Bimodal Tertiary volcanic rocks are exposed along the Tintina Trench in central Yukon in the Glenlyon, Grew Creek, and Ketza areas. Basalt and rhyolite are interbedded with coarse sedimentary rocks and preserved in grabens. The basalt forms both subaqueous and subaerial flows, hydrovolcaniclastic, pyroclastic, and autoclastic deposits. Rhyolite forms intrusions, lava flows, and volcaniclastic deposits. The volcanic and sedimentary rocks probably formed in a series of extensional basins whose original size and shape is yet to be determined. At Grew Creek, gold-bearing chalcedony veinlets and associated argillic alteration are near rhyolite dykes. In the Glenlyon Area, quartz, chalcedony, and fluorite veins and associated silicification are in and near rhyolite dykes.

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 Tintina Gold Province (TGP) of east-central Alaska, Yukon Territory, and the southwestern Northwest Territories comprises a very large number of gold (± base metal) deposits and occurrences that are spatially and temporally related to mid-Cretaceous intrusions. Intrusions in the eastern Selwyn Basin, south of MacMillan Pass and east of Frances Lakes, include some of the largest bodies within the TGP and are the focus of this study. Magmatic rocks of the TGP have been divided into individual plutonic suites on the basis of crystallization age, lithology, mineralogy, geochemistry, and spatial distribution, as well as metallogenic association. From -111 Ma to -99 Ma, magmatism is thought to reflect the formation of a southwest-facing continental magmatic arc, represented by the Whitehorse - Coffee Creek suite, and that the coeval Anvil and Cassiar suites formed in a back-arc environment. The younger Tay River, Tungsten and Tombstone plutonic suites successively stepped inboard between 99 Ma to 89 Ma. However, the processes leading to such volumetrically significant magmatism remains poorly understood. Intrusions within the study area range in composition from granite to granodiorite with subordinate diorite and are characteristically calc-alkaline, peraluminous to weakly metaluminous, relatively reduced, and typically contain only biotite as the dominant mafic phase. Sixteen new U-Pb ages, ranging from ~107 Ma to -91 Ma, constrain a temporal framework for plutonism across the region that is consistent with the progressively "inboard younging" pattern of magmatism observed in the northern and western portions of the TGP. - Geochemical (major, trace and rare earth elements) characteristics, together with geochronology indicate that the Anvil, Tay River, Tungsten, and Tombstone plutonic suites as originally defined farther to the northwest do continue southeastward and into the southwestern Northwest Territories. Initial Sr ratios and epsilon Nd values (n=20; age corrected for T = 100 Ma) range from 0.70853 to 0.72243 and -6.0 to -17.5, respectively. Lead isotopic compositions (n=20) show relatively narrow ranges for 2 0 6Pb/2 0 4Pb, 2 0 7Pb/2 0 4Pb, and 2 0 8Pb/2 0 4Pb ratios of 19.397 to 19.772, 15.697 to 15.829, and 39.461 to 39.883, respectively. All radiogenic isotope systematics indicate that these magmas have interacted extensively with or were derived entirely from continental crust. Several spatial and temporal trends are apparent in the data including an increase in overall REE abundance and sNd values, and a decrease in Srjnit ai, values with decreasing age (broadly moving from west to east). These trends may reflect differences in the nature of the underlying basement, potential magma source(s), and/or the melt producing processes that were involved. Lead isotope compositions of feldspars from various intrusions and sulphides from associated precious- and base metal deposits and occurrences define narrow and overlapping ranges indicating that the metals in many of the mineral deposits (and prospects) in the region are mostly derived from the mid-Cretaceous TGP intrusions.

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.

Restoration of the 450 km of right lateral movement on the Tintina Fault brings the massive sulphide-rich Finlayson area of the Yukon-Tanana Terrane to a location southeast of Dawson. This suggests that the underlying Yukon-Tanana Terrane geology west of the Tintina Fault, in west-central Yukon is prospective for massive sulphide mineralization. This area, west of the Tintina fault, largely escaped Pleistocene glaciation and is deeply weathered and covered by a thick mantle of soil and vegetation. This presents problems for traditional geochemical prospecting using soil samples. Alternatively the vegetative cover can be used as a sampling medium. To test the effectiveness of this technique in the exploration for VMS deposits two areas of the Yukon-Tanana Terrane were chosen for study:: Matson Creek and Bradens Canyon. At Matson Creek, a biogeochemical signature was obtained from a site with known polymetallic sulphides and data compared with known soil geochemistry. At Bradens Canyon soil and biogeochemical samples were collected. At each sample site in the two locations twigs were collected from both black and white spruce trees according to availability, along several transects. Results indicate that biogeochemical sampling is a viable alternative to soil sampling. The data show that the two species of spruce contain different concentrations of elements, therefore care must be taken during the sampling procedure to identify which species is being sampled. From the data a normalization factor was created by multiplying the white spruce Cu and Pb values by a factor of two in order to obtain an approximate equivalent concentration of these elements in black spruce.

