Shaun Barker

The Dawson Range district of west-central Yukon is a 150-km long northwest-trending metallogenic corridor extending from Carmacks to the Yukon-Alaska border. The Dawson Range district has recorded several magmatic episodes associated with the formation of porphyry and associated epithermal deposits that range in age from Late Triassic to Late Cretaceous. Recent studies have identified a link between the most prolific epoch of Cu-Au-Mo mineralization and the Late Cretaceous magmatic episode that emplaced the calc-alkaline Casino suite (79-72 Ma) and the calc-alkaline to mildly alkaline Prospector Mountain suite (72-67 Ma). Despite the economic significance of the Late Cretaceous magmatic episode, many aspects regarding the source of magmas remain unresolved. New spatial (bedrock mapping), temporal (U-Pb zircon ages), and geochemical data (whole-rock and zircon trace element geochemistry) provide constraints on the complex tectonomagmatic processes controlling the petrogenesis of Late Cretaceous magmas. U-Pb geochronology indicates an apparent ~13 m.y. record from ca. 80 to 67 Ma of protracted magmatism and hydrothermal activity with time-dependent compositional changes. Geochemical characteristics of the Casino and Prospector Mountain plutonic suites indicate compositional similarities to older subduction-related plutonic suites (e.g., Whitehorse suite). However, the Late Cretaceous suites show: (1) more LREE enrichment; (2) greater fractionation of HREE (reflecting garnet-bearing sources); (3) a co-magmatic source and intermingling of magmas. The evolution from mid-Cretaceous subduction-related continental arc magmatism to Late Cretaceous small volume calc-alkaline to mildly alkaline magmatism with continental arc signatures is attributed to postsubduction partial melting of the previously metasomatized subduction-modified lithosphere to produce magmas with geochemical characteristics resembling that of earlier plutonic suites. The Casino suite formed in response to slab roll-back and foundering at ca. 83 Ma, which advected heat towards the base of the lithosphere and induced partial melting. Differential roll-back of the slab may have resulted in slab rupture and window opening at ca. 72 Ma, resulting in asthenospheric upwelling. Decompression melting of a common Casino suite reservoir may have generated the slightly alkaline Prospector Mountain suite. The role of post-subduction magmas is considered important to the petrogenesis of fertile magmas that are temporally and spatially associated with porphyry systems in the Dawson Range.

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