Geological and Precious Metal Evolution at Freegold Mountain, Dawson Range, Yukon

Abstract

<p> Freegold Mountain is part of a northwesterly trending volcanoplutonic arc comprising the eastern portion of the Yukon Crystalline Terrane. Plutonic rocks in the area intrude Paleozoic metasediments and consist of a Late Triassic to Early Cretaceous granodiorite intruded by the Big Creek Syenite dated at 138 ± 10 Ma. A suite of K-rich intrusive volcanic rocks comprise an early period of basalt-andesite-dacite volcanism and a later rhyolitic volcanism dated at 78 ± 6 Ma. Volcanic rocks at Freegold Mountain can be correlated with Mt. Nansen Volcanics.</p> <p> Two sets of fracture planes, oriented at 165/70E and 18/80W, show dextral and sinistral displacement, respectively. Rhyolite dykes and gold-quartz veins predominantly oriented NNE have been emplaced along the 18/80W extensional fracture systems. These fractures may be modelled as high-angle Reidel shears, although evidence indicates that fracture orientations and fault movement have been complicated by changing stress fields from middle Cretaceous to Tertiary time.</p> <p> The gold-bearing quartz veins at Freegold Mountain are high grade, low-tonnage deposits with grades ranging from 0.4 to 1.0 oz/t. Fluid inclusions from the lower portions of the Laforma mine homogenize at 185° C with salinity ranging between 2 and 4.5 wt. % NaCl equivalent. A boiling zone has been detected above the deeper ore where fluid inclusions show extreme ranges in homogenization temperatures (165-430° C) and salinity (4 - 43 wt. % NaCl). Fluid inclusion waters have stable isotopic signatures indicative of meteoric water with 𝛿D = -138 and 𝛿^18O = -18.4. Boiling of the Laforma fluid has caused a shift towards heavier hydrogen and oxygen isotopic values, mineral deposition and appears to enhance gold precipitation over non-boiling zones. Stable isotopic evidence indicates that the paleolatitude of Freegold Mountain during late Cretaceous time was south of the present day Yukon-B.C. border.</p> <p> The Emmons Hill (Dart) prospect is a gold-bearing vein-breccia with a mineral assemblages of barite, stibnite, marcasite, cinnabar, orpiment, Fe and Mn carbonates and chalcedonic silica. Fluid inclusions from these deposits homogenize between 140-185° with salinities of 0-0.5 wt. % NaCl. This property shows marked similarities to acid-sulphate type precious metal deposits in New Zealand and western United States.</p> <p> Large (x to xx Mt) northwesterly trending diatremes appear to be structurally controlled by small transcurrent faults parallel to and coeval with displacement along the Tintina fault. The Antoniuk diatreme (3.7 Mt @ 1.14 g/t Au) consists of heterolithic and monolithic breccias intruded by dacite and rhyolite dykes. Diamond drilling indicates that a large body of fine to medium grained alaskite is present beneath the auriferous breccia. The deposit formed by the explosive escape of volatile components evolved from the retrograde boiling of a silicic magma chamber. Gold mineralization in the breccia is associated with pyrite and arsenopyrite formed at temperatures around 300°C and salinities of about 4.5 wt. % NaCl.</p>