Neoproterozoic to lower Paleozoic basin and platform strata that formed during and after rifting along the western Laurentian margin are preserved in the northern Cordillera. Several pulses of magmatism occur within margin strata and are concentrated along the Dawson fault in central Yukon. Magmatism is dated as late Cambrian and Late Ordovician using: 1) U-Pb zircon geochronology of volcaniclastic rocks; and 2) fossil ages from strata interbedded with, and enclosing, volcanic rocks. Volcanic rocks from both pulses are predominantly alkaline and basic and erupted in subaqueous environments. The trace element geochemical compositions of the rocks suggest that they formed from partial melting of enriched lithosphere from the garnet stability field.

The study area is underlain by four stratigraphic successions ranging in age from Middle Proterozoic to Early Paleozoic. From oldest to youngest, they are: Middle Proterozoic Wernecke Supergroup; Middle to Upper Proterozoic Pinguicula Group; Upper Proterozoic Windermere Supergroup; and Uppermost Proterozoic to Lower Paleozoic sandstone and carbonate. Together, they represent about a billion years of intermittent sedimentation punctuated by processes such as deformation, uplift, erosion, magmatism and mineralization. Rocks in the study area record eight phases of contractional and extensional deformation, some of which may be related to strike-slip faulting. Two phases of southwest-verging folds and thrust faults may be related to dextral transpression on the Snake River Fault. Mineral enrichments occur in two general forms:: breccia-related (Middle Proterozoic), and veins (Mesozoic to Tertiary). The breccia-related occurrences have enrichments of Cu ± U, Co, Au and Ag, as dissemminations and veinlets in and near intrusive breccia zones (Wernecke breccia). The vein occurrences comprise Zn-Pb-Ag ± Cu and Au, in veins and related lenses and irregular replacements of carbonate.

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.

Map sheets 95C/5 and 95D/8 in southeast Yukon contain eight successions of sedimentary rocks ranging from Proterozoic to Paleocene in age. Early to middle Paleozoic starta are best preserved. Proterozoic strata have been tentativel correlated with Pinguicula Group, Mackenzie Mountains Supergroup and Windermere Supergroup. Newly defined Cambrian-Ordovician Crow Formation documents shallow water to suaerial siliciclastic sedimentary rocks related to a paleogeographic high.

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The upper Sixtymile River area is located approximately 128 km west of Dawson City, Yukon. Lithology in this area consists of Precambrian to Paleozoic metamorphic rocks, Paleozoic ultramafic rocks, Middle Jurassic pegmatite and aplite dykes, Late Cretaceous porphyritic dykes and volcanic rocks with intercalated sedimentary rocks, Quaternary alkaline basaltic dykes and Quaternary alluvial sediments. Gold bearing, mesothermal quartz-(carbonate)-sulphide veins which trend NNE-SSW are hosted by metamorphic rocks north and south of Sixtymile River. The mesothermal quartz-(carbonate)-sulphide veins are surrounded by successive envelopes of sericitic, K-feldspar and propylitic alteration. Two stages of vein mineralization are recognized in the northern part of the area, and three stages are recognized in the south part. In the northern veins, pyrite, arsenopyrite, pyrrhotite and quartz are intergrown and formed first. These minerals are fractured and healed by second stage minerals, which include galena, sphalerite, chalcopyrite, pyrite, carbonate and minor quartz. Stage I mineralization in the southern veins is represented by quartz, and pyrite containing inclusions of other sulphides. Stage II is the main stage of precious metal enrichment, represented by arsenopyrite and galena which contain tetrahedrite, miagyrite and polybasite exsolutions. Stage I and II minerals are tectonically fractured, and healed by pyrite, sphalerite, chalcopyrite, freibergite and quartz of stage III. In both vein systems, gold enrichment is associated with arsenopyrite and silver enrichment is associated with galena. The evolution of hydrothermal fluids in the northern area is characterized by decreasing temperature (330°C to 280°C), salinity (12.8% wt.-% to 6 wt.-% NaCI equiv.), oxygen activity (log a(O2) = -30 to log a(O2) = -35), and sulphur activity (log a(S2) = -10 to log a(S2) = -12), as well as a slight increase in pH range (from >3.1 - <5.2 to >3.3 - <5.4). In the southern vein system the fluid evolution characterized by a decrease in temperature (330°C to 150°C), salinity (18.3 wt.-% to 10 wt.-% NaCI equiv.), oxygen activity (log a(O2) = -29 to log a (02) = - 52), and sulphur activity (log a(S2) = -9 to log a(S2) = -18, as well as a slight increase in pH range (>3.2 - <5.3 to >4.1 - <5.9). The following conclusions can be drawn about the fluid composition and mineral enrichment process in the fossil geothermal system of the Sixtymile River area. Deep seated fluids which circulated in the metamorphic rocks were characterized by high temperatures (above 300°C), high salinities (about 18 wt.-% NaCI equiv.) and pH values between 3.1 and 5.2. These fluids are similar to alkaline chloride fluids of active geothermal systems. Arsenic was transported as H3AsO3°-complex, gold as Au(HS)2- and lead, zinc, iron, copper as MeCl2° complexes. The first stage mineralization resulted from reaction of this deep-seated fluid with the wall rock. Second and third stage mineralization is believed to result from the mixing of two fluids with different physico-chemical characteristics.

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.

The upper Sixtymile River area is located approximately 128 km west of Dawson City, Yukon. Lithology in this area consists of Precambrian to Paleozoic metamorphic rocks, Paleozoic ultramafic rocks, Middle Jurassic pegmatitic and aplitic dikes, Upper Cretaceous porphyritic dikes and volcanic rocks with intercalated sedimentary rocks, Quaternary alkaline basaltic dikes and Quaternary alluvial sediments. Precious metal occurrences in these volcanic rocks are divided into two types, based on differences in local distribution, petrology and wall rock alteration: a gold-bearing pyrite-arsenopyrite type and a silver-bearing galena-sphalerite type. Both types are characterized by four stages of mineralization.

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