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.

Tin and tungsten veins and skarns in the McQuesten River area occur in the contact zones of Cretaceous feldpspar-porphyritic biotite granite stocks, plugs, and dykes. Most occurrences are located in the exocontact of plutons, in the brittle metasedimentary country rocks known as the Grit Unit of Upper Precambrian to Lower Cambrian age. Cassiterite (+/- silver) occurs in chlorite-, tourmaline-, and quartz-matrix breccias with fragments of quartzite, schist, and vein material of chlorite, tourmaline or quartz; in thin veinlets with little gangue of tourmaline, K-feldspar, or muscovite; and in actinolite-quartz-epidote-axinite-garnet skarn (+/- pyrrhotite, pyrite and chalcopyrite). Scheelite is mainly disseminated in fine-grained, diopside-quartz, or actinolite-quartz (+/- pyrrhotite) exoskarn which is interlayered with white wollastonite-quartz skarn. Scheelite (+/- molybdenite) also occurs in sheeted quartz (+/- feldspar) veins in both the endo- and exocontact regions of two feldspar-porphyritic granite stocks. Rarely do tin and tungsten occur together, however some low tin values are found in some tungsten occurrences, and vice versa. Tungsten occurs closer to the associated intrusion than tin.

not_specified

The Pattison Pluton is a high level Tertiary subaluminous alaskite that is part of a northwest trending chain of igneous intrusions in the Yukon Crystalline Terrane, southwestern Yukon. It is roughly circular in form with a diameter of about 18 km and a funnel-like profile. Crosscutting it are a series of shallowly dipping aplite dykes which intruded into open fractures. Compositionally and mineralogically, the alaskite body and aplite dykes are very uniform but the pluton is texturally and gradationally divided into a fine-grained upper border phase, a medium-grained graphic, strongly miarolitic, weakly porphyritic phase, and a lower coarse-grained phase. Due to eutectic crystallization, no strong major element trends are found within the pluton and dykes. Trace element chemistry, however, shows an apparent liquid line of descent with the fine grained alaskite the least differentiated and the aplite dykes the most differentiated. The medium and coarse-grained alaskites plot in between these. Rayleigh fractional crystallization of alkali feldspar accounts for these trends. Textural zonation evolved by the initial chilling of the originally H2O-poor magma at its upper edges. Crystallization of anhydrous phases from the melt had the effect of increasing the volatile pressure within the enclosed chamber. The volatiles migrated to and concentrated in the upper regions where the melting temperature was depressed, causing the pluton to crystallize from the bottom up. This allowed for the lower regions to reach more advanced stages of phenocryst crystallization, hence forming the lower coarse-grained alaskite. The volatile pressure eventually exceeded the confining pressure, resulting in failure of the surrounding rocks and retorgrade boiling, as well as vertical extension and lateral injection of residual magma due to filter pressing to form the aplite dykes. This final series of events also resulted in the formation of the graphic groundmass, straining of the minerals within the pluton, brecciation of some of the surrounding rocks, as well as quartz veining and molybdenum mineralization.

Between the Swift and Nisutlin rivers, unmetamorphosed granite to ultramafic intrusions of four ages (from Permian through Cretaceous) span the amalgamation of Cassiar Platform with Yukon-Tanana and Cache Creek terranes. The mid-Permian granitic Ram Stock and two plutons cutting the Sylvester Allochthon lie at the edge of the Dorsey Complex, a remnant of an ancient passive margin succession that underlies the volcanic arcs of Yukon-Tanana Terrane. Middle Jurassic, locally foliated granodiorite to gabbro intrusions are metaluminous, and high in Sr and low in Ti compared to the Cretaceous suite. These `I-type volcanic arc plutons may be the remnants of an overlapping arc correlative with the Quesnel Terrane. The Cretaceous (113 to 98 Ma) meta- to peraluminous granites are late orogenic incipient `A-type plutons from highly fractionated F- and Cl-rich magmas. These generated extensive hydrothermal systems that produced tin, tungsten, molybdenum and beryl occurrences.

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.

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.

A sequence of Permo-Triassic rocks within a 260 square kilometre part of the Kluane Ranges, southwestern Yukon, consists of Lower Permian volcaniclastic and sedimentary rocks, disconformably overlain by upper Triassic amygdaloidal volcanic rocks. Mafic to ultramafic rocks occur within the upper part of the Permian section and in the Triassic flows. Folding and faulting of this sequence is intense. The ultramafic rocks are peridotite-dunite complexes in the form of sills. Some gabbroic bodies have intruded along sill boundaries, but most are separate sills or dykes. Nowhere have they been found cutting rocks younger than the upper Triassic volcanics. Ni-Cu mineralization is spatially associated with the gabbros and peridotites. Dunite portions of ultramafic complexes are barren. Sulphide minerals include pyrrhotite, pentlandite and chalcopyrite, locally with pyrite, sphalerite and galena. Occurrences are classified as follows:: 1. Massive to stringer within country rock adjacent to gabbro intruding peridotite. 2. Massive to heavily disseminated at the country rock contact of gabbro intruding peridotite. 3. A) Heavily disseminated to nearly massive within gabbro intruding peridotite. B) Disseminated within separate gabbroic bodies. 4. Very weakly disseminated within peridotite. The mineralized gabbro-ultramafic intrusions are spatially related to the Triassic volcanic flows. The mafic and ultramafic rocks, their Ni-Cu deposits, and volcanic flows are believed to be related genetically. This report is accompanied by five geological maps as follows:: 1) Arch Creek (1::4760 scale; NTS 115 G/5 northeast); 2) Wellgreen area (1::4800 scale; NTS 115 G/5 northeast); 3) Linda Creek (1::2380 scale; NTS 115 G/6 northwest); 4) White River (1::4760 scale; NTS 115 F/15 northeast); and 5) Quill Creek (1::80 000; 115 G/5,6 north and 115 G/11,12 south).

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

Reconnaissance geological mapping in the Coal River map area of southeastern Yukon investigated several small mid-Cretaceous plutons. The intrusions are composed of unfoliated or incipiently foliated, fine to coarse-grained, equigranular and porphyritic, biotite ± hornblende quartz monzodiorite to granodiorite. They are metaluminous to peraluminous and have reduced to oxidized geochemical characteristics. The composition of selected samples is consistent with magma formation from partial melting of infracrustal source rocks.U-Pb ages were obtained for nine plutons from five or six zircon single-grain analyses by the isotope dilution thermal ionization mass spectrometry method with chemical abrasion (CA-TIMS). All interpreted ages are concordant within statistical uncertainty. The plutons range in age from 99.80 ± 0.03 to 97.70 ± 0.03 Ma. Given the primarily unfoliated nature of the plutons, contractional, fabric-forming deformation within the Cordilleran orogeny must therefore have largely ceased at the present level of exposure in the Coal River area by the time of intrusion (ca. 98 Ma).The ages and compositions of the plutons in Coal River map area are consistent with their being part of the Tay River plutonic suite, a northwest-trending belt of coeval and compositionally similar plutons and local volcanic rocks (South Fork volcanic suite) that, when augmented by the addition of the Coal River plutons, extends approximately 465 km with a width of up to 150 km.