SULPHUR ISOTOPE ANALYSES OF MOLYBDENITE IN THE APPALACHIAN-CALEDONIAN OROGEN

Silurian-Devonian (435 to 370 Ma) granite plutons constitute a major portion of the Appalachian-Caledonian orogen and are associated with a wide variety of mineral deposits. This study presents new d³⁴S isotope data from granite hosted molybdenite mineralisation in Newfoundland and Scotland that may constrain the sulphur source. These data are integrated with sulphur isotope data from other granophile Mo, Sn, W, and Cu systems in the Appalachian-Caledonian orogen and used to discriminate between mineralization styles from different tectonic settings.

In Scotland, granites are associated with Mo and Cu-bearing quartz veins (Ballachulish and Etive Granites), Mo-bearing skarns (Etive Granite) and porphyry Cu-Mo deposits (Lagalochan). The d³⁴S composition of molybdenite ranges from -0.64 to +4.28‰ These values are comparable with sulphur isotope data of other Mo-deposits from north of the Iapetus suture (e.g. the Galway Granite) and are consistent with a magmatic sulphur source.

In contrast, molybdenite associated with granites from south of the Iapetan suture are enriched in isotopically heavy sulphur. The Upper Devonian Ackley granite in Newfoundland contains several molybdenite occurrences, which yield d³⁴S values of +6.6 to +7.58‰. Positive d³⁴S values have also been recorded from porphyry Mo-mineralization at Moly Brook (+6.49 to +8.5‰) and Mo-bearing quartz veins from the St. Lawerence Granite (+4.72‰). These Newfoundland data are similar to sulphur isotope values from W-Sn±Mo mineralisation in the Leinster and Skiddaw Granites.

The project suggests that distinctive sulphur isotope signatures can be recognised from opposed sides of the Iapetan suture. Sulphur isotope data from granite-related sulphide mineralization in Britain and Ireland have been used to discriminate between tectonic terranes and, based on the results of this study, and unpublished d³⁴S data from sulphide mineralization in central Newfoundland, correlations may be drawn between terranes along the Appalachian-Caledonian orogen.