| Summary: | 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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