GRANITE-RELATED MAGMATISM AND METALLOGENESIS: TRANSATLANTIC CORRELATIONS ALONG THE APPALACHIAN-CALEDONIAN OROGEN

The North Atlantic sector of the Appalachian-Caledonian orogen is partly comprised of a suite of syn- and post-tectonic granitoid intrusions, episodically emplaced during ~ 50 Ma of crustal shortening and late distensional collapse. Examples of geochemically evolved plutons in Britain, Ireland and Newfoundland display evidence of sporadic and typically sub-economic granophile Mo, Sn, W, Cu and F mineralization. New and existing data are presented which provide constraints on the timing of granitic magmatism and associated mineralization, and the physicochemical nature of metallogenic magmatic-hydrothermal fluids along this segment of the orogen.

Re-Os molybdenite geochronology has helped constrain the temporal evolution of orogenic granite-related mineralisation. In Britain, granite emplacement and molybdenite mineralisation in the Shap (405.2 ± 1.8 Ma), Weardale (398.3 ± 1.6 Ma) and Skiddaw granites (392.3 ± 2.8 Ma; U-Pb emplacement age earlier at 398.8 ± 0.4 Ma) reflect a relatively short-lived phase of spatially coincident granophile mineralization south of the Iapetus Suture. In contrast, Re-Os ages from the Galway Granite, western Ireland, indicate discrete pulses of molybdenite mineralization occurring at ~ 422 Ma, ~ 410 Ma, ~ 400 Ma and ~ 383 Ma. These ages reflect more protracted, episodic plutonism and mineralization north of the suture line. In Newfoundland, new Re-Os molybdenite ages from the post-tectonic Ackley Granite (379.3 ± 1.7 Ma, 2σ, n = 7) indicate that mineralization along the orogen continued into Frasnian times. Thus, the collective ages show that diachronous, orogen-parallel, metallogenic plutonism occurred from NE to SW.

Fluid inclusion studies show that the composition of magmatic-hydrothermal fluids vary with commodity, location and age. Fluids associated with Mo mineralization are generally comprised of aqueous-carbonic fluids of moderate salinity (4-10 eq. wt % NaCl) and trapping conditions ranging from ~ 0.5 to 2 kb. In Newfoundland, fluid inclusions associated with Sn-W±Mo mineralization are comprised of high-salinity brines containing abundant daughter crystals, as well as mixed populations of vapour- and liquid-rich aqueous-carbonic types.