Late Neoproterozoic epithermal alteration and mineralization in the western Avalon Zone: A summary of mineralogical investigations and new U/Pb geochronological results

Investigations into the distribution and mineralogy of late Neoproterozoic epithermal alteration systems of the western Avalon Zone, using visible/infrared spectroscopy (VIRS), provide new information that allows for better definition of spatial zonation patterns at select occurrences. The alteration zones contain alunite, pyrophyllite, dickite, kaolinite and diaspore, and the distribution of these minerals, coupled with compositional variations in alunite and the crystallinity of white micas, locally suggests variations in fluid temperatures. However, systematic spatial zonation cannot be defined in all examples.

The U/Pb geochronological investigations reveal an age of 635 ± 2 Ma for a granitic intrusion on the southern Burin Peninsula, representing a previously unrecognized magmatic episode in the western Avalon Zone. A sample from a unit previously mapped as part of the ca. 550 Ma Cross Hills Intrusive Suite instead gave an age of 581 ± 1.5 Ma, indicating the presence of older plutonic rocks that are not fully defined by existing mapping. At the Stewart prospect, quartz diorite affected by advanced argillic alteration and related mineralization gave an age of 577 ± 1.4 Ma, and a nearby granodiorite (part of a unit referred to as the Burin Knee granite) gave an age of 575.5 ± 1 Ma; both of these ages are close (within error range) to the 577 ± 3 Ma age previously reported for the Swift Current Granite. Felsic volcanic rocks of the Marystown Group were dated at 576.8 ± 2.6 Ma on the southern Burin Peninsula, and at 576.2 ± 2.8 Ma in the vicinity of Tower prospect at the northern end of the Burin Peninsula. New analyses of the archived zircon, derived from a sample of felsic ash-flow tuff from the Marystown Group that had previously given an age of ca. 608 Ma, suggest that the older result reflects inheritance, and the revised crystallization age is 574.4 ± 2.5 Ma. Collectively, these new geochronological data emphasize the importance of the period from ca. 580 to 570 Ma for volcanic and plutonic activity throughout this region; further, it suggests that epithermalstyle alteration and mineralization were broadly synchronous with this activity.


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