Speculation regarding the genetic history of the Minto copper-gold deposit in the Carmacks map area (NTS 115I) has existed since its discovery. Minto copper sulphides are hosted in sheet-like expanses of biotite-rich, variably deformed granitoids surrounded by massive granodiorite. Attempts to explain Minto’s unusual mineralization style have ranged from digested red-bed copper, to aborted and deformed porphyry, and recently to an Iron Oxide Copper Gold (IOCG) type system. Although these commonly used genetic frameworks can explain many aspects of Minto-style mineralization, questions regarding chemistry, paragenesis, and structural controls on mineralization still remain. This paper is part of an MSc thesis project that will focus on characterizing mineral textures, mineral chemistry, mineral paragenesis and micro and macro structural analyses to improve our understanding of the Minto copper-gold mineralized system and to enhance regional exploration potential in the district. This paper summarizes some preliminary observations at the Minto deposit and outlines future research.

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.

The White River copper deposit, in upper Triassic Nikolai Greenstone of southwestern Yukon, is representative of the native copper-basalt association. Native copper and chalcocite are the association. Native copper and chalcocite are the most abundant ore minerals, but a substantial amount of bornite is known, as well as lesser amounts of chalcopyrite, pyrite, digenite, covellite, cuprite and native silver. These minerals are found in crosscutting fractures, amygdules, gas release tubes, small crackle zones, and as local disseminations in basalt; and although concentrated near the margins of a single glomeroporphyritic unit, are neither confined to that unit nor to a single zone within it. Two stages of copper mineralization are postulated: Stage I mineralization is thought to account for most of the native copper as a product of continental weathering of Nikolai basalts. Stage II mineralization is a much later event characterized by copper sulphides in crosscutting structures. Native copper and copper sulphides of Stage II appear to form a stable and primary product of a low grade (regional) metamorphism indicated by such minerals as chlorite, epidote, prehnite, pumpellyite, calcite analcite and apophyllite which have essentially the same mode of occurrence as primary copper minerals. Consequently, metamorphism (prehnite-pumpellyite facies) is interpreted to have been the mineralizing process. Whole-rock potassium-argon dating suggests an age no older than 120 million years for the metamorphic mineralizing event; hence, stage II mineralization post-dated host rock formation by at least 80 million years. It is probabe that many other copper occurrences in Nikolai Greenstone have formed in a similar manner. Also, it is likely that some of these mineralizing fluids could have moved higher in the stratigraphic sequence and precipitated copper minerals in other units.

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

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Conglomerates belonging to the Indian River formation (IRF), south of the Klondike goldfield, have recently become the focus of exploration activity owing to their potential as hosts for paleoplacer gold derived from the Klondike. However, textures within the conglomerate have been interpreted as indicative of hydrothermal activity, and the possibility exists of in situ epithermal gold. Paleocurrents in conglomerates indicate dominant transport from the southeast, incompatible with gold transport from the Klondike. Gold grains from unconsolidated conglomerate at Montana Creek reveal an epithermal signature (20-50% Ag, 0.3 to 3% Hg and opaque inclusion suite containing complex polymetallic sulphotellurides and sulphosalts), distinct from the signature of placer and lode sources in the central and southern Klondike (12-20% Ag, Hg absent and opaque inclusion suite of simple base metal sulphides). Gold grain morphology and alteration textures within unconsolidated conglomerates suggests that Montana Creek gold is derived from in situ epithermal mineralization related to that previously reported at Eureka Dome.

A new widespread, structurally controlled gold mineralizing system has been identified during the 2010 exploration drilling program at the Coffee Project, west-central Yukon. The Coffee Creek area is underlain by a sequence of shallowly to moderately south to southwest-dipping Paleozoic metamorphic rocks that are considered to be part of the Yukon-Tanana terrane and are intruded by the Cretaceous Coffee Creek granite along a west to northwest-trending contact. During the 2010m drilling program, structurally controlled gold mineralization was discovered in all major lithological units underlying the Coffee property. Importantly, these mineralized zones correspond to a number of discrete structural corridors. The gold zones are steeply dipping and characterized by extensive silicification in addition to sericite and clay alteration accompanied by variable As-Ag-Sb-Ba-Mo enrichment. Polyphase breccias of both hydrothermal and tectonic origin, in addition to andesitedacite dykes, are common within the gold-bearing structural corridors. The dominant sulphide is pyrite, although trace arsenopyrite, chalcopyrite and stibnite are observed locally. The similarity of breccia textures and alteration/sulphide mineralogy between all gold zones currently defined on the Coffee property implies a common mineralizing event.

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The Stu copper-gold±silver occurrence (Yukon MINFILE 115I 011) is located midway between the Minto and Carmacks Copper deposits in the eastern, Minto suite portion of the Granite Mountain batholith. The best known mineralization at Stu is in Zone A, where at least four foliated and mineralized bodies strike northwest and dip moderately to steeply to the northeast. These bodies grade 0.2 to 0.6% total Cu, though the best historic diamond drill intersection contains 3.5% Cu over 13.5 m. In plan and cross section view, the foliated and mineralized granodioritic orthogneiss bodies pinch and swell, appearing as lenses surrounded by unfoliated K-feldspar porphyritic granodiorite. Copper mineralization occurs as both fine-grained hypogene bornite and chalcopyrite, and supergene malachite, azurite, tenorite and chrysocolla. This study suggests that Stu mineralization is similar to that at the Carmacks Copper and Minto deposits as it is primarily hosted in multiple, discrete bodies of foliated granodioritic rock. In terms of ore body orientation, the moderate to steeply dipping nature at Stu is more reminiscent of Carmacks Copper than Minto, which likely explains the presence of significant supergene copper mineralization.

The Len porphyry gold prospect is located 47 km north of Mayo, Yukon, in the Tombstone Suite intrusive belt. The area was explored as a Keno Hill-style silver prospect in the 1960s and 1970s. An arsenic-in-soil anomaly first identified in 1980 was followed up by soil geochemistry and excavator trenching in 1996. Multiple sheeted quartz-sulphide veins hosted in a previously unmapped granodiorite stock were discovered during the trenching program. A six-hole program of diamond drilling in 1997 encountered grades ranging up to 2.22 g/t gold across 18.6 m, and showed that gold mineralization is dominantly within, but not restricted to, the intrusive stock.