In the southeastern Yukon Territory, Quaternary continental alkaline basalts have erupted across an important crustal suture, the Tintina Trench, which separates the accreted terranes of the Canadian Cordillera from the ancestral North American craton. The lavas from the Rancheria region from the west side of the Tintina Trench are basanites (BASAN), alkaline olivine basalts (AOB), and hypersthene-normative basalts (HYN). They display fractionated rare earth element (REE) profiles and are enriched in light rare earth elements (LREE) and high field strength elements (HFSE). The compositional spectra of the Rancheria alkaline magmas appears to represent the progressive melting of an amphibole-bearing garnet lherzolite. The involvement of amphibole in the petrogenesis of the Rancheria alkaline magmas indicates that these magmas were generated within the lithosphere. At the eastern end of the Rancheria suite, on the east side of the Tintina Trench, the AOB from Watson Lake have higher Zr contents than Rancheria AOB to the west of the Trench. The high Zr contents of the Watson Lake AOB are similar to those observed in the Hoole Eocene tholeiitic basalts, on the east side of the Tintina Trench, further to the north. The Eocene basalts from the Hoole River region are olivine tholeiites which have experienced closed-system crystal fractionation of olivine at low pressure. The estimated primary magma for these Eocene basalts appears to have been derived by partial melting of an incompatible-element enriched lithospheric mantle source, during which garnet was not a residual phase. The Nb-Zr systematics of the Watson Lake basalts indicate that they may be derived by mixing between melts produced by melting of an amphibole-bearing residue and a lithospheric mantle similar in composition to that of the Hoole basalts. Therefore, these compositional differences in the alkaline basalts across the Tintina Trench appear to reflect the juxtaposition of chemically distinct continental lithospheric mantles, indicating that the Tintina Fault is a steep lithospheric suture.

This paper presents new lead isotope data from intrusive rocks and several mineral deposits from the southeastern portion of the Tintina Gold Province (TGP). Lead isotopic studies of feldspars separated from various mid-Cretaceous intrusions in the study area and sulphide minerals from a number of mineral occurrences that are spatially associated with the intrusions were done in order to investigate possible relationships between the mineralization and magmatism. These data provide insight on metal sources within these systems and hence help to constrain exploration models. Results from this study indicate that: 1) the metals in many mineral deposits and prospects in the region are mostly derived from the mid-Cretaceous TGP intrusions; 2) the Sa Dena Hes deposit is broadly mid-Cretaceous in age; and 3) metals in apparently distal styles of intrusion-related gold mineralization are not entirely derived from magmatic sources.

Tin and tungsten-bearing veins, breccias and skarns occur in a 60 km long belt trending west from Keno Hill to the Tintina Fault. They are hosted by mid-Cretaceous felsic intrusions, or adjacent metasedimentary rocks of Upper Precambrian to Mississippian age. Tin occurrences are mainly associated with two-mica granites in the southern part of the belt, while the tungsten lodes are more commonly associated with biotite-hornblende granitoids. Tin- and silver-bearing veins are associated with the central granite phase of a zoned intrusion in the northwest part of the belt (the Syenite Range). The zoned intrusion ranges in composition from tourmaline orbicular granite to granite to quartz monzonite to syenite. Most skarns are tungsten-dominant, whereas most breccias and veins are tin-bearing. The skarns are calcic and reduced. Three stages of skarn mineral formation and associated minerals are recognized:: 1) isochemical contact metamorphism, including diopside, grossular, wollastonite, and tremolite; 2) metasomatic skarn formation including andradite, idocrase, hedenbergite, axinite, and some sulphide minerals; and 3) retrograde alteration including actinolite, chlorite, clinozoisite, epidote, calcite, biotite, scheelite, cassiterite and sulphide minerals. Sulphide minerals are mostly minor, with pyrrhotite and pyrite predominant. Breccias, veins and sheeted veins of tin and tungsten occur in steeply diping tabular bodies close to felsic intrusions. The veins consist of quartz, tourmaline or chlorite. Tin-bearing veins and breccias contain all three gangue minerals plus pyrrhotite, pyrite, sphalerite, chalcopyrite, arsenopyrite and galena. Tungsten is only found in quartz (~orthoclase) veins which contain minor pyrite and molybdenite. Sheeted vein systems consist of three mineral assemblages:: 1)quartz-orthoclase-scheelite, 2) quartz-orthoclase-cassiterite, and 3) tourmaline-cassiterite. The first assemblage is present both in the endo- and exocontact of felsic intrusions, whereas the second and third occur further away from the granite in metasedimentary rocks which generally lie outside the thermal aureole of the intrusion. Breccia clasts consist of quartzite, schist, and/or vein fragments (quartz, tourmaline, or chlorite). The breccias are either clast-supported with a matrix of rock flour, or matrix-supported with a matrix (groundmass) of crystalline quartz, tourmaline or chlorite similar to vein material. Geochemical studies of the McQuesten River occurrences indicate that:: 1) Some properties are exclusively tin or tungsten properties, but others contain both metals. There is a positive correlation between tungsten and tin in some tin-bearing rocks. 2) Silver is common in veins and skarns which contain over 50 ppm Sn. 3) Gold occurs in significant quantities in most skarns and in several veins. 4) There is a positive correlation between gold and bismuth in the skarns. Bismuth can be used as a pathfinder for gold in these skarns.

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.

The Minto and Williams Creek copper (-gold) deposits in western Yukon are hosted by variably deformed Early Jurassic (198-197 Ma; U-Pb) plutonic rocks and to a lesser extent strongly metamorphosed supracrustal rocks. These rocks are pendants and schlieren within slightly younger (197 Ma; U-Pb), intermediate-composition intrusive phases of the Granite Batholith. Chalcopyrite and bornite are disseminated and also occur as stringers in these rocks. Alteration muscovite associated with late quartz-feldspar-epidote veins gives a 182 Ma Ar-Ar age. Geobarometry on postmineral intrusive phases in the area indicate that they were emplaced at a depth of >9 km. Hornblende geochemical studies of plutonic and meta-plutonic host rocks at Minto and Williams Creek indicate that they formed in a continental magmatic arc setting. Cu/Au ratios and field observations indicate that supergene mobility of copper was more extensive at Williams Creek than at Minto. Our results indicate that the two deposits represent variations on typical copper (-gold) porphyry deposits.