet, matrix biotite in the rock and biotite inclusions in garnet yields temperatures ranging from 411C to 933C 50C for metamorphism of the garnet-biotite schist at the Discovery Outcrop.  Although garnet compositions are fairly consistent, biotite compositions vary from inclusions in garnet cores to inclusions in garnet rims and to matrix biotite, yielding temperatures that increase from the garnet core towards the rim, recording prograde metamorphism from the upper amphibolite to granulite facies during garnet growth.  

To compliment metamorphic parameters and establish metamorphic geochronology, Lu-Hf geochronology of garnet was conducted.  Results suggest that peak granulite facies metamorphism associated with garnet growth took place at ca. 2629 Ma, consistent with earlier estimates of the age of granulite facies metamorphism.

Competency contrasts between the granulite and retrograde amphibolite facies lithologies created heterogeneous strain, ideal for gold mineralization, during ductile deformation at amphibolite facies metamorphic temperatures.  On the macroscopic scale, the relict granulite facies lithons behaved more competently than the reaction-softened retrograde amphibolite.  On the microscopic scale, competent relict orthopyroxene, garnet and pyrite provided an adjacent low-strain site for gold mineralization.  Gold is typically observed in competent lithologies with weakly developed foliation and also in competent units that are bordered by strongly foliated units.  Retrograde metamorphism is critical to the structural control of mineralization at this deposit.  Results indicate an important relationship between gold mineralization, retrograde metamorphism and deformation.  Understanding this relationship will benefit further exploration and development of the Borden gold deposit.