Sulphur isotopic studies of Cu-PGE mineralization in the Coldwell Complex, Ontario: Implications for mineralizing processes

English Abstract:

The Marathon and Geordie Lake Cu-PGE deposits are hosted by the Two Duck Lake (TDLG) and Geordie Lake (GLG) gabbros of the Coldwell alkaline complex, respectively. The TDLG intruded Archean metavolcanic rocks and the GLG occurs within syenite of the Coldwell Complex. The role of sulphur contamination from country rocks in the genesis of these two deposits is a point of debate.

All sulphide δ³⁴S (V-CDT) values for both Marathon and Geordie Lake mineralization fall between -2.0 and +2.0 ‰. There are no significant or consistent differences between different minerals at Marathon, although slight differences exist between some textural variants. Py from the country rocks at Marathon exhibits a wide range of δ³⁴S with an average of 0.2 ‰. At Geordie Lake, Po exhibits a distinctly lower range of δ³⁴S values (<-1 ‰) than other sulphides. At Marathon, there is no systematic variation in δ³⁴S with distance from the Archean country rocks, however, δ³⁴S values of Cpy from mineralized samples at Geordie Lake decrease with increasing distance from the syenite.

Analyses of multiple sulfur isotopes of representative samples from mineralized zones in the Marathon deposit show evidence of mass-independent fractionation, with negative Δ³³S values for most samples (total range = -0.91 to 0.0 ‰). There is a negative correlation between Δ³³S and Δ³⁶S with a slope of ≈ -1, which is characteristic of Archean surficial sulphur. In addition, Δ³³S values approach magmatic values of ≈0 systematically with increasing distance from the country rocks.

These results support genetic models that involve locally derived sulphur for both deposits. In the case of Marathon, this likely reflects crustal contamination and assimilation of during emplacement of the TDLG, whereas at Geordie Lake, sulphur was likely added to the gabbros through subsolidus volatile transport.