Abstract

Kalzas is a sheeted vein and stockwork wolframite deposit, crosscutting continental margin sedimentary rocks of the Proterozoic Windermere Supergroup in central Yukon. Mineralization is synchronous with Cretaceous post-tectonic granites of the Selwyn Plutonic Suite.Parallel sets of planar quartz veins contain coarse euhedral wolframite and are generally oriented perpendicular to southeast-plunging fold axes. Widespread alteration of the host rocks and intense stockwork veining surround the veins. Alteration and mineralization show a distinct concentric zonation across a 2.5 km long southeast-trending oval. The core zone is characterized by orthoclase contained within quartz–tourmaline–wolframite veins. Minor phases include apatite, molybdenite, bismuthinite, pyrite, chalcopyrite, and pyrrhotite. Host rocks are prevasively tourmalinized and sericitized. Hydrothermal alteration here at the core of the system overprints early biotitization of the host rocks. The biotitization appears to have been the result of earlier contact metamorphism from a hidden pluton situated beneath the deposit that converted the chloritic groundmass of the metasediments to biotite. At the fringe of wolframite mineralization, cassiterite occurs with quartz, muscovite, and tourmaline, whereas orthoclase is distinctly lacking. The outer periphery of the concentrically zoned sequence features pervasive sericitization, disseminated pyrite, and the disappearance of tourmaline. Here, quartz veins are barren except for minor galena.Three main stages mark the mineralogical evolution of the deposit: (1) early biotitization of the chloritic host rocks in the core area; (2) quartz-dominated veining with internally complex timing relations, hydrothermal alteration, and zoning; and (3) carbonate veining and replacement of wolframite by scheelite.Fluid inclusions within quartz are dominated by H2O and contain variable amounts of CO2 as well as minor CH4 and NaCl. The CO2 content ranges from approximately 0 to 6 mol%. The fluids are generally dilute but locally contain as much as 7 wt.% NaCl equivalent. Homogenization temperatures are variable: for quartz, 160–340 °C; for cassiterite, 280–350 °C; and for apatite, 220–360 °C. Minimum hydrothermal pressures at the time of mineralization are estimated to have been in the range 300–600 bar (1 bar = 100 kPa). Fluctuations in pressure with cooling appear to have resulted in CO2 immiscibility and the formation of late-stage carbonates at the termination of hydrothermal activity.

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