Epithermal vein and porphyry-related gold-silver deposits in the Mount Nansen area are mainly hosted in Paleozoic Yukon-Tanana Terrane metasedimentary rocks and Early Jurassic Big Creek Batholith intrusive rocks. Mineralization is spatially, and probably temporally, related to a northwest-trending belt of mid-Cretaceous hypabyssal felsic intrusions and dykes along the Mount Nansen Trend. The proximal relationship between the veins and mid-Cretaceous intrusive rocks suggests that mineralization may be genetically related to felsic magmatism. The Dickson stock yields a U-Pb zircon age of 108.3 ± 0.7 Ma, and proximal dykes in the Flex, Dickson, Brown-McDade and Weber zones give ages of 107.9 ± 0.9 Ma to 109.0 ± 0.7 Ma, similar to the age of the Mount Nansen Group volcanic rocks. Granodiorite that hosts the Dickson deposit gives a U-Pb titanite age of 191.5 ± 2.9 Ma, and is likely part of the Big Creek Batholith. Previous studies indicated two periods of mineralization in the Dawson Range: mid-Cretaceous and Late Cretaceous. Dating indicates that Mount Nansen mineralization is associated with the mid-Cretaceous emplacement of the high-level felsic intrusions.

The study area is centred on the 91.2 ± 0.9 Ma Scheelite Dome quartz-monzonite stock of the Tombstone Plutonic Suite (TPS). This stock and associated dykes and sills intrude highly deformed metasedimentary strata of the Yusezyu Formation of the Neoproterozoic to Lower Cambrian Hyland Group. The emplacement of TPS intrusions post-dates regional greenschist-facies metamorphism and multiple phases of ductile deformation related to the Tombstone strain zone. Although the Scheelite Dome stock hosts auriferous, sheeted quartz veins, extensive soil geochemistry indicates that the bulk of the gold resource is hosted in the variably hornfelsed metasedimentary rocks immediately south of the stock. The associated gold-in-soil anomaly forms an east-trending corridor of anomalous gold values (>80 ppb) approximately 6 km long by 1.5 km wide, with a more weakly defined eastern continuation. Where metasedimentary bedrock is exposed in the corridor, gold is hosted in fault-vein arrays, and less commonly as disseminated grains and in replacement zones. The styles and distribution of mineralization are largely controlled by brittle structures; a phase of east-west shortening was largely coeval with gold mineralization. R-mode factor analysis of multi-element geochemical data indicates two geochemically distinct metal suites within the area of the gold-in-soil anomaly at Scheelite Dome. The first suite, characterized by Au-Te-Bi ± W ± As, possesses the stronger gold association and is typical of intrusion-related gold occurrences elsewhere in the Tombstone gold belt. The second suite displays a metal association of Ag-Pb-Zn-Cd-Sb ± Cu ± Au, which is more characteristic of mid-Cretaceous Ag-Pb-Zn mineralization in the Keno Hill district, located approximately 60 km to the east-northeast. Field observations, combined with soil geochemistry, suggest that the different metal associations are paragenetically related. However, the possibility of two distinct hydrothermal events cannot yet be ruled out.

The RAM zinc-lead-silver property in southwest Yukon Territory (105 D/4) is on the contact between metamorphic rocks of the Yukon Crystalline Terrane and foliated granitic rocks of the Coast Plutonic Complex. Both are intruded by an Eocene granitic stock. The metamorphic rocks, predominantly biotite-quartz pelitic schists, with amphibolite, marble, graphitic phyllite and foliated metagranite, may be partly Precambrian and are modified by metamorphism. Foliated granodiorite to quartz diorite of the Coast Plutonic Complex intrudes the metamorphic rocks. Rb-Sr analyses of two of these granitic rocks give a possible whole rock date for foliated granite of 143 Ma (initial 87Sr/86 Sr ratio of 0.7068), and a maximum age of 200 Ma, while K-Ar dating of hornblende and biotite separates from a granodiorite gives 106 ±4 Ma and 53.7 ±1.9 Ma, respectively. An Eocene, porphyritic microgranite stock (Rb-Sr whole rock date:: 43 Ma with assumed initial 87Sr/86Sr ratio of 0.705) discordantly crosscuts the older rocks on the property, and has probably reset the biotite K-Ar date in the granodiorite. Mineralization on the property includes small skarns (maximum size 15m x 6 m) with disseminated sphalerite and galena, in metasedimentary rocks of the Yukon Crystalline Terrane, near the Eocene stock. Assay of grab samples from the showings average 53.8 gm/t silver, 4.35% zinc, and 2.20% lead.

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for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca

Description and analysis of the geology of quartz veins containing gold ores in the Yukon Territory.

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Magmatism in the Nahanni region, which defines the eastern extent of the Tintina Gold Province, is generally associated with tungsten mineralization and/or gold-copper-antimony-bismuth-lead-zinc metal occurrences. Intrusions are subalkaline, granitic to granodioritic, and contain several types of textural variations and highly evolved phases. The intrusions range from large composite batholiths to small stocks with associated felsic dykes and veins. Initial U-Pb and Ar-Ar geochronology reveals ages of 97.5-95 Ma with short (0.5-1.5 m.y.) cooling periods, although the intrusion associated with the Cantung tungsten-skarn orebody cooled over a relatively long period (3 m.y.). Magmatism in the area has been interpreted as crustally derived, however, the rare earth element primitive-mantlenormalized profile revealed negative niobium, tantalum and titanium anomalies suggesting an arctype setting. Furthermore, the granites lack volumetrically significant, primary peraluminous mineralogies characteristic of S-type granites.