HETEROGENEITY IN THE ARCHETYPE OF MESOTHERMAL DEPOSITS IN THE VAL D’OR MINING DISTRICT, QUEBEC, CANADA: MINERALOGICAL ASSOCIATION OF TOURMALINE-PYRITE

The Abitibi subprovince in the Superior Province hosts one of the largest gold concentration on the world. Within this subprovince, the Val d’Or mining district (Quebec, Canada) hosts several major gold deposits, on Cadillac Larder Lake deformation corridor section. The mineralization is hosted by quartz-tourmaline veins. This type of deposit has been largely described and studied. Previous models defined the quartz-tourmaline veins as part of one vein field from one hydrothermal event on the basis of the hydrothermal and structural characteristics (mineralogical assemblage, hydrothermal alteration, no deformation). This district is considered as an archetype for the mesothermal gold deposit.

Although belonging to the same family, the deposits display some difference. Samples from the Goldex, Lamaque, Lac Herbin and Beaufor mines were investigated, using a new methodology and mineralogical approach (monocrystal density and size fraction separation). Crystal fractions are analyzed with the binocular and a SEM-EDX, XRF and XRD techniques. LA-ICP-MS analyse has been run on 63 tourmalines.

The four mines display a similar mineralogical association of pyrite-tourmaline in quartz-tourmaline veins. However, several variations have been observed. Native gold is mostly associated with large pyrites with cubic shape in the < 100 μm size-fractions. Tourmaline displays three types: schorl, uvite and dravite. They are associated with specific size-fractions and pyrite habits. Each specific associations could be associated to a phases of mineralization. First, the crystallization of dravite-cubic gold-bearing pyrite association from a Mg-rich solution, then uvite-pyritohedron pyrite from Ca-Mg-rich fluid and last the schorl-shapeless pyrite from a Fe-rich solution. Each phase of mineralization, linked to different fluids, takes place in three distinct events from a Mg-rich to Fe-rich fluid. The gold mineralization occurred during the first phase with some Dravite in the Mg-rich fluid.

Nevertheless, this heterogeneity between the samples supposedly from the same hydrothermal event allows to call this hypothesis into question. This variability is also supported by Re-Os datings on gold showing different age. The archetypal Val d’Or orogenic golds district is therefore polyphased.