The Logtung large tonnage, low-grade W (scheelite)-Mo porphyry deposit, south-central Yukon Territory

Abstract

The Logtung deposit, discovered in 1976, is an example of large tonnage (162 million metric tons), low-grade (0.13% WO3 and 0.052% MoS2) W(scheelite)-Mo mineralization related to multiple felsic intrusions. Since mineralization is contained in fractures ()95%) rather than being disseminated Logtung is not a skarn deposit. The principal reason is that the quartz-diopside-garnet calc-silicate, biotite-quartzite, and wollastonite-vesuvianite calcsilicate reaction skarn host rocks to mineralization, which were derived from calcareous shales interbedded with graphitic quartzites, are carbonate poor and competent. Hence, permeable mineralization formed in permeability created by hydraulic fracturing, and not by replacement. Logtung shows a scheme of several phases (4) of mineralization related to multiple intrusions (>•2) which is similar to that which is so important in, for example, the evolution of the Climax and Urad-Henderson Mo systems. The earliest quartz-molybdoscheelite vein system, which consists of a typical porphyry-style crackle breccia of thin (•--0.5-4.0 mm), red-brown quartzgarnet-diopside-molybdoscheelite veins, is related to a highSiO2 ('• 76%), seriate-porphyritic monzogranite stock (•--2.0 X 0.5-1.0 km) of Cretaceous age (118 +_ 2 m.y. (2tr); (Stewart, 1983). Later quartz-pyrite-scheelite (fiuorite-K-Na-feldspars-molybdenite) veins, with prominent green, hornblende-bearing alteration selvages, and quartz-molybdenite (-pyritefluorite-scheelite) veins are related to a quartz-feldspar-spessartine-rich garnet porphyritic felsic dike complex •'-500 m across. Both the intrusive phases have anomalously high W and Mo concentrations of •--100 ppm and •'-50 ppm, respectively, which are thought to reflect a primary enrichment (Stewart, 1983). The final stage of mineralization consists of a prominent •'-4.5 X •'-1.0 km zone of polymetallic sheeted veins •--1 cm to •--1 m wide centered on the felsic dike complex. The system is zoned from quartz-scheelite-molybdenite-pyrite-blue-green beryl-fiuorite-bismuthinite veins in the core to Pb-Zn-Ag veins with abundant arsenopyrite and minor Sn (•--0.1%) in the northeast. The focus of recurrent intrusive and hydrothermal activity was located above a flank, or nose, of the monzogranite stock. The final distribution of WO3 and MoS• grades within the '--750 m (across) X '--300 m (evaluation depth) ore zone is a result of the variable superimposition of the four separate vein systems which had different W/Mo ratios. Hence, well-defined W and Mo ore shells, such as those at Climax, Henderson, and Mt. Emmons, did not develop at Logtung. The upper surface of mineralization, as shown by the 0.03 percent MoS• contour, defines an elongate dome with an apical region approximately coincident with the felsic dike complex. Hydrothermal alteration occurs as well-defined vein selvages and not as pervasive zones. The Logtung deposit shows many similarities to porphyry Mo deposits in the western Cordillera and may be classed as a porphyry W(scheelite)-Mo deposit, with a W/Mo ratio of 3.3:...

Stockwork tungsten (scheelite)-molybdenum mineralization, Lake George, southwestern New Brunswick

“…Scheelite (-molybdenite) stockworks. The recent work of Noble et al (1984) has clarified the nature of the Logtung W-Mo deposit, southern Yukon, and has demonstrated that scheelite-rich stockwork, or porphyry, deposits may constitute significant concentrations of tungsten. This is also apparent from Yan et al's (1980) description of the Yanchuling W-Mo deposit, southern China.…”

“…Features shared by Lake George and Logtung (Noble et al, 1984) include: (1) the association with epizonal stocks of biotite (-hornblende) monzogranite, with A/CNK 2: 1, and with porphyritic felsic dikes; (2) the mixed calcareous-noncalcareous nature of the metapelitic country rocks which host the bulk ofthe stockwork mineralization; (3) the premineralization development of calc-silicate hornfels, in part through bimetasomatic reactions during metamorphism; (4) the absence of pervasive hydrothermal alteration; (5) the occurrence of several stages of W-and/or Mobearing veinlets; (6) the emplacement of porphyry dikes between the first and second stage of mineralization; (7) the association of calc-silicate minerals with the earliest mineralization; (8) the lack of a systematic W vs. Mo zonation relative to the center; and (9) the association with late-stage, base metal sulfide veins, containing, among others, sphalerite, galena, arsenopyrite, and bismuthinite. This stage is apparently better developed at Logtung, unless the major Sb veins at Lake George are representative of such mineralization.…”

“…Ishihara, 1977) that scheeliterich stockwork deposits may be generated by both magnetite-and ilmenite-series granitoids (Ishihara, 1981) and in both moderately and strongly fractionated magmatic systems. In the latter context, it is of interest that Lake George lacks the enrichments in F and Sn shown by the Logtung deposit (Noble et al, 1984).…”

“…The Lake George scheelite-molybdenite mineralization is considered to b e a lower Paleozoic (Upper Silurian) example of a W(-Mo) stockwork and is comparable in many respects to the large Logtung deposit (Noble et al, 1984). We prefer to employ the term "stockwork" rather than "porphyry" to describe this type of deposit because of the coarse-grained, only weakly porphyritic nature of the early monzogranite stocks and the absence of significant pervasive hydrothermal alteration.…”

Metallogenic Evolution of the Mackenzie and Eastern Selwyn Mountains of Canada’s Northern Cordillera, Northwest Territories: A Compilation and Review

“…For this reason the Tungsten Suite is considered the most likely to contain tungsten skarn and associated ore in the study area (Rasmussen et al 2007). The intrusion-related skarn deposits in the MSM area are coeval with other Cretaceous examples in the Cordillera, such as the Scheelite Dome deposit in the Yukon (Mair et al 2006b), and the Northern Dancer/Logtung deposit in southern Yukon and northern British Columbia (Noble et al 1984(Noble et al , 1986. Possible North American correlatives to Cantung and Mactung include tungsten skarn deposits associated with ca.…”