Abstract: 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.

Authors: Germann, A., Friedrich, G., and Schattner, R.

Citation: Germann, A., Friedrich, G., and Schattner, R., 1992. Ore mineralogy and formation conditions of vein and replacement-type Pb-Zn-Ag occurrences, Logan and YP properties, Rancheria District, Yukon, Canada. In: Yukon Geology Volume 3, T.J. Bremner (ed.), Exploration & Geological Services Division, Indian & Northern Affairs Canada, p. 37-44.

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