Quantitative mineralogy, U-Pb geochronology of zircon and monazite, 40Ar/39Ar geochronology of muscovite and sericite, and Pb isotopes from galena in veins and feldspar in plutons provide insight into the age of metamorphism, mineralization, intrusion emplacement and the sources of metals at the Plateau South (MINFILE 105N 034, 035, 036) occurrences in central Yukon. Orogenic mineralization and metamorphism is ca. 110 Ma to 100 Ma, and possibly as old as ca. 130 Ma. Following deformation and regional metamorphism, two biotite-muscovite plutons, the Russell stock and Armstrong pluton, were emplaced at 95.39 ± 0.03 Ma and 95.51 ± 0.03 Ma, respectively. These plutons are here reassigned to the Tungsten suite based on mineralogy, chemistry and age. Coeval with these plutons are contact metamorphism and possibly intrusion-related mineralization. Lead isotopic data from galena cluster into two groups: Group 1 is enriched in thorogenic Pb with 206Pb/204Pb values between 18.31 and 18.14, 207Pb/204Pb between 15.62 and 15.55 and 208Pb/204Pb between 38.77 and 38.30. Group 2 is isotopically evolved with 206Pb/204Pb values between 19.13 nd 18.91, 207Pb/204Pb between 15.78 and 15.63 and 208Pb/204Pb between 39.24 and 39.07. We suggest that late Early Cretaceous mineralization is related to large-scale orogenic fluids that tapped primitive (deep?) metal sources and early Late Cretaceous mineralization, coeval with local intrusions, sourced isotopically distinct metals from the intrusions. Alternatively, all mineralization could relate to Early Cretaceous orogenic fluids but with heterogeneous, locally derived metal sources and thermal resetting of Ar ages near the intrusions.

Gold mineralization in the Clear Creek area is associated with ca. 92 Ma Tombstone Plutonic Suite intrusions (TPS) emplaced into metasedimentary rocks of the Neoproterozoic to Early Cambrian Hyland Group. Hyland Group rocks have undergone four ductile deformations (D1-D4 ) in the structurally thick (>10 km) Jura-Cretaceous Tombstone high strain zone. Kinematic features indicate overall top-to-the-northwest movement on shallow shear planes. Four different types of quartz veins developed during ductile deformation and are associated with a progression from ductile to brittle-ductile behaviour. Three major brittle structural trends postdate ductile deformation. A set of sinistral ~165° striking faults developed and are crosscut by secondary east-west fracture zones in Hyland Group rocks. The Tombstone Plutonic Suite was then emplaced in a broadly east-west oriented belt, with some local control exerted by the ~165° oriented faults. Continued development of the east-west fracture set after the Tombstone Plutonic Suite intrusion, resulted in an extensive system of gold-bearing sheeted quartz veins. Finally, sinistral reactivation and associated quartz-tourmaline veining occurred on ~165° oriented structures. Preliminary analysis of fault geometry and connectivity suggests the most favourable sites for mineralization are east-west fracture zones connected to ~165° oriented faults. Other favourable structural sites include misoriented segments of ~165° faults and possibly northeast-striking structures connected to ~165° faults. Mapping has delineated a large area of contact metamorphism suggesting extensive shallowly buried intrusions. The highest priority in any exploration program should be given to sites that are coincident with shallowly buried, or that are near Tombstone Plutonic Suite intrusions and have the above fault/fracture geometries.

The Bear Paw breccia zone is located at the Clear Creek property in the Tintina gold belt, central Yukon Territory. Gold mineralization occurs in hydrothermal breccias with stockwork quartz + potassium-feldspar + sulphide veins that overprint earlier intrusive and tectonic breccias. Grades of up to 2.3 g/t gold over 31.8 m have been intersected in recent drilling. The Bear Paw breccia zone is 1.5 km from any significant-sized granitoid stock and thus is indicative of the high potential for gold mineralization outboard of Tombstone Plutonic Suite stocks.

Structure imparts a significant control on the distribution of Carlin-type gold mineralization recently discovered in the Nadaleen trend, Yukon. An improved understanding of the structural framework for gold mineralization is essential for continued exploration success and interpreting ore fluid controls. Structural observations from the Osiris cluster of the Nadaleen trend indicate that NW-verging F1 folds were refolded in response to later SSW-NNE directed contraction. F2 folds have a subvertical ESE-striking axial plane with subvertically plunging axes on steep F1 limbs and subhorizontal fold axes in shallow F1 limbs. F2 folds have a pervasive axial planar cleavage that is recognized regionally. The steeply dipping Osiris and Nadaleen faults appear to cut all folds. Mineralization is spatially associated with later NW-striking faults in the Conrad zone. Much of the folding within the mineralized Conrad limestone is synsedimentary and its geometry reflects its emplacement as an olistostrome.

