The Logan (MINFILE 105B 099) and YP (MINFILE 105B 001) zinc-lead-silver deposits are located in the Rancheria District, Yukon Territory. These deposits and numerous other occurrences occur in veins and breccia zones which cut clastic sedimentary rocks of Proterozoic and Palaeozoic age, along fracture zones cross-cutting Cretaceous granites and Eocene volcanic dykes of mafic and felsic composition, and form replacement bodies in Palaeozoic carbonates. At the Logan and YP deposits, four phases of mineralization can be distinguished. The main sulphide minerals are sphalerite, galena, pyrrhotite, pyrite, chalcopyrite and arsenopyrite. Silver is mostly confined to galena, but also occurs in tennantite-freibergite group minerals, stannite, Pb-Ag-Bi-sulphosalts of the matildite-galena series and lillianite homologues. Arsenopyrite geothermometry using the method of Kretschmar and Scott (1976) returned maximum formation temperatures of 465° - 490° C at YP and 335° - 385°C at Logan. Microthermometric investigations of fluid inclusions in quartz associated with the YP mineralization showed formation pressures corresponding to a depth of 2500 m. Fluid inclusion data from quartz suggest that mineralization at YP and Logan is caused by a mixture of magmatic and metamorphic fluids. The fluid inclusions have low salinities of 34 weight per cent NaCl equivalent, an unusally low value for fluids which have separated from acid magmas. The metamorphic fuids may have been derived from metamorphic dewatering of Palaeozoic sediments.

Latest (?) Proterozoic to Earliest Cambrian and Devonian to (?) Mississippian strata, and a Cretaceous or Early Tertiary porphyry dyke underlie the Plata-Inca property. The sedimentary rocks are part of the dominantly clastic assemblage that makes up the other part of the northern Cordilleran miogeocline. The sedimentary rocks are folded and cut by thrust faults and younger (?) normal faults. Steep normal faults with a variety of orientations cut all other structures. These faults host most of the veins and are well exposed in the mine workings. Most veins contain galena, sphalerite, and tetrahedrite in a gangue of siderite and quartz with minor barite and calcite. Silver-lead ratios determined from the grade of ore shipments range from 55.5 g/t Ag : 1% Pb to 137.1 g/t Ag : 1% Pb The age and origin of the gold-silver veins in the Plata-Inca camp is unclear. They are most likely related to a buried intrusion, although the only evidence for one is the small porphyry dyke at the northwest end of the property. There are no other intrusions nearby, but the deposits are at the northern margin of the belt of mid-Cretaceous intrusions that belong to the Selwyn Plutonic Suite.

A detailed evaluation of structure and alteration related to gold- and silver-rich, base metal-bearing veins was completed at the Skukum property as part of the 2002 mineral exploration program. The structural setting is an east-trending sinistral strike-slip system bounded by the Berney Creek and Goddell faults to the south and north, respectively. The deposit comprises northeast-trending quartz-sulphide mineral shear veins that formed during syn-tectonic intrusion of rhyolite and andesite dykes related to the Eocene Mount Skukum caldera complex. A genetic relationship between mineralization and certain rhyolite dykes is indicated by patterns of alteration and mineralization. Dilational, northeast-trending structures interconnect and splay off the controlling faults, and host extensional quartz-sulphide mineral veins. At Skukum Creek the main gold-silver-bearing minerals are electrum and freibergite, which precipitated with late galena-stibnite mineralization, whereas refractory gold in arsenopyrite is the main style at Goddell. A geological model is proposed that facilitates identification of prospective structures within the property.

The Montana Mountain area lies at the western edge of the Intermontane Belt, where the Whitehorse Trough overlies the Atlin Terrane. Paleozoic volcanic rocks of Atlin Terrane and clastic rocks of the Laberge Group occur along the margins of the area mapped. Cretaceous intermediate volcanic rocks of the Mount Nansen Group intrude this basement within a roughly circular area about 7 km in diameter and are referred to as the Montana Mountain Volcanic Complex. The northern margin of the complex is metamorphosed by a granite pluton related to the Coast Plutonic Belt. Porphyritic rhyolite dikes intrude the Mount Nansen Group and adjacent Laberge strata. Quartz veins, some with economic mineralization, cut the granite and Mount Nansen volcanic rocks. Although Montana Mountain has a long history of prospecting and mining ventures, no detailed study of the geology is generally available.

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.

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

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This field trip will introduce the bedrock geology, surficial geology and palaeontology of the Klondike District, as well as, gold-bearing orogenic vein systems that are currently being explored in the area and placer gold deposits from which 13(+) million ounces of gold have been mined.

The Flex Gold-Silver deposit is a multiple vein epithermal system found within the Mount Nansen precious metal trend. High-grade gold and silver values are associated with north-northwesterly trending, sulphide-rich quartz veins that infill structures associated with regional shearing. BYG has stripped and mapped the deposit to prepare advanced exploration and mining design plans. The mineralization is typically epithermal with extensive wall rock alteration including argillic and phyllic zones. Gold - silver values occur with sulphide-rich quartz veins, breccia veins, and silicified zones. Gold values up to 34 grams per tonne and silver values up to 1416 grams per tonne were obtained from the sulphide-rich veins.