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Geologic setting, nature, and structural evolution of intrusion-hosted Au-bearing quartz veins at the Longline occurance, Moosehorn Range area, west-central Yukon Territory Joyce, Nancy L.

Abstract

The Longline gold occurrence is located within the Tintina Gold Belt, in the Moosehorn Range area of west-central Yukon Territory. Gold occurs in sheeted, high-grade (-30 g/t), shallowly dipping mesothermal quartz veins hosted within mid-Cretaceous intrusions of the Dawson Range batholith. Felsic to intermediate intrusions in the Moosehorn Range area are probably closely related; they were emplaced at -96-100 Ma, and have similar geochemical and lead isotopic compositions. Geochemistry of the intrusive phases suggests the magmas were mantlederived, subduction-related, and extensively contaminated by continental crust. The goldbearing quartz veins post-date all of these intrusions and are cut by younger mafic dykes. Metallic minerals inside the veins include galena, sphalerite, arsenopyrite, pyrite, boulangerite, tetrahedrite, native gold, and scheelite. The alteration assemblage includes muscovite, sericite, iron carbonate, pyrite, arsenopyrite, minor clay, quartz, and tourmaline. Vein minerals precipitated from moderately saline fluids, containing H₂0-C0₂-CFi₄-NaCl + N₂ , at temperatures of ~260°-300°C, pressure of -1.3 - 1.9 kbar, and depth of 5-7 km, assuming near-lithostatic fluid pressures. Lead isotopic studies indicate the Moosehorn Range area intrusions are not the source of the metals in the veins and the ultimate source remains uncertain. The veins were emplaced between 92 and 93 Ma along NNW-striking, shallowly ENE-dipping brittle reverse fault structures during a WSW-verging contractional event. The veins are 1cm to lm thick, sheeted, lens-shaped, banded, and locally connected by subhorizontal dilational oreshoots, creating a ramp-flat geometry. The long axis of the oreshoots is NNW and slip along the reverse faults was up-dip towards the WSW. Regional structural context for the formation of the veins is unclear. Prominent NNW-trending topographic and magnetic lineaments in the area may be dextral strike-slip faults along which there may be contractional and dilational jogs. Contraction at a jog in the vicinity of the Longline property may have generated the structures that host the veins. If so, dextral strike-slip faults and associated contractional and/or dilational jogs may be an important exploration guide to finding other shallowly dipping auriferous quartz vein systems in the region.

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