The Livingstone Creek area is located 100 km northeast of Whitehorse, Yukon Territory, Canada. Hydrothermal gold-sulphide mineralization (MINFILE 105E 001) occurs in quartz-carbonate veins and veinlets which cut Paleozoic metamorphic rocks of the Teslin Suture Zone. The metamorphic rocks are also cut by Cretaceous(?) feldspar-porphyry dykes with an average thickness of 2 m. The mineralization appears to be structurally controlled by NNE-striking faults and a set of NNW-trending joints. The vein minerals consist of gold, pyrite, chalcopyrite, galena, hessite/stuetzite, tetradymite, Au-Ag tellurides, tennantite, hematite, pyrrhotite, quartz, and carbonate. Gold occurs as: 1) "free gold" in cracks and interstices of quartz gangue, 2) inclusions in galena, usually rimmed by hessite, 3) minute grains associated with chalcopyrite and galena in aggregates of coarse-grained pyrite and 4) individual grains or fracture fillings in iron hydroxides. The coarse-grained gold in Livingstone Creek appears to be derived from gold-quartz veins in the metamorphic bedrock. This is indicated by: 1) similar silver and mercury contents in primary and placer gold 2) identical trace element composition of galena from gold-quartz veins and galena inclusions in placer gold, 3) similar telluride mineral assemblages in both gold-quartz veins and placer gold grains and 4) similar homogenization temperatures and salinities in fluid inclusions from both gold-quartz veins and placer nuggets. A limited amount of gold appears to have formed by supergene leaching and precipitation. This kind of gold occurs as irregular-shapd grains in the stream placers and in iron hydroxide along fractures in quartz veins. Relative to the pirmary gold it is enriched in silver and mercury.

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.

We present new field and U-Pb analytical data from the Sonora Gulch Project that demonstrate a protracted history of polymetallic mineralization (Au-Ag-Cu-Zn ± Mo) associated with several pulses of Cretaceous magmatism. Recent exploration on the Sonora Gulch Project has highlighted the presence of two important mineralized zones: the Nightmusic zone, a mesothermal Au-enriched base metal skarn, and the Amadeus zone, an epithermal Au-Ag system. Four U-Pb age dates determined from each of two feldspar porphyry dykes (ca. 74 Ma), a weakly mineralized quartz porphyry stock (ca. 75 Ma) within the Nightmusic zone and the Au-Ag mineralized Amadeus stock (ca. 75 Ma), demonstrate the widespread occurrence of Late Cretaceous magmatism. The age determinations indicate that mineralization occurring within the Sonora Gulch project area are temporally equivalent to the Casino Cu-Au-Mo deposit, located roughly 40 km to the west-northwest. These new data extend the currently known eastern limit of Late Cretaceous magmatism and associated mineralization.

The Rau deposit in central Yukon is a gold-rich carbonate replacement deposit hosted in mid-Paleozoic carbonate rocks of the Mackenzie Platform in the footwall of the Dawson thrust. Gold-bearing sulphide mineralization is peripheral to a zone of hornfels and local tungsten-bearing skarn that is associated with several small bodies of granitic aplite and pegmatite that have yielded 40Ar/39Ar muscovite ages of 62.3 ± 0.7 Ma, 62.4 ± 1.8 Ma and 59.1 ± 2.0 Ma. These intrusions are geochemically different and slightly younger than the 65.2 ± 2.0 Ma McQuesten plutonic suite farther to the south. Most Pb isotopic analyses of sulphides from the Rau deposit cluster within compositions of igneous feldspars from the associated intrusions; however, some analyses fall on a trend toward more radiogenic compositions that were determined for the host carbonate rocks. The data are consistent with the Rau sulphide mineralization being genetically related to the early Paleocene felsic intrusions, and forming peripheral to more proximal zones of hornfelsing and tungsten-bearing skarns.

Yukon has over a century of placer mining history, predominately in unglaciated regions. However, as these targets are exploited, focus turns to more complex landscapes where glaciation has buried, eroded and incorporated placer gold. This study examines how Early Pleistocene glaciation in the Mount Nansen area, central Yukon, has affected placer gold deposits. Detailed stratigraphic analysis and sample collection has focused on Back Creek, where placer mining has exposed a 22 m section with several gold bearing units. In the section, sediment from two glacial advances cap sporadically preserved pre-glacial gravel. The section is variably dissected by younger placer gold bearing fluvial gravel with enrichment related to intersection of inter-glacial or pre-glacial placer gold deposits. Analysis at Back Creek reveals the potential for deeply buried placer gold deposits in other glaciated regions of Yukon.

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