Emplacement depth and porphyry copper-gold potential of Late Triassic to Early Jurassic granitoids in Yukon Topham, Matthew James
The objective of this study is to provide additional constraints through Al-in hornblende geothermobarometry on the nature and tectonic setting of Late Triassic to Early Jurassic magmatism and associated intrusion-related mineralization in west-central Yukon and eastern Alaska. Late Triassic to Early Jurassic (220-180 Ma) plutonic rocks in west-central Yukon and eastern Alaska represent a pulse of magmatism that emplaced the subvolcanic magma chambers of a continental magmatic arc that was superimposed on the Yukon-Tanana terrane. The Late Triassic to Early Jurassic Granite Mountain batholith hosts the Minto and Carmacks Copper deposits, the two most significant examples of Late Triassic to Early Jurassic intrusion-related copper-gold mineralization in the area. Predominantly granitic rocks of the Late Triassic to Early Jurassic Aishihik plutonic suite are exposed along the contact between the Yukon-Tanana terrane and the Mesozoic intraoceanic volcanic arc terranes, Stikinia and Quesnellia. The granitic plutons form mountain ranges on the southern border of the Yukon Plateau where it joins the adjacent Whitehorse Trough. This intermontane basin is occupied by mainly clastic sedimentary rocks overlapping the Stikine and Quesnel terranes. The Early to Middle Jurassic stratigraphy of the Whitehorse Trough records the uplift and erosion of the batholiths surrounding the basin. Specifically, Late Pliensbachian (183 Ma) and younger strata of the Tanglefoot Formation of the Early to Middle Jurassic Laberge Group contain granitic plutonic clasts with igneous ages and detrital uranium-lead zircon age peaks corresponding to Late Triassic to Early Jurassic plutons, indicating rapid exhumation during emplacement. Samples were collected from plutons of the Aishihik and Tatchun batholiths for the purpose of Al-in-hornblende geothermobarometry to provide estimates of pressures and temperatures of crystallization. Assuming an average density of continental crust, calculated pressures were converted into emplacement depths. Based on the depths corresponding to crystallization pressures, the Tatchun batholith was emplaced on average about 10 kilometers deeper (21-31 kilometers) than the Aishihik batholith (12-17 kilometers). Age-equivalent intrusions in British Columbia were responsible for generating significant copper-gold porphyry deposits that are currently preserved at or near surface in Stikinia and Quesnellia. By contrast, the Late Triassic to Early Jurassic Aishihik and Tatchun batholiths were emplaced at deeper levels in the crust than is believed to be required for the exsolution of a mineralizing fluid phase to form conventional porphyry deposits. The rapid exhumation of these batholiths accompanying emplacement produced a strong preservation bias against rocks hosting porphyry deposits that form at shallow crustal levels, generally in the upper six kilometers. While the emplacement depths and exhumation rates of the Aishihik and Tatchun batholiths do not make them prospective in terms of conventional porphyry deposits, they may still be prospective hosts for other unconventional, deformed intrusion-related copper-gold deposits such as Minto and Carmacks Copper.
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