Duality of magmatism at Kirkland Lake, Ontario, Canada.

Abstract

One of the world's most productive gold camps is located along the major east-trending Larder Lake fault (LLF), which extends approximately 250km from Val d'Or to Kirkland Lake. The western portion of the LLF consists of a 60 km long narrow (less than 5 km wide) and sinuous deformation zone commonly referred to as the Kirkland Lake-Larder Lake fault zone (KLF). The occurence of two distinct suites of magmatic rocks, syenitic and granitic, in and around the KLF indicates a magmatic duality. It is possible to identify three distinct magmatic domains defined by mode of emplacement, minerology and geochemistry. Mineral chemistry indicates that rocks from both suites evolved under oxidizing conditions in domains 1 and 3. Rare earth element compositions indicate a similar upper mantle source for both magmatic suites. Major element compositions confirm the shoshonitic nature of the syenitic suite, while the granitic suite is comparable to the sanukitoid suite (high-Mg,-K, -LILE andesites). Both the syenitic and granitic suites originate from an upper mantle source that was very probably modified a relatively short time before by fluids and/or melts derived from a subducted slab. The Kirkland Lake area is marked by an early transtensive phase when syenitic and granitic rocks were emplaced in extensional fractures associated to sinistral strike-slip faulting. Uplift of domain 3 and reverse faulting along the Kirkland Lake main break (KLMB) and the LLF are associated with a later transpressive phase; it was during this phase that gold deposits were formed. The structural, mineralogical and geochemical evidence fit in a tectomagmatic model where oblique convergence and accretion dominate.